摘要:

J . Chem. SOC., Faraduy Trans. I , 1982, 78, 2101-2109 Infrared Study of the Adsorption of Aromatic Molecules onto Silica and Chlorinated Silica Application of the Charge-transfer Theory to the Data BY WALTER POHLE Academy of Sciences of the G.D.R., Central Institute of Microbiology and Experimental Therapy, Department of Biophysical Chemistry, DDR-6900 Jena, Beutenbergstrasse 11, G.D.R. Received 23rd June, 198 1 The charge-transfer theory of Puranik and Kumar for hydrogen bonding is applicable to the hydrogen bonds formed between a variety of aromatic adsorbates and the silanol groups of both silica and partially chlorinated silica, with the predictable restrictions. From the relative positions of the respective data points in the diagrams correlating either the ionization potentials or the Hammett substituent constants with the infrared spectroscopic data, it can be concluded that, besides benzene and the methylbenzenes, styrene and most halogenobenzenes also interact cia their aromatic n-systems, whereas the fluorobenzenes form H bridges via the fluorine atoms.If there are three or more methyl groups in the benzene derivatives, dual site adsorption of those molecules (mesitylene, hexamethylbenzene) must be taken into consideration. The most important molecular mechanism in the interaction involved in the adsorption of basic compounds onto silica is the hydrogen bonding between surface Si-OH groups and the electron-donor centres of the ads0rbates.l One of the features of hydrogen-bond formation is the shift in wavenumber of the stretching vibration band of the proton-donating group.The displacement of the band of isolated silanols at Po, = 3748 cm-l during adsorption, AC0,, has been tested for possible correlation with several molecular parameters of the adsorbates in order to characterize the nature of such hydrogen bonds. Of all the properties and theories taken into consideration, e.g. polarizabilities,2 dipole moment^,^ Kirkwood-Bauer-Magat functions4 and 0 parameters of the Hammett and Taft equation^,^-^ only the substituent constants could be correlated with AV",,. This led to the successful application of the charge-transfer (c.t.) model to the hydrogen bonding of adsorption systems by Cusumano and LOW,^ van Cauwelaert et aL7. and later by Sempels and Rouxhet,6 who obtained straight lines for plots of the inverse square root of the relative wavenumber shift, the so-called Mulliken-Puranik-Kumar (MPK) parameter, against the ionization potentials of the adsorbed molecules.According to the c.t. theory, these correlations are valid only for various classes of adsorbates with similar donor orbitals.4$ This paper has two aims: (i) Does the c.t. model also fit the corresponding infrared (i.r.) data from the adsorption of aromatic compounds on partially chlorinated SiO,? Owing to the influence of neighbouring C1 atoms, the acidity of the remaining hydroxyl groups is substantially enhan~ed,~ whilst simultaneously some repulsion may occur between surface chloride and adsorbed molecules.1o (ii) If more than one electron-donor centre is present in an adsorbate molecule (e.g.n-system and a heteroatom), use of the c.t. theory may help to clarify which of them is actually involved in the H bond with the silanol groups. 21012102 ADSORPTION OF AROMATIC MOLECULES ON SILICA EXPERIMENTAL Aerosil380 (Degussa, Frankfurt-am-Main) was used as the adsorbent. Self-supporting discs (weight: 15-20 mg cm-2) were treated in air and in oxygen at 500 "C for 1 h and then degassed at 700 OC for 3 h in situ (ca. Pa). The chlorination of the silica surface was carried out by two different methods as described in a previous paper:" (i) Reaction of dehydrated Aerosil with HCl at 600 O C ; the substitution of hydroxyls by C1 atoms can be controlled by the reaction time. (ii) Reaction of dehydrated Aerosil with SiCl, at 400 O C , leading towards the complete removal of the silanol groups, followed by partial hydrolysis of the resulting surface complexes1o with small amounts of water vapour.In both cases a modified surface containing both OH groups and C1 atoms is obtained. The aromatic compounds (cf. table 1) were at least p.a. grade and were used without further purification. Other experimental details concerning the infrared equipment and the procedure for the adsorption process are described elsewhere.I2 RESULTS The wavenumber shifts, AFOH, due to the adsorption of various aromatic adsorbates onto silica and chlorinated silica are given in table 1, together with the MPK parameters calculated from the shifts and the related ionization potentials. In most TABLE 1 .-WAVENUMBER SHIFTS AFOH (FOH = 3748 cm-l) AT 8,, = 0.3 AND CALCULATED MPK (ccl = 2 CI/OH) TOGETHER WITH THE RELATED IONIZATION POTENTIALS AND THE HAMMETT PARAMETERS FOR THE ADSORPTION OF AROMATIC MOLECULES ON SILICA AND CHLORINATED SILICA SUBSTITUENT CONSTANTS 6, noa adsorbate 1 benzene 2 toluene 3 rn-xylene 4 mesitylene 5 hexa- methyl- benzene benzene fluoro- benzene 6 fluoro- 7 hexa- SiO, SO2-C1 119 5.61 124 5.50 9.25 13, 14 0 136 5.25 144 5.10 8.82 13, 14 -0.17 153 4.95 157 4.89 8.56 13, 14 -0.34 171 4.68 178 4.60 8.39 13, 15 -0.51 197 4.36 214 4.19 7.85 15 - 1.02 63 7.71 66 7.54 9.19 13, 15 f0.062 25 12.24 27 11.80 9.97 15 + 0.37 a See fig.3-5. systems, APoH shows a slight linear increase with do, (ix. that part of the isolated silanol groups which is engaged by adsorbate molecules16), in accordance with the findings of other authors.2' 1 7 9 l8 Therefore, all the wavenumber shifts listed in table 1 are adjusted to OOH = 0.3, which is obtained by interpolation from the straight lines in the AGoH against O,, plots.Furthermore, for chlorinated silica, A?,, also depends on the relative chloride content of the su~face,~ which in our experiments was consistently 2 surface C1 atoms per remaining OH group. As the data in table 1 show, the wavenumber shift for a given aromatic compound adsorbed on chlorinated SiO,W. POHLE 2103 exceeds, in each case, the related figure for pure silica, thus indicating a higher acidity for the hydroxyl groups on the surface of chlorinated silica. This was ascribed to the influence of the electron-withdrawing power of chlorine owing to its high electronegativi t ~ .~ Turning to the wavenumber shifts, AV",,, for the adsorption of benzene and its methyl derivatives on silica, our results are in good agreement with those of other authors.lG. 18-21 Table 2 gives a survey of ionization potential correlative data derived from the literature for halogeno- and oxy-aromatics. TABLE 2.-LITERATURE VALUES FOR WAVENUMBER SHIFTS AND MPK PARAMETERS FOR AROMATIC MOLECULES ADSORBED ON SILICA AND POROUS GLASS AT e = 0.5 no.a adsorbate b / e V ref. 6, 6 fluorobenzene 8 a,a,a-trifluoro- 9 chlorobenzene toluene 10 bromobenzene 11 iodobenzene 12 styrene 13 acetophenone 14 benzophenone 59b 87 33b 100 102 110 114 130 320 340 7.97 6.68 10.66 6.09 5.84 5.73 5.36 3.42 3.32 4 20 4 22 20 20 20 23 21,24 26 9.19 9.68 9.07 8.97 8.73 8.42 9.65 9.45 13, 15 +0.062 13, 14 f0.54 13 + 0.227 13-15 +0.252 13, 15 +0.18 14 25 25 - - - a See fig.3-5. 3720 Porous glass, all the rest silica. / / 3720 I I I 1 I 3000 3400 3800 3400 3 800 wavenumber/cm -' wavenumberlcm-' FIG. 1 .-Infrared spectra of the adsorption of mesitylene (a) and hexamethylbenzene (6) onto partially chlorinated silica in the vOH region. Solid line: spectrum before adsorption; dotted line: spectrum after dosage of the adsorbate; O,, was cu. 0.3 in both cases. In the band of two new i.r. spectra of the adsorbed highly methylated aromatics, the 3748 cm-l vOH isolated silanol groups is accompanied not only by one (as is usual) but by bands, both attributed to hydroxyls engaged in H bonds; fig.1 shows two examples. Consistently, one of these displaced OH bands is situated at 3720 cm-l, i.e. the wavenumber shift is AqOH = 28 cm-l; this is not included in table 1. The spectrum of chlorinated SiO, reveals a band near 620cm-l which may be assigned to Si-C1 stretching vibrations.1° As illustrated by fig. 2, neither the wavenumber nor the absorbance of this band is influenced by the adsorption of benzene.2104 ADSORPTION OF AROMATIC MOLECULES ON SILICA L I I I 500 600 700 wavenumber/cm -' FIG. 2.-1.r. spectrum of benzene adsorption on Si0,-Cl in the low-frequency region. The relative weight of the discs was reduced to 4-5 mg cmP2 in order that the SiO, background is diminished and observation of spectral changes in the range of 550-750 cm-I is possible.1 : SO,-Cl; 2: Si0,-Cl after small benzene dosage; 3 : after addition of an excessive amount of benzene (without compensation for the vapour spectrum). DISCUSSION The charge-transfer theory developed primarily by M ulliken for benzene-iodine complexes27 was applied to hydrogen bonding by Puranik and Kumar,28 who obtained the final expression : which correlates the ionization potential of the electron-donor molecule (In) to the MPK parameter (inverse square root of the relative wavenumber shift) of the X-H stretching vibration band by direct proportionality. The meanings of the other terms are as follows: c is a constant, S is the overlap integral between the donor orbital and the negative ion of the acceptor group, A is a measure of the polarity of the acceptor (X-H) bond, E, is the electron affinity of the acceptor molecule and W is the net attraction energy between X- and Y+ in the X--Ha - .Y bond and is essentially a coulombic term.2s Although the c.t.theory applied to hydrogen bonding was not only supported by several fundamental papers,29. 30 but also subject to certain c r i t i c i ~ m s , ~ ~ ~ 32 it has been shown to be compatible with the i.r. spectroscopic data arising from hydrogen bonds in both 34 and heterogeneous (i.e. adsorption) systems,4* 6-8 with the restrictions mentioned in the introduction. Application of the c.t. model to the results of the present paper will demonstrate once more the usefulness of this theory. APPLICATION OF THE CHARGE-TRANSFER THEORY TO THE INFRARED DATA CONCERNING AROMATIC MOLECULES ADSORBED ON SILICA AND CHLORINATED S I L I C A Fig.3 shows that the experimental data for the adsorption of aromatic molecules give linear plots for both the silica and chlorinated silica surfaces. This is well known for the interaction of benzene and its methyl derivatives with the hydroxyls of SiO, and porous g l a s ~ , ~ ? ~ ~ ~ but novel for the adsorption of aromatics on Si0,-Cl. Both curves are approximately parallel, there being no significant differences between the respective slopes and intercepts, i.e. c.t. theory is too insensitive to reflect quantitatively the change of polarity of the silanol groups caused by the influence of adjacent electron-withdrawing C1 atoms.W. POHLE 2105 t ' O t I 1 I 1 1 I 1 1 I I ) I 6 8 10 12 1 FIG. 3.-Application of charge-transfer theory to the wavenumber shifts of the silanol groups of SiO, (a) and Si0,-Cl (A) caused by the adsorption of aromatic molecules at O,, = 0.3. The numbers of the data points correspond to those given in tables 1 and 2; 0 refer to literature data.With respect to the fluoroaromatics, a linear plet is established for the first time for three adsorbates of this species by the additional investigation of hexafluorobenzene; in the paper by Cusumano and Lowg the plot for the fluorobenzenes was supported by a value due to cyclohexane only. The position of the data for the Si0,- Cl/fluoroarornatics system suggests, as expected, that this plot (dotted line) is parallel to that for the unmodified silica. In conclusion, we can state that the c.t. theory is also applicable to hydrogen bonds between aromatic molecules and the hydroxyls of SO,-Cl in spite of the higher polarity of the OH groups and the fact that some repulsion may occur between the aromatic adsorbates and the surface chloride, as concluded from the dramatic decrease of the monolayer capacity for benzene adsorption going from SiO, to SiO,-Cl.lo This is supported by the results in fig. 2, which shows that the i.r.band of surface Si-Cl groups is changed in neither wavenumber nor intensity during benzene adsorption, indicating the non-existence of an interaction -Cl- -aromatic. T H E NATURE OF T H E ELECTRON-DONOR CENTRES OF T H E ADSORBATES Whereas for benzene and its methyl derivatives the formation of 0-H * ' n bridges can be established in accord with related papers,1.4T 6~ 17-19 this question is not so trivial for ' bifunctional ' adsorbates which, besides the n-system, have certain heteroatoms which may also act as electron-donor centres.If these heteroatoms are N or 0 (as in or a n i ~ o l e ~ ~ ) , dual-site adsorption is observed with a preferred binding via the heteroatoms. Regarding, however, the halogen-substituted benzenes, a decision about the electron-donor centre in the H bond n-electrons or non-bonded electrons of the halogens is more problematical because the wavenumber shifts observed for the interaction with surface OH groups are of the same order of magnitude for both methylbenzenes and halogenated alkyl corn pound^.^^^ In the literature, the role of an active centre is ascribed to the n-system in both fluoroaromatics4t338 and2106 ADSORPTION OF AROMATIC MOLECULES ON SILICA bromoaromatics.6 Our results demand, however, an alternative interpretation : the fluoroaromatics take an exceptional position in so far as in these species the heteroatom is the primary electron-donor site, whereas in the other halogenoaromatics this role is exerted by the n-ring.Inspection of fig. 4 may help to rationalize this assignment. P NH3 ds 1 I I I 1 I I I 1 I 1 I * 2 L 6 8 10 12 FIG. 4.-Survey of the relative positions of the MPK plots due to several classes of adsorbates. A, CH,-aromatics; B, halogenoaromatics; C , fluoroaromatics; D, ketoaromatics; E, amines; F, chloromethanes. From the magnitude of their slopes, the straight lines can be subdivided qualitatively into three classes containing, in each case, two species of adsorbates.These classes consist of the lines D and E, A and B, and C and F, respectively (in order of decreasing slope). Fortunately, one representative of each type is a priori determined concerning the nature of its electron-donor centre included in the H bond; in detail these sites are: N in the amines (E), the n-systems in the methylbenzenes (A) and the halogen in the chloromethanes (F). From this, the electron-donor sites for the remaining adsorbate classes can be determined from conclusions by analogy (similar slope means that because of the constancy of c and ;1 in the above equation, the overlap integral S, stipulated by similar electron-donor orbitals, is also similar) as follows: 0 atoms in the ketoaromatics (as expected, cf anisole), fluorine atoms in the fluoroaromatic (C) and the aromatic n-system for the other halogenoaromatics (B).With respect to bromobenzene, this interpretation is in agreement with the results of Sempels and Rouxhet,6 but our findings for fluoroaromatics are in contrast to findings in the literature.4* 38 Putting aside the arguments of Low and coworker^,^^ 38 there is another item contradicting the conception of 0-H - - . n bridges existing in fluoroaromatics: if such bridges exist, line C would have, from a logical point of view, to intersect line A (methylbenzenes) at the value of benzene, because this adsorbate would be equivalent in both families. Obviously, this is not the case. The small difference in the respective positions of lines A and B may be caused by the fact that both classes of adsorbates have substituents with opposing inductiveW. POHLE 2107 effects (negative for halogens and the vinyl group, positive for methyl groups).Furthermore, note that methylchloride and chlorobenzene, in spite of giving rise to very similar wavenumber shifts due to the interaction with silanol groups (namely 10637 and 100-102 cm-1,22, 2o respectively), belong to different species when taking the geometry of their hydrogen bonds into account (inclusion of C1 atoms or not). This example may illustrate the usefulness of the application of the c.t. theory to the determination of the electron-donor site in special cases, as this would hardly be possible for chlorobenzene without such correlations.From the proposed modes of interaction, the expectation is that all these systems, which include aromatic adsorbates forming H bonds via their n-systems, should be correlated if their wavenumber shifts, AFOH, are plotted against the Hammett o parameters (see tables 1 and 2), due to the pertinent substituents of the phenyl ring.4 Correlation with the data points corresponding to those systems which involve fluoroaromatics, i.e. which do not interact uia n-electrons, should occur only by chance. This expectation is, in fact, realized, as fig. 5 demonstrates. The occurrence of one straight line for the data from both methylbenzenes and halogenoaromatics and the significant deviation of the data points for the fluoroaromatics can be taken as proof of the interpretation given above, i.e.whether the n-system of heteroatom- containing adsorbates is included in the hydrogen bonding or not. 7 8 o o I n I 1 I -1 -0.5 0 0.5 a FIG. 5.-Correlation of wavenumber shifts, AGoH, arising from the interaction between hydroxyl groups of silica and several aromatic adsorbates with the related Hammett polar substituent parameters [d,, the values shown in tables 1 and 2 were taken from ref. (4)-(6)]. The numbers of the data points correspond to those given in tables 1 and 2. A is for anisole; the wavenumber shift, AijoH, for anisole due to the interaction via the n-system (anisole is bound by dual-site adsorption) is 150 ~ r n - ' . ~ ~ D U A L-SI TE A DSO R P TI ON Several aromatic adsorbates showing dual-site adsorption onto silica or porous glass have been already mentioned (anisole, aniline etc.).In these cases, two i.r. bands, which are to be attributed to the perturbed (i.e. involved in H-bonding interactions) hydroxyl groups, always emerge in the vOH region of the spectra owing to the fact that these molecules are bound twice, via the n-systems and, even more strongly, via suitable heteroatoms, such as 0 and N. Two displaced OH bands can also be observed in the infrared spectra of highly methylated benzenes adsorbed onto SiO, and SiO,-Cl (see fig. 1). This is true for hexamethylbenzene adsorption on both surfaces [fig. 1 (6): Si0,-Cl] and for mesitylene adsorption on highly chlorinated silica [Aerosil modified by the SiCI, method, cf the Experimental section; fig. 1 (a)]. The rather low value of the wavenumber shift (28 cm-l) indicates a very weak interaction. The most2108 ADSORPTION OF AROMATIC MOLECULES ON S I L I C A probable interpretation is that, in addition to the 0-H- n bridge represented by the more displaced vOH bands, a second interaction occurs between surface hydroxyls and adsorbate methyl groups.CONCLUSIONS The charge-transfer theory of hydrogen bonding has been shown to be consistent with data arising from the adsorption of a variety of simple aromatic molecules not only onto silica (as previously known), but also onto partially chlorinated silica. I H I 0 I ,Si \ I ‘ ( a ) 0 I I /Si\ I I FIG. 6.- of silica -Schematic representation of different possible modes of interaction between the hydroxyl groups and several aromatic adsorbates.X=H. CH,, CH=CH,, C1, Br, I ; Y=F (also in side chains), 0- and N-containing substituents. In the scheme given in fig. 6, part (I) involves those adsorbates with the aromatic n-system as the most prominent centre of ‘transferable’ electron density. Part (11), on the other hand, shows that n-electrons are overwhelmed if the molecules have substituents containing the heteroatoms 0, N or F. Depending on the surface OH concentration, single-site (a) or dual-site (b) adsorption is then possible. For methyl- and halogeno-benzenes, this assignment is strongly supported by finding a correlation for molecules belonging to part (I) of fig. 6 in the plot of AfOH against (T (from the Hammett equation) (cf. fig. 5). Drs P. Fink and H. Fritzsche are thanked for helpful discussions and critical reading of the manuscript.H. Knozinger, Hydrogen Bonding in Systems of Adsorbed Molecules, in The Hydrogen Bond (North Holland, Amsterdam, 1976). R. S. McDonald, J. Am. Chem. Soc., 1957, 79, 850. A. V. Kiselev, Zh. Fiz. Khim., 1964, 38, 2764. J. M. Cusumano and M. J. D. Low, J . Catal., 1971, 23, 214. H. Knozinger, Surf Sci., 1974, 41, 339. R. E. Sempels and P. G. Rouxhet, Bull. SOC. Chim. Belg., 1975, 84, 361. F. H. van Cauwelaert, F. Vermoortele and J. B. Uytterhoeven, Discuss. Faraday Soc., 1971, 52, 66. P. Fink and W. Pohle, Z . Chem., 1976, 16, 32. lo W. Pohle, unpublished results. l 1 W. Pohle and P. Fink, Z . Chem., 1972, 12, 394. l 2 W. Pohle and P. Fink, 2. Phys. Chem. N.F., 1978, 109, 77. l 3 K. Watanabe, J .Chem. Phys., 1960, 26, 542. l4 D. W. Turner, in Advances in Physical Organic Chemistry, vol. 4, Ionization Potentials, ed. V. Gold (Academic Press, London, 1966), pp. 3 1-69. ’ F. H. van Cauwelaert, J. B. van Asche and J. B. Uytterhoeven, J. Phys. Chem., 1970, 74, 4329.W. POHLE 2109 l5 R. Bralsford, P. V. Harris and W. C. Price, Proc. R. SOC. London. Ser. A , 1960, 258, 459. l 6 W. Hertl and M. L. Hair, J . Phys. Chem., 1968, 72, 4676. M. R. Basila, J. Chem. Phys., 1961, 35, 1151. l 8 G . A. Galkin, A. V. Kiselev and V. I. Lygin, Trans. Faraday Soc., 1964, 60, 431. l9 A. Zecchina, C . Versino. A. Appiano and G. Occhiena, J . Phys. Chem., 1968, 72, 1471. Lo P. G. Rouxhet and R. E. Sempels, J . Chem. Soc., Faraday Trans. I, 1974, 70, 2021. 21 P. A. Elkington and G. Curthoys, J . Phys. Cheni., 1968, 72, 3475. 2 p G. A. Galkin, A. V. Kiselev and V. I. Lygin, Zh. Fiz. Khim., 1967, 41, 40. '':{ W. Kuhn, Thesis (Jena, 1973). A. Kohler, unpublished results. 25 F. J. Vilesov, Dokl. Akad Nauk SSSR, 1960, 132, 632. 2fi A. Kohler, Thesis (Jena, 1973). 27 R. S. Mulliken. J . Am. Chem. SOC., 1952, 74, 81 1 . P. G. Puranik and V. Kumar, Proc. Indian Acad. Sci., Sect. A , 1963, 58, 29. 2g P. Schuster, Theor. Chim. Acta, 1970, 19, 212. 3 o H. Ratajczak and W. J. Orville-Thomas, J . Chem. Phys., 1973, 58, 911. u L E. Clementi and J. N. Gayles, J . Chem. Phys., 1967, 47, 3837. 32 P. A. Kollman and L. C. Allen, Chem. Rer., 1972, 72, 283. 33 M. R. Basila, E. L. Saier and L. R. Cousins, J . Am. Chem. SOC., 1965, 87, 1665. 34 K. Szepaniak and A. Tramer, J. Phys. Chem., 1967, 71, 3035. 35 M. J. D. Low and V. V. Subba Rao, Can. J . Chem., 1969, 47, 1281. :16 M. J. D. Low and J. A. Cusumano, Can. J . Chem., 1969, 47, 3906. :j7 L. H. Little, Infrared Spectra of Adsorbed Species (Academic Press, New York, 1966). B . K. Sahay and M. J. D. Low, J . Colloid Interface Sci., 1974, 48, 20. (PAPER 111016)

