摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 287-291 Electron Transfer from Aromatic Compounds to Phenyliodinium and Diphenylsulfinium Radical Cations Y. Yagcit and W. Schnabel* Hahn-Meitner-lnstitut Berlin GmbH, Bereich S, Glienicker Str. 100,D-14 109 Berlin, Germany A. Wilpert and J. Bendig Humboldt-Universitat zu Berlin, Fachbereich Chemie, Hessische Str. 1-2,D-10115 Berlin, Germany Phenyliodinium (I*+)and diphenylsulfinium radical cations (Il'+)have been generated by flash photolysis (Ainc = 347 nm) of diphenyliodonium ions (I+)and diphenyl(4-phenylthiophenyl)sulfonium ions (II+)in acetonitrile solu- tions at room temperature. I*+and II'+were found to undergo electron-transfer reactions with benzene deriv- atives resulting in the formation of radical cations of the aromatic compounds.A study involving 25 compounds including various methyl- and methoxy-benzenes, biphenyl, phenol and cresols revealed that electron transfer is independent of the ionization energy Ei provided that the rates are encounter-controlled. This applies to cases where Ei does not exceed a critical value: Ei,crit x 820 kJ mol-' (I*+)and 780 kJ mol-' (ll*+).Bimolecular rate constants decrease with increasing Ei in the case of aromatic compounds having ionization energies exceeding the critical values. A Marcus-inverted region was not detected. This paper reports a study concerning the reaction of phenyl- + +iodinium (I' ) and diphenylsulfinium (11' ) radical cations with various aromatic compounds (see Table 2, later) in ace- tonitrile solution.1-II '+ I" and II'+ are formed in the photolysis of diphenyliodonium ions (I+) and diphenyl(4-phenylthio-pheny1)sulfonium ions (11') according to reactions (1) and (2).1-8 r rr In the present study I" and 11" were generated by flash photolysis of acetonitrile solutions of I+ and II+ (Ainc = 347 nm). The reactivity of the radical cations I*+ and 11" towards aromatic compounds is of importance for the elucidation of the mechanism of photo-crosslinking of polymers bearing aromatic pendant groups. According to Crivellog various polystyrene derivatives containing diaryliodonium or tri-phenylsulfonium salts act as negative tone photoresists. It was suggested' that insolubilization, i.e. intermolecular cross- linking of the polymers occurs uia coupling through aromatic nuclei, as is depicted in Scheme 1.-I +2H+ Scheme 1 Photo-coupling of polystyrene derivatives after the attack of aryliodinium radical cations Experimental Materials Diphenyliodonium hexafluorophosphate (I) and diphenyl(4- phenylthiopheny1)sulfonium hexafluoroarsenate (11) were pre- pared according to procedures described in the literature.' The aromatic compounds were commercial products. They were purified by distillation or recrystallization from solu- tions in appropriate solvents. Acetonitrile was refluxed over P,O, and distilled. Laser Flash Photolysis Solutions containing an onium salt and an aromatic com- pound were freed from oxygen by bubbling with purified argon prior to irradiation with 20 ns flashes of 347 nm light.This light was produced with the aid of a ruby laser (Korad, model K1 QS2) operated in conjunction with an ADP fre- quency doubler. Actinometry was performed with a benzene solution containing both benzophenone and naphthalene as described earlier." Dabs,the dose absorbed per flash by the solution was in the order of 6 x lo-' einstein dmP3. Determination of Ionization Energies t On leave from Istanbul Technical University, Department of Ionization energies, Ei , of