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. 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