ISSN:0300-9599    

DOI:10.1039/F19827802101

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faruduy Trans. I , 1982, 78, 21 11-21 19 Infrared Study of Hydrogen Adsorption on MgO, CaO and SrO Possible Mechanism in Promoting 0; Formation BY SALVATORE COLUCCIA,* FLORA BOCCUZZI, GIOVANNA GHIOTTI A N D CLAUDIO MORTERRA Istituto di Chimica Fisica dell’universita di Torino, Corso Massimo D’Azeglio 48, 10125 Torino, Italy Received 23rd July, 1981 Hydrogen adsorption on high-surface-area alkaline-earth oxides has been studied by i.r. spectroscopy at 300 and 78 K. In the former case heterolytic dissociation of H, molecules produces hydride and hydroxy groups which are partially reversible upon evacuation. Low-temperature hydrogen adsorption appears to be undissociative. The active sites are cation-anion couples in low coordination on the surface. On the basis of i.r.evidence a mechanism is proposed for the interaction of oxygen with preadsorbed hydrogen envisaging an electron-transfer process from H- ions to oxygen molecules to give 0; paramagnetic species. Highly dispersed alkaline-earth oxide powders show widespread surface activity. In particular it has long been known that unirradiated MgO exhibits remarkable catalytic activity in promoting the isotopic hydrogen-deuterium equilibration at temperatures as low as 78 K.l This topic has been widely investigated and paramagnetic centres, formed in extremely low concentration during the pretreatment of the sample, were shown to be the catalytic sites., Recently hydrogen preadsorption has been demonstrated to promote the subsequent production of 0; radicals upon oxygen adsorption on Mg03 and Ca0.4 In these cases non-stoichiometric excess oxygen, forming 0- pairs or larger clusters, has been suggested to be responsible for hydrogen adsorption through homolytic dissociation of H, m01ecules.~$~ The possibility that ‘intrinsic’ sites such as metal cations and 0,- anions present at the surface in low coordination (e.g.M2L+C and OL2) could be active for hydrogen uptake on these solids has not been taken into account. A reason may be the absence, up to now, of direct evidence of the nature of the adsorbed hydrogen species. However, preliminary measurements have shown that new bands, although weak owing to the low concentration of surface species,3 appear in the i.r. spectra when hydrogen is adsorbed on Mg0.5 Such data, together with photoluminescence studies,6 suggest that heterolytic dissociation of H, molecules is likely to occur over MgiA>-O‘& surface couples.6 This i.r.study is now extended to confirm the above suggestion and to check if a similar mechanism also takes place in the case of other alkaline-earth oxides. Moreover, a detailed analysis of the hydrogen adsorption could be of some help in the interpretation of more complex reactions, such as the dimerization of pyridine,’? the production of organicg and inorganic3? * radicals,1° and exchange and related reacti0ns.l 211121 12 HYDROGEN ADSORPTION O N ALKALINE-EARTH OXIDES EXPERIMENTAL MgO, CaO and SrO were obtained by thermal decomposition in U ~ C U O ( lop4 N m-2) of either the parent hydroxides or carbonates which had been compressed into pellets of good mechanical resistance and placed in the i.r.cell.12 The oxides were then outgassed up to 1 123 K. The surface areas were 200, 70 and 8 m2 g-l, respectively, for MgO, CaO and SrO. High-purity gases were admitted cia a trap at 78 K without further purification. The i.r. spectra in the 4000-600 cm-l region were obtained with a Beckmann IR 12 spectrophotometer using reference-beam screening to obtain a suitable 100% base-line in ail spectral ranges. The relevant spectra were repeated with a Perkin Elmer 580 spectrophotometer equipped with an i.r. data station, whose computer facilities allowed precise background subtraction and accumulation of spectra. Identical results were obtained in the two cases. The bands being weak, great care was taken in positioning the cell, which was not moved over each adsorpti on-desorp tion cycle. B.E.T.surface areas and hydrogen adsorption on MgO were measured by a small-dead-space conventional volumetric apparatus. RESULTS The bands shown below are weak, and this could well be the reason why they have escaped detection so far. The possibility that the changes observed in the i.r. spectra upon contact with hydrogen were just variations of the intrinsic spectra of the oxides has been checked by allowing onto the samples a number of different gases (He, Ar, N2), and no effect was detected. This definitely demonstrated that the bands appearing upon hydrogen absorption are associated with adsorbed species, as confirmed by the isotopic shift described in the following.HYDROGEN ADSORPTION ON MgO Fig. 1 ( a ) gives the spectrum of the MgO pellet in the 1400-800 cm-1 range. The high-frequency region is not shown in the figure for the sake of brevity. No remaining OH groups are detected in the i.r. spectrum after the standard thermal pretreatment. When the spectrum is run in the presence of 26.6 kN m-2 hydrogen after 20 min contact at beam temperature (b.t.), two broad and complex absorptions are observed in the 1350-800 [curve (b)] and 3800-3000 cm-l ranges. Their intensities increase with time, reaching a maximum within 16 h [curve (c)]. Most components of each absorption are broad, but sharper bands are observed at 1326 and 3750 cm-l. This is more clearly seen in the spectra given in absorbance units obtained by background subtraction from spectrum (c), as shown for the low-frequency region in the inset of As shown elsewhere,j when the pressure of H, is progressively reduced, the overall intensity of the spectrum in the 1350-800 and 3800-3000 cm-l regions decreases, and this effect is enhanced by pumping off at b.t.Most of the variation occurs within 1 min outgassing. The 1326 cm-l band completely disappears, and the better defined, although still broad, components, such as that at 1130 cm-l, tend to decrease preferentially, leaving a much broader band. 1.r. spectra run in the presence of H, at 78 K show that the intensities of the bands already observed at b.t. do not change significantly and that no new bands appear. Hydrogen uptake on MgO is at the limit of sensitivity of the volumetric apparatus and this prevented systematic experiments from being carried out.However, the amount of hydrogen adsorbed at 298 K was found to be ca. molecules nm-2. This is of the same order of magnitude as the data of Cordischi et al.,3 but probably higher than those of Tanaka et al.13 At 78 K the uptake is more than one order of fig. I .S. COLUCCIA, F. BOCCUZZI, G. GHIOTTI AND C. MORTERRA _? ~ ~- __- - 00 ._T ~- -~ I I *.\\ I ! ! 13 1300 1200 1100 1000 900 800 wavenumber/cm-' 21 13 I ---, , -__i_-L 1300 1200 1100 1000 900 wavenumber/cm-' FIG. 1.-1.r. spectra of hydrogen, adsorbed at b.t. on MgO. ( a ) MgO outgassed at 1123 K (-); (h) in the presence of 26.6 kN m-' H, after contact for 20 min (---); (c) as (b) after 16 h ( . - . -).Inset: spectrum (c) in absorbance units, having subtracted the background (a). I I I I I I 1200 1100 1000 900 800 700 2 5 . 1200 1100 1000 900 800 wavenumber/cm-' FIG. 2 . 4 . r . spectra of hydrogen adsorbed at b.t. on CaO. (a) CaO outgassed at 1123 K (-); (b) in the presence of 26.6 kN rn-, H, after contact for 16 h (---); (c) after outgassing 1 min at b.t. ( . -. -). Inset: spectra (6) and (c) in absorbance units, having subtracted the background (a).21 14 HYDROGEN ADSORPTION O N ALKALINE-EARTH OXIDES magnitude larger (ca. 0.3 and ca. 0.4 molecules nmP2 at 2.66 and 13.3 kN mP2, respectively). HYDROGEN ADSORPTION ON CaO AND SrO The effects observed in the spectra of calcium and strontium oxides parallel those described for MgO. After hydrogen adsorption on CaO two broad and complex absorptions are observed at 1200-700 [fig.2(b)] and 3000-3800 cm-l. The intensity increases with pressure (up to 26.6 kN mP2) and time of contact (up to 16 h) and decreases by outgassing [fig. 2(c)]. A sharp component is observed at 3700 cm-'. In the case of SrO the spectrum above 2000 cm-l could not be explored because of high light scattering. Upon hydrogen adsorption [fig. 3(b)J a broad and complex 1oc is 5c K 0 10 1000 900 800 700 wavenumber/cm-' 1000 900 I 800 700 wavenumber/cm-' FIG. 3.-1.r. spectra of hydrogen adsorbed at b.t. on SrO. (a) SrO outgassed at 1123 K ; (b) in the presence of 26.6 kN m-* H, after contact for 16 h; (c) after outgassing 1 min at b.t. Inset: spectra (b) and (c) in absorbance units, having subtracted the background (a).absorption appears at lower frequencies (1000-600 cm-l) than those found with MgO and CaO. The absorbance spectra (fig. 3, inset) show that by outgassing the narrower components are depleted preferentially, this tendency being common to the three oxides.s. COLUCCIA, F. BOCCUZZI, G. GHIOTTI AND c. MORTERRA 21 15 DEUTERIUM ADSORPTION ON MgO, CaO AND SrO When 2H2 is allowed onto the three oxides absorption bands appear in the i.r. spectra, which are similar in shape to those observed upon hydrogen uptake, but shifted to lower frequencies. The observed shifts agree well with the expected values and the sharp band observed at 1326 cm-l in the MgO spectrum upon hydrogen uptake shifts to 950 cm-l when deuterium is adsorbed. INTERACTION OF PREADSORBED HYDROGEN WITH OXYGEN If oxygen is allowed onto a CaO sample which has previously been contacted with hydrogen and then outgassed for 1 min [fig.4(b)] the intensity of the low-frequency 1 1 I I wavenumber/cin-' FIG. 4.--Oxygen interaction with preadsorbed hydrogen on CaO. (a) CaO outgassed at 1123 K; (b) after contact with 26.6 kN m-2 H, and subsequent outgassing at b.t.; ( c ) after allowing 1.33 kN m-2 0,. hydrogen absorption strongly decreases, whereas the absorption above 3000 cm-l is not affected. Similar behaviour is observed in the case of SrO. Parallel experiments with MgO have shown very small, if any, effects of oxygen on the i.r. spectrum of preadsorbed hydrogen. DISCUSSION On the basis of the i.r. spectra at beam temperature two types of adsorbed species can be recognized: one is reversible at b.t.and is related to the bands whose intensity depends on the pressure of gas; the other is irreversible and is related to the bands still present after outgassing at b.t. The latter is desorbed at high temperature. As both species absorb in the same spectral regions, their nature and structure must be similar, their stability only being different. Therefore we shall discuss them together.21 16 HYDROGEN ADSORPTION O N ALKALINE-EARTH OXIDES NATURE OF THE ADSORBED SPECIES The i.r. spectroscopic features associated with hydrogen species on alkaline-earth oxides can be summarised as follows: (a) a broad and complex absorption band at high frequencies (above 3000 cm-l), (6) a broad and complex absorption band in the low-frequency region, observed at 1350-800 cm-l in the spectrum of MgO and at progressively lower frequencies in the spectra of CaO and SrO.As already suggested for the interaction of H, with Mg0,5 absorption (a) can be assigned to stretching modes of surface hydroxy groups and absorption (b) to surface hydride species. The former assignment is straightfor~ard,~~~ l5 whereas the latter requires some comments as, after the original observation by Eischens et aZ.16 on ZnO, the formation of surface hydride groups has not been detected on other highly dispersed oxides. Comparison of our results with spectroscopic data available for molecular complexes support the proposed assignment. Stretching modes of hydride groups in metal complexes are observed at 1900+ 300 cm-' when H is in a terminal position and at 1 loo+ 300 cm-l when H is in a bridged position.17 Similar correlations have been determined for hydride groups on ZnO, a sharp band at 1708cm-l being ascribed to Zn-H and a broad one extending down to 1200 cm-l to Zn-H-Zn surface species, respectively.ls The assignment of bands of type (h) to hydride species is further supported by their shifts to lower energies in the series MgO, CaO, SrO.In fact this is expected for stretching modes of hydride groups, whose frequency moves to lower values as the bond polarity increase~.'~ The enhanced basicity of the matrix in the order MgO, CaO, Sr020 as a function of the cationic radii is bound to bring about increasing polar character of the M-H bond". Indeed, it has been determined that, among alkali metal hydrides, CaH, and SrH, are definitely ionic, whereas some contribution of covalency is present in the case of MgH,.,O From the fact that the stretching-mode frequencies of hydride groups on MgO, CaO and SrO are below 1400 cm-l, hydrogen could be inferred to be shared by two or more surface cations.However, because of the strongly ionic character of the matrix, the M-H bond on alkaline-earth oxides must be much more polar than the analogous one in molecular metal hydrides17 and that of hydride groups on the surface of ZnO.ls Therefore, even a non-bridged surface hydride on the surface of MgO, CaO, SrO is bound to absorb at lower energies19 as compared with the terminally bonded homogeneous complexes and surface Zn-H, which have a high degree of The above consideration, together with the fact that the bands associated with terminal hydride groups are generally sharper than those associated with bridge structure,17 helps in interpreting the complex features of absorption bands of type (b) in the spectra of the three alkaline-earth oxides.Moreover, it has been shown that outgassing at beam temperatures tends to deplete preferentially the sharper components, so that, for all the three oxides, the spectrum run in the presence of H, can be considered as the superimposition of two spectra whose features can be summarised as follows: (1) in the spectrum still present after b.t. outgassing broad components are predominant which must be associated with bridge structures M-H-M ; (2) in the other spectrum, strongly pressure-dependent, sharper compo- nents are present which are probably associated with ' terminal' surface hydrides Me-H.Although many parameters, e.g. kinetic, can play a role in determining the reversibility of the surface species, the different stabilities of the two species seem to support the above assignment, since in the former species, irreversible at b.t., the l8S. COLUCCIA, F. BOCCUZZI, G. GHIOTTI AND C. MORTERRA 21 17 hydrogen anion is stabilised by the electrical field of two or more surface cations, whereas in the latter, reversible at b.t., only one cation is involved. Two stretching modes (symmetric and asymmetric) should be observed for each bridge structure,21 but the weakness of the bands and their heavy overlapping prevents more thorough correlations.However, as the relative intensities of several components vary from sample to sample of the same oxide, the presence of families of slightly different surface hydride species can be inferred. Analogous conclusions can be drawn from a detailed analysis of the high-frequency absorption band A which we have already assigned to stretching modes of the hydroxy groups. On the basis of previous i.r. studies of hydrated Mg02, and Ca023 powders, the sharp bands in the 3700-3750 cm-l region are assignable to ‘free’ OH groups and the broad absorption extending down to 3000 cm-l to hydroxy groups whose H atom is engaged in hydrogen bonding with other anions on the surface. Finally hydrogen adsorption at low temperature (78 K) must be discussed.It is necessary in this case to make use of the following information from different techniques. (1) Photoluminescence spectroscopy has shown that interaction with hydrogen at low temperature enhances quenching of the intrinsic emission of alkaline-earth oxides as compared to the effect produced by hydrogen adsorption at room temperature.6 (2) Gas-volumetric data quoted in the results show a larger uptake of hydrogen at 78 than at 300 K . (3) Neither a significant intensity increase of the bands observed at room temperature nor new bands are shown in the i.r. spectra upon hydrogen adsorption at 78 K. From this the conclusion must be drawn that the extra amount of hydrogen present on the surface at 78 K is adsorbed in a molecular form not detectable in the infrared.ADSORPTION MECHANISM A N D STRUCTURE OF ACTIVE SITES In the above section it has been shown that hydride and hydroxy groups are produced at the same time on the surface of highly dispersed alkaline-earth oxides upon hydrogen adsorption at room temperature. This is consistent with the heterolytic dissociation of hydrogen molecules already suggested in the case of Mg053 6 l 24 and shown here to be operative on CaO and SrO as well: H,-+H++H-. (1) The uptake of molecular hydrogen at low temperature can be thought to occur through H--H bond polarization, not large enough to cause dissociation of the molecule but sufficient to stabilise at 78 K a weak surface complex: Both processes are coherent with the nature of intrinsic sites on the surface of alkaline-earth oxides, e.g.metal cations and oxygen anions whose acidity and basicity are highly enhanced by the low coordination states characteristic of exposed ions.25 The fact that couples of metal and oxygen ions in low coordination states are involved in the hydrogen adsorption has been demonstrated by photoluminescence spectroscopy in the case of Mg0,6 and similar results have been obtained for CaO and Sr0.26 Moreover, by comparison of photoluminescence spectra of MgO powders from different origins, and from the effect of hydrogen adsorption on the different emission bands, idealised models of the surface structure can be drawn.6, 27 Hydrogen has been shown6 to be adsorbed at room temperature only by: ( a ) couples containing both the cation and the anion in the lowest coordination (e.g.three-coordinated magnesium and oxygen ions: Mg:GO;,), (6) regions of high irregularity likely to occur21 18 HYDROGEN ADSORPTION O N ALKALINE-EARTH OXIDES on the very rough surface of MgO ex-hydroxide,28 and possibly (c) a fraction of Mg,2;0;,, although usually this requires U.V. irradiation to react. The surface heterogeneity that can be envisaged from the models6t27 accounts for the multiplicity of adsorbed species which are evidenced by this infrared study. For example, dissociation occurring on a couple whose ions are at the two corners of a microstep protruding over a large (100) plane generates ‘free’ hydride and hydroxy groups. In fact neither H+ nor H- can interact respectively with more than one anion or cation in very low coordination. These OH- and (Mg-H)+ groups may well be associated with the sharpest components in the i.r.spectra. By contrast, when dissociation occurs in a region of high irregularity, where three O$j ions are present in a triangular array which simulates a (1 11) microplane, the H+ fragment is shared by the three anions. This is a bridge structure which contributes to the intensity of the broad bands in the high-frequency region (3700-3000 cm-l). Analogous structures in which three Mg;; are present would generate bridge hydride groups associated with the broader components of band (b). The molecular undissociative adsorption of hydrogen at 78 K occurs on surface couples whose ions are in states of higher coordination. These are certainly the four-coordinated ions on extended edges (e.g.Mgi&0iC)6 and possibly even some of the five-coordinated ions on (100) planes. Such a scheme, although derived from idealized models,6t 27 illustrates situations which are plausible for the real highly irregular and heavily stepped surface of Mg0.28 They can be extended to CaO and SrO, which have the same crystal structure and are obtained by the same decomposition route. Other mechanisms have been proposed involving homolytic dissociation of hydrogen molecules over 0- ions to give hydroxy We believe that those mechanisms may contribute to the hydrogen adsorption on alkaline-earth oxides, but cannot account for the overall process, as the hydride species shown by the i.r. spectra can only be produced through a heterolytic pathway. Very recently such a model has been used to interpret t.p.d.data relative to hydrogen adsorption on Mg0.29S. COLUCCIA, F. BOCCUZZI, G . GHIOTTI AND C. MORTERRA 21 19 PROMOTING ROLE OF PREADSORBED HYDROGEN FOR 0, FORMATION It has been shown by e.s.r. that 0; species are formed when oxygen is allowed onto alkaline-earth oxides which have preadsorbed hydr~gen.~. On the other hand, the infrared spectra indicate that a large fraction of hydride groups is destroyed by interaction with oxygen (fig. 4), whereas no significant changes are observed in the hydroxy bands. These observations suggest that the paramagnetic species could be formed following (3) the reaction by which an electron-transfer process occurs between preformed negative ions (H-) and newly adsorbed oxygen species, as already observed with other molecules.lo According to other mechanisrn~~~ 0; species are produced by direct electron-transfer from the surface to the oxygen molecule, but we feel that the promoting role of hydrogen preadsorption is better understood on the basis of eqn (3), which is supported by spectroscopic evidence. + ( 3 2 ' . . .O-+ 1H Ca2+-. .H-+O, 2 2 2 J. H. Lunsford and T. W. Leland, J. Phys. Chem., 1962, 66, 2591. M. Boudart, A. Delbouille, E. G. Derouane, V. Indovina and A. B. Walters, J . Am. Chem. SOC., 1972, 94, 6622 and references therein. D. Cordischi, V. Indovina and M. Occhiuzzi, J. Chem. SOC., Faraday Trans. I , 1978, 74, 456. D. Cordischi, V. Indovina and M. Occhiuzzi, J. Chem. SOC., Faraday Trans. I , 1978, 74, 883. S. Coluccia, F.Boccuzzi, G . Ghiotti and C. Mirra, Z . Phys. Chem. (N.F.), 1980, 121, 141. fi S. Coluccia and A. J. Tench, 7th Int. Congr. Catalysis, Tokyo, 1980, preprints, paper B35. T. Iizuka and K. Tanabe, Bull. Chem. SOC. Jpn, 1975, 48, 2527. S. Coluccia, J. F. Hemidy and A. J. Tench, J. Chem. SOC., Faruday Trans. I , 1978, 74, 2763. S. Coluccia, A. Chiorino, E. Guglielminotti and C. Morterra, J . Chem. SOC., Faraday Trans. 1, 1979, 75, 2188. lo E. Garrone, A. Zecchina and F. S. Stone, J. Catal., 1980, 62, 396. M. Utiyama, H. Hattori and K. Tanabe, J . Catal., 1978, 53, 237. l2 E. Borello, A. Zecchina and M. Castelli, Ann. Chim. (Rome), 1963, 53, 690. lB Y. Tanaka, Y. Imizu, H. Hattori and K. Tanabe, 7th Int. Congr. Catal., Tokyo, 1980, preprints, paper B42. L. H. Little, Infrared Spectra of Adsorbed Species (Academic Press, London, 1966). l5 A. V. Kiselev and V. I. Lygin, Infrared Spectra of Surface Compounds (John Wiley, New York, 1975). l6 R. P. Eischens, W. A. Pliskin and M. J. D. Low, J. Catal., 1973, 1, 180. l 7 H. D. Kaesz and R. B. Saillant, Chem. Rev., 1972, 72, 231. I * F. Boccuzzi, E. Borello, A. Zecchina, A. Bossi and M. Camia, J. Catal., 1978, 51, 150 and references l9 L. J. Bellamy, Infrared Group Frequencies (Methuen, London, 1968). 2o K. M. Mackay and R. A. Mackay, Introduction to Modern Inorgunic Chemistry (Intertext Books, 21 M. W. Howard, U. A. Jayasooriya, S. F. A. Kettle, D. B. Powell and N. Sheppard, J. Chem. SOC., 22 P. J. Anderson, R. F. Horlock and J. F. Oliver, Trans. Faraday SOC., 1965, 61, 2754. 23 M. J. D. Low, N. Takezawa and A. J. Goodsel, J. Colloid Interface Sci., 1971, 37, 422. 24 H. Praliaud, S. Coluccia, A. M. Dean and A. J. Tench, Chem. Phys. Lett., 1979, 66, 44. 25 S. Coluccia, A. M. Dean and A. J. Tench, J. Chem. SOC., Faraday Trans. 1, 1978, 74, 1973. 26 S. Coluccia and A. J. Tench, to be published. 27 S. Coluccia, R. L. Segall and A. J. Tench, J. Chem. SOC., Faraday Trans. 1, 1979, 75, 1769. 2H A. F. Moody and C. E. Warble, J. Cryst. Growth, 1971, 10, 26. 29 T. Ito, T. Sekino, N. Moriai and T. Tokuda, J . Chem. SOC., Faraday Trans. 1, 1981, 77, 2181. therein. London, 1972). Chem. Commun., 1979, 18. (PAPER 1 / 1 174)

ISSN:0300-9599    

DOI:10.1039/F19827802111

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1482, 78, 2121-2128 Photoinduced Metathesis Reaction of C,H, on Supported MOO, Catalyst BY MASAKAZU ANPO,* ICHIRO TANAHASHI AND YUTAKA KUBOKAWA* Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 59 1 , Japan Received 11 th August, 198 1 Ultraviolet irradiation of MOO, supported on porous Vycor glass (PVG) in the presence of C,H, has been found to induce the formation of approximately equimolar amounts of C,H, and 2-C4H, as well as a small amount of CH,CHO, suggesting that the metathesis reaction of C,H, occurs. The dependence of the yields upon the excitation wavelength is in good agreement with the excitation band of the phosphorescence of MoO,/PVG, which arises from the charge-transfer transition hv hvr (Mo6+=O2-) (Mo5+-O- ).* The quenching of the phosphorescence of MoO,/PVG and the decrease in the yield of C,H, metathesis on adding 0, and CO suggest that the photoinduced metathesis reaction is closely associated with the charge-transfer excited triplet state of MoO,/PVG.From these results, together with those of e.s.r. experiments reported previously, the mechanism of the photoinduced C,H, metathesis, especially the primary process of metalkarbene formation, is discussed. We have recently investigated the photoluminescence of metal oxides supported on porous Vycor glass (PVG) containing 1 wt % metal, as well as its correlation with the photoreactions on these oxides.' The charge-transfer excited states (Mo5+-O- ) * Play a significant role in the photoreactions, the trapped holes (0-) as well as electrons (Mo5+) being involved in the reactions; e.g.the presence of electrons in the neighbourhood of holes is necessary for the fission of the C=C bond in alkenes.2 It has been proposed by a number of workers that photolysis of liquid water on TiO,, SrTiO,, ZnO etc. proceeds via a photoelectrochemical mechanism whereby reduction and oxidation of the species take place on different surface^.^ On the other hand, Somorjai et aL4 have proposed that a photocatalytic mechanism is applicable to the photolysis of water on SrTiO, without added metal; i.e. the surface sites active for oxidation and reduction exist close together. Such a situation is similar to that with the excited complex 0-)*, with the close existence of electrons and holes.In the present work, as an extension of previous studies, the photoreaction of C,H, on MoO,/PVG has been investigated. As has been shown by Kazansky et a/.,5 studies of e.s.r. and optical spectra of supported metal oxides with low metal contents can give valuable information on the structure and reactivity of low-coordinated metal ions on the support. Although the importance of ' surface coordinative unsaturation ' in heterogeneous catalysis appears to be established, the role of surface ions in low coordination in photocatalytic reactions on oxides is still unclear. Studies of supported metal oxides with an extremely low metal content are expected to provide information on the role of surface coordinative unsaturation in photocatalysis.69 2121 FAR 12122 REACTION OF C,H, ON SUPPORTED MOO, EXPERIMENTAL The gases used were of 99.5% purity (Takachiho Kogyo Co.) and were used after purification by low-temperature fractional distillation. MOO, supported on PVG (MoO,/PVG) (0.007 wt %) was prepared by impregnation of PVG (Corning no. 746685-7930, 160 m2 g-', 9.0 x 3.0 x 1.0 mm3) with an aqueous solution of (NH,),Mo,O,,. The catalysts were dried at 350 K and heated in 0, at 773 K followed by evacuation at 613 K. The metal content was determined by atomic absorption and/or calorimetry. The photoluminescence spectra were measured with a Shimazu RF-501 spectrofluorophotometer with filters to eliminate scattered light between 77 and 300 K. E.s.r. measurements were carried out at 77 K using a JES-ME-X (X-band) spectrometer. Mn2+ in MgO powder was used for g-value and sweep calibrations.U.V. irradiation was carried out at 290 K using a 75 W high-pressure mercury lamp with a colour filter (A < 280 nm) and a water filter. The reaction products were separated by low-temperature fractional distillation and analysed by gas chromatography or by a Shimazu quadrupole mass-spectrometer. Details of the experimental procedures have been described previously.', 2* RESULTS AND DISCUSSION QUENCHING OF THE MoO,/PVG PHOSPHORESCENCE WITH C,H, Fig. 1 shows the photoluminescence of MoO,/PVG and its spectral change caused by added C,H, together with the corresponding excitation spectrum. As described previously,'? the excitation spectrum of the photoemission having A, = 295 nm can be attributed to a charge-transfer process; i.e.hv (M06+=02-) = (M05+-0-)* hV' where Mo5+ is tetrahedrally coordinated.? The emissions at 440 and 340 nm are assigned to phosphorescence from the charge-transfer excited triplet states and to 320 400 500 600 0 wavelength/ n m FIG. l.-(a) Photoluminescence of MoO,/PVG at 298 K (excitation, 290k7.5 nm): (i) O,, 0.03 Torr; (ii) 0,, 0.13 Torr; (iii) C,H,, 0.42 Torr. (6) Excitation spectrum of 500 nm emission (slit width for excitation, 5.0 nm; slit width for emission, 7.0 nm). t According to our recent studies, PVG outgassed at higher temperatures shows emission at 400 nm with an excitation band at 255 nm, arising from the presence of specific sites of low coordination. Little or no emission of PVG is observed with light in the range > 280 nm.Since the measurements in the present work have been carried out with light in this region, it is concluded that PVG makes no contribution to the observed emission (e.g. fig. 1).M. ANPO. I. TANAHASHI A N D Y. KUBOKAWA 2123 fluorescence from the excited singlet states, respectively, since on decreasing the temperature from 300 to 77 K, the former increased while the latter scarcely changed in intensity. As seen in fig. 1, only the phosphorescence is quenched by added C3H6, suggesting that C3H6 molecules interact with the excited triplet states. As described previously,6a the possibility that collisional quenching, whereby gaseous molecules interact with the excited states, takes place may be excluded, since the quenching became irreversible at lower temperatures.Accordingly, quenching in the present system is attributable to the formation of an adsorption complex between added gases and the oxide surface. As shown in fig. 1, 0, quenches the phosphorescence more efficiently than C3H6, i.e. the interaction of MoO,/PVG with 0, is stronger than with C,H6. As regards such a discrepancy, there appears to be some difference in the nature of adsorption complexes for 0, and C3H6. As will be shown later, 0; anion radicals are formed under U.V. irradiation of MoO,/PVG in the presence of 0,. Formation of anion radicals would not be expected for C3H6. PHOTOINDUCED METATHESIS REACTION OF C3H6 U.V. irradiation (3, > 280 nm) of Mo03/PVG in the presence of C3H6 was found to induce the formation of approximately equimolar amounts of C,H, and 2-C4H,. This suggests that the metathesis reaction of C3H6 occurs, its yield increasing with pressure of C3H6 (fig.2). As shown in fig. 3, on U.V. irradiation of MoO,/PVG in the presence of C3H6 the metathesis of C3H6 takes place immediately, its yields increasing with irradiation time. As soon as U.V. irradiation ceases, the reaction stops. As expected from this lack of metathesis Of C3H6 in the dark, no interaction O f C3H6 with MoO,/PVG occurred at room temperature in the dark, since the C3H6 introduced was completely recovered by desorption up to 323 K. 0.5 1.0 1.5 2.0 2.5 initial C, H, pressure/Torr FIG. 2.-Effect of pressure of C,H, upon the yields of photoinduced metathesis reaction at 290 K. Fig. 4 shows the dependence of the yield of photoinduced metathesis upon the excitation wavelength.From a comparison with the results shown in fig. 1, it is seen that the dependence of the yields upon excitation wavelength is in good agreement with the excitation band of the photoemission of MoO,/PVG. This suggests that the photoinduced metathesis reaction is closely associated with the charge-transfer excited states of MoO,/PVG. After the photoreaction, the temperature of the Mo03/PVG was raised stepwise, and the desorption products were analysed. As shown in fig. 5, in addition to C,H, 69-22124 REACTION OF C,H, ON SUPPORTED MOO, v, v, .- 2 30- (3 c-’ \D 2 r, : 20- v, al h 22 .- 5 10- - 5 I 1 reaction time/min FIG. 3.-Time course of photoinduced metathesis reaction of propene on MoOJPVG.C,H,, 0.92 Torr; excitation, 3 10 f 9 nm; temperature, 290+ 2 K. FIG. excitation wavelength/nm -Effect of excitation wavelength upon the yield of photoinduced metat..esis reaction of propene on MoOJPVG. C,H,, 1.2 Torr; temperature, 298 f 2 K ; slit width 9 nm. and 2-C4H,, CH,, 1-C,H, and CH,CHO were desorbed, the sum of the three minor products being ca. 3% of the total desorption products. As regards the desorption of CH,CHO, essentially the same thermal desorption pattern was obtained with the desorption after adsorption of CH,CHO on MoO,/PVG at 300 K. This suggests that CH,CHO is formed from the photoreaction at 290 K and not by thermal reaction due to the temperature rise of MoO,/PVG. EFFECT OF THE ADDITION OF 0, AND co UPON THE PHOTOINDUCED METATHESIS REACTION Fig.6 shows the effect of added 0, upon the yield of C,H, metathesis reaction under U.V. irradiation at 290 K. Both C,H4 and 2-C4H, yields decrease with increasingM. ANPO, I. TANAHASHI AND Y. KUBOKAWA 2125 h % 5 5.0- 4 ”, c - 3 *? 1 0 e . r/: + 0 e c 2 . 5 - L 0 3 c2 H4 desorption temperature/ K FIG. 5. pressure of oxygen/Torr FIG. 6. FIG. 5.-Photoformed products and their desorption pattern. C,H,, 0.66 Torr. FIG. 6.-Effect of the addition of oxygen upon the yield of photoinduced metathesis of propene on MoO,/PVG at 290 K. C,H,, 0.66 Torr. pressure of 0,. In addition to CH,CHO, new oxygen-containing compounds such as C,H,CHO were produced in the presence of 0,. E.s.r. studies shows that U.V. irradiation of MoO,/PVG at 77 K in the presence of 0, leads to the formation of (o;),,, anion radicals.Such 0; formation has been reported for various oxides by several workers.l9 Formation of new oxygen-containing compounds described above appears to be caused by the presence of (o;),,, species. Details will be reported in the near future. The decrease in the yield of C,H, metathesis on adding 0, is attributed to the quenching of the excited triplet states of MoOJPVG by 0,, since 0, quenches the excited triplet states more efficiently than C,H,, as shown in fig. 1. The effect of added CO upon the C,H, metathesis has been similarly investigated. As shown in fig. 7, a similar decrease in metathesis yield occurred, its extent being smaller than that of 0,. This less efficient quenching with CO could result from the fact that the interaction of CO with the excited states of MoO,/PVG is weaker than that of 0,.Fig. 1 shows that in the metathesis of C,H, more C,H, is formed than 2-C4H,, the extent being more significant in fig. 6 and 7. Although the true nature of such features is unclear, the adsorption of C,H,, which includes its dimerization, was found to be induced by U.V. irradiation while the 2-C4H, adsorption was not. Similar results have been reported by Morikawa and coworkers.8 U.V. irradiation of MoO,/PVG in the presence of C,H, is expected to cause the photoreduction of MOO,, i.e. formation of Mo5+ and/or Mo4+ ions which are known to be the active sites for alkene metathe~is.~ If the photoreduced Mo ions were responsible for the photoinduced metathesis, such an immediate response for the intermittence of U.V.irradiation (fig. 3) would be unexpected, since the photoreduced Mo ions should exist even in the dark. Furthermore, the decrease in the yield of C,H,2126 REACTION OF C3H6 ON SUPPORTED MOO, t 0 1.0 2.0 3.0 4.0 pressure of CO/Torr FIG. 7.-Effect of the addition of carbon monoxide upon the yield of photoinduced metathesis of propene on MoOJPVG at 290 K. C,H,, 0.66 Torr. metathesis on the addition of CO (fig. 7), which is expected to accelerate the photoreduction of MOO,, would be unexpected on such a basis. This again confirms that the excited state (Mo5+-O-)* is responsible for the photoinduced metathesis reaction. MECHANISM OF THE PHOTOINDUCED METATHESIS OF C3H6 It has been generally accepted that with homogeneous catalytic reactions alkene metathesis proceeds via metal-carbene and metallocyclobutane intermediates.1° A number of workers have proposed that a similar mechanism is applicable to heterogeneous catalyst^.^ However, the mechanism of carbene formation is still unclear.As has been reported recently, U.V. irradiation of MoOJPVG at 77 K in the presence of C2H4 brings about the formation of C3H6, a bridged-type x-complex (C,H,=qH2)- \ I \\ ' \d being formed simultaneously.2 From these results it has been concluded that the interaction of C,H, with photoformed 0- hole centres leads to fission of the C=C bond in C,H,, resulting in the formation of methylene and HCHO. The C=C bond fission has been supposed to proceed as follows. The photoinduced increase in electron density in the Mo ions which constitute the excited states (Mo5+-O-)* enhances back-donation of electrons to the n* orbital of the bridged-type n-complex with consequent easier rupture of the C=C bond.A similar mechanism appears to be applicable to the reaction of C3H6 on U.V. irradiation of MoO,/PVG. However, no formation of the bridged-type ;n-complex occurred. This might be attributed to the low stability of the complex formed from C3H6, which results in a higher efficiency of formation of methylene as compared to the case of C2H,. Thus, it may be concluded that carbene formation in theM. ANPO, I. TANAHASHI AND Y. KUBOKAWA 2127 photoinduced metathesis of C,H, proceeds via an oxometallocyclobutane intermediate following the bridged-type n-complex: (Mo5+-0-)* + C,H, C,H2-FH-CH, excited complex Mo5+-O- n-complex \ I '\ J Tl H2 H2 H II I I Mo MO~+-O- -kC3H, I I I metathesis -C + O=CH-CH, +- C-C-CH, methylene ethanal.Simultaneous formation of CH,CHO and methylene is supported by the difference in the pressure dependence of the yields between C2H4 (or 2-C4H,) and CH,CHO. In contrast with the increase in yield of C2H4 (or 2-C4H,) with increasing C,H, pressure, as shown in fig. 2, the yield of CH,CHO was essentially independent of C,H, pressure above 0.7 Torr.* As has been discussed by a number of worker^,^ alkene metathesis is a chain reaction, with carbenes as the chain carriers reacting with alkenes to generate new carbenes and alkenes via intermediate metallocyclobutanes. From the ratio of C2H4 to CH,CHO in the C,H, metathesis, it is concluded that the photoformed methylenes recycle ca.500 times at 2.5 Torr of C,H,. An immediate response to the intermittence of U.V. irradiation (fig. 3) suggests that the photoformed carbenes are unstable, their concentration at steady state being negligibly small after interruption of U.V. irradiation. Although it is difficult to exclude the possibility that U.V. irradiation is essential for the propagation step, e.g. photoexcitation of the alkene-metallocarbene complex occurring, it seems unlikely that such a photoexcitation step plays a significant role in the metathesis reaction, since the dependence of its yield upon the excitation wavelength is in good agreement with the excitation band of the photoemission of MoO,/PVG.Formation of the oxometallocyclobutane intermediate shown above has already been proposed for carbene formation by Rooney and StewartlOa as follows: 0 CHR' 0-CHR1 O=CHR' M CHR2 M-CHR2 M=CHR2. II+II * I I * I I However, they consider that the strength of the Mo=O bond compared to that of the Mo=C bond is probably too large for carbene formation to proceed appreciably. In the charge-transfer excited states of MOO, formed under U.V. irradiation, the bond length of Mo=O becomes longer, i.e. its bond strength becomes weaker compared to that in the ground state.' Accordingly, direct involvement of the Mo=O bond in carbene formation described above is expected to occur more easily under U.V. irradiation. Thus, formation of the oxometallocyclobutane intermediate is also supported by the concept proposed by Rooney and Stewart.* The reduction of the MOO, proceeds with increasing amount of CH,CHO. However, its extent is negligibly small compared to the amount of MOO,, having little or no effect upon its catalytic activity. 1 Torr z 133.322 Pa.2128 to the Asahi Glass Foundation for financial support. REACTION OF C,H, ON SUPPORTED MOO, Thanks are due to the Ministry of Education of Japan (grant no. 56219019), and M.Anpo, I. Tanahashi and Y.Kubokawa, J. Phys. Chem., 1980, 84, 3440, Y.Kubokawa and M. Anpo, Shokubai (Catalyst), 1981, 23, 189, M. Anpo, I. Tanahashi and Y. Kubokawa, J. Phys. Chem., in press. M. Anpo and Y. Kubokawa, J. Catal., accepted for publication. A. J. Bard, J . Photochem., 1979,10,59; M. D. Archer, in Photochemistry (Specialist Period.Rep., The Chemical Society, London, 1979), vol. 10, p. 613; S. Sat0 and J. M. White, J. Phys. Chem., 1981, 85, 592; K. Koga, H. Yoneyama and H. Tamura, J. Phys. Chem., 1980, 84, 1705; H. V. Damme and W. K. Hall, J. Am. Chem. Soc., 1979, 101, 4373; T. Kawai and T. Sakata, Chem. Phys. Lett., 1980, 72, 87. F. T. Wagner and G. A. Somorjai, J. Am. Chem. SOC., 1980, 102, 5494; Sur- Sci., 1980, 101, 462. V. B. Kazansky, Proc. 6th int. Congr. Catal. (TheChemicalSociety, London, 1976), vol. 1-50; J . Catal., 1980, 64,426. (a) M. Anpo, C. Yun and Y. Kubokawa, J. Chem. Soc., Faraday Trans, 1,1980,76, 1014. (b) M. Anpo, C. Yun and Y. Kubokawa, J. Catal., 1980,61,267; Y. Kubokawa, M. Anpo and C. Yun, Proc. 7th Int. Congr. Catal. (Tokyo) (Elsevier, Amsterdam, 198 l), part B, p. 1 170. ' M. Formenti and S. J. Teichner, in Catalysis (Specialist Period. Rep., The Chemical Society, London, 1979), vol. 2, 87, and references therein. T. Nakajima, A. Morikawa and K. Otsuka, unpublished results. R. Nakamura, Y. Morita and E. Echigoya, Nikkashi, 1973, 244; P. P. O'Neill and J. J. Rooney, J. Am. Chem. Soc., 1972, 94,4383; Y. Iwasawa, S. Ogasawara and M. Soma, Chem. Lett., 1978, 1039; E. A. Lombardo, M. Houalla and W. K. Hall, J. Catal., 1978, 51, 256; K. Tanaka, K. Tanaka and K. Miyahara, J. Chem. SOC., Chem. Commun., 1979, 314; J. Engelhardt, J. Catal., 1980, 62, 243. lo (a) J. J. Rooney and A. Stewart, in Catalysis (Specialist Period. Rep.. The Chemical Society, London, 1977), vol. 1, p. 277 and references therein: (b) A. K. Rappe and W. A. Goddard 111, J . Am. Chem. SOC., 1980, 102, 5 1 15 and references therein. (PAPER 1/1287)