the aromatic compounds D were Chemistry, Maslak, TR-80626 Istanbul, Turkey. determined according to the method described by Foster' ' 288 and by Zweig et al." in the following way: Optical absorp- tion spectra of charge-transfer complexes formed by the aro- matic compounds and tetracyanoethene (TCNE) were recorded in dichloromethane solution at [TCNE] = mol dmP3 and [D] = 10-2-1.0 mol dm-3.From the fre- quency, vct, of the maximum of the charge-transfer band at 25 "C,Ei values were calculated with the aid of eqn. (I).13 c1 = 6.06 eV and c2 = 0.32 (eV)2.14 Results Transient Absorption Spectra formed by Irradiation of I and I1 Upon irradiation of I+PF, and II+AsF, in acetonitrile solu- tion with 20 ns flashes of 347 nm light the transient optical absorption spectra shown in Fig. 1 were formed during the flash. These spectra which possess strong maxima at 660 nm (in the case of I) and at 750 nm (in the case of 11)are attrib-uted to the radical cations I.+ and II.', In the presence of aromatic compounds the decay of the absorp- tion of the radical cations was accompanied by the develop- ment of new absorption spectra indicating the formation of new species.Typical kinetic traces demonstrating the decay of the absorption of phenyliodinium radical cations at i= 660 nm and the simultaneous formation of the absorp- tion of the radical cation of p-methoxytoluene at A = 430 nm are shown in Fig. 2. At relatively high concentrations of the aromatic additive the spectra of the new species had already formed during the flash and the spectra of the radical cations I" and 11" were no longer detectable. Typical optical absorption spectra of the new species are presented in Fig.3. They are attributed to radical cations of the aromatic com- 400 600 800 A/nm Fig. 1 Irradiation of (a) I+PF, (2 x rnol dmP3) and (b) II'AsF; (2.3 x loP4 rnol dm-j) in Ar-saturated acetonitrile solu- tion at room temperature. Transient absorption spectra recorded at the end of the 20 ns flash. Ainc = 347 nm. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 AA = YO2/= flash I Fig. 2 Irradiation of p-methoxytoluene (1.5 x rnol dm-3) in acetonitrile solution containing I+PF, (5 x mol dmP3) at I = 347 nm. Kinetic traces demonstrating changes in the absorbance (A)at 660 nm (a)and at 430 nm (b) during and after the laser flash. pounds, Actually, the absorption spectra of the radical cations reported in the literature closely resemble the absorp- tion spectra shown in Fig.3. This can be seen from Table 1, where the wavelengths of the maxima of the absorption spectra of the radical cations are listed. t 400 480 560 A/nm Fig. 3 Optical absorption spectra of radical cations of various aromatic compounds (c = lo-' mol dm-3) recorded in acetonitrile solution containing I+PF, (c = 2.5 x rnol dm-3) at the end of the flash (Ainc = 347 nm, Dabs= 5.8 x lo-' einstein dm-3). (a) Toluene, (b) 1,2,4,5-tetramethylbenzene,(c) pentamethylbenzene, (d) hexamethylbenzene, (e) 1,2,3-trirnethoxybenzene, (f) 1,2,4-trimethoxybenzene, (9) 1,3,5-trimethoxybenzene,(h) 1,2-dimethoxy-benzene, (i) 1,4dimethoxybenzene, (j) 1,3-dimethoxybenzene. (Since the spectra are not comparable with respect to the amplitude the ordinates have not been labelled.) J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 289 Table 1 Maxima of absorption spectra of radical cations of various do not absorb light at A 2350 nm. Cyclohexadienyl-type aromatic compounds formed by electron transfer to phenyliodinium radicals have an absorption maximum at about 320 nm.22 radical cations (I") Actually, changes in the optical absorption at 45 350 nm could not be measured because of the strong absorption of ~~ the onium salts in this wavelength range. compound this worku other work* ~ ~ ~~~~ ~ ~~~~ ~~ methoxybenzene 435 m 430,' 435,6*' 445/ Determination of Rate Constants of the Reaction of I*+and 1,4dimethoxybenzene 450 m 430-4W II'+ with Aromatic Compounds 1,2-dimethoxybenzene 410 m rn 1,3-dimethoxybenzene 470 m 4W The reactivity of the radical cations I'+ and II'+ towards the 1,2,3-trimethoxybenzene 400 s, 470 m 4 10-42W aromatic compounds listed in Table 2 was investigated in the 1,2,4trimethoxybenzene 430 s, 450 m 410-45W following way: The rate of decay of the absorption of I" and 1,3,5-trimethoxybenzene 590 m 580" II'+ at 660 and 750 nm, respectively, was measured in the 1,3,5-trimethylbenzene 450 m, 470 s, 485 s 455,' 4758 absence and presence of aromatic compounds.The decay rate 1,2,4,5tetramethylbenzene 455 m 46Y8 was accelerated by the aromatic additives and at sufficiently hexamethylbenzene 490 m, 510 s 495'4 5Wh high additive concentrations the decay followed first-order et h ylbenzene 380 s, 450 m p-chlorotoluene 380 s, 470 s, 490 m kinetics.Therefore, rate constants of the reaction of I*+ and toluene 380 m, 450 s II'+ with donors D could be determined via pseudo-first-1,2-dimethylbenzene 400 s, 440 m, 460 s order kinetic data treatment : 1,3-dimethylbenzene 430 s, 460 s 1 ,Cdimethylbenzene 400 s, 450 m, 470 s k, = k + k,,[D] p-met hoxytoluene 435 m, 450 s where k, is the pseudo-first-order rate constant, k the rate a m, maximum; s, shoulder or second maximum. Absorption constant in the absence of aromatic compound D and k,, the maxima. Ref. 15. Ref. 16. 'Ref. 17. Ref. 18. Ref. 19. Ref. 20. bimolecular rate constant of electron transfer from the donor ' Ref. 21. D to the radical cation. Typical results obtained with the system p-methoxytoluene-1' are presented in Fig.4, which shows a Principally, phenyl and 4-phenylthiophenyl radicals, plot of k, us. [D]. All k,, values determined in this work formed according to reactions (1) and (2), respectively, or (error limit f10%) are compiled in Table 2. This table also reaction products of these species can also give rise to tran- contains the ionization energies of the aromatic compounds sient absorptions. These transient absorptions are considered which have been determined in this work. In Fig. 5, In k,, is not to interfere substantially with the absorption bands at plotted as a function of Ei, the ionization energy of the aro- long wavelengths of the radical cations I*+ and 11". For matic compounds. Obviously, In k,, increases with decreasing instance, phenyl radicals are very likely to abstract hydrogen values of Ei and becomes constant when the values of k,, are from the solvent or to add to phenyl rings of I+ or 11'.The of the order of magnitude of encounter-controlled (diffusion- resulting solvent radicals and cyclohexadienyl-type radicals controlled) reactions. Table 2 Electron transfer from aromatic compounds to I*+ and 11'' : bimolecular rate constants and ionization energies of aromatic com- pounds k,,*/dm mol -s -' compound no. compound E:/eV molecule -I' + 11' + methoxy benzene 8.35 (8.20) 1.1 x 1o'O 6.8 x lo6 1,4-dimethoxybenzene 7.8 9.3 x 109 1,2-dimethoxybenzene 7.96 1.1 x 1O'O 1,3-dimethoxybenzene 8.16 3.9 109 1,2,3-t rimet hox ybenzene 8.3 1.5 x 109 1,2,4-trimethoxybenzene 7.36 1.2 x 1O'O 1.2 x 1o'O 1,3,5-t rimet hox ybenzene 8.11 9.4 109 1,3,5-trimethylbenzene 8.58 (8.39) 