ISSN:0300-9599    

DOI:10.1039/F19827802121

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Furada-v Trans. I , 1982, 78, 2129-2145 Effects of Chain-transfer Agents on the Kinetics of the Seeded Emulsion Polymerization of Styrene BY GOTTFRIED LICHTI? AND DAVID F. SANGSTER AINSE and the Australian Atomic Energy Research Establishment, Sutherland, New South Wales 2232, Australia A N D BARRY C. Y. WHANG, DONALD H. NAPPER* AND ROBERT G. GILBERT Departments of Physical and Theoretical Chemistry, The University of Sydney, New South Wales 2006, Australia Receiued 14th August, 1981 The kinetics of the seeded emulsion polymerization of styrene have been studied in the presence and absence of the chain-transfer agents carbon tetrachloride and carbon tetrabromide. Initiation was achieved by both a chemical initiator (potassium peroxydisulphate) and irradiation with prays.The latter permitted relaxation studies to be performed. A combination of 7-ray initiation, relaxation and particle size distribution studies allowed the fate of the exited free radicals generated in the presence of carbon tetrabromide to be determined. Cross-termination in the aqueous phase was found to be operative in pray initiated systems when the free radical concentration in the aqueous phase was relatively high. In contrast, re-entry of the exited free radicals into the latex particles was found to be important in relaxation studies when the free radical concentration in the aqueous phase was comparatively low. These results show that the exited free radical fate parameter can vary between - 1 and + 1. The exit rate coefficient was found from relaxation measurements to increase linearly with increasing concentration of chain-transfer agent; this result is consistent with a diffusion/transfer mechanism for exit.The increase in the exit rate coefficient paralleled the increase in the chain-transfer constant for the additives : CBr, > CCl, > styrene. On the other hand, the efficiency of exit from the latex particles of free radicals formed by chain transfer follows the inverse order: CBr, < CC1, < styrene. This order may well reflect the relative reactivities with monomer of the low-molecular-weight free radicals formed by atom abstraction. As expected from the increase in exit rate coefficient, the presence of carbon tetrabromide reduced the rate of polymerization of chemically initiated systems.At high initiator concentrations, for which the average number of free radicals per particle i was ca. 0.5, the rate reduction was small but increased monotonically with increasing concentration of carbon tetrabromide. This shows that any effect of carbon tetrabromide on the propagation rate constant was small in these studies. At lower initiator concentrations, however, a much larger reduction in rate was observed, as expected theoretically for values of i < 0.5. The rate in this case did not decrease monotonically with increasing concentration of carbon tetrabromide but passed through a minimum. This minimum was caused by the enhanced rate of entry of free radicals into the latex particles counterbalancing the rate reduction arising from the increased exit rate.The increase in the entry rate in the presence of carbon tetrabromide was explained by the production of hydrophobic free radicals by chain transfer in the aqueous phase and/or a colloidal contribution to the measured entry rate. In principle, chain-transfer agents should not influence the kinetics of the free radical polymerization of monomers in homogeneous bulk or solution systems, although they do reduce the molecular weight of the polymer formed. We have recently reported1 that under certain heterogeneous conditions, such as those pertaining to emulsion t Present address: I.C.I. Australian Central Research Laboratories, Ascot Vale, Victoria, Australia. 21292130 SEEDED EMULSION POLYMERIZATION OF STYRENE polymerizations, a chain-transfer agent may significantly reduce not only the molecular weight of the polymer formed but also the rate of polymerization.This reduction in rate was shown' to arise, at least in part, from the breakdown in the extent of compartmentalization of the free radicals in the latex particles. The presence of the chain-transfer agent promoted the exit (desorption) of free radicals from the latex particles. In what follows, we present quantitative data, obtained from seeded kinetic studies of the emulsion polymerization of styrene, on the role of chain-transfer agents. The results cast light upon the fate of the exited free radicals generated in the presence of chain-transfer agents and especially upon the influence of the free radical concentration in the aqueous phase. They also show that chain-transfer agents can influence the rate of entry of free radicals into the latex particles. Inter aka, our results show that the exit rate coefficient is linearly dependent on the concentration of added chain-transfer agent.This finding is consistent with a diffusion/transfer mechanism for the exit of oligomeric free radicals, a result which is also suggested by previous studies on the dependence of the exit rate coefficient on particle size.2 The planning and interpretation of experiments involving exit of free radicals in emulsion polymerization systems is particularly complex, owing to the plethora of mechanisms which must be taken into account: organic free radicals entering into, propagating within and exiting (desorbing) from the latex particles, and hetero-termi- nation and re-entry (into the latex particles) of oligomeric free radicals in the aqueous phase.Thus a unique interpretation of, for example, a single dilatometric kinetic run is impossible, since so many rate coefficients are involved that many sets of apparently physically reasonable values may fit the data. In addition, refinements such as changing the concentration or nature of the initiator are (when considered alone) only of limited help, since (for example) the rate coefficients for entry and aqueous-phase hetero- termination will then be altered. In order to resolve this problem, we first employ a large range of experimental procedures, as follows. (i) For initiation, we use both a chemical initiator of varying concentrations and y-radiolysis.With both techniques we obtain the rate of approach to steady state for Interval I1 of a seeded styrene emulsion polymerization system; for the y-radiolysis studies, we also obtain the rate of relaxation after removal from the radiation source. (ii) In addition to obtaining rate data from the overall kinetics (followed dilatometrically)2*3 we also use data on the time evolution of the particle size di~tribution.~ (iii) Studies are carried out varying the nature and concentration of initiator and also the nature and concentration of chain-transfer agent. The data obtained using the above experimental methods are then interpreted by the following means. (i) We may reasonably assume that while the rate coefficient for exit will vary with the nature and concentration of chain-transfer agent (since the process seems to be diffusion/transfer controlled),2 it should be invariant to the nature and concentration of initiator.(ii) We may interpret the complexities of the fate of oligomeric radicals in the aqueous phase by a recent theory5 which enables the kinetics of aqueous-phase hetero-termination and of re-entry into the latex particles to be expressed in terms of a single parameter. This parameter will depend on the nature of the oligomeric species (i.e. on the nature and concentration of the chain-transfer agent) and on the nature and concentration of the aqueous-phase initiator. (iii) With the two preceding simplifications, we derive a treatment which permits a unique set of internally consistent rate coefficients to be obtained.LICHTI, SANGSTER, WHANG, NAPPER AND GILBERT 2131 EXPERIMENTAL As stated above, all studies were carried out using seeded latex systems in Interval 11.The seeds employed were two monodisperse polystyrene latices (labelled R17/5 and R007f 15) which were prepared and characterized as described ea~1ier.l.~ For this characterization, the mean particle radius in both latices was determined to be ca. 48 nm by both electron microscopy and sedimentation velocity. The concentration of particles in these studies was ca. 5 x 10l6 particles dm-3. The polymerization rate was found dilatometrically. All studies were carried out at 50 O C . THEORETICAL FRAMEWORK Prior to presenting our experimental results, we give a brief outline of the theory which will be used for their interpretation; details have been presented el~ewhere.~.~ Consider the kinetics of an Interval I1 seeded system.The overall rate of formation of polymer is given by: where x is the fractional molar conversion of monomer, N , is the concentration of latex particles, C, is the concentration of monomer within these particles, k , is the propagation rate coefficient and ri is the average number of free radicals per particle, which will be determined by the values of the rate coefficients for free radical entry ( p ) , exit ( k ) and any other microscopic events which need to be taken into account. Since x is observable directly (e.g. as in our dilatometric experiments), it can be seen that one way of determining ti is to know C , k,. This would then give information which could be used to determine the values of p and k , and in particular the dependence of these parameters on such controllable variables as the nature and concentration of the initiator, etc.However, a major problem which arises is that (at least for styrene systems of the type studied here) small errors in the value of C M k p , and hence of ti, can lead to errors of orders of magnitude in the derived values of p or k . Thus one needs either an accurate value of C,k, for a given system or an alternative means of determining ti. In styrene systems of the type studied here, it has been well established' that ri cannot exceed 0.5 in Interval 11. In such a system, to determine ti from values of p, k , etc. it is sufficient to determine the (time-varying) values of No and N , , which are defined as the relative numbers of latex particles containing zero or one free radical, respectively. One has by definition No + N , = 1, and thus ri = N , .The requisite rate equation for determining N , is: -- dN1 - p ( 1 - N , ) - ( p + k ) N , . dt Note that p is an effective rate coefficient for the entry of free radicals into the latex particles and must incorporate all relevant aqueous-phase processes; p itself will thus in general depend on ri. Aqueous-phase processes which need to be considered are ( a ) entry of species originating directly from aqueous-phase processes, (b) re-entry of oligomers which have exited from latex particles into the aqueous phase and (c) aqueous-phase hetero-termination of these exited oligomers before they undergo re-entry, this hetero-termination being specifically with an entity which would otherwise have entered a particle as in (a).It has been shown5 that one may write:21 32 SEEDED EMULSION POLYMERIZATION OF STYRENE where pA is the free radical entry rate coefficient from process (a) above and a is a dimensionless ‘fate parameter’. One has - 1 ,< a < 1 , and a can be written as a function of the rate coefficients for processes (b) and (c) above (and also processes such as direct production of free radicals from initiator and aqueous-phase bimolecular homo-termination of these). In this formalism, complete re-entry of exited free radicals into latex particles is characterized by a = + 1, whereas complete hetero- termination in the aqueous phase corresponds to a = - 1 .Such cross-termination is postulated to occur between exited free radicals and free radicals (or derivatives thereof) generated by process ( a ) above. Note that the general formulation encom- passed by eqn (1) and (2) incorporates various theoretical refinements suggested by other workers in this field,6 at least as far as a description of systems where fi cannot exceed 0.5 is concerned. We emphasize that extensive experimental studies have shown this last requirement to be met for the present A complete kinetic description of the system would thus be furnished by a knowledge of the dependences of pA, a and k (as well as k, and C,) on the experimentally controllable parameters : the size and nature of the latex particles, the nature and concentration of the initiator and the nature and concentration of any chain-transfer agent.In the present studies, we keep the size and nature of the latex particles fixed. The other controllable variables will be expected to affect the microscopic rate parameters as follows. (i) One expects pA to depend only on the nature and concentration of the initiator I ; we denote this dependence by pA(1, [I]). (ii) One expects the exit rate coefficient k to be dependent on the nature and concentration of any chain-transfer agent T (since it is controlled by diffusion and transfer); we denote this by k(T, [TI). (iii) The fate parameter a may depend on all the controllable variables, since, for example, the rate coefficient for hetero-termination will depend both on the initiator and on the exiting species; we write a (I, [I], T, [TI).(iv) Both k , and C, may depend slightly on the nature and amount of chain-transfer agent. In addition to the time variation of the fractional conversion x, another experimental observable is the time evolution of the particle size distribution : n( V, t), the relative number of particles with volume V at time t. This quantity will be determined by p and k ; the governing equations have been presented previ~usly.~ The object of the present study is to obtain values for the rate parameters pA, a and k , or rather, to show how they depend on the controllable variables. This will enable us to make deductions concerning the mechanisms governing the microscopic processes.The above-mentioned plethora of rate coefficients requires careful data analysis in order to obtain unique values for the desired quantities. Detailed discussions of the means of effecting this are given in the sections on the individual sets of experiments. However, important general assumptions which are relevant to mention at this point are as follows. (i) We assume that the presence of the chain-transfer agent does not significantly affect the value of k , CM at the concentrations employed here. This will be justifed in a later section. For k,, we have assumed (where required) a value of 258 mol-1 s-l dm3, as obtained from previous studies on these latex systems.2 (ii) In Interval I1 of a seeded system, fi goes from ri = 0 at time t = 0 to some steady-state value which we denote by tiss.From eqn (l), one may readily write down fi,, as a specific function of pA, a and k (see below). Now, as indicated by extensive studies of similar systems,3 we assume that for systems where initiation is by y-radiolysis, fiss = 0.5 in the absence of chain-transfer agent, i.e. when I = y and [TI = 0. Assuming then that k, CM is essentially unchanged by the presence of chain-transfer agent, for y-radiolysis in the presence of chain-transfer agent we may then obtain ti,, (or indeed ti at any time) simply by comparison of the rate in the presence of chain-transfer agent with that in the absence of chain-transfer agent.LICHTI, SANGSTER, WHANG, NAPPER AND GILBERT 21 33 The data which we have at our disposal are thus as follows. (a) For y-radiolysis, the steady-state value of ri in the radiation cavity, ti,, (I = y); note that because of experimental limitations, we cannot readily either alter the intensity of the radiation ( i e .for y-radiolysis, [I] = constant) or find the time dependence of approach to steady state within the radiation cavity. (b) For y-radiolysis experiments, the relaxation of the system after removal from the cavity, i.e. in the absence of initiator. Note that we are able to obtain an absolute value of ri through assumption (ii) given above. (c) For chemically activated systems (all with persulphate initiator of varying concentrations) we are able to obtain the time dependence of the fractional conversion, including the rate of approach to steady state, with varying initiator concentration; we may denote this x(t, I = C, [I]). This must be 'normalized' (by means which wilI be discussed later) in order to determine 2.Note that because of experimental limitations, we are able to extract only two meaningful pieces of data from a given x(t) curve: the slope of the straight line portion (i.e. the steady-state value) and the intercept that it makes with the t = 0 axis, expressing the rate of approach to steady state ; under unfavourable circumstances, the errors involved in the value of the intercept may be so large as to render this quantity unusable. 0.14 0.12 2 0.10 r 53 2 0.08 > c - 3 c 0 .- 5 0.06 it" 0.04 0.02 0 20 40 60 80 100 time/ m in FIG. 1.-characteristic kinetic curve for relaxation measurements on latex R17/5 in the absence of chain transfer agents.An illustration of typical kinetic data is given in fig. 1. This is for y-radiolysis initiation both inside and outside the radiation cavity. After an approach to steady state which is too rapid to observe experimentally, the x(t) curve inside the cavity shows a constant polymerization rate. The sample is then rapidly removed from the cavity, and the system then relaxes with a measurable transient decay to a final period of constant rate. This particular experiment was for polymerization in the absence of chain-transfer agent; further typical raw data curves (including those in the presence of chain-transfer agent) may be found in ref. (1).2134 SEEDED EMULSION POLYMERIZATION OF STYRENE RESULTS AND DISCUSSION We first discuss the data obtained from relaxation studies.Table 1 lists the values of the three measured quantities for such a system, as discussed in points (a) and (b) in the preceding section: the steady-state rate inside the cavity, the steady-state rate outside the cavity and the intercept of the out-of-cavity steady-state line; note that we present these data as rates, this being the quantity which is measured. These rates TABLE 1 .--~-RADIOLYSIS RESULTS IN THE PRESENCE AND ABSENCE OF CHAIN-TRANSFER AGENTS (SEED LATEX R17/5) steady-state rate/ min-l lo3 x intercept additive % added (w/w) in-cavity out-of-cavity out-of-cavity nil CBr, CBr, CCl, CCI, CCl, CCI, nil 3 6 1 3 6 10 3.6 1.3 20 2.3 0.29 4.5 1.6 0.27 2.1 3.4 1.2 16 3.6 1.3 14 3.7 1 . 1 13 3.7 1 . 1 8.9 TABLE 2.-KINETIC RESULTS FOR SEEDED (R17/5) EMULSION POLYMERIZATIONS OF STYRENE INITIATED BY )'-RAYS nss 0 3 6 0.50 0.3 1 0.22 are then converted to ii values, using the rate in the absence of chain-transfer agent to give ii,, = 0.5 and thus a normalizing factor, as given in assumption (ii) in the preceding section. Some typical ti values are shown in table 2, with CBr, as chain-transfer agent. ANALYSIS OF D A T A FOR CARBON TETRABROMIDE Using the values of ii in table 2, we wish to obtain the value of the exit rate coefficient k .This apparently requires that appropriate values of pA and a are also known. We resolve this problem as follows, using both the out-of-cavity and in-cavity results, with the condition (as set out in the previous section) that although pA and a will be different inside and outside the cavity, the value of k must be the same. We consider first the out-of-cavity results: the slope and intercept of the curve. From eqn (1) and (2) we may then use these to obtain pA and k as functions of a; the precise equations required for this have been given previ~usly.~ Values of pA and k are given in table 3 and in fig.2, for data obtained with 3 and 6% added CBr,. Note that the values of pA in table 3 refer to the rate of entry of thermally generated free radi~als~9~ after the system has withdrawn from the cavity. Secondly, we consider the in-cavity results. Since we are unable to observe the rateLICHTI, SANGSTER, WHANG, NAPPER AND GILBERT 21 35 TABLE 3.-cALCULATED VALUES OF THE ENTRY AND EXIT RATE COEFFICIENTS FOR DIFFERENT VALUES OF THE FATE PARAMETER COMPUTED FROM Y-RADIOLYSIS RELAXATION DATA 3% ( W W ) CBr, 6% (w/w> CBr4 a PA/ 10-4 s-1 k / 10-3 s-1 pA/10-4 s-1 k/10-3 s-1 - 1.0 -0.5 0 + 0.5 + 1.0 3.4 4.2 3.2 5.2 2.8 6.7 2.2 9.7 0.67 20 4.2 5.7 4.0 7.2 3.7 9.6 3.0 15 0.96 35 6 4 2 16 12 8 4 0 -1 0 ff FIG.2.-Calculated values for the exit rate coefficient for various values of the fate parameter: (a) 3% CBr,; (b) 6% CBr,; curves 1 and 2 refer to the in-cavity and relaxation methods, respectively. The dashed lines define the lower and upper bounds for k estimated from the precision of the data points.2136 SEEDED EMULSION POLYMERIZATION OF STYRENE of approach to steady state, we know only the steady-state rate, i.e. fi,,, which we use as follows. From eqn (1) and (2) in the steady state (when dN,/dt = 0, N , = is,) we obtain: PA( - 2, k = (1-a+2afiS,)' (3) The value of p A in eqn (3) (which of course will be different inside and outside the cavity) was found for in-cavity conditions from the time evolution of the particle size distribution, as described previ~usly,~ in the absence of chain-transfer agent.The experimental details have been given el~ewhere.~ For in-cavity conditions in the absence of chain-transfer agent, we have ii,, = 0.5. Since this implies that akii,, < p A , the analysis of the particle size evolution data given in ref. (4) is straightforward (in the absence of chain-transfer agent). We thus obtain pA (I = y) = (2.740.8) x s-l. From this value and from eqn (3) we may then determine k as a function of a for our in-cavity data.The results are shown in fig. 2. Since k must be the same for both in-cavity and out-of-cavity data (for a fixed concentration of chain-transfer agent), even though the values of p A and a will be different, fig. 2 allows us to establish the value of k and the value of a under the two different conditions. The ranges of k values given by the two methods overlap only in a relatively narrow zone (see fig. 2). The values of k obtained with 3 and 6% carbon tetrabromide present were (1.6f0.5) x low2 and (3.2k0.6) x lo-* s-l, respectively. These values will be discussed below. It is also apparent from fig. 2 that inside the radiation cavity the exit fate parameter is negative and lies in the range - 1 .O d a < - 0.75, irrespective of the concentration of carbon tetrabromide. This is in accord with the value of a = - 1 obtained previously5 for similar systems in the absence of carbon tetrabromide but initiated by potassium peroxysulphate.In contrast, the results obtained outside the cavity suggest that a is positive and lies in the range 0.75 < a < 1 .O, again irrespective of the concentration of carbon tetrabromide. The foregoing results suggest that the value of a changed from ca. - 1 for systems in the radiation cavity to ca. + 1 for polymerization outside the cavity. The former value corresponds to almost complete cross-termination of the exited free radicals in the aqueous phase whereas the latter value corresponds to almost complete re-entry. This change in the value of a we attribute as being due to the decrease in the concentration of free radicals in the aqueous phase when the system is removed from the radiation source.As shown in table 3, the background thermal entry rate coefficient ( p A ) is very small outside the cavity so the value of p (= p A - kii) would be negative if a = - 1 .O. This is physically unacceptable because a negative entry rate coefficient has no meaning in the context of eqn (2). This apparent anomaly is explained by a breakdown of the various assumptions concerning the relative magnitudes of the various rate coefficients for production and termination of free radicals in the aqueous phase used to derive eqn (1) and (2). We see from the foregoing results that, in the cavity, the high concentration of free radicals in the aqueous phase results in the exited free radicals being annihilated almost completely by cross-termination.This we ascribe to the known rapidity of cross-termination reactions compared with the corresponding self-termination reaction^.^ The enhanced rate of cross-termination, typically one or two orders of magnitude, may be a consequence of the differences in polarity of the exited free radical species (presumably -CBr,) and the initiating species (presumably -OH or an oligomeric derivative thereof).* Stated differently, in the presence of a chemical initiator or for initiation by y-rays,LICHTI, SANGSTER, WHANG, NAPPER A N D GILBERT 21 37 the rate of production and concentration of free radicals in the aqueous phase is high and the exited free radicals undergo rapid cross-termination in the aqueous phase.In relaxation studies, the concentration and rate of production of free radicals in the aqueous phase fall rapidly to comparatively low values. The exited free radicals are then more likely to undergo re-entry into the latex particles than to cross-terminate in the aqueous phase. These results suggest that for styrene the value of a can vary from + 1 to - 1 according to the concentration and rate of production of free radicals in the aqueous phase. ANALYSIS O F D A T A FOR CARBON TETRACHLORIDE When carbon tetrachloride was added to the styrene it was found that the calculated values of k were relatively insensitive to the assumed value of a. It was therefore adequate to calculate k using the slope-intercept method2* applied to the relaxation data with a = + 1 (see table 1).Note also that we have previously presented both relaxation results3 and approach to the steady-state data2 for the seeded emulsion polymerization of styrene that were interpreted using an exited free radical fate parameter equal to zero. From the present studies, the values of a that should have been used previously are + 1 and - 1, respectively. Fortunately, the differences in the numerical values that would be obtained using a = 0 and the correct values for a lie within the precision of the data for almost all of the experiments reported. [CC141 / 10 mol d ~ n - ~ [CBr4],’10 rnol d ~ n - ~ F CC14 7 CBr4 FIG. 3.--Ef€ects of different concentrations of chain transfer agents on the exit rate coefficient for latex R17/5: (a) carbon tetrachloride; (h) carbon tetrabromide.EXIT RATE OF FREE RADICALS FORMED FROM CHAIN-TRANSFER AGENTS Having extracted the rate coefficients for exit from the radiolysis experiments, we will now discuss their variation with the amount and type of chain-transfer agent. The exit rate coefficients from relaxation studies in the presence of differing amounts of the chain-transfer agents carbon tetrachloride and carbon tetrabromide are displayed in fig. 3. These results were obtained as described above using a = + 1. As shown in table 4, the chain-transfer rate constant for carbon tetrachloride is2138 SEEDED EMULSION POLYMERIZATION OF STYRENE TABLE 4.-TRANSFER, SOLUBILITY AND EXIT DATA FOR THE SYSTEMS STUDIED transfer % chain-transfer rate constant free radicals water solubility agent /dm3 mol-l s-l escaping /mol dm-3 styrene 1 .4 ~ 2 4 x 10-3 carbon tetrachloride 2.3 0.5 5 x 10-3 carbon tetrabromide 9.5 x 104 0.003 7 x 10-4 approximately two orders of magnitude larger than that for transfer to the monomer styrene; that for chain transfer to carbon tetrabromide is approximately seven orders of magnitude greater than that for transfer to rn~nomer.~ The results presented in fig. 3 show that the exit rate coefficient increases linearly with increasing concentration of both carbon tetrachloride and carbon tetrabromide. This result is consistent with a diffusion/transfer mechanism for the exit process, as is also suggested by the dependence of the exit rate coefficient on particle size;2 two important cautions with regard to this result will however be mentioned below.Note also that, at comparable molar concentrations in the latex particles, the increase in the exit rate coefficient produced by carbon tetrabromide is approximately forty times that produced by carbon tetrachloride. This may be attributed qualitatively to the greater chain-transfer rate coefficient for CBr, compared with that for CCl,. More low-molecular-weight free radicals are generated in the presence of CBr, than in the presence of CCl,, and some of these escape from the latex particles into the aqueous phase. One caveat with regard to a simplistic diffusion/transfer mechanism for exit is as follows: the increases in the exit rate coefficient induced by CBr, and CCl,, while linear in the concentration of these substances, are not, however, commensurate with those expected if the probability of exit of the free radicals produced by transfer to these chain-transfer agents was the same as that for free radicals generated by transfer to monomer.This is illustrated by the following calculation. The mean interval between successive chain transfers to monomer is l/ktr, , C,, where ktr, is the rate constant for chain transfer to monomer and C, is the concentration of monomer in the latex particles. Setting ktr, dm3 mol-l s-l and C, = 5.8 mol dm-3,27 trans- fer to monomer is calculated to occur once every 12 s. An exit rate coefficient k = 1.6 x lop3 s-l (see fig. 3) in the absence of chain-transfer agents implies that an exit event occurs at average intervals of I l k = 625 s.This corresponds to ca. 2% of the free radicals generated by transfer to monomer actually escaping from the latex particles. By way of comparison, consider a seeded emulsion polymerization of styrene containing 3 % carbon tetrabromide. The rate constant for chain transferlo ktr,cHr, = 9.5 x lo4 dm3 mol-l s-l implies that ca. 520 chain-transfer events occur every second. This assumes that the carbon tetrabromide is distributed equally between the emulsion droplets and the particles and that chain transfer to monomer can be ignored under these conditions. The measured exit rate constant k = 1.6 x lop2 s-l corresponds to an exit event occurring once every 63 s. Accordingly, only ca. 0.003% of free radicals generated by chain transfer to carbon tetrabromide actually escape from the latex particles.This is only approximately one seven- hundredth of the probability of exit of free radicals generated by transfer to monomer. The linearity of the data in fig. 3(6) implies that this probability remains constant as the concentration of carbon tetrabromide increases. A corresponding calculation for = 1.4 xLICHTI, SANGSTER, WHANG, NAPPER A N D GILBERT 21 39 carbon tetrachloride shows that ca. 0.5% of the free radicals generated by chain transfer underwent exit from the latex particles. This is only one-quarter of the value observed for styrene but one hundred and fifty-times greater than that calculated for carbon tetrabromide (see table 4). The results demonstrate clearly that the rate of chain transfer by the propagating chains is only one of the factors governing the observed exit rate coefficient. Several other factors might be expected to be important in determining the value of k : (a) the relative solubilities of the low-molecular-weight free radical species in both the aqueous phase and the swollen latex particles; (b) the relative speed at which monomer molecules add to the low-molecular-weight free radicals generated by chain transfer ; (c) the viscosity inside the latex particles (this was probably similar in all the experiments described above).As shown in table 4, the solubilities of styrene and carbon tetrachloride in water are not dramatically different, although the solubility of carbon tetrabromide is smaller. This suggest? that the solubilities of the corresponding atom-abstracted free radicals would not be greatly different.Accordingly, it would appear that the primary factor that controls the exit rate once the low-molecular-weight free radicals are generated by chain transfer may be their rate of reaction with monomer. Addition of styrene molecules to the low-molecular-weight free radicals reduces their solubilities in water so significantly as to render the resulting growing free radicals incapable of undergoing exit from the latex particles. One simple explanation for the data shown in table 4 is that *CBr, is significantly more reactive with styrene than is *CCl,, which in turn is significantly more reactive than the hydrogen abstracted monomeric species, presumably CH,=C-C,H,.The greater reactivity of CBr, and *CCI, could well be analogous to the postulated complex formation between polystyryl free radicals and carbon tetrahalidesll Note that this explanation requires that, for these three free radical species, the following principle be operative: the more easily is a free radical formed, the faster is its rate of reaction with styrene monomer. Whether this principle has more general validity remains to be determined. EFFECTS OF C HA I N-TR A N SFER AGENTS ON CHEMICALLY INITIATED POLYMERIZATIONS As mentioned in the theoretical section of this paper, the interpretation of data in chemically initiated systems is more complex than in the radiolysis studies, because it is now more difficult to go from the experimental observable (i.e.the time dependence of x) to the quantity necessary to extract values of k from the data, i.e. the value of $1) together with some independent information on pA and/or a. We first present a qualitative discussion of the observed behaviour of x(t) with different chain-transfer agents in chemically initiated systems. Some effects due to the presence of the chain-transfer agent carbon tetrabromide on the kinetics of the seeded emulsion polymerization of styrene initiated by potassium peroxydisulphate are displayed in fig. 4. At the higher initiator concentration (5.0 x lo-, mol dm-3), the presence of the carbon tetrabromide led to a relatively small reduction in the rate of polymerization at all concentrations studied [see fig. 4(a)]. ?‘he rate decreases monotonically with increasing concentration of chain-transfer agent. However, the presence of 1 % carbon tetrabromide appears to have a disproportion- ately large effect on the polymerization rate.In the presence of 6% carbon tetrabromide, the rate of polymerization at this higher initiator concentration was never less than one-half of the rate observed in the absence of chain-transfer agent. In contrast, at the lower initiator concentration (5.0 x mol dm-,), the initial rate of polymerization was only one-seventh of that in the absence of additive at the same2140 SEEDED EMULSION POLYMERIZATION OF STYRENE concentration of the chain-transfer agent. At this initiator concentration it was also found that the addition of 3 and 6% carbon tetrabromide produced almost identical reductions in rate at early times, although the rate of polymerization in the presence of 3% carbon tetrabromide was greater than that for 6% carbon tetrabromide at longer times.The addition of 10% carbon tetrabromide reduced the rate of polym- erization but the reduction in rate at early times was only approximately one-half that observed with 3 and 6% carbon tetrabromide. The shape of the fractional conversion against time curve was also different in this case: whereas the 3 and 6% carbon tetrabromide curves displayed a characteristic ‘knee’ point, this was absent from the 10% carbon tetrabromide curve. 0.8 0.6 0 . 4 g 0.2 $ 0 .... ;n a, C - cd 0 0 .d +4 $ 0.4 0.3 0.2 0 . I 0 t (=) 1 / I- / - 0 50 100 150 time/min FIG. 4.-Effects of different concentrations of carbon tetrabromide on the rate of the seeded emulsion polymerization of latex R007/ 15 initiated by potassium peroxydisulphate.Initiator concentration : (a) 5.0 x and (b) 5.0 x mol dm-3; % CBr,: curve 1, 0; 2, 1; 3, 3; 4, 6; 5 , 10. RETARDATION EFFECT OF CARBON TETRABROMIDE Before proceeding further with the discussion of the chain-transfer effects of carbon tetrabromide on the heterogeneous polymerization kinetics, it is first necessary to consider its role as a retarder of the homogeneous polymerization of styrene. It has long been knownl1-l6 that carbon tetrabromide may decrease the rate of polymerization of styrene, the reduction in rate reaching a plateau at a ratio of additive to monomer concentrations of ca. 5 x lod4. Several different explanations for the observed retardation have been proposed: the low reactivity of radicals such as CBr,,14 or theirLICHTI, SANGSTER, WHANG, NAPPER AND GILBERT 2141 adduct with styrene,14~17 towards styrene; the formation of a retarder such as hydrogen bromide;I3 or, most likely, the formation of a complex between the growing polystyryl radicals and carbon tetrabromide.l1 Such complexed radicals are assumed to be less reactive towards propagation than the uncomplexed species. In these heterogeneous experiments, 1 % carbon tetrabromide in the styrene corresponds to a retarder to monomer concentration in the particles ratio of 3 x assuming ideal mixing. This is almost an order of magnitude greater than that required in bulk systems for maximum retardation to be exhibited.Consequently the maximum decrease in the apparent propagation rate constant due to the presence of carbon tetrabromide ought to be manifested by the addition of 1 % additive. As shown in fig. 4(a), the effect of 1% carbon tetrabromide on the rate of polymerization at high initiator concentration was relatively small ( 5 1273, although it was disproportion- ately large compared with the effects of 3 and 6% carbon tetrabromide. Note that not all of the reduction in rate at 1% concentration must necessarily be attributed to a decrease in the apparent propagation rate constant; an increase in exit rate constant may also contribute to a decrease in ti and, hence, to a decrease in rate. We conclude that although a retardation effect was probably operative in these experiments, as witnessed by the disproportionately large effect of the addition of 1 carbon tetrabromide, the overall reduction in rate due to the decrease in the apparent propagation rate constant was relatively small (probably 5 12%).For this reason, the discussion set forth below concerning the larger reductions in rate at the lower initiator concentration will ignore this small perturbation due to an apparent decrease in the propagation rate constant. C HA I N-T R A N S FE R EFFECTS OF CAR B ON TETRA B R 0 MID E It was pointed out in the theoretical section that for chemically initiated systems, the precise determination of fi was difficult. However, in view of the discussion given above, we may make a semi-quantitative data interpretation by assuming that k , C , for these systems is the same in systems containing chain-transfer agent and in those that do not, and (for the latices used in the present study) we take the value of k , C, from earlier work.* We emphasize that, because of this, the following treatment is only semi-quantitative; nevertheless, certain obvious trends will enable mechanistic deductions to be made from the values of ti found by invoking the above assumption.We have two controllable variables in the systems under study: the concentration of chemical initiator and the concentration (and nature) of chain-transfer agent. We now make qualitative mechanistic deductions based on trends in ti,, observed by changing each of these variables. HIGH INITIATOR CONCENTRATION Some understanding of the reasons for the differing effects of carbon tetrabromide on the polymerization kinetics at different initiator concentrations becomes apparent from estimates of ti,,.In the absence of CBr,, tiss values of 0.50 and 0.17 were found for initiator concentrations of 5.0 x mol dm-3, respectively. Now, from eqn (l), we have ti,, = 1 /(2 + k / p ) , irrespective of the fate of exited free radicals. From this, it is clear that ti,, = 0.5 only if p 9 k . To decrease the value of ti,, in this case to below 0.5 would require a dramatic increase in k since ?is, falls below 0.5 only if k 2 p. Carbon tetrabromide, of course, would not be expected to reduce significantly the value of p. Indeed, as shown below, the value of p may well be increased in the presence ofcarbon tetrabromide.Hence, if tiss is close to 0.5, the effects ofchain-transfer agents are muted because the entry rate coefficient is relatively large. This is what was observed at the higher initiator concentration. and 5.0 x2142 SEEDED EMULSION POLYMERIZATION OF STYRENE LOW INITIATOR CONCENTRATION At low initiator concentration, ti,, < 1 so that from eqn (3), ti,, z p / k . Any significant increase in k will be manifested by a significant decrease in ti,, and thus by a significant reduction in the rate of polymerization. Carbon tetrabromide would therefore be expected to be more effective in reducing the rate of polymerization at lower initiator concentrations than at higher concentrations. Fig. 2(b), however, shows that this explanation of the effects of carbon tetrabromide is incomplete.At early times, the addition of 6% carbon tetrabromide at the lower initiator concentration was no more effective in reducing the rate of polymerization than was the addition of half that amount. The addition of 10% carbon tetrabromide was even less effective than the addition of either of the two lower concentrations. Clearly, at least one additional phenomenon must intrude. For this reason, it is convenient to establish first the mechanistic phenomena in the simpler cases of 3 and 6% added CBr,, and then consider the more complex case of 10% CBr,. 3 AND 6% ADDED CARBON TETRABROMIDE The following discussion shows, unexpectedly, that the comparable rates observed at low conversions with 3 and 6% carbon tetrabromide can be attributed, at least in part, to an increase in the rate of entry of free radicals into the latex particles in the presence of carbon tetrabromide. The rate of polymerization is directly proportional to k , C, 5, where k, is the propagation rate constant.As discussed above, the value of k,, may be reduced by the presence of carbon tetrabromide. The swelling of the latex particles by monomer is unlikely to have been significantly increased in the early stages of the reaction for the 3 and 6% carbon tetrabromide runs since the additive was added to the monomer and few, if any, oligomeric species formed by chain transfer would be present. Accordingly, the comparable rates observed in these runs imply that ti was similar in both cases. The relaxation results presented above show that k was significantly greater in the presence of 6% chain-transfer agent and, since ti = 1 /(2 + k/p), this implies that p must also have been increased by the presence of the larger amount of carbon tetrabromide. Perhaps the simplest explanation for the surprising conclusion that p is increased by the presence of the chain-transfer agent resides in the production of hydrophobic free radicals in the aqueous phase.Using the data shown in table 4, together with the value of k,, = 258 dm3 mol-l we calculate that in the aqueous phase the ratio of the probability of transfer to carbon tetrabromide to the probability of propagation (kt,,cB,4 [CBr,]/k,[St]) is ca. 4 for the 6% additive system. This high probability of transfer to carbon tetrabromide could result in the production of CBr, radicals which, because of their hydrophobic character, might more readily enter the latex particles than the growing oligomers, thus increasing p.The foregoing calculation is, however, far from conclusive because the assumption of ideal mixing might not be correct; moreover, the values of the rate constants for transfer and propagation are those for high-molecular-weight species, not oligomers. Further, CBr, free radicals might be less hydrophobic than the oligomeric free radicals. For these reasons, we provide an alternative or additional mechanism for the influence of carbon tetrabromide on p , based upon our studies18 of the nucleation mechanism in emulsion polymerization. These experiments suggest that there is a range of oligomeric species that enter the seed latex particles, including colloidal precursor particles.These particles, which are composed of aggregates of ' insoluble' oligomeric species, are characterized by a very slow growth rate, probably arising from poor swelling of the aggregate by monomer due to their residual hydrophilic character.LICHTI, SANGSTER, WHANG, NAPPER AND GILBERT 2143 Free radicals associated with such precursor particles propagate slowly relative to those in mature latex particles. They also enter seed particles more slowly than surfactant-like oligomeric species. The presence of chain-transfer agents could increase the rate of exit of free radicals from the precursor particles and thus stimulate the rate of entry of surfactant-like oligomeric species. Additional experiments will be required to clarify the details of the mechanism whereby chain-transfer agents influence the entry rate of free radicals into latex particles.One further piece of evidence that supports the conclusion that carbon tetrabromide increases p is the unexpected linearity of the rate curves obtained at low conversions with 3 and 6% additive. It might be expected that as the carbon tetrabromide in the latex particles is consumed at early times, so the exit rate constant for the particies would decrease. Thus ti, and consequently the rate of polymerization, should increase as polymerization proceeded. The fact that no such rate increase was observed at early times is presumably a consequence of the reduction in p due to the depletion in the aqueous phase of the carbon tetrabromide.Under the conditions of these experiments, the two effects on ii, viz. a decrease in both k and p, apparently cancelled approximately at low conversions with 3 and 6% carbon tetrabromide present. The incorporation of chain-transfer agents into an emulsion polymerization can lead, under certain conditions, to enhanced swelling of the latex particles by monomer.19 This effect, however, is readily discounted in the present study. First, in these experiments the supposition that all of the monomer initially present was inside the latex particles would only increase the concentration of monomer in the latex particles from ca. 6 to ca. 7 mol dm-3. Thus enhanced swelling can increase the rate of polymerization by ca. 15 % at most.Secondly, to achieve a significant enhancement effect, the weight of oligomeric species produced by chain-transfer agent must be at least comparable to the weight of polymer present. In these experiments, the ratio of the weight of seed polymer to monomer is approximately one-half. Consequently, at fractional conversions below 0.15, which is primarily what is discussed here, there is insufficient weight of oligomeric species present to induce enhanced swelling. One additional feature of the 3 and 6% curves that requires explanation is their general shape. Both show two essentially linear regions interconnected by a fairly sharp ‘knee’ point that occurs at approximately the same fractional conversion (ca. 0.04). It was previously1 proposed that this knee point represents the point at which the carbon tetrabromide in the latex particles is consumed by rapid incorporation into the polymer chains.Beyond the knee point, carbon tetrabromide diffuses from the monomer droplets into the latex particles and for this reason the rate of polymerization after the knee point is always less than that observed in the absence of chain-transfer agents. This depletion explanation for the knee point is supported by the calculation of the relative rates of consumption of monomer and carbon tetrabromide at 3% additive. The data presented in table 4, together with the value of k , adopted above, imply that the relative rate of consumption of carbon tetrabromide to that of monomer is ca. 0.6 and so the chain-transfer agent should be depleted within the particles at a fractional conversion of 0.05, in fair agreement with the observed value (0.04). Note that the oligomeric species produced by chain transfer in the early stages of the reaction could cause enhanced swelling of the latex particles by the monomer.This might contribute to the more rapid polymerization rate at higher conversion^.^^^ 2o Experimentally, however, it is found that enhanced swelling actually reduces the rate of polymerization owing to an abnormally low value for the entry rate parameter.213 22 One corollary of the foregoing theory is that the conversion at which the knee point occurs should be relatively independent of the amount of chain-transfer agent present. Thus, for example, doubling the initial amount of carbon tetrabromide in the latex2144 SEEDED EMULSION POLYMERIZATION OF STYRENE particles would also double the rate of chain transfer and thus double the rate of consumption of carbon tetrabromide.The data presented in fig. 4(b) for 3 and 6% chain-transfer agent show that this prediction of the independence of the position of the knee point is confirmed experimentally. The alteration in p observed in chemically initiated systems in the presence of chain-transfer agent is probably unimportant in the radiolysis studies because of the very high flux of aqueous-phase free radicals with the latter technique. However, the complications occurring in chemically initiated systems warn against a too simplistic interpretation of the linear dependence of k on the concentration of chain-transfer agent which was observed with the radiolysis studies.While such a linear dependence is not inconsistent with a diffusion/transfer mechanism for exit, a quantitative interpretation of this linearity would require the complexities which appear to be operative in chemically initiated systems to be taken into account. 10% A D D E D C A R B O N TETRABROMIDE The shape of the rate curve obtained with the addition of 10% carbon tetrabromide is different from that for 3 and 6% additive. It displays no knee point and for the first few minutes corresponds to a rate of polymerization marginally in excess of that observed in the absence of chain-transfer agents. The rate of polymerization shows a monotonic decline as polymerization proceeds. It was established above that the addition of carbon tetrabromide increases the rate of entry of free radicals into the latex particles.It is suggested that the observed monotonic decline in rate is a consequence of the progressive reduction in the entry rate concomitant with a decreasing concentration of carbon tetrabromide in the aqueous phase. The latter is occasioned by the consumption of carbon tetrabromide in the latex particles as polymerization proceeds. Note that the Mayo equation23 for the degree of polymerization of the species produced at this high concentration of carbon tetrabromide predicts a value less than one, which is physically impossible and suggests that chemical reactions may well occur prior to the commencement of polymerization. CONCLUSIONS It has been shown that the exit rate coefficient k for the seeded emulsion polymerization of styrene increases linearly with increasing concentration of chain- transfer agent for both CCI, and CBr,.Moreover, CBr, is significantly more effective in increasing k than is CCl,, as would be expected from its much larger value for the chain-transfer rate coefficient. While this is consistent with a diffusion/transfer mechanism for exit, as also suggested by previous studies2 on the size dependence of k , it is important to note that diffusion and transfer alone are insufficient completely to describe the exit process, since the efficiency of exit of free radicals formed by chain transfer follows the inverse of the chain-transfer order: CBr, < CCl, < styrene. This may well be a consequence of the relative rates of reaction with monomer of the free radicals formed by chain transfer.Moreover, the consumption of chain-transfer agent and the effect of exited oligomeric species originating from the chain-transfer agent must also be taken into account in a detailed mechanistic description of the exit process. It was found that for systems initiated by potassium peroxydisulphate, the presence of carbon tetrabromide reduced the rate of polymerization. The reduction in rate was significantly greater for ii < f than for ii = 8, as expected theoretically. At high concentrations of carbon tetrabromide, the rate of entry of free radicals into the latex partides was increased significantly by the presence of the chain-transfer agent.LICHTI, SANGSTER, WHANG, NAPPER AND GILBERT 2145 As a result, the rate of polymerization passed through a minimum owing to the counterbalancing of the increased exit rate by the increased entry rate.Finally, a combination of steady-state and relaxation runs in the presence of carbon tetrabromide allowed the fate of the exited free radicals to be determined. When the seeded emulsion polymerization occurred in the presence of the y-ray source, exited free radicals underwent cross-termination in the aqueous phase. This is in agreement with what was found previously5 for chemically initiated systems. When the system was removed from the pray source, as in the relaxation studies, re-entry of exited free radicals into latex particles occurred. This change in the fate of the free radicals appears to be associated with the relative concentration and rate of production of free radicals in the aqueous phase. When these are relatively high, the exited free radicals undergo cross-termination in the aqueous phase. When low, the exited free radicals undergo re-entry. We gratefully acknowledge the financial support of both the Australian Research Grants Committee (for B. C. Y. W.) and AINSE (for G. L). The Electron Microscope Unit of the University of Sydney is thanked for their provision of facilities. We also thank Professor C . H. Bamford F.R.S. for helpful discussion. B. C. Y. Whang, G. Lichti, R. G. Gilbert, D. H. Napper and D. F. Sangster, J. Polym. Sci., Polym. Lett. Ed., 1980, 18, 711. B. S. Hawkett, D. H. Napper and R. G. Gilbert, J. Chem. SOC., Faraday Trans. I, 1980, 76, 1323. S. W. Lansdowne, R. G. Gilbert, D. H. Napper and D. F. Sangster, J . Chem. Soc., Faraday Trans. I, 1980, 76, 1344. 4 G. Lichti, B. S. Hawkett, R. G. Gilbert. D. H. Napper and D. F. Sangster, J. Polym. Sci., Polym. Chem. Ed., 1981, 19, 925. B. C. Y. Whang, D. H. Napper, M. J. Ballard, R. G. Gilbert and G. Lichti, J. Chem. SOC., Faraday Trans. I, 1982, 78, 11 17. D. T. Birtwistle and D. C. Blackley, J. Chem. SOC., Faraday Trans. I, 1981, 77, 397 and references therein. M. J. Bowden, in Macromolecules, ed. F. A. Bovey and F. H. Winslow (Academic Press, New York, Polymer Handbook, ed. J. Brandrup and E. H. Immergut (Wiley, New York, 2nd edn, 1975). ' H. G. Elias, Mucromolecules (Plenum Press, New York, 1977), vol. 2, p. 784. 1979). lo C. H. Bamford and S. N. Basahel, J. Chem. SOC., Faraday Trans. I, 1978, 74, 1020. l 1 C. H. Bamford, J. Chem. Soc., Faraday Trans. I , 1976, 72, 2805. Iz H. Suess, K. Pilch and H. Rudorfer, Z. Phys. Chem., Teil A, 1937, 179, 361. l 3 J. W. Breitenbach and H. Karlinger, Monatsh, 1951, 82, 245. l 4 A. S. Dunn, B. D. Stead and H. W. Melville, Trans. Faraday Soc., 1954, 50, 279. l 5 R. A. M. Thomson and I. R. Walters, Trans. Faraday Soc., 1971, 67, 3046. l7 H. N. Friedlander and M. S. Karasch, J. Org. Chem., 1949, 14, 239. lR G. Lichti, R. G. Gilbert and D. H. Napper, to be published. l 9 J. Ugelstad, P. C. Msrk, K. Herder Kaggerud, T. Ellingsen and A. Berge, Adv. Colloid Interface Sci., 2o B. S. Hawkett, D. H. Napper and R. G. Gilbert, J. Chem. SOC., Faraday Trans. 1 , 1981, 77, 2395. '' B. Chamberlain, D. H. Napper and R. G. Gilbert, J. Chem. SOC., Faraday Trans. I, 1982,78, in press. 22 D. Wood, B. C. Y. Whang, G. Lichti, D. H. Napper and R. G. Gilbert, to be published. 23 F. Mayo, J . Am. Chem. SOC., 1943, 65, 2324. D. A. J. Harker, R. A. M. Thomson and I. R. Waters, Trans. Faraday Soc., 1971, 67, 3057. 1980, 13, 101. (PAPER 1/1311)