7.5 x 109 2.8 x lo6 1,2,4-trirnet hylbenzene 8.62 (8.27) 9.5 109 2.2 105 1,2,4,Stetramethylbenzene 8.32 (8.41) 9.4 x 109 3.5 x 107 pen tamet hy lbenzene 8.31 (7.92) 8.6 x lo9 2.6 x lo8 hexamet h y lbenzene 8.18 (7.85) 1.1 x 1o'O 5.5 x 109 ethylbenzene 8.91 (8.77) 1.4 x 109 p-methoxytoluene 8.09 1.1 x 10'O 2.7 109 p-chlorotoluene 8.73 (8.69) 2.1 x 109 toluene 8.95 (8.82) 1.0 109 1,2*dimethylbenzene 8.77 (8.56) 6.3 109 1,3-dimethylbenzene 8.77 (8.56) 4.5 109 1,4dirnethylbenzene 8.73 (8.44) 4.6 109 cumene 8.92 (8.69) 8.6 x 10' biphenyl 8.39 (8.35) 5.0 x lo6 phenol 8.47 (8.50) 1.1 x 1O'O 3.5 107 o-cresol 8.30 2.2 x 109 rn-cresol 8.39 7.0 x lo8 p-cresol 8.22 9.2 x 109 7.8 x 109 Ionization energy, in brackets: values compiled in ref.23. Error limit lo"?. 0 0 1 2 [PMT]/10-4 mol dm-3 Fig. 4 Reaction of p-methoxytoluene (PMT) with I*+ in Ar-saturated acetonitrile solution. Dependence of the pseudo-first-order rate constant of the decay of the optical absorption at 660 nm on the concentration of p-methoxytoluene. p+PF;] = 2.5 x lo-* mol dm-3, Dabs= 5.8 x einstein dmP3. 24 I-I 23 *;22 C-15' 21 16 I 2o t (a) I 19 7 8 9 EJeV molecule-' I 7 8 9 EJeV molecule-' Fig. 5 Dependence of the rate constant of the reaction of I" (a)and 11" (b) with various aromatic compounds, D, on the ionization energy of D.Plot of In k,, us. Ei. The dotted lines are straight lines of slope rn = -(RT)-l. Discussion The important feature of this paper concerns the high reacti- vity of phenyliodinium (I*+)and diphenylsulfinium radical cations (II'+) towards many aromatic compounds. In all J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 cases studied the radical ions undergo electron-transfer reac- tions as indicated by the formation of radical cations of the aromatic compounds. For example, I*+ reacts with p-methoxytoluene as depicted by reaction (3): Note that the intensity of the spectra of I" and II" decreased with increasing concentration of the aromatic com- pounds. At suficiently high concentrations the absorption spectra of I*+and 11" were not detectable anymore, because the radical cations reacted very quickly with the aromatic compounds. Therefore, it is concluded, that most of the radical cations (1.' and 11") underwent electron-transfer reactions according to the type represented by reaction (3).Inspection of the kinetics of these reactions reveals the fol- lowing: plots of In k,, us. E, of the aromatic compounds (see Fig. 5) demonstrate the independence of k,, of Ei provided that the rates are encounter-controlled. This applies to com- pounds having ionization energies lower than the critical values of E,: Ei,crit = 8.5 eV molecule-' (820 kJ mol-I), in the case of I*+, and Ei,,rit= 8.1 eV molecule-' (782 kJ mol-') in the case of 11". At Ei values exceeding the critical values, In k,, decreases with increasing E,.As can be seen from Fig. 5(a) and (b) the curves representing most experi- mental points merge, as Ei increases, into straight lines with slope m = -(RT)-'. Notably, this kind of dependence corre- sponds with the behaviour described by Rehm and Weller24,25 for electron-transfer reactions if the dependence of AG, the Gibbs energy of the reaction, on the ionization energy according to eqn. (111) is taken into account: AG= Ej -E, + C (111) where E, is the ionization energy of the aromatic compound acting as donor, E, the electron affinity of the radical cation and C is a constant. Actually, k,, should be related to AG according to eqn. (IV) in the case of activation-controlled reactions with a transfer constant a = 1.0,21i.e. at E, > E,,crit: k,, = k, exp( -g) aAGIn k,, = In k, --RT On the basis of eqn.