ISSN:0300-9599    

DOI:10.1039/F19827802129

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. Soc., Furuduy Trans. I, 1982, 78, 2147-2156 Formation of Monodispersed Colloidal Cubic Haematite Particles in Ethanol + Water Solutions BY SHUICHI HAMADAT AND EGON MATIJEVIC* Department of Chemistry and Institute of Colloid and Surface Science, Clarkson College of Technology, Potsdam, New York 13676, U.S.A. Received 18th August, 1981 Cubic particles of colloidal haematite of narrow size distributions were prepared by aging ferric chloride solutions in water+ethanol mixtures. It was shown that a-Fe,O, formed by phase transformation from 8-FeOOH precipitated first. The kinetics of this conversion were followed at different temperatures, pH and chloride concentrations. The role of alcohol in the studied system is discussed and the rate of growth explained in terms of the theory of Burton, Cabrera and Frank.Numerous studies have shown that the precipitation of metal (hydrous) oxides from corresponding salt solutions is strongly affected by a variety of parameters, among which the pH, temperature, concentration of the reacting species and the nature of anions play dominant roles. Despite this sensitivity to experimental conditions, procedures have been established which result in the formation of colloidal dispersions of these materials consisting of particles exceedingly uniform in size and of different shapes, including spheres.' Specifically, ' monodispersed ' ferric basic sulphates are obtained by hydrothermal aging of ferric sulphate solutions2 and P-FeOOH and haematite from ferric chloride solution^.^ While P-FeOOH always appears in the form of acicular particles, a-Fe203 crystals of a variety of geometries can be prepared by minor changes in the experimental conditions.Only a few quantitative investigations on the effect of mixed solvents on the modification of the crystal habit have been reported in the literat~re.~ Since the composition and the shape of metal (hydrous) oxide particles depend on the medium from which they precipitate, it is to be expected that an addition of a miscible organic liquid to an aqueous solution should have a considerable effect on the properties of the resulting solids. Indeed, recently it was shown that on aging at elevated tem- peratures, ferric chloride solutions in mixed water + alcohol solvents may yield, under certain conditions, cubic haematite particles of great ~niformity.~ This process is investigated here in greater detail and the effect of the alcohol (ethanol) is discussed.It is shown that these dispersions are generated by phase transformation from a-FeOOH. The growth rate of cubic particles can be explained in terms of the theory of Burton, Cabrera and Frank. The availability of well defined dispersions of haematite is of considerable interest in various fundamental studies of this important material. EXPERIMENTAL MATERIALS Stock solutions of ferric salts at high concentrations (2.1-3.6 mol dm-3) were prepared by dissolving the corresponding salts in doubly distilled water at room temperature (except for t On leave of absence from Science University of Tokyo, Japan. 21472148 FORMATION OF CUBIC HAEMATITE PARTICLES ferric sulphate which was dissolved at ca.40 "C) and then filtering through a Millipore membrane of 0.2 pm pore size before storage. These solutions showed no visual changes on standing for 3 months. All chemicals used were reagent grade of highest purity. Millipore filters were carefully washed before use. SOL PREPARATION All sols were generated by heating solutions of ferric salts at specified temperatures (80, 90 or 99 "C) for varying periods of time (up to 200 h). The stock solutions were first mixed with the desired amounts of the corresponding acids (to prevent hydrolysis on dilution) and then with water and ethanol to give an alcohol content of 50% by volume: 40 cm3 samples of the thus prepared solutions were pipetted into 50 cm3 screw-cap culture tubes, tightly stoppered and placed in a preheated laboratory constant temperature oven or in an oil bath.The samples were spaced as far apart as possible to secure uniform heating. After aging, the tubes were quenched to room temperature and the particles were separated by centrifugation and washed with doubly distilled water. pH MEASUREMENT The acidity of each system was determined at room temperature (ca. 25 "C) with a Radiometer model PHM 26 pH meter before and after aging. An operational pH* unit (where * denotes pH of the mixed solvent systems) is defined as6 E - E(s) pH* = ~H*(s) + ~ 2.3 RT/F where pH*(s) is the value for a selected buffer solution of the same mixed solvent and E(s) and E are the corresponding e.m.f. values of the buffer and the sample solution, respectively.The pH*(s) values of buffers of mixed solvent systems6, were used to calibrate the pH meter. The pH* at higher temperatures (90 and 99 "C) in mixed solvents was evaluated from the expression8 (2) K,* (25OC) was taken from the work of Woolley et al.;9 for 50% (by volume) ethanol the extrapolation gave a value of pK,* = 14.67. Using for the heat of ionization of water in a 50% ethanol+water mixturelo AHo = 51.0 kJ mol-', and taking AH" to be constant over the temperature range of interest, pK,* at 90 and 99 "C was calculated to be 13.07 and 12.89, respectively. This estimate is based on the assumption that eqn (2) applies to mixed solvents. PH*(T,) = PH*(T,) x PK,*(T,)IPK,*(T,). ELECTRON MICROSCOPY For transmission and scanning electron microscopy the particles were redispersed in an ultrasonic bath and then deposited on corresponding sample holders.Particle size histograms were obtained from calibrated transmission micrographs. DETERMINATION OF FERRIC AND CHLORIDE CONTENTS The content of iron in the solutions and in the precipitated solids after aging was determined spectrophotometrically using 1,lO-phenanthroline as the complexing agent. For this purpose the particles were dissolved in hydrochloric acid. The chloride content in the solutions was analysed by the Volhard method. RESULTS FORMATION OF HAEMATITE PARTICLES IN WATER+ETHANOL MIXTURES To evaluate the precipitates formed by forced hydrolysis at 99 OC, ferric chloride solutions of concentrations from 1 x to 2 x 10-1 mol drnp3, which contained hydrochloric acid in concentrations between 5 x and 1 x 10-1 mol dm-3 and ethanol to give a final content of 50% (by volume), were aged for 48 h in a preheated laboratory oven. Precipitation occurred in all systems, except those containing theJ. Chern.SOC., Faraday Trans. 1, Vol. 78,part 7 S. HAMADA AND E. MATIJEVIC Plate 1 (Facing p . 2149)s. HAMADA AND E. MATIJEVIC 2149 -2.0- highest concentration of HCl. The solid phase always consisted of two kinds of particles, i.e. P-FeOOH and a-Fe,O,, which were identified by the X-ray powder diffraction technique.ll9 l2 Fig. 1 displays the FeC1,-HCl concentration domain covered in these experiments with the indication of the particle morphology in each system studied. The upper and the lower symbols refer to the P-FeOOH and haematite species, respectively.In most cases, P-FeOOH particles were rod-like with varying acicularity, whereas a-Fe,O, particles differed considerably in shape, depending on the I I R R S I N 5 E E I'E I I i N - I I d -0.5 I I I I I R C C S !I ? U F! RS R 5 5 R EC EC ic EC EC c E -1.0 R C R c R r! r! c ' R R R c c S S,R N c E N S I N 1 R I t i I N -3.5 -3.0 - 2.5 - 2.0 -1.5 -1 .o log( [ HC11 /mol dm-3) FIG. 1.-Concentration domain of FeCl, and HC1 in 50% (by volume) ethanol+water solutions which were aged for 48 h at 99 OC. The upper and the lower symbols indicate the morphology of the precipitated P-FeOOH and haematite particles, respectively: C, cubic; E, ellipsoidal; I, irregular; S , spindle; R, rod-like; N, no precipitation.concentration of the reacting components. Cubic particles of narrow size distribution were obtained under a limited set of conditions within the dashed boundary in fig. 1. The initial pH* of the solutions giving these uniform sols was 1.7; on aging the pH* dropped to 1 .O-1.4. Because of the great difference in their size, cubic haematite particles were readily separated from P-FeOOH needles by centrifugation (for 30 min at ca. 3000 r.p.m.) or by free settling (for 1-5 days). Plate 1 shows a transmission and a scanning electron micrograph of the separated cubic haematite particles. Such particles could be obtained under essentially the same concentration conditions (of FeCl, and HCl) as long as the temperature exceeded 80 OC and the ethanol content was > 40%.Additional experiments were carried out to investigate the effects of chloride ion concentration and of pH* on the formation of cubic haematite particles. For this purpose reference systems were chosen, which under the conditions shown in fig. 1 give uniform dispersions. In one series the concentration of C1- was adjusted with NaCl to between 5.7 x lo-, and 1.5 x 10-1 mol dm-, at a constant ferric chloride concentration (1.9 x lop2 mol drn-,) and a constant ionic strength of the supernatant liquid of 0.2 mol dmP3. The latter was adjusted with NaClO, after all the other ionic species in the solution were taken into consideration. In another series of experi- ments the ferric chloride concentration was varied between 1.9 x loP2 and 3.5 x lo-, mol dm-, at a constant C1- content of 0.2 mol dm-, adjusted with NaCl.In these experiments the initial pH* was kept at 1.6 _+O. 1. All the systems described21 50 FORMATION OF CUBIC HAEMATITE PARTICLES yielded cubic haematite particles (in addition to P-FeOOH) on aging at 90 O C for up to 150 h. When ethanol was substituted with methanol only irregularly shaped particles were generated. However, solutions containing propan-2-01 or t-butyl alcohol yielded cubic particles when the alcohol content was > 30% (by volume). Aging solutions of ferric salts other than the chloride (nitrate, perchlorate and sulphate) in the presence of ethanol did not produce well defined precipitates under the same conditions. Solids from systems containing nitrate and perchlorate anions mostly consisted of irregular haematite particles, while ferric basic sulphate formed in solutions of iron(1Ir) sulphate.RATE OF CONVERSION OF P-FeOOH INTO HAEMATITE The formation ofcubic haematite particles in acidified ferric chloride water + ethanol solutions was studied over extended aging times at two different temperatures (90 and 99 "C). In all cases p-FeOOH appeared first as the solid precipitate, while haematite resulted from a phase transformation process. The precipitation of 8-FeOOH was essentially completed after ca. 20 min of aging, whereas the formation and growth of cubic haematite particles proceeded over a longer period of time. The conversion of 8-FeOOH into a-Fe,O, was followed by chemical analysis of the amount of each of the two ferric compounds in the solid phase.For this purpose the precipitate was removed from the dispersion by ultracentrifugation of samples at 20000 r.p.m. for 1 h and subsequent filtration through a Millipore membrane of 0.08 pm pore size. The filtrate was completely free of solids, as verified by the absence 10, I 0 5 0 100 150 tlh FIG. 2 . 4 ~ ) Percentage of soluble Fe"' species remaining in the supernatant solution after aging at (0) 90 and (0) 99 "C for various periods of time. Initial solution contained FeCl, (1.9 x lop2 mol dm-,) and HCl (1.2 x mol dm-3) in 50% (by volume) ethanol+water mixture. (b) Percentage of iron as j?-FeOOH in (a). ( c ) Percentage of iron as a-Fe,O, in (a).s. HAMADA AND E. MATIJEVIC 2151 I I I 0 5 0 100 150 200 t l h FIG. 3.-Change in the length (l/pm) of the edge of cubic haematite particles as a function of time of agmg of 50% (by volume) ethanol+water solutions of FeCI, (1.9 x mol drn-,) and HCl (1.2 x lo-, mol dm-,) at (A) 80, (0) 90 and (0) 99 OC.O.* I 0 L--dldu 0.2 0.4 0.6- 0.8 1.0 1.2 1.4 UCtm FIG. 4.-Histograms of cubic haematite particles precipitated in a 50% (by volume) ethanol + water solution of FeC1, (1.9 x mol drn-,) and HC1 (1.2 x lo-, mol drn-,) aged at 99 O C for (a) 16, (b) 40, (c) 64 and ( d ) 90 h, (i= average length of the edge of the cube in pm).21 52 FORMATION OF CUBIC HAEMATITE PARTICLES of a Tyndall cone. The P-FeOOH and a-Fe,O, particles were then separated as described in the experimental section. The time dependence conversion of rod-like P-FeOOH into cubic a-Fe,O, particles is shown in fig.2. It is clear that the rate of appearance of haematite is the same as the rate of disappearance of p-FeOOH from the mixture. The process was followed for periods longer than shown in this figure and the conversion continued until all the P-FeOOH had disappeared. As one would expect, haematite formation was considerably faster at the higher temperature. The concentration of unchanged ferric species in the supernatant solution remained unchanged during the phase transforma- tion process. The pH* dropped only during the first ca. 20 min and then remained constant. No change in the chloride content in the supernatant solution was noted during the entire duration of the aging experiment. The increase in the size of the cubic haematite particles with the time of heating at 80,90 and 99 OC is shown in fig.3. Except at the very beginning of the conversion process, the average length of the edge of the cubes grows linearly with time. The size distribution at different aging times is illustrated in fig. 4, which gives the histograms obtained from transmission electron micrographs for a sample kept at 99 O C . The apparent rate constants calculated from the linear portions of fig. 3 obey the Arrhenius equation from which the energy of activation for the conversion process of P-FeOOH into haematite is calculated to be E, = 110 kJ mol-l. Finally, fig.5 (a) shows that the apparent rate constant (R) of the phase transforma- tion is independent of the Cl- concentration. Fig. 5(b) indicates that R decreases with increasing acidity.The pH* was varied by altering the FeCl, concentration. Since /?-FeOOH precipitates rapidly, and haematite is formed by phase transformation, we assume that pH* is the only parameter affecting the rate constant in this case. - I s: (0 1 1 -1.3 -1.2 -1.1 -1.0 -0.9 -0.8 log( [ C1-I /mol dm-3 1 I I I I -1.3 -1.2 -1.1 log( [ H'I /mol dm-3 ) FIG. 5.--(a) Dependence of the apparent growth rate constant (R/pm h-*) of cubic haematite on the chloride concentration. Particles were precipitated in 50% (by volume) ethanol + water solutions of FeC1, (1.9 x mol dm-,) and HCl+NaCl (5.7 x 10-2-1.5 x LO-' mol dm-,) aged at 90 OC. The pH* of the supernatant solutions was 1.2. (b) Influence of pH* of the supernatant solution on the apparent growth rate constant (R/pm h-l) of cubic haematite particles precipitated in 50% (by volume) ethanol +water solutions of FeC1, (1.9 x 10-2-3.5 x mol drn-,) at a constant chloride content of 0.2 mol dm-, (adjusted with NaCl), aged at 90 OC.s.HAMADA AND E. MATIJEVIC 2153 DISCUSSION Precipitation of haematite from ferric chloride solutions may proceed either directly or through phase transformation. If a-Fe203 is formed by conversion from amorphous ferric hydroxide, nucleation and crystallization take place within the existing solid phase.13 On the other hand, a crystalline precursor (e.g. P-FeOOH) requires the original solid phase to dissolve first, followed by crystal growth of haematite.l49 l5 This work shows that cubic haematite particles in water +ethanol solutions of ferric chloride are generated through conversion from the originally precipitated P-FeOOH.There is strong evidence that the latter dissolved on longer aging at elevated temperatures with a simultaneous nucleation and growth of a-Fe20,. The effect of increasing acidity (fig. 5) also favours a dissolution mechanism. It is of interest to discuss the reasons why a simple alcohol exerts such a profound effect on the morphology of this common ferric oxide. Lower alcohols enhance the hydrolysis of ferric ions16* l7 and even more so the ferric chloride solute complexation.18-21 These complexes are known to be precursors in the formation of P-FeOOH. Thus, the first effect of alcohol is to promote the precipitation of the rod-like ferric oxyhydroxide. The generation of this solid phase is completed much more rapidly (within 10-20 min) in water+ethanol mixtures than in corre- sponding pure aqueous ferric chloride solution^.^ The fast precipitation of P-FeOOH is also indicated by the sharp decrease in pH* which occurs only during the initial period of aging. The phase transformation of p-FeOOH to a-Fe203 involves no change in pH*, as verified by the experiments.Furthermore, it is not affected by the amount of chloride ions in the solution (at least not within the concentration range studied). The monodisperse nature of the final product is probably due to the slow dissolution of P-FeOOH, which allows for uniform growth of haematite particles after nucleation by incorporation of solute complexes. Thus, the necessary supersaturation in the particle-forming species is maintained until the entire amount of the initial solids is used up.The influence of alcohol on the conversion of P-FeOOH to haematite is most pronounced with respect to particle morphology. The resulting a-Fe203 particles of cubic morphology showed two strong X-ray powder diffraction peaks at d = 0.366 and 0.184 nm, which are characteristic of haematite, while the normally observed strongest peak at 0.269 nm was much weaker.12 Such structure modification reflects a different growth mechanism at crystal faces, which depends on the nature of the solute/solvent interactions. The latter are greatly affected by the presence of alcohol molecules. The influence of mixed solvents on the crystal morphology and on the crystal growth mechanism was examined in terms of the surface entropy factor, a, which is defined as22 24 a = 4 ~ / k T (3) (4) where 4ss, q5ff and dsf represent potential interaction energies between solid blocks, fluid blocks and solid/fluid blocks, respectively.Eqn (3) may be approximated by a relationship which shows that a is proportional to the entropy of phase transition, AS. 23 where k is Boltzmann’s constant and E = 0.5 4,,+0.5 4ff-@,f An estimate of the surface entropy factor can be obtained fromz4 a = 4y/kT ( 5 ) F A R 1 702154 FORMATION OF CUBIC HAEMATITE PARTICLES in which the edge energy y is given as y = d2F (6) F = AHs/2ba2 (7) where d is the height of the monomolecular step for a given crystal face and F is the surface energy25 AH, is the enthalpy of dissolution, a is the lattice spacing and b is the ratio of the binding energy of a molecule in a kink site to the energy required to form a new surface by cleavage of the crystal.Eqn ( 5 ) is strictly applicable only for a cubic lattice, but should offer estimates for other The surface entropy factor a < 3.2 should indicate an inherently rough surface with no energy barrier to crystal growth at low supersaturation. If 3.2 < a < 4.0, the surface is assumed to be smoother but with surface nucleation still possible. Finally, values of a > 4 are characteristic of smooth surfaces, in which case the crystal growth at low supersaturations is only possible in the presence of steps and the diffusion law of Burton, Cabrera and Frank should be applicable.26 Addition of alcohol to an aqueous solution influences q5ff and dsf and causes a given face to grow by a different mechanism.The calculation of a is difficult in general2' and even more difficult for haematite particles in ethanol + water mixtures because of insufficient information in the literature on the necessary parameters. However, a rough estimate yields for this system a very high a value (> 30). Apparently highly hydrated solute species in the presence of alcohol undergo dehydration, causing an increase in entropy. Alcohol acts as an impurity, destroying the normal interfacial structure and providing an easier transition from solution to crystal, thus affecting the growth rate. According to Burton et a1.26 and Bourne et a1.,28 at high supersaturations (0 4 ol, where 0 and o1 are the supersaturation and the critical supersaturation) a crystal may grow linearly according to : In the former case b assumes values between ca.2 and 3. where R is the linear growth rate constant, is the retardation factor for entry of a molecule into a step, l2 is the volume of the molecule, no is the number of molecular positions per unit area on a given surface, h is Planck's constant and AGdesolv. is the activation free energy for desolvation of the constituent species and its entry into the adsorbed layer. Assuming that 0 is reasonably constant at three different temperatures (80, 90 and 99 "C), eqn (8) can be expressed as follows: C = In PQnoa(k/h). (10) The value of AGdesolv. can thus be estimated from eqn (9) using the experimental values of the growth rate at different temperatures.Fig. 6 shows that the rate data given in fig. 3 yield a linear relationship when plotted in terms of eqn (9), from which AGdesolv. is calculated to be 110 kJ mol-l. Note that this value is very close to several listed total evaporation energies, W, for crystal growth from the vapour phase of different materials.26 Using AGdesolv. = 110 kJ mol-1 and reasonable values for the other parameters in eqn (lo), /? is estimated to be 3 x This value is lower than that obtained for crystal growth from the vapour phase (0.1-1. 1),26 which is to be expected if chemical reactionss. HAMADA AND E. MATIJEVIC 2155 -27.5 - 28.0 n - ;C -28.5 - I m 5 . - I -29.0 v E - - 29.5 -30.0 I I I 2.70 2.75 2.80 2.8: 1 0 3 ~ 1 ~ FIG. 6.-Plot of the data shown in fig. 3 according to eqn (9).at the crystal/solution interface take place, as is the case when ferric solute complexes are condensed to form an 0x0-bridge network. The effects of alcohols on crystal growth from aqueous solutions studied before24$ 2 8 y 29 involved simpler systems than that described in this work. It is therefore of interest that the same fundamental principles may be applied to a case as complicated as the conversion of P-FeOOH into a-Fe,O,. The existing theories give a reasonable explanation for the formation of haematite particles of cubic morphology. This work was supported by the Electric Power Research Institute, contract no. RP-966-2. E. Matijevid, Acc. Chem. Res., 1981, 14, 22. E. Matijevid, R. S. Sapieszko and J.B. Melville, J. Colloid Znterface Sci., 1975, 50, 567. E. MatijeviC and P. Scheiner, J. Colloid Interface Sci., 1978, 63, 509. J. W. Mullin, Chem. Ind. (London), 1980, 9, 372. S. Hamada and E. MatijeviC, J. Colloid Interface Sci., 1981, 84, 274. R. G. Bates, Determination of p H (John Wiley, New York, 1964), p. 222. ’ M. Paabo, R. A. Robinson and R. G. Bates, J. Am. Chem. SOC., 1965, 87, 415. R. T. Lowson, Aust. J. Chem., 1974, 27, 105. E. M. Woolley, D. G. Hurkot and L. G. Hepler, J . Phys. Chem., 1970, 74, 3908. lo G. L. Bertrand, F. J. Millero, C. Wu and L. G. Hepler, J. Phys. Chern., 1966, 70, 699. l1 J. D. Bernal, D. R. Dasgupta and A. L. Mackay, Clay Miner. Bull., 1959, 4, 15. l2 V. Kastalsky and M. F. Westcott, Aust. J . Chem., 1968, 21, 1061. l3 W. R. Fischer and U. Schwertmann, Clays Clay Miner., 1975, 23, 33. l4 R. J. Atkinson, A. M. Posner and J. P. Quirk, J. Znorg. Nucl. Chem., 1968, 30, 2371. l5 R. J. Atkinson, A. M. Posner and J. P. Quirk, Clays Clay Miner., 1977, 25, 49. G. Popa, C. Luca and E. Josif, Z. Phys. Chem. (Leipzig), 1963, 222, 49. 70-22156 FORMATION OF CUBIC HAEMATITE PARTICLES l7 E. J. Bowers and H. D. Weaver, Proc. Indiana Acad. Sci., 1961, 17, 101. la K. Bridger, R. C. Pate1 and E. Matijevid, Polyhedron, in press. Is G. Wada and Y. Kobayashi, Bull. Chem. SOC. Jpn, 1975, 48, 2451. 2o G. D. Brykina, N. L. Filippova and T. A. Belyavskaya, Zh. Neorg. Khim., 1976, 21, 2936. G. S. Murty and M. N. Sastri, J . Indian Chem. SOC., 1977, 54, 783. 22 K. A. Jackson, in Liquid Metal and Solidijication (Am. SOC. Metals, Cleveland, Ohio, 1958), p. 174. 23 P. Bennema and G. H. Gilmer, in Crystal Growth: An Introduction, ed. P. Hartman (North Holland, 24 J. R. Bourne and R. J. Davey, J. Cryst. Growth, 1976, 36, 278. 25 W. J. Dunning, in Physics and Chemistry of the Organic Solid State, ed. D. Fox, M. M. Labes and A. Weissberger (Interscience, New York, 1963), vol. 1, p. 412. 26 W. K. Burton, N. Cabrera and F. C. Frank, Philos. Trans. R. SOC. London, Ser. A , 1951, 243, 299. 27 B. Simon and R. Boistelle, J. Cryst. Growth, 1981, 52, 779. 28 J. R. Bourne and R. J. Davey, J. Cryst. Growth, 1976, 36, 287; 1977, 39, 267; 1978, 43, 224; 1978, 29 J. R. Bourne, R. J. Davey and J. McCulloch, Chem. Eng. Sci., 1978, 33, 199. Amsterdam, 1973), p. 263. 44, 613. (PAPER 1 / 1335)