(111) and (IVb) the following expressions are obtained for the dependence of In k,, on E, for the region Ei > Ei,crit: In k,, = In k, + mE,+ n (V) The parameter m corresponds to the slope of the linear por- tions of the curves in Fig. 5(a) and (b). For example, m = -(RT)-', i.e. for T = 298 K, rn = -38.7 molecule eV-' = -0.4 mol kJ-'. Obviously, not all of the experimental points fit the curves in Fig. 5(a) and (b).Although this could be due to experimen- tal errors it also might reflect an influence of the chemical nature of the aromatic compounds on their reactivity towards the radical cations I*+ and 11" which is not prop- erly describable in terms of a dependence of k,, on the ioniza- tion energy or oxidation potential. J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 29 1 Notably, a Marcus-inverted region corresponding to k,, values decreasing with decreasing AG (in the region of high exothermicity, i.e. at Ei < Ei,,,i,)26*27was not found in this work. Y.Y. expresses his sincere gratitude to the Alexander von Humboldt-Stiftung organization which supported him finan- 12 13 14 15 16 A. Zweig, W. G. Hodgson and W. H. Jura, J. Am. Chem. SOC., 1964,86,4124. F. A. Matsen, J. Chem. Phys., 1956,24602. G. Briegleb, Elektronen-Donator-Akzeptorkomplexe,Springer-Verlag, Heidelberg, 196 1. P. ONeill, S. Steenken and D. Schulte-Frohlinde, J. Phys.Chem., 1975,79,2773. S. Takamuku, S. Komitsu and S. Toki, Radiat. Phys. Chem., cially by a research grant. 17 1989,34,553. E. K. Kim, T. M.Bockman and J. K. Kochi, J. Am. Chem. SOC., 1993,115,3091. References 18 T. Shida, Electronic Absorption Spectra of Radical Ions, Elsevier, 1 2 3 4 5 6 7 8 9 10 J. V. Crivello and J. H. W. Lam, Macromolecules, 1977,10, 1307; J. Polym. Sci., Polym. Chem. Ed., 1980, 18, 2677. J. V. Crivello, Ah. Polym. Sci., 1984,63, 1. J. V. Crivello, J. L. Lee and D. A. Coulon, Makromol. Chem., Makromol. Symp., 1988,13114,145. S. P. Pappas, UY Curing: Science and Technology, Technology Marketing Corp., Norwalk, CT, 1985, vol. 11. S. P. Pappas, L. R. Gatechair and J. H. Jilek, J. Polym. Sci., Polym. Chem. Ed., 1984,22, 77.S. P. Pappas, B. C. Pappas, L. R. Gatechair, J. H. Jilek and W. Schnabel, Polym. Photochem., 1984,5, 1. S. P. Pappas, B. C. Pappas, L. R. Gatechair and W. Schnabel, J. Polym. Sci., Polym. Chem. Ed., 1984,22, 69. Y. Yagci and W. Schnabel, Makromol. Chem., Makromol. Symp., 1988, 13/14, 161. J. V. Crivello, J. Electrochem. SOC., 1989, 136, 1453. T. Sumiyoshi and W. Schnabel, Makromol. Chem., 1985, 186, 19 20 21 22 23 24 25 26 27 New York, 1988, p. 254. K. Sehested, J. Holcman and E. J. Hart, J. Phys. Chem., 1977,81, 1363. J. M. Masnovi, S. Sankaraman and J. K. Kochi, J. Am. Chem. SOC.,1989, 111, 2263. P. P. Dymerski, E. Fu and R.Dunbar, J. Am. Chem. Soc., 1974, 96,4109. E. J. Land and M.Ebert, Trans. Faraday Soc., 1967,63,1183. W. J. Wedenejew, L. W. Gurwitsch, W. H. Kondratjew, W. A. Mewedew and E. L. Frankewitsch, Energien Chemischer Bind- ungen, Ionisierungs-Potentiale und Elekttonemfinitaten, VEB Deutscher Verlag fir Grundstofindustrie, Leipzig, 1971. D. Rehm and A. Weller, Ber. Bunsenges. Phys. Chem., 1969, 73, 834. D. Rehm and A. Weller, Zsr. J. Chem., 1970,8, 259. R. A. Marcus, J. Chem. Phys., 1956,24,966. R. A. Marcus, Discuss. Faraday SOC., 1960,29,217. 1811. 11 R. Foster, Organic Charge Transfer Complexes, Academic Press, Paper 3/01864D;Receioed 1st April, 1993 London, 1969.