ISSN:0300-9599    

DOI:10.1039/F19827802147

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1982, 78, 2157-2165 Hydrogen Bonding and Proton Transfer in Hydrido-bis-phenolate Complexes in Acetone BY ZENON PAWLAK AND BOGUSLAW NOWAK Institute of Chemistry, University of Gdansk, 80-952 Gdansk, Poland AND MALCOLM F. Fox* School of Chemistry, Leicester Polytechnic, Leicester LE 1 9BH Received 2nd September, 198 1 The homoconjugation, (ArO),H-, and heteroconjugation, Ar’O- . . * HOAr, (where Ar is aromatic) with proton transfer have been determined in acetone at 298 K. Tetra-alkylammonium phenolates were titrated with a variety of phenols to given homocomplexes and heterocomplexes. Potentiometric data give the overall equilibria constants, KO, proton-transfer constants, KpT and formation constants, K f . Two types of heterocomplexes were studied.When ArO- is a weaker base than Ar’O-, the complexation occurs without proton transfer, as confirmed by the low KO values for the reaction. The overall equilibrium constants, KO, are large when ArO- is a stronger base than Ar’O-, as both the equilibrium proton-transfer constant (KpT) and equilibrium formation constant (Kf) of the hydrogen bond are included in the measurement of KO = KfKPT. It has been shown by many authors studying the proton transfer in molecular B - HA BH+A- complexes that symmetrical hydrogen bonding occurs when the difference of ApK, (H,O) (ApKa being the difference between pK, values for the acceptor and donor) falls within the range - 2 to +7.5. For instance, symmetrical hydrogen bonds in the systems phenol-substituted anilines in cyclohexane, benzoic-acid-substituted pyridines in acetonitrile, and carboxylic acid with base as solvent have been observed at the ApK, (H,O) values of - 1.75,’ 3.752 and 2.33 in water, respectively.The conductiometric, spectrophotometric and potentiometric paH measurements (where paH is the hydrogen-ion activity) in mixtures of phenols with their tetra- alkylammonium salts4> shows stable complex formation of hydrido-bis-phenolate. Formation constants for homo- and hetero-complexes in acetonitrile have been determined. In conductivity studies some phenols in acetone6 exhibited a considerable associative ability to form complex anions, (ArO),H-. In this study the phenols were found to exhibit a stronger interaction in acetone [anion (ArO-)-molecule (ArOH)] than carboxylic acids of approximately the same strength in water.The enthalpy changes for the formation of hydrogen-bonded complexes of the form (RCOO),H- and (ArO),H- have been determined in propylene carbonate as solvent by a calorimetric m e t h ~ d . ~ The hydrogen-bond energy of homocomplexes decreases almost linearly with decreasing acidity of the proton donor. The ratio of the slopes of the curves for hydrido-bis-carboxylate and hydrido-bis-phenolate is ca. 4.5. This result may mean that substituent effects in aromatic acids are attenuated to a large degree through charge delocalization on hydrogen bonding to the negative phenoxide or benzoate ions. 21572158 PROTON TRANSFER I N PHENOL COMPLEXES I N ACETONE Anions with a localized charge can be stabilized in polar aprotic solvents either by homoconjugation : ArO-+ HOAr + (ArO),H- or by heteroconjugation without proton transfer or, alternatively, with proton transfer ArO- + Ar’OH ArO- * * * HOAr or ArOH * - * -0Ar’ (2) where Ar’OH is a weaker or stronger acid than ArOH.8!9 Studies on paH in acetone solution showed that the interaction between phenols (Ar’OH) and phenolates resulted in two distinct types of products.One would be a hydrogen-bonded complex, Ar’OH - - .-OAr, with the proton still attached to the oxygen of the original Ar’O-, whilst for the other, the complex would be formed by proton transfer to the oxygen of ArO-, ArOH. * .-OAr. The hydrogen is transferred to the equilibrium position of proton-transfer anionic bridges, (-OH - . . -0-) + (-0- - - - HO-), and is determined from a study of the hydrogen- ion activity, paH.were confined to the determination of the formation constants, K,, of the homo- and hetero-complexes (hydride-bis-carboxylate). In this paper we progress further to the determination of additional equilibrium constants, overall equilibrium constants, KO, and the proton-transfer constants, KpT, and the relationship between formation constants, Kf. We give special attention to proton transfer between anion-proton donors and the interpretation of these data in terms of the acidity scale of non-aqueous solvents. Previous studiess$ EXPERIMENTAL Acetone was purified and rigorously dried.I0 All phenols used (table 1) were recrystallised 2-3 times from methanol or methanol+water mixtures and dried in vacuum over P,O,.The tetra-n-alkylammonium salts of the substituted phenols were prepared by potentiometric titration of a weighed sample of the phenol with a methanolic solution of the corresponding tetra-alkylammonium hydroxide. After evaporation of the solvent under reduced pressure the salts were recrystallised from ethyl acetate and dried in a vacuum over P,O,. Their purity was checked potentiometric titration with 0.1 mol dm-3 perchloric acid in glacial acetic acid. All results fell in the range 99.5-100.5%. Electromotive-force measurements were made with a PHM-52 digital pH meter (Radiometer, Copenhagen) using an S-60 glass electrode (Gliwice, Poland). The reference half-cell was a saturated calomel electrode with a double junction, and the salt bridge was filled with a 1 .O x mol dmP3 acetone solution of tetra-n-butylammonium perchlorate.The electrode was checked every day in picric-acid-picrate buffers. All measurements were carried out at 298k0.1 K. RESULTS CALIBRATION OF THE GLASS ELECTRODE The reversibility of the glass electrode was checked by e.m.f. measurements in buffer solutions containing CBu,NPi = 4 x 1 OP3 mol dm-3 + picric acid, CHPi = 1 x 1 0-1 mol dmP3. The paH values of these solutions were calculated, assuming complete dis- sociation of Bu,NPi in dilute solution,ll and pKgspne = 6.3,” PaHref = P&piflog& where the subscript f stands for CHPi = Csalt. The activity coefficient was calculated from the expression -lOgf= 3.76 d I .TABLE I .-HETERO- AND HOMO-CONJUGATION OF PHENOLATES (ArO-) WITH SUBSTITUTED PHENOLS (Ar'OH), OVERALL EQUILIBRIUM CONSTANTS (KO), FORMATION CONSTANTS (Kf) AND PROTON-TRANSFER CONSTANTS (KPT) IN ACETONE AT 298 K log KPT log KOb 1% Kf calcd calcd calcd phenol, ArOH (pK,AC)" quaternary salt, R,N+ArO- [eqn (8)l [eqn (711 [eqn (91 1 3,5-dichlorophenol 2 2-nitrophenol 3 2,4,6-tribromophenol 4 3,5-dichlorophenol 5 2-nitrophenol 6 2-nitrophenol 7 3,Sdinitrophenol 8 2,4,6-tribromophenol 9 pentachlorophenol 10 2,4,6-tribromophenol I I 2-nitrophenol 12 3,5-dichlorophenol I3 2,4,6-tribromophenol 14 pentachlorophenol I5 2,4,6-tribromophenol 16 pentachlorophenol 17 pentachlorophenol 18 2,4-dinitrophenol 19 pentachlorophenol 20 2,5-dinitrophenol 2 1 2,4,6-tribromophenol 22 2-nitrophenol 23 2,4,6-trichlorophenol 24 3,5-dichlorophenol 25 2,6-dichlorophenol ( 1 5.7) ( 1 8.3) (19.8) (22.3) (22.5) (22.7) (23.9) (21.1) heterocomplexes, (Ar'OH0Ar)- (C4H9),N 2,4-dinitrophenolate - 7.0 (C,H,),N 2,4-dinitrophenolate - 6.6 (C,H,),N 2,4-dinitrophenolate - 5.4 (C2H5),N pentachlorophenolate - 4.4 (C,H,),N 2,5-dinitrophenolate - 3.5 (C,H5),N pentachlorophenolate - 4.0 (C,H,),N 2,5-dinitrophenolate - 2.9 (C2H5),N pentachlorophenolate - 2.8 (C,H,),N 2,4-dinitrophenolate - 2.6 (C,H,),N 2,5-dinitrophenolate - 1.3 (C,H,),N 2,4,6-trichlorophenolate + 0.2 (C,H,),N 2,6-dichlorophenolate + 1.2 (C,H,),N 2,4,6-trichlorophenolate + 1.4 (C,H,),N 2,5-dinitrophenolate + 1.5 (C,H,),N 2,6-dichlorophenolate + 2.8 (C,H,),N 2,4,6-trichlorophenolate + 4.2 (C,H,),N 2,6-dichlorophenolate + 5.6 homocomplexes, ArOHOAr- (C,H,),N 2,4-dinitrophenolate 0 (C2H5),N pentachlorophenolate 0 (C,H,),N 2,5-dinitrophenolate 0 (C,H,),N 2,4,6-tribromophenolate 0 (C,H,),N 2-nitrophenolate 0 (C,H,),N 2,4,6-trichlorophenolate 0 (C,H,),N 3,5-dichlorophenolate 0 (C,H,),N 2,6-dichlorophenolate 0 3.04 k 0.08 2.95 0.07 2.98 & 0.07 3.27 0.08 3.52 & 0.08 3.23 f 0.06 3.78 f 0.09 3.71 kO.08 3.42 f 0.07 3.79 5 0.09 3.58 f 0.06 4.33 & 0.10 4.70 & 0.12 4.30 k 0.10 4.85 & 0.1 1 5.42 & 0.14 6.25 f 0.12 3.41 f 0.07 4.28 f 0.09 4.59 & 0.10 4.32 f 0 .12 4.12 f 0.09 4.20 f 0.09 4.02 f 0.12 3.50 f 0.10 3.04 2.95 2.98 3.27 3.52 3.23 3.78 3.71 3.42 3.79 3.38 3.13 3.30 2.80 2.05 1.22 0.65 3.41 4.28 4.59 4.32 4.12 4.20 4.02 3.50 a Values from ref. (13); if log KPT < 0, log KO = log Kf for systems 1-10 and 18-25.w 0 X2160 PROTON TRANSFER I N PHENOL COMPLEXES I N ACETONE The glass electrode was calibrated every day in a picrate buffer. For our electrode13 paH = (E,’ - E )/ W = (765 - E ) / 42.5 where Wis the Nernst slope, and E,’ and E are the apparent potential of the reference electrode and the measured potential, respectively. DETERMINATION OF THE PROTON-TRANSFER CONSTANTS, Kpp THE FORMATION CONSTANTS, Kf, A N D THE OVERALL EQUILIBRIUM CONSTANTS, KO The reaction between the proton donor Ar’OH and proton acceptor ArO- in an aprotic solvent may lead to the formation of hydrogen-bonded complexes with proton transfer (PT) or without proton transfer. A general scheme for the formation of the heterocomplexes can be written as follows : KPT ArO- + Ar’OH + ArOH + Ar’O- (3) K € Ar’O- + HOAr S Ar’O- - HOAr (4) KO ArO- + Ar’OH f: Ar’O- - * HOAr where KO, KPT and Kf are the equilibrium constants of the overall reaction, the proton-transfer constant and the formation constant, respectively.The overall equilibrium constant KO is related to KPT and Kf by KO = KPTKf (6) and was calculated from the potentiometric data using eqn (7) adapted by us14 from the study by Kolthoff and Chant~oni:~ Ko = CR4NCArO-r2 - r(cAr’OH + CR4NCArO-) + CAr’OH/r(CR,N+ArO- - CAr’OH) (7) where r = aH f / a ; h and a4 a n d 4 are values at midpoint (CR,N+ArO- = CAro.H). quaternary ammonium salts are presented in fig. 1 (a) and (b). by then Plots of paH against log CAr’oH/CR4N+ArO- of mixtures of phenols with different If we express the ionization constant, K,, of an acid ArOH in acetone medium (S) ArOH + S f SH+ + ArO- Ka = [SH+l EAro-1 fSH +fArO-/[Sl [ArOH1 f S f A r O H - We assumefAr,, andf, to be equal to 1 at low concentrations and By replacing [SH+]f,,+, [ArO-] and [ArOH] by aH+, CR,N+ArO- and CArOH, respec- tively, and taking logarithms we obtain For a medium point (subscript t, at CR4N+ArO- = CArOH) the equation may be written as follows13* l4 This equation is correct for the homosystems R4N+ArO- + ArOH.pK,Ac = paH4 - log &.Z . PAWLAK, B. NOWAK AND M. F. FOX 1 I I I Lr - 0.6 - 0.3 0 0.3 0.6 log (CA~'OH/~K~N+A~O- 1 2161 -0.6 -0.3 0 0.3 0.6 log (CA~'OH /CR~N+A r o - ) FIG. 1 .-Relationship between log CAr,OH/CR,N+ArO- and paH in acetone at 298 K. Numbers identify the systems listed in table 1.2162 PROTON TRANSFER I N PHENOL COMPLEXES I N ACETONE 14 2 2 - I 1 I I I 1 I n - - m 2 20 9 ; t" 2 16 \ 0 9 0 18 -6 - 4 -2 0 2 4 6 AC 'PKtc = PK;&eptor) - pKa (donor) FIG.2.-Relationship between paHt(Cphenol = Cquaternary salt) in homocomplexes and heterocomplexes in acetone at 298 K plotted against ApK,Ac, where ApK,AC = PK%cceptor)- ~Kaq2oonor). Numbers identify the systems listed in table 1 . Hence, we consider the reaction of an acid, Ar'OH, which involves the following ArO- + Ar'OH $ ArOH + Ar'O- reaction for which the equilibrium constant, KpT is KpT = KAc a ( A r ' 0 H) 0 H) * (8) Hence the values of K,, the equilibrium constants for the formation of hydrogen bonding of heterocomplexes with proton transfer, may be found, since log Kf = log KO -log K p T .(9) In the case when KPT is close to, or less than, unity, the value of the overall equilibrium constant, KO, is equal to that of the formation constant, KO = K,. For instance, this case if found in systems where In mixtures of R,N+ArO- with a non-conjugated phenol Ar'OH in acetone, where the proton transfer is not complete, the paH change is relatively small, but the decrease in paH is sharp where the proton transfer is complete, fig. 1 (a). For systems in which the proton is attached to the proton-donor group Ar'OH (ApK,Ac < 0), the plot of the function is linear. function is non-linear, and the curve has a sigmoidal shape. The plots of paH of a mixture of > pKfGrOH), table 1. psH =f(log CAr/OHICArO-) In heterosystems in which proton transfer occurs (ApKkC > 0) the plot of the PaH = Alog CAr/OHICArO-)Z.PAWLAK, B. NOWAK A N D M. F. FOX 2163 4.5 2 00 - 4.0 3.5 I I I I I I I I I * 16 17 18 19 20 21 22 23 AC pKa(pheno1) FIG. 3.-Formation constants, log K,, of homocomplexes, (ArO),H-, in acetone at 298 K plotted against pK$&,,. Numbers identify the systems listed in table 1. an acid and salt without proton transfer, pK&ceptor) < p K ~ ~ o n o r , are linear in fig. l ( a ) and (b), curves 1-10. Some heterosystems in which pK$&.ceptor, > pK$&,nor) [fig. l(a) and (b), curves 13-17] are indicative of the proton-transfer reaction, e.g. curve 17, where ApK,Ac = 5.6: C,Cl,OH + Cl,C,H,O- + C,Cl,O- * * HOC,H,C1,. Consequently, the paH greatly decreases, and the plot assumes a sigmoidal shape.Calculated values of KpT, KO and Kf obtained from the plots of fig. 1 (a) and (b) are given in table 1 for each system. Plots of paH+ at the point (CR,N+ArO- = CArfOH) in the systems studied as a function ApK,Ac, wherg of mixtures of these phenols with different tetra-alkylammonium salts are presented in fig. 2, and exhibit a pronounced maximum around ApK,Ac = 0. Let us consider the two systems: (1) without proton transfer [3,5-C12ArOH + 2,5(N02),Ar0-, ApK,Ac = - 2.901 and (2) with proton transfer [2,4,6-Br3ArOH + 2,6-Cl2Ar0-, ApK,Ac = + 2.801. The observed peH values are comparable, 22 0.2. From the paHi values (at the point CR,N+ArO- = CAr,OH) as a function of ApK,Ac, fig. 2, the proton concentration has a minimum at ApK,Ac = 0, as a result of the formation of homoconjugate (ArO),H- ions.In fig. 3 the stability constants of the homocomplexes, log K(Aro)?F-, are plotted against pK4irOH) on the acetone scale. The largest increase in stability is observed in the region of 20pK,Ac units. In acetone, the Kf values of homocomplexes of substituted phenols are of the order of lo4 or less. In the series considered in fig. 3, the stability of the homocomplexes increases with pK,Ac up to pK, = 20, and then drops with the pK,Ac of phenols. As can be seen in fig. 4, the plot of the overall equilibrium constant, log KO against ApK,Ac, is linear over the ApK,Ac range from -7 to 0. Above the latter value, log KO markedly increases. Linear plots of log KirOHOAr- against ApK,Ac were obtained for heterocomplexes.An increase in the formation constant, K,, in acetone was found with decreasing ApK,Ac, whereas for ApK,Ac > 0 the stability decreased more markedly. Systems characterised by pK$gonor) (Ar'OH) > pK$!&.eptor) (ArOH) are represented2164 PROTON TRANSFER IN PHENOL COMPLEXES IN ACETONE t1 Y L a g Kf -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 APIEBAC = PK:&ceptor) - PK3ionor) FIG. 4.-Plots of overall equilibrium constants, log KO, and formation constants, log K,, for homocomplexes (0) and heterocomplexes (0) in acetone, at 298 K, as a function ApK,Ac on the'acetone scale. Numbers identify the systems listed in table 1 . by entries 1-10 in table 1 and in fig. 4. For these ApK,Ac is negative. As shown in fig. 4 the overall constants, KO, of the reaction are low under these conditions, as proton transfer does not take place.The intermediate region, where ApK,Ac = 0, corresponds to the formation of homocomplexes, (ArO),H-, with log KO ranging from 3.41 to 4.59. The largest change of overall equilibria occurs in the region where ApK,Ac > 0. In this case, the proton is transferred from the less basic donor Ar'OH to the more basic acceptor ArO-. The overall constant KO does not represent equilibria for the formation of the hydrogen bond alone (Kf), but also includes that of the proton transfer (KpT). The overall equilibrium constant, KO, is related to K p T and Kf by log KO = log K ~ T + log Kf. This difference is illustrated by comparison of systems 8 and 15. The values for system 15 are 2,6-C12C,H,O- 4- 2,4,6-Br,C6H20H -+ (2,6-C1,C6H30H.a * OC6H,Br3-2,4)- where log KO = 4.85 and ApK,Ac = 2.80, while for system 8 one has C6C15O- + 2,4,6-Br3C6H,0H -+ (C6C150 * ' . HOC6H2Br,-2,4,6)- where log KO = 3.71 and ApK,Ac = -2.80.Z . PAWLAK, B. NOWAK AND M. F. FOX 2165 Similar results were obtained for other systems with fApK,AC, namely systems 3, 17 and 5, 16: viz. (3) 2,4,6-tribromophenol+ 2,4-dinitrophenolate, log KO = 2.98 (1 7) pentachlorophenol+ 2,6-dinitrophenolate, log KO = 6.25 ( 5 ) 2-nitrophenol+ pentachlorophenolate, log KO = 3.52 (1 6) pentachlorophenol + 2,4,6-trinitrophenolate, log KO = 5.42 CONCLUSIONS The main interaction in our study of phenolate-phenol by hydrogen bonding shows stable homocomplexes with Kf 2 lo4 and heterocomplexes with Kf = 102-103. The most important implication of this work is that contained in fig.4, showing KO, Kf and ApKi0lvent to have a complex relationship. When ApK is negative or zero, then log KO = log Kf. Formation values Kf at whole range ( -ApK,) are not changed as much. However, when ApK is positive for the systems, then log KO diverges rapidly from Kf. Complexes formed after proton transfer at ApK,AN in the positive range undergo a change information constant more markedly. In future, simple statements concerning ApKi0lvent and log KO for proton transfer should either not be made or should carry the qualification that ApK, is negative or zero. When ApK;Olvent is positive, then both KO and Kf must be given. Further, it is clear that the ApK, scale used must be that for the relevant solvent.Whereas comparisons made in the past have used the water scale for ApK,, maxima in the measured quantities, e.g. in the proton chemical shift, have been taken as showing symmetrical hydrogen-bond formation between acid and base at ApK,Hzo values ranging between - 2 and + 7.5. It is self-evident that symmetry ofhydrogen-bond formation will occur for equal basicity/acidity of the two components, at ApK:OIVent = 0. This has been demonstrated in another paper5 for phenolate complexes in acetonitrile when ApK,Ac was used. Therefore, we urge that KO, Kf and ApKPlvent for proton transfer systems should be interpreted in a more meaningful manner and that ApKi0lvent values used should be those for the relevant solvent. It should no longer be acceptable to use ApK,Wzo values when discussing proton-transfer equilibria in non-aqueous systems. l G. Dobecker and P. Huyskens, J. Chim. Phys., 1971, 68, 295. * S. L. Johnson and K. A. Rumon, J. Phys. Chem., 1965, 69, 74. R. Lidemann and G. Zundel, J . Chem. Soc., Faraday Trans. 2, 1978,73, 788. I. M. Kolthoff and M. K. Chantooni Jr, J. Am. Chem. SOC., 1965, 87, 4428; 1966, 88, 8430. J. Magonski and Z. Pawlak, J . Mol. Struct., in press. Z . Pawlak, T. Jasinski and B. Nowak, Zesz. Nauk. Wydz. Mat., Fiz. Chem., Uniw. Gdariski, Chem., 1972, 2, 5 . Z. Pawlak and R. G. Bates, J. Chem. Thermodyn., in press. Z . Pawlak, Roczn. Chem., 1973,47, 641; 1972, 46, 2069. 2. Pawlak and J. Magonski, J. Mol. Struct., 1980, 60, 179; 1978, 47, 329. lo J. F. Coetzee and D. K. McGuire, J. Phys. Chem., 1963, 67, 1810. l1 M. B. Reynolds and C. A. Kraus, J . Am. Chem. SOC., 1948, 70, 1709. l2 C. M. French and I. G. Roe, Trans. Faraday SOC., 1953, 49, 314. l3 B. Nowak and Z. Pawlak, J . Pol. Chem., 1981, 55, in press. l4 Z. Pawlak, Z. Szponar and G. Dobrogowska, Roczn. Chem., 1974, 48, 501. (PAPER 1 / 1378)