ISSN:0956-5000    

DOI:10.1039/FT9949000287

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 293-295 Raman Spectroscopic Monitoring of Oxygen Clathrate Hydrate Formation from Microporous Amorphous Solid Water Andreas Hallbrucker lnstitut fur Allgerneine, Anorganische und Theoretische Chemie, Universitat lnnsbruck, A4020 lnnsbruck, Austria The 0, stretching vibration of 0, adsorbed by vapour-deposited amorphous solid water, after crystallization of amorphous to cubic ice, and after formation of oxygen clathrate hydrate, has been studied by Raman spectros- copy: its peak maximum is shifted increasingly to lower frequency and evidence is given that only the large cages of the clathrate hydrate structure are occupied by 0, molecules. Vapour-deposited amorphous solid water has the ability to trap considerable amounts of 0, when it is heated above its formation temperature of 77 K in the presence of moderate pressures of oxygen gas.On further heating, the sample largely crystallizes to cubic ice at ca. 150 K, most of the 0, remaining in the ice matrix. At ca. 193 K oxygen clathrate hydrate of structure I1 is formed. The 0,-stretching vibration was recorded by Raman spectroscopy during the various steps of clathrate hydrate (CH) formation. Its peak maximum is shifted by 8.3 cm-' to lower frequency compared with the gas-phase value by adsorption of the oxygen gas by the amorphous solid water. Crystallization to cubic ice and for- mation of the clathrate cause additional downshifts of 0.7 cm-' each. Only the large cages of the clathrate hydrate are occupied by 0, molecules as only one peak occurs at the appropriate position. Slow deposition of water vapour at low base pressure on a substrate cooled to 77 K leads to the formation of an amor- phous form of ice (amorphous solid water, ASW).'v2 This highly adsorbent microporous solid has a large surface area up to 400 m2 g-'.3,4 Heating of ASW in the presence of moderate pressures of oxygen (<1 bar) to ca.120 K results in the reduction of the surface area and occlusion of consider- able amounts of gas which cannot be pumped off any further in uacuo. On further heating in uacuo the ASW largely crys- tallizes to cubic ice (I,) at CQ. 150 K releasing only a part of the trapped oxygen. With rising temperature the remaining gas exerts increasing internal pressure to the surrounding matrix of cubic and residual amorphous ice, and at ca.193 K oxygen clathrate hydrate (0,-CH) of structure type I1 is formedsp7 besides some hexagonal ice. In this study the stretching vibration of oxygen was re- corded by Raman spectroscopy during the various steps of clathrate hydrate formation, i.e. after enclosure -of 0, in ASW, after partial crystallization of the sample to I,, and after formation of 0,-CH. The various transformations in the sample were characterized and controlled by X-ray diffrac- tion measuremen ts. Experimental ASW samples were prepared as described before in detail7 in a high-vacuum apparatus. Briefly, water vapour from a reservoir held at room temperature was admitted through a fine needle valve and a tube of 13 mm inner diameter acting as a nozzle into a high-vacuum system.The vapour con-densed there on the flat surfaces of small brass plates acting as Raman sample holders or, in another experiment under otherwise identical conditions, on an X-ray sample holder. These were mounted on a copper plate cooled to 77 K. Deposition of water vapour for 6 h gave an ASW layer of ca. 1 mm thickness. The apparatus was pressurized then with 0, close to atmospheric pressure and the sample heated to 120 K in the presence of gas. After recooling to 77 K the Raman or X-ray sample holders were transferred to the precooled cryostat and X-ray camera, respectively.' Raman spectra were recorded on a computerized Coderg PHO instrument with double monochromator and photon counting.488 nm argon laser excitation was used with a power of 500mW. Plasma lines of the Ar-ion laser were used for frequency calibration which should be better than cm-'. The slit width was 2 cm-' for the 0, gas sample [Fig. 2A(a)] and 0.5 cm-' for all other samples. Three scans were coadded for each spectrum. Spectra were reproducible within f0.1 cm-'. The maximum likelihood restoration program 'SSRes' of Spectrum Square Associates, Inc., was used to improve peak separation.8 Corresponding to the manufac- turers recommendation, spectra were not recorded at still smaller slit widths because the advantage of a higher signal- to-noise ratio exceeds the disadvantage of diminished resolution.X-Ray diffractograms were taken on a Siemens Kristallo- flex 4 powder diffractometer using Cu-Ka radiation. Results and Discussion Measurements of the ASW sample with enclosed 0, were carried out at 130 K. The X-ray diffractogram in Fig. l(a), exhibits