ISSN:0300-9599    

DOI:10.1039/F19827802157

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. 1, 1982, 78, 2167-2181 Quenching of Fluorescence from Ru(digy)i+ and Ru(dipy),(CN), in Solutions of Sodium Dodecyl Sulphate B Y STEPHEN J. ATHERTON, JOHN H. BAXENDALE* AND BRIDGID M. HOEY Department of Chemistry, University of Manchester, Manchester M 13 9PL Received 7th September, 198 1 The kinetics of the fluorescence quenching of Ru(dipy)i+ and Ru(dipy),(CN), by Cu2+ and MVZ+ are reported. At SDS concentrations above the c.m.c. the kinetics of Ru(dipy)i+ quenching by both quenchers are simple first order with constants which increase linearly with quencher concentration and which decrease with increasing SDS concentration. The results quantitatively agree with those predicted by the model in which the emitter is completely micellised and the quenching is slow enough to allow equilibration of the quencher between solution and micelles.With Ru(dipy),(CN), above the c.m.c., the quenching by MV2+ has fast and slow components, the latter being that of the natural emission decay. The results are shown to be consistent with almost complete micellisation of the quencher and a high micellar quenching rate constant. With Cu2+ the micellar quenching is too fast to follow so that ‘quasi-static quenching’ is seen, i.e. there is a decrease in the initial emission intensity followed by the slower unquenched decay. However, analysis shows that specific interaction of Cu2+ with the emitter on the micelle must be assumed to account for the abnormally high efficiency of Cu2+. Enhanced quenching of Ru(dipy)i+ emission by Cu2+ and MV2+ is also observed below the c.m.c.This is explained in terms of the formation of clusters of the emitter and SDS with which the quenchers associate. With Ru(dipy),(CN), only Cu2+ gives enhanced quenching below the c.m.c. It is suggested that Cu2+ complexes with SDS to form a micelle-like species with which the emitter can associate. The kinetics of fluorescence quenching in micellar systems are determined by a number of factors such as equilibrium constants for the distribution of emitter and quencher between micellar and aqueous phases, the rates of transfer of emitter and quencher to and from the micellar and aqueous phases, the rate of quenching in the micellar phase and the natural lifetime of the emitter. For the simpler case where the emitter is completely micellised and the quencher only partially so, these processes can be represented: P* -+ P P*+nQ, + P k+ Qa+M*Qm k- ko nkq, quenching with n quenchers in the same micelle K , k,, k-, exchange of Q between water and the micelles where P is the emitter, Q, and Qm are the quencher in aqueous and micellar phase, M is the micelle and K = k + / k - = [Qm]/[Q,][M], where [MI is the total micelle concentration.Infeltal has derived an equation for quenching kinetics which takes account of the above processes and which has been amended recently by Dederen et al.2 to include an additional process, viz. the transfer of quencher directly from one micelle to another: WnQ) + MbQ) -+ W z Q ) + M(P + 1Q) k , 21672168 QUENCHING OF FLUORESCENCE FROM Ru(dipy)t+ AND Ru(dipy),(CN), where M(nQ) means a micelle containing n quenchers.With this modification the kinetic equation for quenching is: In I / I , = - (k, + A[Q]) t - B[Q] [ 1 - exp ( - Ct)] where I, and I are the emission intensities at t = 0 and t, C = k , + ke[M] + k- and [Q] is the total concentration of quencher present. In the derivation of this equation it is assumed that in a micelle with n quenchers, the quenching rate constant is n times that with one quencher, as indicated above. In the general case eqn (1) gives a logarithmic decay curve with two distinguishable parts, viz. an initial faster decay determined by the second term, followed by a portion linear in time with the form of the first term. This case arises when the quenching constant k, is high w.r.t.the exchange of the quencher, so that the emitters in micelles already containing quenchers are rapidly quenched, the remainder having to await the equilibrium of quenchers among the micelles. Several examples of this behaviour have been r e p ~ r t e d . ~ If k, is sufficiently large the initial decay may be too fast to be observed in which case ‘static quenching’ apparently occurs, i.e. there is an apparent decrease in the initial emission intensity followed by the logarithmic decay term. The quenching of excited Ru(dipy)i+ by methyl anthracene was reported to show this static quench- ing phen~menon,~ but recent work5 with better time resolution has shown the apparent decrease in initial intensity to be a fast initial decay. A third variation occurs if k, is small and the second term of eqn (1) becomes negligible.In this case the decays are exponential and show Stern-Volmer behaviour, the Stern-Volmer constants varying with the micelle concentration. Many examples of this have been r e p ~ r t e d ~ ? ~ and the situation arises from the fact that the slow quenching allows equilibration of the quencher among the micelles. It has recently been reported7 that even at sub-micellar surfactant concentrations the quenching of charged emitters is considerably enhanced. This has been attributed to the formation of emitter-surfactant clusters which behave like micelles in that they have an affinity for the quencher. In the present work we have examined the quenching of excited Ru(dipy):+ and Ru(dipy),(CN), by Cu2+ and methyl viologen (MV2+) in the presence of sodium dodecyl sulphate (SDS) at concentrations above and below the critical micelle concentration (c.m. c .). EXPERIMENTAL Ru(dipy)i+ was precipitated as the perchlorate from a solution of the chloride (G. F. Smith). Ru(dipy),(CN), was kindly supplied by the Ciamician Institute, University of Bologna, and used without further treatment. Cu2+ was added as the sulphate (analytical reagent quality), MV2+ as the chloride (B.D.H.) and SDS was B.D.H. specially purified grade. Stock solutions of the latter were freshly made for the day of their use. Solutions of the mixed reagents were used immediately after making up in triply distilled water and were deaerated using pure nitrogen. Concentrations of Ru(dipy):+ between 20 and 150 pmol dmP3 and of Ru(dipy),(CN), of ca. 20 pmol dm-3 were used.Experiments were done at room temperature, i.e. 295 & 2 K.S. J. ATHERTON, J. H. BAXENDALE AND B. M. HOEY 2169 Emissions were generated using 530 nm 15 ns light pulses from a System 2000 J.K. neodymium laser, frequency-doubled and moni tored at 680 nmusing a standard monochromator- photomultiplier arrangment feeding a Tektronix 79 12A digitiser. The latter was controlled by a Commodore PET microcomputer which was also used to store the data on disk, to analyse the data and to give hard copies of the primary signals and functions derived from them. It has been showns that in some conditions high exciting light intensities can lead to anomalous kinetics for emission decays of Ru(dipy)g+ in SDS solutions.Care was taken to ensure that the intensities used were such that these effects were absent. RESULTS AND DISCUSSION QUENCHING OF EXCITED Ru(dipy)t+ BY Cu2+ ABOVE THE C.M.C. The decay constant for Ru(dipy)t+ in aqueous solution, 1.6 x lo6 s-l, is well established and we find the same value in SDS solutions below the c.m.c. However, at 12 mmol dm-3 SDS and higher we find the lifetime increases slightly and k = 1.25 x lo6 S-'. The Cu2+ quenching reaction in simple aqueous solution has been showng to be an electron transfer *Ru(dipy):+ + Cu2+ + Ru(dipy):+ + Cu+ with rate constant 7.7 x lo7 mol-1 dm3 s-l, and it was also shownlo that the quenching rate increases in the presence of 0.02 mol SDS. 8 - 1 I I I 0 2 I 6 8 [Cu2+] /mrnol drn-' FIG. 1 .-Quenching of Ru(dipy)i+ by Cu2+.Variation of quenching constant, kobsr with Cu2+ concentration. Numbers on lines are the SDS concentrations in mmol dm-3. Using SDS concentrations from 12 to 40 mmol dm-3 we have measured the effect of Cu2+ up to concentrations which increased the emission decay approximately fourfold at each SDS concentration. The decays were all first order over at least 90% of the reaction and the decay constants, kobs, were found to be linear with Cu2+ concentration, as shown in fig. 1. The general behaviour is consistent with the third variation of eqn (1) mentioned above when k , < k,[M]+k- and is small, in which case the first term is dominant and eqn (1) becomes In I l l , = - kobs t = - {k, + k,K[Cu2+]/( 1 + K[M])) t . (2)2170 QUENCHING OF FLUORESCENCE FROM Ru(dipy):+ AND Ru(dipy),(CN), The measured quenching constant, kobs, should thus be linear with [Cu2+] as in fig.1 giving lines of gradient S where 1 / S = (1 + flM])/k, K . Calculating [MI using c.m.c. = 8.2 mmol dm-3 and aggregation number 62 the plot in fig. 2 shows this to be true and from the line in fig. 2 we obtain k, = 2.1 x lo5 s-l and K = 2.0 x lo4 mol-1 dm3. The latter compares with 6 x lo4 mol-1 dm3 obtained from pyrene quenching3d and 1.0 x lo4 mol-1 dm3 from methyl pyrene quenching.3c 100 200 300 400 500 [ M I /pmol dm-3 FIG. 2.-Quenching of Ru(dipy)i+ by Cu2+ and MV". Variation of S, the gradients of lines in fig. 1 and 3, with micellar concentration. QUENCHING OF EXCITED Ru(dipy):+ BY MV2+ ABOVE THE C.M.C. This is also an electron-transfer reaction in simple aqueous solution *Ru(dipy):+ + MV2+ -+ Ru (dipy):+ + MV+ with6 rate constant 1.8 x lo8 mol-1 dm3 s-l.Measured at one MV2+ concentrat' n (2 mmol dmP3) and SDS concentrations up to 82 mmol dm-3, the rate constant was found in earlier work to be much increased at low concentrations of SDS and fell as the concentration of SDS increased.6 We have measured the decay in SDS concentrations up to 40 mmol dm-3 with MV2+ concentrations which give an increase of up to approximately fourfold in the decay rate. In these conditions the decays were found to be first order over at least 90% of the reaction and the first-order constants, kobs, were linear with [MV2+], as shown in Treating the data in the same way as for Cu2+, the plot of 1/S (from fig.3) against [MI shown in fig. 2 gives k, = 6.6 x lo5 s-l and K = 7 x lo4 mol-l dm3, although the latter could be in considerable error since it is determined by the small intercept in fig. 3. fig. 2.S. J. ATHERTON, J. H. BAXENDALE AND B. M. HOEY 2171 [ MV2'l/mmol dm-' FIG. 3.-Quenching of Ru(dipy)i+ by MV2+. Variation of quenching constant, kobs, with MV2+ concen- tration. Numbers on lines are SDS concentrations in mmol drnp3. QUENCHING OF EXCITED Ru(dipy),(CN), BY MV2+ ABOVE THE c.M.C. *Ru(dipy),(CN), + MV2+ -+ Ru(dipy),(CN)i + MV+ and the rate constantll is found to be 5.3 x lo9 mol-1 dm3 s-l. No experiments are reported with MV2+ in the presence of SDS. Several sources11$ l2 have given k = (3.8 f 0.1) x lo6 s-l for the excited-state decay constant in water and we have confirmed that this value also holds in SDS solutions up to 6 mmol dm-3, but at 16 mmol dm-3 and higher the decay constant decreases to (2.220.1) x lo6 s-l.At these latter SDS concentrations we have shown by pulse-radiolysis experiments, in which the reaction of e;, with the complex was followed, that > 96% of the complex is in SDS micelles. The longer lifetime in the micelles is paralleled by that in methano112 where k = 2.5 x lo6 s-l and presumably the less polar environment is responsible. The quenching kinetics in micellar solutions are not first order like those for Ru(dipy)g+, but over a range of SDS and MV2+ concentrations the logarithmic decay plot has fast and slow components (see fig. 4), as has been found for example in the quenching of pyrene by methylene iodide,3a ~ y r e n e ~ ~ by Cu2+, methyl p ~ r e n e ~ ~ by Cu2+ and Ru(dipy)g+ by methyl anthra~ene.~ Qualitatively this phenomenon is observed when the quencher is strongly bound to the micelles and has a high quenching rate constant so that there is rapid quenching in micelles containing both emitter and quencher followed by the slower emission from micelles containing emitter only.In simple aqueous solution this also is quenching by electron transfer In terms of eqn (l), when k, % k-+k,[M] this reduces to In I / I , = fi[exp ( - k, t) - 11 ( k , + B(k- + k,[M])) t (3 4 where fi = K[Q]/( 1 + K[M]), the average number of quenchers occupying a micelle. Analysing the slow components using only the latter part of the decays (fig.4) we find that these have decay constants within 10% of k, so that ~ ( k - + k,[M]) is negligible (3 b) and hence In I / I , = R[exp ( - k, t) - 11 - k , t an equation first derived in this form by Rodgers and Wheeler.362172 QUENCHING OF FLUORESCENCE FROM Ru(dipy)i+ AND Ru(dipy),(CN), 5*43 h 4.60 g 3 . 7 7 \ 5 E 4 2 . 9 5 2.12 110 308 506 70 4 tlns FIG. 4.-Logarithmic decay of emission from solution containing 20 pmol dm-3 Ru(dipy),(CN),, 500 pmol dm-3 MV2+ and 30 mmol dme3 SDS excited at 530 nm observed at 680 nm. 0 1 2 3 4 5 6 7 [MV2*l/10-' mmol dm-3 FIG. 5.-Quenching of Ru(dipy),(CN), by MV2+. Variation of extrapolated value of In Z/I, to t = 0 [ = N by eqn (3 c)] with MV2+ concentration. It will be seen from this equation that the form of the slow component is In I / I , = -Ha-k,t (3 4 so that extrapolation to t = 0 gives values of A{ = K[MV2+]/( 1 +KIM])}. We obtained values of A in this way for 40 mmol dm-3 SDS using 100-600 pmol dm-3 MV2+ and these are plotted against [MV2+] in fig.5. The inverse of the slope of this line, which should be given by (1 + K[M])/K, is 550 50 pmol dmF3. Using accepted values of 8 mmol dm-3 for the c.m.c. and 60 for the micelle aggregation number, [MI = 530pmol dm-3, the slope to be expected if K[M] B 1. The value 7 x lo4 mol-1 dm3 obtained above for K is entirely consistent with this, i.e. the MV2+ is entirely micellised in these conditions. We have also analysed the fast initial portion of the decays by computer fitting aS. J. ATHERTON, J. H. BAXENDALE A N D B. M. HOEY 2173 TABLE VA VALUES OF k , FOR QUENCHING OF Ru(dipy),(CN), BY MV2+ [SDS]/mmol dm+ [MV*+]/pmol dm-3 kq/107 S-1 20 20 20 30 30 30 40 40 40 200 300 400 200 400 500 200 400 600 1.09 & 0.12 1.21 kO.11 1.12 f 0.1 1 1.55k0.25 1.21 k0.09 1.07 +O.13 1.65 _+ 0.25 0.93 k0.22 1.18 k0.15 Values of k, obtained by computer fit of fast component (see fig. 4) to eqn (3b). value of k, to eqn (3b) using the experimental value of k,. The results are given in table 1, from which it can be seen that k, = 1.2 x lo7 s-l satisfies a range of MV2+ and SDS concentrations. QUENCHING OF EXCITED Ru(dipy),(CN), BY cu2+ ABOVE THE C.M.C. *Ru(dipy),(CN), + Cu2+ -+ Ru(dipy),(CN)z + Cu+ for which12 k = 3.9 x lo8 mol-I dm3 s-l. No work on quenching in the presence of SDS has been reported. For SDS concentrations from 20 to 40 mmol dm-3 and Cu2+ concentrations up to 150 pmol dm-3, we find that over at least 90% of the decay the reaction is first order and the rate constants increase slightly from 2.5 x lo6 s-l as the Cu2+ concentration increases but by only ca.20% at the highest Cu2+ concentration. However, at each SDS concentration the emission intensity at t = 0 decreases appreciably as the Cu2+ concentration is increased, as shown in fig. 6, i.e. there is 'quasi-static quenching'. This is clearly a system which follows eqn (3a) above, where k, is so large that the fast component cannot be observed with the time resolution of our equipment, and the n(k- + k,[M]) term is not entirely negligible w.r.t. k,. In these circumstances In I / I , = - A - { k, + n(k- + k,[M])} t In simple aqueous solution this is oxidative quenching and A gives the extent of quasi-static quenching.The data are not sufficiently accurate to allow a meaningful analysis to obtain k- and k, as has been done for other but n = K[Cu2+]/(1 +K[M]) can be obtained from the intercepts of the In I / I , against t plots. Examples of these are given in table 2 for 20 mmol dm-3 SDS. Compared with MV2+ in the same conditions Cu2+ is much more efficient, requiring only ca. 20% of the MV2+ concentration to produce the same extent of the fast quenching. We see, however, that the MV2+ observations are understandable if the MV2+ is almost entirely micellised and, in terms of the model used, no matter how big the value of k, this should give the maximum extent of quasi-static quenching possible with any quencher. Moreover, the maximum value of A( = K[Q]/(l +K[M])} which can be attained is [Q]/[M], whereas it will be seen from table 2 that for each Cu2+ concentration the experimental value is approximately three times this.This pattern is followed for 25, 30 and 40 mmol dm-3 SDS solutions. It is clear that far more Cu2+ is associated with the emitter on the2174 QUENCHING OF FLUORESCENCE FROM Ru(dipy):+ AND Ru(dipy),(CN), 0 0.2 0.4 0.6 0.8 tlns FIG. 6.-Quenching of Ru(dipy),(CN), by Cu2+. Logarithmic decay of 20 pmol dm+ emitter in 25 mmol dm-3 SDS showing quasi-static quenching. The numbers on the lines are Cu2+ concentration in pmol dmP3. TABLE 2.-QUASI-STATIC QUENCHING OF Ru(dipy),(CN), BY CU2' ~~~~~~ ~ ~~~ [Cu2+]/pmol dm-3 15 20 30 50 60 ECu2+l/[M1 0.079 0.105 0.158 0.263 0.315 A 0.40 0.41 0.61, 0.71 0.88 1.09 Solution contains 20 mmol dm-3 SDS. A = In I / & at t = 0 obtained from data as in fig.6 . [MI calculated using c.m.c. = 8.2 mmol dm-3 and aggregation number 62. micelle than can be accounted for by the simple model and some more specific affinity for the emitter must be evoked. There is evidence for this in simple aqueous solution12 based on abnormally efficient kinetic quenching and an association constant of 200 mol dm-3 has been derived. There can be no similar association in solution in the present system since little Cu2+ or emitter are in solution. However, it is not unreasonable to suppose that this affinity may cause micelles containing the emitter to have a higher affinity for Cu2+ than the unoccupied micelles.We then would have two equilibria: Cu2++M$CuM K Cu2+ + RUM + CuRuM KR where CUM and RUM represent micelles containing Cu2+ and emitter, respectively.S. J. ATHERTON, J . H. BAXENDALE AND B. M. HOEY 2175 From these equilibria we have: By making some approximations we will now use this equation to relate the extent of the emission left after static quenching, I,, to the quencher concentration. We will neglect multiple occupancy of the micelles which is reasonable in view of the low concentrations of Cu2+ and emitter used in these experiments. Also, we will assume that there is no Cu2+ free in solution which, from the micelle association constant, will be approximately the case. Now from the extent of static quenching we can calculate how much of the emitter is associated with Cu2+.Thus from table 2, 60 pmol dmP3 Cu2+ quenches 66% of the emitter, i.e. Cu2+ is associated with 13 of the 20 pmol dm-3 emitter present. Assuming single occupancy this means that ca. 13 % of the Cu2+ is present in the species CuRuM. A similar extent is found over the range of Cu2+ concentrations. In view of the complexity which is necessary to take this into account, we shall assume that this is negligible and equate [CUM] to the total Cu2+ present. With these approximations and remembering that Ir/Io = [RuM]/([RuM] + [RuCUM]) the above equation leads to Io/Ir = KR[Cu2+]/K[M] + 1 (4) where [Cu2+] is the added Cu2+. Fig. 7 shows that I J I , is reasonably linear with [Cu2+] for a range of SDS concentrations, and fig.8 shows that the reciprocal slopes of these lines are linear with the calculated micellar concentration. From fig. 8 we obtain K,/K = 6.3, which is a measure of the preference of Cu2+ for micelles containing the emitter. QUENCHING OF Ru(dipy):+ BY MV2+ BELOW THE C.M.C. MV2+ is an even more efficient quencher at SDS concentrations below the c.m.c. We have investigated the system using a 4 mmol SDS solution. At all concen- trations of Ru(dipy)$+ used, viz. 29- 114 pmol dm-3, the emission kinetics are first order over at least 90% of the decay and there is no quasi-static quenching. The MV2+ is more effective as a quencher the smaller the Ru(dipy)i+ concentration (fig. 9), and over the above range 20pmol dm-3 MV2+ causes substantial increases in the quenching rate constants.This compares, for example, with several hundred micro- molar required at 12 mmol dmP3 SDS. As for the micellar solutions, rate constants are linear with MV2+ concentration, as shown in fig. 9. There is clearly an association of MV2+ and the emitter, as is the case for micellar solutions, but at 4 mmol dmP3 SDS there can be no micelles of the conventional type. However, it has been shown7 that at 4 mmol dmP3 SDS there is an association of Ru(dipy)t+ with SDS ions to form a cluster containing several Ru(dipy)$+, and that very efficient quenching occurs with methyl anthracene, in this case quasi-static. Those results were explained quantitatively by treating the cluster as a micelle able to associate with methyl anthracene in the manner outlined above.It seems likely that MV2+ behaves similarly. However, here there is no quasi-static quenching and the kinetics are of the same type described above for MV2+ in micellar solutions, which originate from a rapid equilibrium of quencher with micelles and a relatively small quenching constant k,.2176 QUENCHING OF FLUORESCENCE FROM Ru(dipy)$+ AND Ru(dipy),(CN), FIG. 7.-Quenching of Ru(dipy),(CN), by Cu2+. Variation of residual emission after quasi-static quenching, I,, obtained from data as in fig. 4 with Cu2+ concentration. The numbers on the lines are SDS concentrations in mmol dm-3. [Ml/10-' mmol dm-3 FIG. &-Quenching of Ru(dipy),(CN), by Cu2+. Variation of gradients, S, from fig. 7 with micellar concentration to compare with eqn (4).S. J.ATHERTON, J. H. BAXENDALE AND B. M. HOEY 2177 9- I - 1 - I I 1 I 0 5 10 15 20 [ MV2+] /pmol dm-3 MV2+ concentration. Numbers on lines are Ru(dipy)z+ concentrations in pmol dm-3. FIG. 9.-Quenching of Ru(dipy)i+ by MV2+ below the c.m.c. Variation of quenching constant, kobs, with 1 1 I 1 1 I I 0 LO 80 120 [Rul /pmol dm-’ FIG. 10.-Quenching of Ru(dipy)g+ by MV2+ below the c.m.c. Variation of gradients of lines in fig. 9 with Ru(dipy)i+ concentration at 4 mmol dm-3 SDS. Formulating this in terms of a Ru(dipy)i+-SDS cluster C, we have C + MV2+ e (CMV2+) K,. Assuming that multiple occupancy by MV2+ can occur this gives which is of the same form as eqn (2) above. Hence the observed quenching constants should be linear with [MV2+] as found (fig.9) and the slopes of the lines are given by k , Kc/( 1 + K,[C]). Since the concentration of SDS is large compared with that of Ru(dipy)E+, it would be expected that the composition of the clusters would not change2178 QUENCHING OF FLUORESCENCE FROM Ru(dipy),2+ AND Ru(dipy),(CN), and that the cluster concentration would be proportional to the concentration of Ru(dipy)i+. Hence, we should expect that the reciprocal of the gradients of the lines in fig. 9 would be linear with Ru(dipy)g+ concentration. Fig. 10 shows this to be the case and from the line we may obtain k, and K,. We find K , = 1 . 1 x lo5 mol-l dm3 and, if each Ru(dipy)g+ gives rise to only one cluster, k, = 2.6 x lo7 s-l. However, previous experiments have indicated that as many as eight Ru(dipy);+ may be contained in one c l ~ s t e r , ~ in which case we have k, = 3.3 x lo6 s-l and K, = 8.8 x lo5 mol-1 dm3. These values compare with 6.6 x lo5 s-l and 7 x lo4 mol-l dm3 obtained above for the micellar system.The higher value of k, is perhaps to be expected since it is likely that the clusters are smaller than micelles and the emitter and quencher are in closer proximity as a result. The higher value of K, suggests that the structure of the clusters differs from that of micelles. 0 0.5 1 .o 1.5 [Cu2+]/mmol dm-3 FIG. 1 1 .-Quenching of Ru(dipy)i+ by Cu2+ below the c.m.c. Variation of quenching constant, kobs, with Cu2+ concentration at 4 mmol dmP3 SDS. e, x , 0 and + are values at 29, 57, 86 and 114 pmol dmP3 Ru(dipy)i+, respectively. QUENCHING OF Ru(dipy)i+ BY c U 2 ' BELOW THE C.M.C.Cu2+ also quenches in these conditions but it is not as effective as MV2+. Thus at 6 mmol dm-3 SDS concentrations, ca. 1 mmol dm-3 Cu2+ are required to give appreciable quenching (20 pmol dm-3 for MV2+). Nevertheless for Cu2+ this is approximately five times more effective than at 20 mmol dm-3 SDS. The kinetics are also different from those of MV2+ quenching, for although the emission decays are first order over at least 90% of the reaction and there is no quasi-static quenching, the first-order constants are not linear with Cu2+ concentration, as shown in fig. 11. Instead the quenching constants appear to approach a limiting value and moreover are seen to be independent of the Ru(dipy)i+ concentration over the range 29-1 14 pmol dm-3.Again there must be association of quencher and emitter since the rates are well in excess of those for simple aqueous solutions, but the concentration dependence suggests that it is not the micellar or cluster type of association encountered so far in this work. The approach to a limiting value of the quenching constant suggests thatS. J. ATHERTON, J. H. BAXENDALE AND B. M. HOEY 2179 it is a 1 : 1 association of emitter and quencher rather than the micellar type which allows the emitter to be associated with many quenchers. The reason for this difference is probably due to the fact that, unlike MV2+, Cu2+ is strongly associated with SDS below the c.m.c. The evidence for this, presented in detail below, is that Cu2+ is a very efficient quencher for excited Ru(dipy),(CN), below the c.m.c.whereas MV2+ is no more effective than in simple aqueous solution. We conclude that Cu2+, like Ru(dipy):+, forms a micellar type species, which can take up Thus with Cu2+ and Ru(dipy)!+ neither emitter nor quencher exist as simple ions in SDS solution and the quenching observations are consistent with the association of the two SDS-complexed ions C and R to form CR: Ru(diPY),(CN)z. C + R e C R KR. 0 5 10 [Cu2'] /mmol dm-3 15 FIG. 12.-Quenching of Ru(dipy)g+ by Cuz+ below the c.m.c. Variation of quenching constant, kobs, with Cu2+ concentration plotted to compare with eqn ( 5 ) . SDS concentration 6 mmol dm-3, 0, (data obtained from smoothed observations given in fig. 1 1 at various emitter concentrations), and 4 mmol dm-3, x , (using 114 pmol dm-3 emitter).The quenching occurs in the species CR, and since there is no static quenching, this equilibrium must be rapidly established within the lifetime of the excited emitter. In this situation it is readily shown that if the lifetime of the emitter in the species CR is Ilk, and the Cu2+ concentration is such that C % R then where ko is the natural lifetime of the emitter. tration but is given by the equation The observed quenching constant kobs is no longer linear with quencher concen- kobs - k O = (kq - k o ) KR[cl/(l + KR[c]). ( 5 ) If [C] is proportional to [Cu2+] then 1 /(kobs - k,) should be linear with 1 /[Cuz+]. As shown in fig. 12 this is the case for 6 mmol dm-3 SDS and a range of Ru(dipy):+2180 QUENCHING OF FLUORESCENCE FROM Ru(dipy):+ AND Ru(dipy),(CN), concentrations and for 4 mmol dm-3 SDS with 114 mmol dm-3 Ru(dipy):+.If C contains only one Cu2+ the lines in fig. 12 give k , = 8.7 and 10.3 x lo6 s-l and KR = 4.5 and 4.1 x lo3 mol-1 dm3 for 6 and 4 mmol dm-3, respectively. If, as seems likely, C contains more Cu2+, then KR is proportionately larger. Note that k, is approximately fifty times the value obtained with the micellar systems, which again may be due to the closer proximity of quencher and emitter and/or the presence of more than one Cu2+ in C. QUENCHING OF Ru(dipy),(CN), BELOW THE C.M.C. Unlike Ru(dipy)i+, below the c.m.c. Ru(dipy),(CN), does not show an increased sensitivity to quenching by MV2+, which is consistent with our conclusion that Ru(dipy):+ as a consequence of its charge forms micelle-like aggregates by association 0 5 10 15 20 [ Cuz+l /pmol dm-3 FIG.13.-Quenching of Ru(dipy),(CN), by Cu2+ below the c.m.c. Variation of I, the initial intensity remaining after quasi-static quenching obtained from data as in fig. 6, with CU*+ concentration. Plotted according to eqn (6). 0 and x are for 6 and 4 mmol dm-3 SDS, respectively. with SDS ions. The uncharged cyanide complex would not be expected to behave in this way. However, at 4 and 6 mmol dm-3 SDS, Cu2+ is still a very effective quencher in the concentration range below 20 pmol dm-3 and behaves kinetically as at SDS concentrations above the c.m.c. (see fig. 6), i.e. there is quasi-static quenching followed by exponential decay with the unquenched aqueous rate constant 3.7 x lo6 s-l.At 20 pmol dmP3 Cu2+ the quasi-static quenching removes > 70% of the initial emission and over a range of Cu2+ concentrations there is little difference between 4 and 6 mmol dm-3 SDS. This again indicates a strong affinity of the emitter for Cu2+ but clearly at these low concentrations it cannot involve the simple ion Cu2+ referred to above.12 We suggest that it is the association of the emitter with a Cu2+-SDS cluster, the latter behaving as though it were a micelle. We then have the equilibrium C + E e R E K , where C is the cluster and E the emitter. In the associated species RE there will be quasi-static quenching as in a micelle and all the emission originates in the free emitter,S. J. ATHERTON, J. H. BAXENDALE AND B. M. HOEY 2181 E.Assuming a micellar-type association with multiple occupancy of C by E then [El I 1 [E]+[RE] =lo= l+K,[C] where I is the initial emission intensity at a concentration [C] of the Cu2+-SDS species and I, is the intensity in the absence of Cu2+. If all the Cuz+ is present as C then I,/I should be linear with [Cuz+], and fig. 13 shows this to be reasonably so. The fact that there is little difference between 4 and 6 mmol dm-3 SDS supports the assumption that the Cu2+ is present entirely as C. From fig. 13, we find KE = 1.25 x lo5 mol-1 dm3. If C contains more than one Cu2+ then K , is proportionately larger. Note that the absence of similar behaviour by MV2+ indicates that the latter is unable to form micelle-like clusters in these conditions. Note also that similar association of SDS with multiply charged metal ions in solution is to be expected and that as a result the ion-micelle association constants usually obtained do not necessarily refer to the free ion in solution.It would seem that the high charge density of the small metal ions is required for such association, since MV2+ does not behave like Cu2+ in the above conditions. CONCLUSIONS To summarise, we find the observations are explicable by Infelta's general kinetic equation' and in particular : (a) The enhanced quenching of the emission from Ru(dipy)i+ by Cu2+ and MV2+ in micellar SDS solutions is accounted for quantitatively in terms of a completely micellised emitter and rapid equilibration of the distribution of the quencher between aqueous and micellar phases.The quenching is slow compared with the equilibration. (6) In the quenching of Ru(dipy),(CN), emission by Cu2+ and MV2+ in the same conditions, the quenching rates are faster than the equilibration. Cu2+ has a specific affinity for micelles containing the emitter. (c) The quenching of Ru(dipy)i+ emission by Cu2+ and MV2+ is still enhanced by concentrations of SDS below the c.m.c. ( d ) The quenching of Ru(dipy),(CN), emission by MV2+, in contrast to that by Cu2+, is not enhanced by SDS below the c.m.c. The emitter seems to associate with Cu2+-SDS clusters as it does with micelles. Values of the quenching and association constants, k, and K are given in table 3. TABLE 3.-vALUES OF QUENCHING AND ASSOCIATION CONSTANTS cu2+ MV2+ k,/s-' Klmo1-l dm3 kq/ss1 Klmo1-l dm3 micellar systems Ru(dipy):+ 2.1 x 105 2.0 x 104 6 . 6 ~ lo5 7~ 104 Ru(dipy),(CN), large 6.3a 1.2x 107 large Ru(dipy)z+ 9.5 x 106 4.3 x 104b 3.3 x 106 8.8 x Ru(dipy),(CN)z large 1.25 x below the c.m.c. - - a Ratio of association constants for Cu2+ and micelles with and without emitter. K for K for association of Ru(dipy), (CN), association of quencher with Ru(dipy)t+-SDS cluster. with Cu2+-SDS cluster.2182 QUENCHING OF FLUORESCENCE FROM Ru(dipy)t+ A N D Ru(dipy),(CN), We thank Dr E. J. Land for the use of the laser at the Paterson Laboratory, Christie Hospital, the Royal Society for financial support for the development of our computerised data acquisition and analysis facility, and Mr P. R. Baxendale who interfaced the Tektronix 7912A digitiser to the Commodore PET and con- tributed substantially to the programming. P. P. Infelta, Chem. Phys. Lett., 1979, 61, 88. J. C. Dederen, M. Van der Auweraer and F. C. De Schryver, Chem. Phys. Lett., 1979, 68, 451. (a) P. P. Infelta, M. Gratzel and J. K. Thomas, J. Phys. Chem., 1974, 78, 190. (b) M. A. J. Rodgers and M. F. S. Wheeler, Chem. Phys. Lett., 1978,53, 165. (c) J. C. Dederen, M. Van der Auweraer and F. C. De Schryver, J. Phys. Chem., 1981,85, 1198. ( d ) F. Grieser and R. Tausch-Treml, J. Am. Chem. SOC., 1980, 102, 7258. N. J. Turro and A. Yekta, J. Am. Chem. SOC., 1978, 100, 5951. M. A. J. Rodgers and J. H. Baxendale, Chem. Phys. Lett., 1981,81, 347. M. A. J. Rodgers and J. C. Becker, J. Phys. Chem., 1980, 84, 2762. ’ J. H. Baxendale and M. A. J. Rodgers, Chem. Phys. Lett., 1980, 72, 424. U. Lachish, M. Ottolenghi and J. Rabani, J. Am. Chem. Soc., 1977, 99, 8062. D. Meisel, M. S. Matheson, W. A. Mulac and J. Rabani, J. Phys. Chem., 1977, 81, 1449. lo D. Meisel, M. S. Matheson and J. Rabani, J. Am. Chem. SOC., 1978, 100, 117. l 1 G. L. Gaines, J. Am. Chem. Soc., 1979, 83, 3088. l2 J. N. Demas, J. W. Addington, S. H. Peterson and E. W. Harris, J. Phys. Chem., 1977, 81, 1039. (PAPER 1 / 1403)

ISSN:0300-9599    

DOI:10.1039/F19827802167

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I , 1982, 78, 2183-2194 Decomposition Reactions in the Flame Ionization Detector BY ANTHONY J. C. NICHOLSON CSIRO Division of Chemical Physics, P.O. Box 160, Clayton, Victoria, Australia 3 168 Receiued 8th July, 198 1 A computer simulation has been made of reactions postulated as occurring in the flame ionization detector, f.i.d. Upstream of the luminous zone, alkanes, alcohols and ethers decompose in an atmosphere of hydrogen to give methane. For each additive the calculated yield of methane equals the experimentally determined relative ionization yield. The response characteristics of the f.i.d. follow from this, provided that the additive decomposition and the ionization reaction occur in separate regions of the flame. For the very different response of an f.i.d.using carbon monoxide as fuel instead of hydrogen, the methane yield and the ionic yield are also equal. Although the flame ionization detector, f.i.d., has been used in gas chromatography' as a sensitive and versatile detector of organic molecules for over twenty years, the chemical reactions which give it its particular electrical response are still not established. The f.i.d., as normally operated, is a hydrogen diffusion flame burning in air. The most detailed and satisfactory attempt to fit the experimental observations on the f.i.d. into current theories of hydrocarbon decomposition and of ionization in flames was that of Blades., He proposed that the 'equal-per-carbon' response of the f.i.d. to hydrocarbon additives followed if all additives were converted to the same distribution of single-carbon hydrides prior to ionization or oxidation.Nicholson and Swingler3 suggested that this distribution was set by the equilibrium of H,, H, CH,, CH,, CH and C. The similar suggestion had been made by Hayhurst and Vince4 that a number of phenomena in hydrogen flames, including ionization, could be correlated by assuming a series of stripping reactions CH, +H+CH,-, +H, of which one was the rate-determining step for production of CH. Blades2 proposed that after the first reaction producing radicals the hydrocarbon chain was degraded by a series of H-atom cracking reactions such as C,H5+H+2CH,. The availability of a computer program5 for simulating the kinetics of a number of coupled chemical reactions provided an opportunity for testing these postulates in a quantitative manner.A mechanism for the degradation process in the f.i.d. was set up using rate constants from the literature. In spite of uncertainties in our knowledge of some of these rate constants, the calculation gives an insight into which reactions contribute to the overall decomposition and which reactions are too slow to be relevant. It also shows whether equilibria can be attained in the time available. A mechanism involving methane production that does not require the equilibrium hypothesis will be shown to fit the facts adequately. 21832184 THE FLAME IONIZATION DETECTOR The experimentally observed f.i.d. relationships that must be deducible from a mechanistic theory are as follows. (a) If & is the ion yield per additive molecule, then K is constant for additive concentrations from to of the concentration of the carrier gas.6 (b) If RK is the yield relative to methane taken as one, then Rx equals the carbon number for hydrocarbon additives (the ‘equal-per-carbon ’ response).Blades2 showed that this relationship only holds if the additive is carried up to the flame in a gas stream containing hydrogen; this observation has recently7 been questioned. The value for acetylene, R ( c ) For a carbon attached to a hetero-atom an ‘effective carbon number’ less than one can be determined.6 RX for a molecule then equals the sum of the actual and effective carbon numbers, the latter being roughly transferable from molecule to molecule. Quantitative values (C=O = 0, CH30 = 0.3 in dimethyl ether but zero for its homologues, COH = 0.75 and so on) must be reproduced by the mechanism.( d ) When carbon monoxide replaces hydrogen as fuel in an f.i.d., q varies as the additive concentration at low concentrations and is constant at high concentrations,s for a carbon monoxide flame in the high-concentration region is ca. 0.08 of that for a hydrogen flame (from a comparison of the figures of McWilliam8 and Sternberg et a1.6 for C,H16). FLAME MODEL = 2.6, is exceptional.2g A representative f.i.d. flame, as described in our earlier experirnent~,~ had a blue luminous zone in the shape of a blunt-ended cylinder ca. 1.2 mm in diameter and 4 mm long. From the measurements of Ohline et aL9 and of Mitchell et al.l0 on similar diffusion flames, a reasonable estimate of the temperature 0.1 mm inside the luminous zone is 1800 K.For a cold gas flow of 2 cm3 s-l the velocity of the gas as it passes through a 1 mm diameter x 4 mm long cylinder at 1800 K is ca. 1 m s-l. The time for the gas to travel 0.1 mm is then 100 ps; the program was arranged to calculate the products formed in this time from various concentrations of the chosen additive reacting at 1800 K. The assumption is made that additive decomposition occurs in a hydrogen atmosphere in a region separate from that in which the ionizing and combustion reactions involving oxygen take place. Calcote’l has shown that the maximum rate of ionization occurs downstream from the luminous zone where the temperature is also at a maximum. Mitchell et al.l0 give plots of [HCN] and [NO] as a function of position in a diffusion flame of methane burning in air. They show that [HCN] peaks at ca.2 mm upstream of the luminous zone, where the temperature is 1400 K, whereas [NO] is almost separated from [HCN] and peaks at the temperature maximum, 2000 K, ca. 1 mm downstream from the luminous zone. In other words, the reducing and oxidizing regions of such flames are physically separate. The calculations given below are for the reducing region, and the half-lives obtained for the additive decomposition are a further justification for neglecting reactions between an additive and 0 or 0,. COMPUTER PROGRAM The program, developed by Davis,5 uses Gear’s algorithm for integrating a group of ‘stiff’ coupled differential equations.It has an option in which a system is considered at constant pressure with variable temperature and volume which corres- ponds reasonably to a flame. Preliminary runs showed that the compositions after 150 ps differed little regardless of whether a temperature rising in steps of 50 ps at each of 1600,1700 and 1800 K or a fixed temperature of 1800 K was used, so the latterA. J. C . NICHOLSON 21 85 was chosen for convenience. The total gas concentration, [MI, was taken as 4 x 10l8 molecule ~ r n - ~ , of which 3 x 1018 molecule cm-3 was hydrogen. Concentrations of various additives from 10l6 molecule ~ m - ~ downwards were used. The program does not accept zeros so all free-radical and atom concentrations were set initially at of the additive concentration.All reasonable reactions of the additive were written into the program, which calculated the concentrations of all the atoms, radicals and molecules as a function of time. RATE CONSTANTS The reactions and rate constants used for alkane pyrolysis [reactions (1)-(27)] are given in table 1. In general the rate constants used were taken from the review of Jensen Y and Jones.12 This-is a self-consistent set chosen for their applicability to TABLE RATE CONSTANTS FOR THE PYROLYSIS OF PARAFFINS k = ATn exp (-E/RT), A/cm3 molecule-' s-l or s-l, T/K, E/kJ mol-1 - initiation (unimolecular) 1 M+H,+ H+H +M 2 3 CzH, CH, + CH, 4 6 M +CH, e CH,+ H + M C,H, + C,H5 + H C3H8 * C3H7 + H 5 C3Ha + CH3 +C,H5 7 C4HIo s CzH5 + CzH5 8 9 10 C,H,, s C,H, + CH, C4H,, e C,H, + H M +C,H, =C,H, +H +M initiation (bimolecular) 11 13 14 H + C,H5 + CH, + CH, H, + C,H, + CH, + CH, H, + C3H, + C,H, + CH, 12 H, + CZH, G= CzH3 + H 15 CzH4 + CzH4 + CzH5 + CH3 radical decomposition 16 M+C,H, +C,H,+H+M 17 M +C,H, -P C,H,+H + M 18 C3H7 + C,H, + H 19 C3H7 -+ CzH4 + CH, 20 C4H, + C3H6 + CH, 21 C4H, + C2H4 + C,H5 22 23 CzH, +CH3eCH4+C,H,.hydrogen abstraction H, + C H , g CH, + H 24 H, + CzH3 S C,H4 + H 25 28 29 H, + C2H5 + CzHe + H M +HCO+ CO+H + M H, +CO =$ HCO +H 26 27 H, + C,H, + C3H8 + H H, + C4H, + C,H,, + H forward _ _ _ _ _ log A n E log A __________ -5.10 5.70 16.04 16.04 16 16 15.30 15.30 15.30 -6.22 -9.3 - 1 1.28 -9.3 -9.3 -8.96 - 8.92 -9.47 13 13 13 13 - 10.52 - 24.04 -11.32 - 16.64 - 1 1 -11 -9.4 - 10.85 438 454 36 1 403 343 393 323 356 376 41 1 X O 269 234 222 268 126 126 173 139 137 120 62.4 34.6 24 38.6 32 32 66 400 - 29.52 -20.70 - 11.00 -9.3 - 9.3 -9.3 -9.3 - 9.3 - - 32 - -11.15 - - - - 33 - 32 -9.3 -9.3 - - -9.15 -24.52 - 10.52 - 15.30 - 10 - 10 - 32.7 -9.7 back n E - 1 0 -3 0 - 4.2 __ - - - 40 - - 62.4 4 60.3 29 2 29 32 32 7 21 - - - - - flame ref.12 12 12 13 13 14 13 13 15 16 12 __ - - 15 12 15 13 13 13 13 12 15 12 12 - - 12 12 a Ref. (12) gives log A,, = - 17.64 and log A-25 = - 16.30. The values used here are a t the upper limit of their error bounds. temperatures. More recently Baulch and Duxburyl' have recommended a slightly faster rate for reaction (3) but consider that its rate is falling off at ca. 1019 molecule ~ m - ~ as it moves towards second-order kinetics. This makes little difference to the rate of 71 FAR I2186 THE FLAME IONIZATION DETECTOR reaction (3) at 1800 K.On the other hand, reaction (17) is in its transition region at this total gas concentration, so the second-order rate constant given by Koike and Gardner15 is preferred. Uncertainties in knowledge of rate constants of hydrocarbons increase with increasing molecular weight; the values chosen here are rounded off values from the review by Benson and O’Neal.13 They are slightly smaller than those given in a more recent review.lS Provided that the rate constant for unimolecular decomposition is > 7 x lo4 s-l (99.9% decomposition in 100 ps) the results obtained below are not sensitive to the value of this rate constant. The back reactions between radicals given in table 1 are unimportant compared to the reactions of radicals with hydrogen.They only become significant when hydrogen is replaced by carbon monoxide, with which there is no equivalent metathetical reaction. Values of these rate constants were calculated from the equilibrium constants given by Jensen and Jones12 or from those in the JANAF Tables,19 or put equal to 5 x 1O-lo cm3 molecule-’ s-l. Decompositions involving the breaking of C-H bonds in hydrocarbons, reactions (4), (6) and (9), are rarely mentioned in the literature because they are usually overshadowed by reactions involving breaking of C-C bonds. If these rate constants are estimated by taking the same pre-exponential factor and correcting for the higher bond dissociation energy, D, then the C-H breaking rates are slower than the C-C breaking rates by exp[(D,,-D,,)/RT].This factor has a value of ca. 10 at 1800 K, and these reactions are in fact significant. In table 2 are given rate constants for a mechanism with methanol and dimethyl ether as additives. TABLE 2.-RATE CONSTANTS FOR THE PYROLYSIS OF CH30H AND CH30CH3 k = ATnexp( -E/RT), A/cm3 molecule-l s-’ or s-l, T / K , E/kJ mol-l forward back log A n E log A n E ref. initiation 30 31 32 33 34 35 36 37 CH, + CH,OCH, CH,OCH, + CH, 39 41 M + CH,OH =$ CH, + OH + M M +CH,OH s C H , O + H + M CH,OCH, + CH,O + CH, CH,OCH, + CH,OCH, + H M +CH,O g CH,O + H + M M +CH,OCH, g CH,O + CH, + M radical decomposition hydrogen abstraction OH + CH,OH g CH,O + H,O 38 OH + H,+ H,O+H 40 CH,O + H, + CH,OH + H CH, + CH,OH + CH,O + CH, CH,OCH, + H, + CH,OCH, + H - 5.3 -3 15 15 - 10 - 10.4 - 11.18 - 10.3 - 14.72 -9.3 -9.3 - 12.52 - 335 -32 - __ 418 -32 - - 318 -11.7 - - 418 -11.7 - - 121 -11.7 - - 60 -11.7 - - 8.4 -9.3 - 63 -9.3 - 1.3 15 - 14.08 1.3 - 48 -10.7 - 20 -9.3 - - 41 -11.2 - - - 0 20 0 0 21 0 - - 0 20 0 21 71 20 75 20 77.6 12 29 16 55.6 22 - .- . RESULTS ETHANE, PROPANE A N D BUTANE Fig. 1 shows the products formed as a function of time when lo9 molecule ~ r n - ~ of C,H6 is taken as the initial concentration of additive in the scheme of table 1. This figure shows that: (i) The major molecular product is CH,, and in 10 ps it has reached a limit set by the amount of C,H6 initially present. Little change in products occurs between 10 and 100 ps. (ii) For reactions (I) and (- 1) the equilibrium valueA. J.C. NICHOLSON I I I I I H2 18 t \c*&- C2H6 2187 0 20 40 60 80 100 time/ps FIG. 1 .-Plots of the logarithm of the concentrations of the products from log molecule cmW3 of C,H, as additive against time at 1800 K. Hydrogen flame. of [HI (1.25 x 1015 atoms cmW3 at 1800 K) has not been reached. Dissociation of H, is negligible. (iii) C,H, is present to ca. 0.05% of the CH,. Its amount is decreasing but so slowly as not to be visible on this scale. The amount is lo5 times greater than it would be if the equilibrium C,H, + 2H, 2CH, had been established. (iv) Reactions (22) and (-22) are almost in equilibrium: [CH,][H,]/[CH,][H] = 47, whereas the equilibrium value is 23. Other products have reached ' steady states' that are not equilibria.Fig. 2 gives similar plots for an initial ethane concentration of 1015 molecule ~ m - ~ . Once again CH, is the main product, although the relative proportions of the minor products have changed. [HI is higher than the equilibrium value, and equals [CH,] throughout. The equilibrium of reactions (22) and (-22) is established. [C,H,] at 100 ps is now 0.4% of [CH,]. Reactions (3) and ( - 3 ) are almost in equilibrium. If initial concentrations of propane or butane are substituted for ethane into the scheme of table 1, the general shape of the concentration against time curves remains the same and methane remains the dominant product. Product concentrations after 100 ps for equivalent concentrations of the first four alkanes are given in table 3. The postulate that every alkane produces the same distribution of minor products is wrong, but the quantities of minor products are so small relative to the quantities of methane that their effect on the yield of methane is negligible.Fig. 3 is a log-log plot of the concentrations of some of the products against the initial additive concentration. This particular plot is for C3H, additive, but the plots are the same for C,H, and C4H1, except for the absolute values of the ordinate. Over a dynamic range of at least lo7, [CH,] is linear with initial additive concentration, 71-22188 - 14 m . 2 12 .- Y E 4 d c 10 s 9 8 W 2 00 THE FLAME IONIZATION DETECTOR - \ \ C4HIO 0 20 40 60 80 100 timelps FIG. 2.-Plots of the logarithm of the concentrations of the products from I O l 5 molecule of C,H, as additive against time at 1800 K.Hydrogen flame. TABLE 3.-DECOMPOSITION PRODUCTS AFTER 100 / l S FOR PARAFFIN ADDITIVES initial concen- tration /lo14 molecule product after 100 p s concentration/molecule ~ r n - ~ additive ~ r n - ~ CH4 H CH, ‘ZH4 CZHZ C3H6 ‘ZH5 ~~ ~ ~ C4H10 1 3.7 x 1014 5.7 x 1014 1.6 x lo’* 1.5 x 1013 1.0 x 1012 4.4 x 109 3.1 x 107 CZH, 2 4.0 x 1014 4.2 x 1014 5.3 x 1011 3.8 x 1011 1.2 x 1011 6.2 x 108 2.7 x 105 CH4 4 4.0 x 1014 4.0 x 1012 1.9 x 1010 3.5 x 109 1.3 x 109 1.1 x 109 4.0 x 104 C3H8 1.3 3.8 x 1014 5.4 x 1014 4.0 x lo1* 6.8 x 10l2 3.2 x 10l2 1.6 x log 9.3 x los , CH4 H CH3 C2H2 C2H4 r 8 10 12 14 16 log,,(initial concentrati~n/cm-~) FIG. 3.-Log-log plot of concentration of products after 100 p s against initial concentration of additive, C,H,, at 1800 K.Hydrogen flame.A. J . C. NICHOLSON 2189 a, 0 4 E l 2 . ... x 22 - ... x t: a, m ... - .3 Y c 2 I I 1 I J 8 10 12 14 16 2 1 ' u log,,(initial concentrati~n/cm-~) FIG. 4.-CH, yield, molecule at 100 p s per initial additive molecule plotted against the logarithm of the initial concentration. The circles give the ion yield, ions per additive molecule relative to CH, = 1 . Hydrogen flame. i.e. its yield per additive molecule, YCH4, is constant, and as shown in fig. 4 this yield is almost equal to the carbon number of the additive. Methane is the only product for which this is true, and neither relationship holds for methyl radical concentration. Rather, over the range 1012-1014 molecule cm-, of additive the slope of the log [CH,] line is 2.The point (ca. lo1, molecule ern-,) where this curve steepens corresponds to the point where the [HI curve approaches the [CH,] straight line. At this point also the equilibrium of reactions (22) and (-22) is established, i.e. [CH,] = [CH,][H]/K,,[H,], and the square law follows since both [CH,] and [HI vary as the initial additive concentration. The only liberty taken in selecting rate constants was to put k,, at the upper limit of the range given in ref. (12). YCH, is sensitive to the ratio of the rates of reaction (25) to reaction (7) and, if this is reduced by a factor of 10, YCH, for C,Hl, is reduced to 3.2 although the yield from C,H, or C,H, is barely altered. CARBON MONOXIDE FLAME Reactions in a carbon monoxide flame are simulated by the reactions for alkanes plus reactions (28) and (29) with the initial H, replaced by CO.The distribution of products, fig. 5, is now quite different (cf. fig. 1). CH, is the major product, CH, a minor one and it is the equilibrium of reactions (3) and (- 3) which is reached. In fig. 6 a log-log plot of the products against initial additive concentration also demonstrates a reversal since [CH,] is linear with additive at low additive concentra- tions, whereas over the range 1011-1014 molecules of additive per cm3, [CH,] varies as the square of the initial additive concentration. Thus for both flames it is the methane yield which parallels the RT behaviour given in the introduction under (a), (b) and ( d ) . In the CO flame at low concentrations [H,] is also proportional to the square of the additive concentration, but once the absolute amount of hydrogen becomes appreciable the system moves towards that of a hydrogen flame, the [CH,] and [CH,] curves cross and the [CH,] tends towards being proportional to the first power of the additive concentration.This behaviour is also paralleled by Rq in CO flames,, although the changeover occurs at a lower concentration than in the model system. The ratio of the methane yields at initial [C,H,] = lo1, molecule cm-, in CO and H, is 0.025, whereas the corresponding ratio of ion yields is 0.08. Agreement is fair when one considers that experimental data on CO flames are scarce and that there2190 THE FLAME IONIZATION DETECTOR 0 time/ps FIG. 5.-Plots of the logarithm of the concentrations of products from los molecule ~ m - ~ of C,H, as additive against time at 1800 K.Carbon monoxide flame. FIG. 6.-L0g-l0g plot 4 ' IOJ 1; Id 1's I; ' log,,(initial concentrati~n/cm-~) of concentration of products after 100 ,us against initial concentration of additive, C,H,, at 1800 K. Carbon monoxide flame. are doubts8 about the purity of the carbon monoxide used in some of the published experiments. O X Y GEN-CONTAIN ING MOLECULES CH+O+HCO The reaction is so exothermic that there is enough energy available to ionize HCO. Conversely, the reactions reducing a carbonyl group to CH are so endothermic, and accordingly slow, that a carbonyl group gives zero ionic response in the f.i.d. It follows that anyA. J . C. NICHOLSON 2191 reaction of the O-containing molecule that produces a carbonyl group will lower the value of Rq if it is fast enough to compete with the reactions producing ions.If, as indicated above, CH, production is an essential step in the path to ion production, this possibility can be further tested by seeing whether YCH, is lower than the carbon number for dimethyl ether and methanol. Formaldehyde is the obvious carbonyl compound that could be formed from these additives. The question is whether its rate of formation is comparable with that of CH, formation. These decompositions have been modelled by adding the reactions of table 2 to reactions (1)-(4), (1 l), (1 3), (1 7), (22), (23) and (25) of table 1 ; the rate constant values of table 2 are more uncertain than those of table 1. DIMETHYL ETHER The yields of methane, YCH,, for CH,OCH, over a range of additive concentrations are shown in fig.4. The value k,, = 1 x exp (- 121 000/RT) gives YCH, = 1.3 in exact agreement with R for CH,0CH3.2 For all concentrations, the sum of YCH, and YHpco is two. The agreement is only of limited significance since YCH, is sensitive to the value of k,,; specifically, YCH, is reduced to 1 by increasing k,, by a factor of 40. However, this sensitivity means that a slight increase in the rate constants of the equivalent of k,, (e.g. CH,CH20+CH3 + H,CO for diethyl ether) would account for the zero Rq of the ether group in all ethers other than dimethyl ether.6 METHANOL For CH,OH decomposition, CH,O has been used to represent the two radicals CH,O and H2COH, since no quantitative data are available to determine any difference in their reactivities.The hydrogen abstraction reactions, (36), (- 39) and (41), are not fast enough to produce H2C0 via CH30 in appreciable quantities in the times considered here. Accordingly, reaction (31) is postulated with a rate making it competitive with reaction (30). The YCH, value calculated is then 0.93. This is higher than the RF value of 0.75 given by Sternberg et al.6 However, YCH, falls off at high additive concentrations more sharply than the yield from alkanes (fig. 4) and, since the measurements of Sternberg et al. were made over the range 1013-1015 molecule ern-,, the discrepancy is not large. As for CH,OCH,, the sum of YcH, and YHzC0 equals the carbon number of the additive. ETHYLENE A N D ACETYLENE In an earlier paper3 we assumed that ethylene and acetylene would be reduced to alkanes by the excess of hydrogen present sufficiently rapidly for degradation to methane to occur. The calculations of the present paper make this unlikely, but the values chosen for the rate constants are uncertain.The only measurements of gas-phase hydrogenation date from the 1930s, of Pease23 on C2H, and Taylor and van Hook2, on C2H,. For such chain reactions it is possible to calculate the rate constant for the initiation reaction from the overall rate of the reaction; SemenofF gives such a calculation for Pease’s measurements. Even if the assumed mechanism is correct, the initiation reaction contributes only a small fraction to the overall reaction and cannot be specified very accurately.Reactions (1 I)-( 15), grouped under the heading of ‘ Bimolecular Initiation’ have been assigned rate constants at the upper limit of what seems reasonable in an endeavour to find a reaction path from unsaturated to saturated that could be significant in 100 ps. All are inadequate and their omission, along with reaction (10) and all reactions involving C2H2 and C,H,, makes a negligible alteration to the calculations described above for the alkanes. The fastest reaction2192 THE FLAME IONIZATION DETECTOR removing C,H, is reaction (lo), and its half-life is 0.36 s at 1800 K and 2.3 x s at 2200 K, which is about the highest temperature reached in f.i.d. flames. On present knowledge of rate constants, C2H4 and C,H, must be regarded as exceptions which do not degrade to CH4 within 100 ps at 1800 K as do alkanes, alcohols and ethers.METHANE Methane, like C2H4 and C,H,, decomposes by breaking a C-H bond which is stronger than the C-C bonds in its homologues. The lifetime of CH4 for decomposition by reaction (2) is 4.3 x lo-, s at 1800 K and 3.9 x lo-, s at 2200 K. CH,, C,H, and C,H, are unique among the gases considered here in that their lifetimes are long with respect to the time-scale used in the calculations. A calculation of products after 100 p s is given in table 3. For the same final amount of CH, the amounts of minor products are markedly different for CH, itself than for its homologues decomposing to CH,. DISCUSSION The calculations given here were planned to give quantitative expression to the mechanism given in a previous paper,3 which was itself an extension of the mechanism given by Blades., Two assumptions made there3 are shown here to be wrong.The first is that equilibrium is reached in the H, f 2H reaction [reactions (1) and (- l)]. In 100 ps, [HI is above its equilibrium value at high additive concentrations and below it at low concentrations. The equilibrium CH3+H, *CH,+H is only attained at the high end of the additive concentration range. The calculations made here give concentrations reached after 100 ps, and whether these are equilibrium values or not is irrelevant. The second, made originally by Blades, that 'all additives give the same distribution of single carbon hydrides before ionization' is not quite correct. All additives give CH, as a major product, while the variable amounts of minor products make a negligible contribution to the decomposition.It has been shown that YcH, from C,H,, C,H, and C4H1" parallels the relative ionization yield in the usual f.i.d. (H, flame) in its major characteristics; the constancy with additive concentration, the wide range of this constancy and its equality to the carbon number. Considerable alterations to the rate constants given in table 1 would be needed to cause a departure from this parallelism. In the f.i.d. with CO as fuel, R F is linear with concentration over a small range and then becomes almost constant; this is also the behaviour of YCH,. Although the rate constants needed are less accurately known, the calculation also correlates YCH, from oxygen-containing compounds with the effective carbon numbers in the f.i.d.of the COH and CH30 groups. A complete explanation of the f.i.d. relationships then requires a mechanism yielding ions in amounts proportional to CH, concentration. Experimentally this was established by Peeters et a1.26 for a premixed flame with H, : 0, = 7 : 3 and it holds for CH, in the f.i.d. diffusion flame. A surprising result of the calculations given above is that while [CH,] is linear with additive concentration over the whole range over which ionization is linear, [CH,] is at first linear, then follows a square law and finally requires an intermediate exponent as the additive concentration rises (fig. 3). This suggests that the reaction path giving ionization (whether via CH or in any other way) proceeds from CH, without passing through CH,.A possible reaction by-passing CH, formation is CH, CH, + H, for which Chen et al.27 give a rate constant of 6.3 x 1014exp (-474000/RT) s-l, stating that this is probably a maximum value. We used a value of one quarter ofA. J. C . NICHOLSON 2193 this in a simuIation of the reactions of the single-carbon hydrides with H, H, and C and found that [CH,], [CHI and [C] were proportional to the initial [CH,] while [CH,] was proportional to the square of the initial [CH,]. No more is claimed for this calculation than that it provides a possible mechanism, since most of the rate constants used were estimated. This calculation also shows that the hydrogen-atom cracking mechanism postulated by Blades is relatively unimportant.For example, in the reaction plotted in fig. 2 the rate of removal of C,H, by atom cracking, reaction (1 I), is 1 O6 [C,H,] molecule ~ m - ~ s-l, whereas by hydrogen abstraction, reaction (25), it is 1.7 x lo7 [C,H,] molecule ~ m - ~ s-l. At lower additive concentration reaction (1 1) is even slower, 10, [C,H5] molecule ~ r n - ~ s-l for the reaction of fig. 1, whereas k25 [H,] does not change with additive concentration. The calculations show that it is reactions with the hydrogen molecule that are important and that alkane decomposition is complete before an appreciable amount of hydrogen has dissociated. This high concentration of hydrogen, in suppressing radical-radical reactions, is essential to give total conversion to the single-carbon compound, CH,.Recently Wagner et al.’ have examined the response of alkanes in the hydrogen atmosphere flame ionization detector, h.a.f.i.d, in which the additive is carried in an oxygen stream, in the inner of two concentric jets, to burn in an atmosphere of hydrogen. They observed an equal-per-carbon response, which seems to be in direct contradiction to measurements of Blades,, and state that ‘an explanation unifying the results reported in this study and in that of Blades remains unclear’. They also state that ‘if only one mechanism is to be proposed as the origin of the equal-per-carbon response it must operate in both hydrogen-rich and oxygen-rich environments ’. The mechanism involving CH, formation advanced here cannot operate in an oxygen-rich flame and a postulate that more than one mechanism exists seems more likely.A difference worth noting is that while the response and methane formation are linear over seven orders of additive concentration in the f.i.d., the h.a.f.i.d. response is only linear over two. Response- per-carbon is only meaningful in the linear region. Additive decomposition in oxygen atmosphere might resemble the carbon monoxide experiments modelled in this paper, where methane formation exhibits both linear and square-law variations with concentrations. Since the mechanism involving CH, production cannot be true for C,H, and C,H, an ad hoc postulate must be made that some other route to ionization is available for these two gases. Blades pointed out the anomalous position of C,H, referring to ‘ the embarrassing situation of basing the mechanism of ion formation on work on C,H,, the very compound for which there is evidence of unique behaviour’.He emphasized that C,H, catalyses H-atom recombination28 rather than being itself reduced, that ionizing reactions other than CH +O could not be ruled out with certainty, and that a C,H +O,* ionizing reaction might be peculiar to C,H,. Later, Blades29 compared ionization rate with CH emission rate and showed that their ratio was higher for C,H,, C,H, and CH, than for other alkanes. He argued that this required an additional ionization reaction to the generally accepted reaction via CH. The most detailed study of the chem-ionization of C,H, by 0 atoms, that of Vinckier et goes no further than saying that ‘an intermediate’ gives HCO+, either in a reaction with 0 or via CH which reacts with another 0 atom.These observations and the still unexplained R x value of 2.6 for acetylene are not inconsistent with the view that acetylene is a special case. It would seem that C,H, is also, although this has not been apparent since its R value of 1.98 fits into the normal pattern. The differences between CH, as additive and CH, produced from higher alkanes might be associated with the differences in minor products shown in table 3.2194 THE FLAME IONIZATION DETECTOR I am indebted to Dr T. McAllister who pointed out the program to me and taught me how to use it, and to Dr H. L. Davies for information about the program. I also thank the referees for helpful suggestions that have improved the presentation of this paper.I. G. McWilliam and R. A. Dewar, Nature (London), 1958, 181, 760 A. T. Blades, J. Chromatogr. Sci., 1973, 11, 251. A. J. C. Nicholson and D. L. Swingler, Combust. Flame, 1980, 39, 43. A. N. Hayhurst and I. M. Vine, Nature (London), 1977, 266, 524. H. L. Davies, CSIRO Division of Computing Research Program Simulation of Chemical Kinetics: H. L. Davies and M. Y. Smith, Proc. 8th Australian Computer Conference, Canberra, 1978 (Australian Computer Society, Canberra, 1978), p. 277. J. C. Sternberg, W. S. Gallaway and D. T. L. Jones, in Gas Chromatography, ed. N. Brenner, J. E. Callen and M. D. Weiss (Academic Press, New York, 1962), p. 231. I. G. McWilliam, Nature (London), 1970, 228, 356. R. W. Ohline, E. Thall and Ping Hwat Oey, Anal. Chem., 1969, 41, 302. ' J. H. Wagner, C. H. Lillie, M. D. Dupuis and J. J. Hill Jr, Anal. Chem., 1980, 52, 1614. lo R. E. Mitchell, A. F. Sarofim and R. Yu, Combust. Sci. Technol., 1980, 21, 157. * l H. F. Calcote, 8th Symp. (Int.) on Combustion (Williams and Wilkins, Baltimore, 1962), p. 184. l2 D. E. Jensen and G. A. Jones, Combust. Flame, 1978, 32, 1. l3 S. W. Benson and H. E. O'Neal, Kinetic Data on Gas Phase Unimolecular Reactions (NSRDS-NBS21, U.S. Department of Commerce, Washington, D.C., 1970). l4 D. G. Hughes, R. M. Marshall and J. H. Purnell, J. Chem. Soc., Faraday Trans. 1, 1974, 70, 594. l5 T. Koike and W. C. Gardiner Jr, J. Phys. Chem., 1980, 84, 2005. l6 P. Camilleri, R M. Marshall and J. H. Purnell, J. Chem. Soc., Faraday Trans. I, 1974, 70, 1434. l7 D. L. Baulch and J. Duxbury, Combust. Flame, 1980, 37, 313. l8 D. L. Allara and R. Shaw, J. Phys. Chem. Ref. Data, 1980, 9, 523. l9 D. R. Stull and H. Prophet, JANAF Thermochemical Tables (NSRDS-NBS37, U.S. Department of 2o C. R. Westbrook and F. L. Dryer, Combust. Sci. Technol., 1979, 20, 125. 21 P. D. Pacey, Can. J. Chem., 1974, 53, 2742. 22 D. Aronowitz, R. J. Santoro, F. D. Dryer and I. Glassman, 17fh Symp. (Int.) on Combustion (The Combustion Institute, Pittsburgh, 1979), p. 633. 23 R. N. Pease, J. Am. Chem. Soc., 1932, 54, 1877. 24 H. A. Taylor and A. van Hook, J. Phys. Chem., 1935, 39, 81 1. 25 N. N. Semenov, Some Problems of Chemical Kinetics and Reactivity, transl. J. E. S. Bradley 26 J. Peeters, C. Vinckier and A. van Tiggelen, Oxid. Combust. Rev., 1969, 4, 93. 27 C-J. Chen, M. H. Back and P. A. Back, Can. J. Chem., 1975, 53, 3580. 28 J. V. Michael and €3. Niki, J. Chem. Phys., 1967, 46, 4969. 29 A. T. Blades, Can. J. Chem., 1976, 54, 2919. 30 C. Vinckier, M. P. Gardner and K. D. Bayes, J. Phys. Chem., 1977, 81, 2137. Commerce, Washington D. C., 1971). (Pergamon, London, 1958), p. 218. (PAPER 1/1417)

ISSN:0300-9599    

DOI:10.1039/F19827802183

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. Soc., Faraday Trans. I, 1982, 78, 2195-2203 Thermolyses of 2-Methyloxetan and 2,2-Dimethyloxetan BY PAUL HAMMONDS AND KENNETH A. HOLBROOK* Department of Chemistry, University of Hull, Hull HU6 7RX Received 17th September, 1981 The thermolysis of 2-methyloxetan has been reinvestigated at temperatures between 429.5 and 483.2 O C and initial reactant pressures from 6.0 to 14.2 Torr. Two parallel unimolecular reactions occur producing either propene and formaldehyde or ethene and acetaldehyde. The ratio of propene to ethene is 1.31 kO.03 within this temperature range. The total rate constant for disappearance of reactant is given by log,,(k/s-l) = (14.89 *0.36)-(249.8 k4.7 kJ mol-l)/2.303 RT. The thermolysis of 2,2-dimethyloxetan was studied in the same apparatus at temperatures between 402.2 and 471.3 O C and at reactant pressures from 7.2 to 9.2 Torr.The reactant disappears by two parallel unimolecular paths to give either isobutene and formaldehyde or ethene and acetone. Minor products of methane, propene and isobutane were also detected. The ratio of isobutene to ethene was found to differ considerably from unity and to be strongly temperature-dependent. The following rate expressions were derived for the overall rate of disappearance of reactant (k2, 2 ) and for the rate constants for the individual reactions producing isobutene (k,) and ethene (k4) 1og10(k2,2/s-1) = (13.78 f0.24) -(226.0+ 2.7 kJ mo1-')/2.303 RT 1og10(k3/s-1) = (13.48 k 0.25) -(222.1 f 2.9 kJ mol-')/2.303 RT 1og10(k4/s-1) = (15.56kO.34)-(270.6f4.4 kJ mol-')/2.303 RT.The differences observed between the thermolyses of these two compounds are noted and compared with previously obtained results for other methyl-substituted oxetans. A possible explanation for the difference in Arrhenius parameters for the two paths in the case of the 2,2-disubstituted compound is suggested. Recent work on the thermolysis of 2-ethyloxetanl has revealed some differences in the Arrhenius parameters for the reactions of oxetans substituted in the 2- and 3-positions. In order to extend the comparison we decided to re-examine the thermolysis of 2-methyloxetan for which only the unpublished data of Cohoe2 and of Shirazi3 existed previously and also to examine the effect of gem di-substitution. The compound 2,2-dimethyloxetan has not been studied previously, nor are there any detailed results for other compounds gem-substituted in the 2-position, although Searles3 has commented on the low rate of decomposition of 2,2-diethyloxetan in comparison with oxetan itself.EXPERIMENTAL The sample of 2-methyloxetan used in most of this work was kindly given to us by Dr M. Bartok (University of Szeged, Hungary) and was found to be > 99.0% pure by g.1.c. Identical results were obtained in some runs using a commercial sample (Aldrich Chemical Co. Ltd, stated purity 98%) which was distilled under vacuum before use. 2,2-Dimethyloxetan was prepared by ring closure of the corresponding bromoalcohol using tributyl tin ethoxide. After an initial fractional distillation, the product was purified by preparative g.1.c.to > 99% purity. 21952196 THERMOLYSES OF METHYL-SUBSTITUTED OXETANS APPARATUS The apparatus and procedure were similar to that described previous1y.l A Pyrex glass vessel was used with a surface/volume ratio of 1.08 em-' and volume 269 em3. The dead volume was 13.6 em3 and this was taken into account in calculating rate constants. The vessel was packed with lengths of glass tubing for some experiments. This produced a surface/volume ratio of 1 1.6 cm-l and volume of 147.6 cm3. Methane, ethene, propene and isobutene were determined by g.1.c. using a 12 ft column of Chromosorb P coated with 14% bis-2-methoxyethyladipate plus 7% di-2-ethylhexylsebacate, operated at 60 O C . Tests for CO and H, were made using a 6 ft molecular sieve column and helium carrier gas.Minor products were also analysed using A Perkin-Elmer Sigma 1 g.1.c. system containing a 7 ft Durapak C column (phenyl isocyanate on Porasil C). The reaction was followed by pressure changes monitored by an S.E.L. pressure transducer type 1150 D. RESULTS 2- METHY LO X E T A N Thermolysis experiments with 2-methyloxetan were carried out in the reaction vessel seasoned with hexamethyl disilazane. The major products were found to be ethene, acetaldehyde, propene and formaldehyde as would be expected from the two reactions The overall pressure change was found to correspond closely to the sum of the amounts of propene and ethene formed at all temperatures, thus confirming the stoichiometry. The thermolysis obeyed first-order kinetics up to 70% reaction, as can be seen from a typical log plot shown in fig.1. The first-order rate constants were found to be independent, within experimental error, of initial pressure (up to 47 Torr), of added nitric oxide (up to 11 %) and of the surface/volume ratio of the vessel (increased by a factor of 10.7) (see table 1). A series of 75 runs at initial pressures normally in the range 7-9 Torr was carried out between 483 and 429.5 O C to derive the Arrhenius parameters. The data obtained are given in table 2. Where multiple runs were done at a single temperature the rate constants quoted are average values with a standard deviation of typically & 1 %. The following rate expression was derived, where the error limits represent the 95% confidence limits: log,,(k/s-l) = (14.89kO.36)-(249.8 k4.7 kJ mol-l)/2.303 RT. The ratio k J k , was found from the observed ratio of propene/ethene.A mean value of 1.3 1 0.03 was found from a total of 56 measurements made over the temperature range 426-477 OC. Minor products of methane, ethane and propane were detected by g.1.c. 2,2-DIMETHYLOXETAN The expected modes of decomposition of 2,2-dimethyloxetan would be according (3) to reactions (3) and (4): (CH,)*=CH, + CH,O L--0 (CH,),CO + C,H, (4)P. HAMMONDS AND K. A. HOLBROOK 2197 TABLE 1 .-EFFECT OF VARIATION OF CONDITIONS ON THE THERMOLYSIS OF 2-METHYLOXETAN effect of initial pressure T/OC Po/Torr k / lop4 s-' (calc.)" k / 10-4 s-1 458.7 7.01 11.78 11.60 458.6 13.06 11.82 11.54 458.7 25.30 11.54 11.61 458.4 47.37 11.60 11.41 effect of added nitric oxide T/OC Po/Torr % NO k/10-4 S-1 (calc.)" k/10-4 S-1 458.8 8.12 5.7 11.17 11.67 458.7 7.86 10.6 11.21 11.61 458.4 7.57 11.2 10.23 11.41 effect of packed vessel ( S / V = 11.6 cm-l) T/OC P,,/Torr k / 1 0-4 s-' (calc.)a k/10-4 S-1 444.5 7.01 5.06 5.15 444.6 7.1 1 4.83 5.18 444.1 7.04 4.67 5.03 444.1 6.94 4.71 5.03 444.1 7.30 4.92 5.03 a Calculated from the observed rate expression for the reaction in the absence of nitric oxide in the unpacked vessel ( S / V = 1.08 cm-').TABLE 2.-RATE CONSTANTS FOR THE THERMOLYSIS OF 2-METHYLOXETAN T/OC T/OC T/OC T/OC T/OC k/10-4 S-1 k/io-4 s-1 k/io-4 S-1 k/10-4 S-1 k/10-4 S-1 483.2 458.8 457.6 446.5 439.5 42.86 11.17 10.77 5.601 3.858 482.4 458.7 457.4 446.4 439.0 41.86 11.59 10.31 5.975 3.941 477.0 458.6 457.3 446.3 430.2 32.45 10.38 10.70 5.1 10 2.223 476.8 458.5 457.2 446.2 430.1 34.47 11.53 10.94 5.407 2.261 470.4 458.4 446.8 446.0 429.7 21.17 11.20 5.567 5.228 2.236 470.3 458.3 446.7 445.8 429.5 20.98 11.15 5.498 5.099 2.294 459.1 458.2 446.6 439.6 9.89 12.01 5.570 3.809 The overall stoichiometry was confirmed by the fact that the total pressure increase was found to be equal, within experimental error, to the sum of the isobutene and ethene pressures as measured by g.1.c.The overall reaction [according to steps (3) and (4)] followed by pressure change was first order up to ca. 70% decomposition, as can be seen from fig. 2. The first-order rate constants were independent, within experimental2198 THERMOLYSES OF METHY L-SUBSTITUTED OXETANS time/min FIG. 1 .-First-order plot for 2-methyloxetan thermolysis at 458.2 OC.11 10 9 0 - 7 I 6 5 . c 55 Z L c - 3 2 1 0 0.5 1.0 1.5 2.0 2.5 timelmin FIG. 2.-First-order plot for 2,2-dimethyloxetan thermolysis at 471.1 "C.P. HAMMONDS A N D K . A. HOLBROOK 2199 error, of initial pressure (up to ca. 40 Torr) and of added nitric oxide (up to 15%) (see table 3). On carrying out the thermolysis in the packed vessel, a small increase in rate constant occurred which can be attributed to a small heterogeneous contribution to path (3) which amounts to 1-2% in the unpacked vessel. TABLE 3.-EFFECT OF VARIATION OF CONDITIONS ON THE THERMOLYSIS OF 2.2-DIMETHY LOXETAN effect of initial pressure at 445 OC P,,/Torr 8.03 14.61 18..26 23.02 30.33 41.20 k2, 2/ 1 OP3 s-l 2.241 2.210 2.300 2.256 2.219 2.3 10 effect of added nitric oxide T/OC Po/Torr % NO k2, 2/ s-l (calc.)b k2, 2/ 1 0-4 s-I ~~ ~~ 419.6 8.00 5.0 5.300 5.532 41 8.1 8.54 7.5 5.300 5.080 419.5 8.78 9.8 5.400 5.500 418.3 8.77 10.3 5.100 5.138 418.2 8.61 15.1 5.100 5.109 effect of packed vessel ( S / V = 11.6 cm-l) k,(calc.) T/OC Po/Torr k2,,/10-4 s-' Ra l~,/lO-~ s-l /lo-* s-lb % k, het.c 443.6 443.6 443.6 443.6 443.6 442.9 443.0 443.0 8.1 1 7.55 8.73 7.42 8.77 7.34 7.5 1 7.27 24.0 24.3 23.7 23.7 23.2 24.8 24.5 23.5 42.7 41.8 40.4 43.0 42.4 44.3 37.5 42.0 23.45 23.73 23.13 23.16 22.67 24.25 23.86 22.95 19.84 19.84 19.84 19.84 19.84 19.13 19.23 19.23 1.9 2.0 1.7 1.7 1.5 2.7 2.5 1.9 a R = k,/k4.heterogeneous contribution to path (3) calculated from Calculated from the appropriate rate expression for the unpacked vessel.[k, -k, (calc.)] 1.08 x 100 10.52 k,(calc.) * The Arrhenius parameters were obtained from a total of 42 runs in the temperature range 402-471 O C and at pressures from 7.2 to 9.2 Torr. The data for these are pre- sented in table 4. The value of R (the ratio of isobutene to ethene from g.1.c.) at each temperature was used to calculate the individual rate constants k, and k, from the overall rate constant k,t 2. The results are summarised by the following rate expressions: log,,(k,, ,/s-l) = (1 3.78 0.24) - (226.0 _+ 2.7 kJ mol-l)/2.303 RT 1og10(k3/s-1) = (1 3.48 k 0.25) - (222.1 f 2.9 kJ molF1)/2.303 RT loglo(k4/s-l) = (1 5.56 k0.34) - (270.6 4.4 kJ mol-')/2.303 RT.2200 THERMOLYSES OF METHYL-SUBSTITUTED OXETANS TABLE 4.-RATE CONSTANTS FOR THERMOLYSIS OF 2,2-DIMETHYLOXETAN T/OC TIOC k2, 2/10-4 s-' ki, 2/ S-l T/OC ki, 2/ 10-4 S-1 T/OC k2, 2/10-4 s-' T/OC k2, 2/ 1 0-4 s-l 471.3 457.4 430.5 41 1.0 402.2 80.77 44.9 1 10.32 3.399 2.128 471.1 447.2 430.3 410.9 81.16 26.01 9.720 3.171 471.0 447.1 418.6 410.7 80.92 25.41 5.175 3.370 458.3 447.0 418.5 403.8 45.54 25.63 5.203 2.154 457.9 438.7 41 8.4 402.8 44.72 15.30 5.145 2.156 457.7 438.4 41 8.0 402.3 45.68 15.05 5.236 2.136 Small amounts of methane, propene and isobutane were detected by g.1.c.From the shape of the appearance/time curves for these products it is clear that isobutane is a secondary product and that methane and propene are produced in part by secondary processes. DISCUSSION The thermolysis of 2-methyloxetan has been confirmed to proceed mainly by the unimolecular paths (I) and (2).The overall Arrhenius parameters are in agreement, within experimental error, with those found previously by Cohoe,2 who reported a lowering of the rate constants by 6-7% in the presence of nitric oxide. This effect was not noted in our work, although a small amount of the minor product methane could possibly have arisen by a primary process involving free radicals. This, however, never constituted > 3% of the total products. The ratio of the amounts of propene and ethene formed was 1.31 0.03, invariant with temperature. Cohoe reported a similar lack of temperature dependence and a slightly higher ratio (1.50). The Arrhenius parameters for 2-methyloxetan are very similar to those reported for 2-ethyloxetan' and show a lowering of both A and E parameters in comparison with 3-substituted compounds.The thermolysis of 2,2-dimethyloxetan was investigated principally to study further the effect of substitution at the 2-position in the oxetan ring and to seek an explanation for the effect of such substitution upon the observed Arrhenius parameters. The observed rate constants are compared with those of analogous cyclobutane compounds in table 5. In most cases it is seen that the oxetans decompose more slowly than the corresponding cyclobutanes, but the 2,2-dimethyl compound is an exception. The effect is large and difficult to reconcile with the observation of Searles4 concerning the 2,2-diethyl compound. In table 6, the Arrhenius parameters for 2,2-dimethyloxetan show a marked contrast with those for the mono-substituted oxetans.There is in this case a definite predominance of the path producing the more highly alkylated olefin [isobutene, path (3)] and a strong temperature dependence of the ratio k J k , was observed which is reflected in the difference in Arrhenius parameters for paths (3) and (4). It is seen that in comparison with the other substituted oxetans, the Arrhenius parameters for path (3) are low whereas those for path (4) resemble the 'normal' values associated with oxetan itself and the 3-substituted compounds. The kinetic features of the thermolysis of both oxetan and 2-ethyloxetan can be reconciled with a biradical mechanism similar to that proposed for the decomposition of cyclobutanes. ' Benson- type ' calculations for the probable biradical intermediates involved in the thermolysisP.HAMMONDS AND K. A. HOLBROOK 220 1 TABLE 5.-cOMPARISON OF RATE CONSTANTS FOR OXETANS AND CYCLOBUTANES AT 450 'Ca oxetan k/io-4 S-1 ref. cyclobutane k/ 1 0-4 s-l ref. 4.7 1 5 (a) 2.70 (b) 2.34 1 d 6 (a) 2.06 (b) 4.01 (a) 1.78 (b) 2.24 6 I3 trans 17.5 7 this work (a) 27.7 (b) 1.04 5.18 8 8.07 9 7.33 10 17.7 1 1 9.56 12 18.6 13 a Path (a) leads to the more highly alkylated olefin and path (b) leads to the less highly alkylated olefin. of 2,2-dimethyloxetan cannot however explain the large difference between the rates of the two reaction paths for this compound. In particular, the observed activation energy for path (3) is lower than the estimated heat of formation of the probable biradical intermediate.A possible alternative explanation could be that path (3) occurs by a concerted rather than a biradical process. The normal concerted decomposition of oxetan in company with the reverse suprafacial addition of ethene and formaldehyde would be symmetry forbidden.'* An allowed process, although normally expected to be of higher energy, would be the addition of ethene to formaldehyde via a twisted conformation, producing suprafacial attack at one carbon atom and antarafacial attack at the other. The reverse process, i.e. the decomposition, could in principle proceed via such a distorted transition state. Normally it would be expected that this process could not compete with the biradical mechanisms. There are, however, reasons for believing that the concerted process could occur for the 2,2-disubstituted compound in the case of path (3).These can be most easily seen in terms of the frontier orbital theory applied to the cycloaddition of the aldehyde and olefin. Fleming15 has discussed in detail the effect of alkyl substitution on the HOMO and LUMO energies of the molecules involved. In principle, one should consider all possible HOMO/LUMO interactions but the most important in the case of olefin/carbonyl compound reactions are likely to be those of the HOMO of the carbonyl compound with the LUMO of the olefin. The minimum energy separation2202 THERMOLYSES OF METHYL-SUBSTITUTED OXETANS TABLE 6.-A RRHENIUS PARAMETERS AND RATE CONSTANT RATIOS FOR THERMOLYSES OF UNSYMMETRICAL OXETANS log10 log10 E J k J E b / k J ( k u / k b ) compound (Au/sP1) (Ab/S-l) mol-l mol-l 450 "C ref.a 14.64 14.52 249.8 249.8 1.31 this work 14.20 14.14 246.5 246.5 1.15 1 15.24 15.70 261.5 254.5 0.510 6 16 15.49 15.91 255.4 270.5 0.720 6 13.48 15.56 222.1 270.6 26.6 this work (15.71) QI (263.7) (1 .OO) 5 LUMO (carbonyl) - HOMO (olefin), and hence the maximum rate enhancement, is achieved when the LUMO (carbonyl) energy is lowered and the HOMO (olefin) energy is raised. Increased methyl substitution in the olefin (by increasing the electron density) is likely to have this effect, hence the concerted addition of isobutene to formaldehyde is favoured over that between ethene and acetone. By microscopic reversibility the decomposition path (3) would similarly be favoured over path (4) if these were concerted processes.Similar arguments have recently been advanced by Imai and Nishida16 to explain product ratios found in the thermolysis of 3-alkyl-2-phenyloxetans. In this system as in theirs it may be that concerted and biradical processes occur concurrently, but clearly the biradical mechanism alone cannot explain the observed experimental facts. We are currently examining other substituted oxetans to clarify these matters. Some preliminary molecular orbital calculations (carried out by Dr D. F. Ewing) have confirmed the general conclusions concerning the energies of the orbitals concerned. We thank Mrs Brenda Worthington for assistance with the preparation of 2,2-dimethyloxetan and Mrs M. Vickers for some confirmatory analytical results. M. J. Clarke and K. A. Holbrook, J . Chem. SOC., Faraday Trans. I , 1977, 73, 890. G. F. Cohoe, Ph.D. Thesis (University of Rochester, 1965). Z. H. Shirazi, Ph.D. Thesis (University of Swansea, 1973) and C. A. Wellington, personal communication. S. Searles Jr, The Chemistry of Heterocylic Compounch with 3- and 4-Membered Rings, ed. A. Weissberger (Interscience, New York, 1964), part 2, p. 983.P. HAMMONDS A N D K. A. HOLBROOK 2203 K. A. Holbrook and R. A. Scott, J. Chem. SOC., Faraday Trans. I , 1975, 71, 1849. K. A. Holbrook and R. A. Scott, J. Chem. SOC., Faraday Trans. I , 1974, 70, 43. C. T. Genaux, F. Kern and W. D. Walters, J. Am. Chem. Soc., 1954, 75, 6196. M. N. Das and W. D. Walters, 2. Phys. Chem. (Frankfurt am Main), 1958, 15, 22. lo M. N. Das and W. D. Walters, Z. Phys. Chem. (Frankfurt am Main), 1958, 15, 23. l 1 H. R. Gerberich and W. D. Walters, J. Am. Chem. SOC., 1961, 83, 3935. l 2 H. R. Gerberich and W. D. Walters, J. Am. Chem. SOC., 1961, 83, 4884. l 3 P. C. Rotoli, M.Sc. Thesis (University of Rochester, 1963). ’ G. F. Cohoe and W. D. Walters, J . Phys. Chem., 1967, 71, 2326. R. B. Woodward and R. Hoffmann, The Conservation of Orbital Symmetry (Academic Press, New York, 1970). l 5 I. FIeming, Frontier Orbitals and Organic Chemical Reactions (Wiley, London, 1976). l 6 T. Imai and S. Nishida, Chem. Lett. (Japan), 1980, 1, 41. (PAPER 1/1456)

ISSN:0300-9599    

DOI:10.1039/F19827802195

出版商:RSC    

年代:1982

数据来源: RSC

摘要:

J . Chem. Soc., Furuduy Trans. 1 , 1982, 78, 2205-2214 Radiolysis of Tetrachloromethane? BY MARTYN C. R. SYMONS* Department of Chemistry, The University, Leicester LEI 7RH AND EMANUELE ALBANO, TREVOR F. SLATER AND ALDO TOMASI Department of Biochemistry, Brunel University, Uxbridge, Middlesex UB8 3PH Received 25th September, 1981 Electron spin resonance studies of tetrachloromethane after exposure to 6oCo y-rays at 77 K reveal the formation of sCC1, and CCl:: radicals. On warming in the presence of spin-traps, or on irradiating fluid solutions, nitroxide radical adducts have been detected that are characteristic of CCl, and chlorine atom adducts. In the light of this evidence and that of other investigators a mechanism for the radiolysis of tetrachloromethane is postulated. In the presence of oxygen, *CCI, radicals are converted into C1,COO- radicals. The use of spin-traps to detect these radicals is described and evaluated. Mishra and Symons have analysed the solid-state e.s.r.spectra of 12CCl, and 13CC13 in various frozen media and the results make it clear that these radicals are non-planar, with extensive delocalization of the unpaired electron onto chlorine.’ Hesse et a1.2 have shown that CCl, radicals give rise to complicated e.s.r. spectra in pure tetrachloro- methane, and have suggested that there are two trapping sites for these radicals, which undergo ‘free’ rotation at 185 K. Other studies have concentrated on pulse radiolysis with optical dete~tion,~? and on the use of spin-traps to give stable nitroxide radicals where e.s.r. spectra give information concerning the original radical^.^? ti The former studies are not clear-cut because of the difficulty of assigning broad structureless ultraviolet absorption bands to specific intermediates.In the most recent work, it is suggested that a band at ca. 340 nm is due to CCl,+ radicals, another at ca. 475 nm is due to CCli+-Cl- ion-pairs, and a third in the 370 nm region is due to C1; ions., In contrast, only neutral radicals are detected in spin-trapping experiments. In this case, however, evidence for .CC13 radicals is clear cut because of the detection of well resolved 13C hyperfine ti Apparently the *CCl, radical has no absorption maximum above 230 nm,7 and hence is not readily detected in pulse-radiolysis studies on pure tetrachloromethane, which is optically black in this region.Our present aim is to use e.s.r. spectroscopy to distinguish between ionic and neutral radical mechanisms for the radiolysis of tetrachloromethane. Our interest in tetrachloromethane arises because it is a powerful hepatotoxic agent used in studies of liver CCl, radicals being implicated as significant intermediate^.^. 6 y lo EXPERIMENTAL Chemicals were of the highest grade available and were used as supplied by the manufacturers without further purification: tetrachloromethane, (B.D.H. AnalaR); N-t-butyl-a-phenyl- nitrone (PBN), (Aldrich); 2-methyl-2-nitrosopropane (MNP), (Aldrich) ; tetrachloromethane, 13C- CC1, (90.2 atom %), diluted as indicated, (British Oxygen Co Ltd). t Taken as Radiation Mechanisms, Part 22.22052206 RADIOLYSIS OF TETRACHLOROMETHANE The radiolysis of tetrachloromethane in the presence of the spin-trapping agent was performed in silica e.s.r. tubes exposed to a source of “Co y-rays either at 77 K or at room temperature. The mean dose was 10 krad. When MNP was used as the spin trap, experiments were performed in the dark, to avoid photolysis. Solutions were degassed by the freeze-thaw technique, or oxygenated by passing a stream of oxygen through the solutions prior to freezing. In some experiments liquid oxygen was added to the fine powder obtained by dropping tetrachloromethane and a solution of the spin-trapping agent into liquid nitrogen. Photolysis was performed using the full arc of a 400 W mercury source focused on the e.s.r. cavity at room temperature.Spectra were measured on a Varian E-3 spectrometer fitted with a variable-temperature cavity . RESULTS AND DISCUSSION SOLID-STATE STUDIES The most direct method of probing the mechanism of radiolysis is solid-state e.s.r. spectroscopy. Two well separated sets of features result, one set being that previously assigned to *CC13 radicals,’. and the other, shown in fig. 1 (a), being tentatively assigned to CC&+ radicals. The central features are due to *CC13 radicals and radicals in the e.s.r. tube, but the other features are due to a species containing strongly coupled 35Cl and 37Cl nuclei (35Cl and 37Cl have I = 3/2: the abundance of 35Cl is 75.4% and that of 37Cl is 24.6%). The possibility that this species is atomic chlorine can be rejected because the highest field line shows at least three features for 35Cl and 37Cl combinations.A two-chlorine radical such as C1; could be responsible for these features, but if it was we would expect only seven parallel features and gll would be ca. 1.95, whereas for Cl; or (RCFClR)+ radicals, gI1 should be close to the free-spin value (2.0023). If three equivalent chlorine atoms are involved then there should be ten sets of features. On this interpretation, gll = 2.005; this is a reasonable value for gll and lends support to the interpretation that the species CCli+. In this case, the high-field parallel (-9/2) line should have four components (3 35Cl; 2 35Cl + 1 37Cl; 1 35Cl + 2 37Cl; 3 37Cl) in the approximate ratios 9: 7: 5: 3. The first three components I 3200 G (9JOL GHz) FIG.1.-For legend see facing page.M. C. R . SYMONS, E. ALBANO, T. F, SLATER A N D A. TOMASI 2207 + 3200 G (9.109 GHrI t FIG. 1 .-First derivative X-band em-. spectra for tetrachloromethane after exposure to ‘j0Co y-rays at 77 K. (a) Showing features assigned to CCla cations; central features are due to .CCI, radicals and radicals in the e.s.r. tube. (b) Showing features for RO, radicals formed in aerated solutions after annealing. g1 are well defined, but the last is largely hidden under the more intense -7/2 feature. The nossibilitv of four eauivalent chlorine niiclei can he riiled niit hemiice then there W V U l U UL. JV111b lVw-llblu palallcl llllL.3 V U L 3 l U C LllC lllLC113G ytlycllulLulal I c a L U l c J , which were not detected.As is frequently the case for powder spectra of such radicals, analysis of the ‘perpendicular’ feature is more difficult. However, the analysis suggested in fig. 2 does accommodate the form of the major low-field lines quite satisfactorily, and, in particular, it accommodates the nearly isotropic nature of the + 5 / 2 set of lines, which is the most characteristic feature of the spectrum. We suggest that this species is CCl,+. The only other reasonable species having three equivalent chlorine atoms is *CCl,, the spectrum of which is quite different.l? Following the loss of an electron, the tetrahedral CCl; cation is bound to be distorted, according to the Jahn-Teller theorem, and our results suggest that this distortion is such that three chlorine atoms move so that the ‘hole’ is shared between them, leaving the fourth chlorine uncoupled.The resulting orbital is a linear combina- tion of 3p (n) orbitals on chlorine which are non-bonding with respect to carbon. A possible form for this orbital is shown, viewed along the axis of the unique chlorine, as structure I. structure I2208 RADIOLYSIS OF TETRACHLOROMETHANE I 3290 G ?) ( 0 "2) (- P ;2) !. FIG. 2.-For legend see p. 221 1 . Clearly the measured parallel and perpendicular components are not principal values for the individual 3p orbitals, but are intermediate between the true parallel and perpendicular values. Hence the estimated anisotropy should be smaller than the true anisotropy. If the data given in table 1 are analysed in the usual way" we obtain Aiso = 46.5 G and 2 B = 12.9 G, after correcting for the shift in gl. The correction for orbital magnetism is no greater than our experimental errors and is therefore neglected.(This analysis is based on like signs for A,, and A l . The choice of opposite signs leads to impossible values for calculated orbital populations.) These results can be converted into approximate orbital populations,ll giving ca. 2.3% s character and 13 % p character on each chlorine atom. The former value is high, but possible for spin polarisation. The latter value leads to a total spin density of 39%, which is far too low. As stressed above, this value should be low, owing to the fact that the experimental data are the values along the symmetry axes and not principal values. We conclude that the assignment of these features to CCI,+ cations is well supported and this is accepted in the following discussion.These features are much weaker thanM. C. R. SYMONS, E. ALBANO, T. F. SLATER A N D A. TOMASI 2209 1 3240 G ii r FIG. 2.-For legend see p. 221 1 . those for *CCl, radicals, which explains why they were not detected by others.2 Nevertheless, since the lines are far broader than those for *CCl,, the two species are present in comparable concentrations. Brede et al. have assigned a band at ca. 340 nm to the CCl,+ cation., That there should be a relatively low-lying optical band is supported by the presence of ultraviolet or visible transitions for the isostructural radicals Poi2- and SO;-. They stress that the species must be distorted in order to prevent rapid charge transfer CCl, + + CCl, z$ CCl, + CCl, +.(1) Our results establish the nature of this distortion. The nature of the 475 nm species is less clear. Their assignment is to the ion-pair CCl,+. * .Cl-, which might well give rise to an intense charge-transfer band. Alternatively, reaction with C1- could give the cr* radical, structure 11, which could exhibit an intense cr-+o* absorption.2210 RADIOLYSIS OF TETRACHLOROMETHANE I I 3LOO G FIG. 2.-For legend see facing page. TABLE 1 .-E.s.R. PARAMETERS hyperfine coupling constants/Ga radical spin trap 14N 'H other CClfb CC13C .13cc1, - 13cc1,d '2CC13 13cci3e c1 Clf Cl,COO Cl,COO - g { or C13C0 Cl3C0O ROO or ROii R0.j ROij - '2CC1,k PBN PBN MNP MNP PBN PBN MNP MNP MNP PBN PBN PBN PBN MNP 13.5 14.10 13.1 12.7 12.2 12.12 27.0 27.5 27.0 13.5 13.7 13.5 13.5-1 3.6 6.75 1.5 1.74 - 0.7 0.75 35Cl, 59.4, 40, is0 46.5 35Cl, 20, ca.0, is0 6.7 13c, 9.4 13C, 9.68 35c1, 2.25 35c1, 2.4 35c1, 6.1 35c1, 6.05 - 1.6 1.75-1.85 2.1 1.3 13c, 5.7 35c1, 0.6 a 1 G = Ref. (6). Ref. (13). f E. G. Janzen, B. R. Knauer, L. T. Williams and W. B. Harrison, J. Phys. Chem., 1970, 74, 3025. N. Ohto, E. Niki and Y. Kamiya, J. Chem. Soc., Perkin Trans. 2, 1977, 1770. i Ref. (17). IC Probably ClCONO(CMe,). T. Isotropic unless indicated. * g,, = 2.005, gL = 2.043. Ref. (1). Ref. (14). Ref. (15).M. C. R. SYMONS, E. ALBANO, T. F. SLATER A N D A. TOMASI 221 1 + 3290 G \ 1 + ' + '2C 0 0 1 (:I) C;:2JI2) (I,* .1J G2) FIG. 2.-First derivative X-band e.s.r. spectra for various spin-trap nitroxide radicals: (a) PBN + C1* [(PhCH(Cl)NO(CMe,)J, (b) PBN and '2CC1,, (c) PBN + * 13CC1,, ( d ) MNP+ - 12CC1, and(e) MNP + * 13CCl, in the presence of oxygen.Features a (a) and /? (a), (c) and ( d ) are due to PhCH(CCl,)NO(CMe,) and acyl nitroxides, respectively. However, we have not obtained any evidence for such species, nor have we been able to detect C1; anions, which were also postulated by Brede et al.3 C1 \ ci - CCl I c 1 / c1 structure I1 If oxygen is not removed from the system, a new species is detected after annealing,2 the e.s.r. spectrum of which is given in fig. 1 (b). This has gll = 2.037 and gl = 2.003, these values being characteristic of ROO- radicals. These must be C1,COO radicals since (Cl,C-CIOO*)+ or ClOO would have quite different spectra.12 USE OF S P I N TRAPS Two different traps were used, PBN and MNP (structures I11 and IV).PBN usually adds radicals ( R e ) to carbon to give a nitroxide, structure V, and MNP usually adds to nitrogen, giving structure VI. Ph CMe, \ / C=N '0 / H Me,C-NO PBN, structure I11 MNP, structure IV2212 RADIOLYSIS OF TETRACHLOROMETHANE 0 \ a / / \ R H-C-N Ph CMe, /* R-No \ CMe, structure V structure VI Radicals of structure V display hyperfine coupling to 'H, 14N and to atoms directly bonded to carbon that are P to the electron on nitrogen and interact via 0--7t overlap (hyperconjugation). Radicals of structure VI give a 14N triplet with hyperfine features due to coupling to a and nuclei in R. Thus, for example, l3Cc1, radicals of structure V should exhibit strong coupling to 13C but none to 35/37C1, whereas radicals of structure VI should give resolved coupling to both types of nuclei.Our results are summarised in table 1. Only PBN gave an adduct with chlorine atoms, and this was unstable above ca. 270 K. The spectrum is, however, quite distinctive [fig. 2(a)], showing features for 35Cl and 37Cl, and a large value of AiSO, as expected for D chlorine atoms with good a-n overlap. Thus the formation of chlorine atom intermediates is established. This accords with the optical detection of C1; radicals,, presumably formed from C1. and C1-. Also present in the e.s.r. spectrum are features for *CCl, adducts (a), which are stable and can be seen clearly at room temperature. This spectrum shows only coupling to 14N and lH [fig. 2(b)], but when 13C-enriched CCl, was used, another set of lines displaying coupling to 13C was also resolved [fig.2(c)]. The coupling of 9.4 G is again characteristic of a P-carbon atom with good a-n overlap. In contrast, with MNP the 12CCl, adduct gave clear coupling to all three chloride nuclei [fig. 2 ( d ) ] , a result which is diagnostic of * CCl, radicals. The coupling to 35Cl and 37Cl nuclei of 2.25 G agrees reasonably with the literature value1, (table 1). These results with spin traps confirm the formation of *CCl, radicals and establish the presence of C1* atoms. Taken alone, they would seem to support the homolysis (2) mechanism CCI, -+ CCl, + + e- (3) CCl; + + e- --+ (CCl,)* --+ *CCl, + C1 in which (CC14)* is an electronically excited molecule. However, the solid-state results show that *CCl, radicals are formed together with CCl;+ cations in the primary process, so this sequence is inadequate.To explain the presence of CClt cations, electrons must react elsewhere and we suggest that the reaction CCl, + e- --+ CC1, + C1- (4) well known in protic media, must also occur. However, this should prevent the occurrence of step (3). In that case, the simplest way of explaining yields of chlorine atoms comparable with those of *CCl, radicals is by the dissociation ( 5 ) CClt * cc1; + c1 This may be an overall reaction occurring by more complicated routes, but we stress that CCl; is isostructural with stable molecules such as SO,, so reaction ( 5 ) may not be too unfavourable. Also CCl; cations would be rapidly removed by reaction with C1- ions formed in step (4). In view of the evidence for CClgCl- ion-pairs,, and the expected rapid formation of ion-pairs in this low dielectric medium, the step (6) CCllCl- --+ CCl, + C1 .M.C. R. SYMONS, E. ALBANO, T. F. SLATER A N D A. TOMASI 2213 is a source of C1* atoms that avoids the formation of CCl; cations. Steps (2), (4) and (6) give all the required products, and so we favour these as the most probable process for the radiolysis of tetrachloromethane. REACTION WITH OXYGEN Our solid-state results confirm the rapid and irreversible reaction of CCl, radicals with oxygen. Reactions of ROO and ROO* radicals with spin traps show that MNP is by far the most useful, as a characteristic species with a greatly increased coupling to 14N is produced (ca.27 G). However, for ROO* radicals there is some confusion about identification, since the 14N coupling constants obtained in reactions with ROO. radicals14 are similar to those obtained with RO* radi~a1s.l~ The 14N isotropic coupling is greatly increased for nitroderivatives because these radicals are pyramidal at nitrogen whereas simple dialkyl nitroxides are planar, or nearly so.16 We expect that RO- and ROO- derivatives will have very similar properties in this respect, and hence that A(14N) will be similar. Evidence for RO- rather than ROO- adducts came from the observation of only one coupled 170 nucleus. However, it is not certain that both oxygen nuclei should give rise to detectable coupling. Whatever the correct identification, the detection of radicals having a large coupling to 14N (ca.27 G) remains diagnostic of ROO radicals in the present systems. In contrast, reaction of PBN with ROO* radicals gave rise to a species having coupling constants to 14N and lH similar to those for many other radicals, including *12CC1,, and we would hesitate to claim that the slight differences are diagnostic of ROB formation. However, using *13CC1, radicals plus oxygen, no 13C splitting was observed, showing that the trapped species was not CCl;. Merritt and Johnson17 have suggested that in this case also, RO- rather than ROO. radicals are trapped. Their results for these two types of radicals (table 1) are quite similar, and since our results lie between them it is impossible for us to decide which derivative we are studying.We have attempted to make the result more positive by using l'o-enriched oxygen. Unfortunately, the lines are broadened by spin exchange with dioxygen in these experiments, and we were unable to detect any coupling to 170 nuclei. FORMATION OF RCONO(R) RADICALS In many cases, especially in the presence of oxygen, we have detected radicals having A(14N) z 7G (/? in fig. 2). These radicals are almost certainly acyl nitroxide RCONO(R), radicals. These are frequently detected in reactions with spin traps and a variety of mechanisms have been proposed to explain their formation. We do not intend to discuss this further except to mention our results for *13CC1, radicals with MPN. In addition to a coupling of 6.75 G to 14N, we detected a small coupling of ca.0.6 G to one chlorine and a clear coupling of ca. 5.7 G to 13C [fig. 2(e)]. The radical is almost certainly C113CONO(CMe,), but we hesitate to propose a route for its formation. PHOTOLYSES In order to check the e.s.r. spectra of some of these spin-trap adducts, we also studied the photolysis of tetrachloromethane in the presence of PBN and MNP. The major species formed were CCl, adducts, but considerable yields of acyl nitroxides were also obtained. We thank the National Foundation for Cancer Research, the Bossolasco Foun- dation and the Wellcome Trust for financial assistance. We are also indebted to Dr K. A. K. Lott for providing e.s.r. facilities and for helpful advice.2214 RADIOLYSIS OF TETRACHLOROMETHANE S. P. Mishra and M. C. R. Symons, Radiat. Phys. Chem., 1975, 7, 617. C. Hesse, N. Leray and J. Roncin, Mol. Phys., 1971, 22, 137. 0. Brede, J. Bos and R. Mehnert, Ber. Bunsenges. Phys. Chem., 1980,84, 63. R. E. Buhler, Radiat. Res. Rev., 1972, 4, 233; Helv. Chim. Acta, 1968, 51, 1558. A. Tomasi, E. Albano, K. A. K. Lott and T. F. Slater, FEBS Lett., 1980, 122, 303. J. L. Poyer, P. B. McCay, E. K. Lai, E. G. Janzen and E. R. Davis, Biochem. Biophys. Res. Commun., 1980, 94, 1 154; J. L. Poyer, R. A. Floyd, P. B. McCay, E. G. Janzen and E. R. Davis, Biochim. Biophys. Acta, 1978, 539, 402. ’ B. Lesigne, L. Gilles and R. J. Woods, Can. J. Chem., 1974, 52, 1135. * R. 0. Recknagel, Pharmacol. Rev., 1967, 19, 145. @ M. U. Dianzani, in Biochemical Mechanisms of Liver Injury, ed. T. R. Slater (Academic Press, London, 1978), pp. 45-95. M. C. R. Symons, Chemical and Biochemical Aspects of Electron Spin Resonance (Van Nostrand Reinhold, London, 1978). R. S. Eachus, P. R. Edwards, S. Subramanian and M. C. R. Symons, J. Chem. SOC. A , 1968, 1704. lo T. F. Slater, Nature (London), 1966, 209, 36. l4 I. H. Leaver, G. C. Ramsey and E. Suzuki, Aust. J. Chem., 1969, 22, 1891. 1Q J. Pfab. Tetrahedron Lett., 1978, 843. l5 A. Mackor, Th. A. J. W. Wajer and Th. J. de Boer, Tetrahedron Lett., 1967, 385. l6 J. H. Sharp and M. C. R. Symons, Nature (London), 1969, 224, 1297. M. V. Merritt and R. A. Johnson, J. Am. Chem. Soc., 1977,99, 3713. (PAPER 1 / 1492)

ISSN:0300-9599    

DOI:10.1039/F19827802205

出版商:RSC    

年代:1982

数据来源: RSC