two broad peaks centred at 20 = 24" and 43" char-acteristic for ASW. Within the accuracy of the instrument 'impurities' of crystalline ice must be <2%. The Raman spectrum of 0, adsorbed in the micropores of ASW is depicted in Fig. 2A(b) C1546.8 cm-', full width at half-maximum (FWHM) is 2.4 cm-']. The peak maximum is shifted by 8.3 cm-' to lower frequency compared with the band of 0, gas in Fig. 2A(a), which was recorded at 130 K with a similar instrumental setup (except a slit width of 2 cm-' instead of 0.5 cm-').After heating of the samples to 163 K and recooling to 130 K the X-ray diffractogram in Fig. l(b) shows sharp reflections due to the formation of I, super- imposed on the broad features of residual ASW or disordered cubic ice.g The Raman band of 0, occluded in this sample and also recorded at 130 K [Fig. 2A(c)] is further shifted to lower frequency by 0.7 cm-' (1546.1 cm-', FWHM 3.0 cm-'). Owing to loss of 0, during crystallization the peak area of this band is diminished by ca. 43%compared with the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 band of 0, in ASW [Fig. 2A(b)]. However, the peak was drawn at the same height in Fig. 2(a) for clarity. In order to form 0,-CH the sample then was heated to 195 K and re- ! i,.->!c..I 20 30 40 50 201degrees Fig.1 X-Ray diffractograms (Cu-Ka) of vapour-deposited amor-phous solid water treated with oxygen and heated to (a) 120, (b) 163 and (c) 195 K. The diffractograms were recorded at 130 K. Sharp peaks in (b) are due to cubic ice (13. Additional reflections in (c) (marked with *) indicate the formation of oxygen clathrate hydrate (0,-CH), structure 11. The diffractograms are drawn on the same scale with arbitrary y-offset. I I I 1560 1550 1540 cooled to 130 K. Apart from the intensified signals of I, the X-ray diffractogram of Fig. l(c) exhibits additional reflections at 20 = 27.0, 29.5, 30.9 and 44.8" (marked by *), which indi- cate the formation of 0,-CH, type 11, to an extent of about 15%.6*7 The 0, stretching vibration of this sample is rep- resented in Fig.2A(4 (1545.4 cm-l, FWHM 2.2 cm-')at the same height; the real peak area is ca. 59% smaller than that of the 0, band in ASW [Fig. 2A(b)]. Looking at the 0, Raman bands of Fig. 2A more closely by different resolution enhancement techniques, splitting of the 0, vibration becomes evident at least for the partially crystallized and the 0,-CH containing samples. Spectra treated with 'maximum likelihood spectral restoration' of Spectrum Square Associates, Inc.,' gave the best resolution and are depicted in Fig. 2B. Adsorption of 0, gas by micro- porous ASW causes the most significant shift to lower fre- quencies compared with the free molecule.The environment of 0, molecules trapped in ASW seems to be quite unique because little indication is given for more than one com- ponent in the Raman band [Fig. 2B(b)]. Although band split- ting is very small, the influence of the changes in the ice matrix surrounding the O2 molecules on the 0, stretching B 1546.8; I I I 1560 1550 1540 wavenumber/cm -Fig. 2 A, Raman spectra of 0, gas (a), of 0, adsorbed by the micropores of amorphous solid water after heating to 120 K (b), after formation of cubic ice (13 at 163 K (c), and after formation of oxygen clathrate hydrate (0,-CH) at 195 K (d). All spectra were recorded at 130 K. Bands in curves (c) and (d) are brought to the same height by multiplying with factor of 2.1 and 2.4, respectively.The dotted line is drawn to indicate the shift of the 0, stretching vibration during the various steps of 0,-CH formation. B, Resolutionenhanced spectra. Raw data of A (b)-(d) were treated with the 'maximum likelihood spectral restoration' algorithm of Spectrum Square Associates, Inc. The peak shape function was defined by applying the half-peak option to the low-frequency side of the 0, vibration in Fig. 2A(b). Defining a pure Gaussian as peak shape function gave similar results. The SSRes calculated uncertainty estimate is 1.9% for (b), 1.4% for (c) and 1.7% for (d). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 vibration becomes obvious in Fig. 2B(c) and (4.The com- ponent at low frequency at 1545.3 cm-in Fig. 2(4 (marked by a vertical line) can probably be assigned to 0, encaged in 0,-CH.The appearance of a shoulder at similar frequency in the 0,band of the I, sample [Fig. 2(c)] suggests that CH-like structures which are not detectable by X-ray diffraction are already formed at the temperature of the ASW +I, trans-formation. The dominant peak at 1546.2 cm-' in Fig. 2(c) is due to 0, trapped in I,. After 0,-CH formation 0, in the residual I, shows up as a shoulder at similar position in Fig. 2(4.Nakahara et al.," in their study of naturally occurring 0,-CH, have calculated, by a model of a spherical cavity in a dielectric medium, peak frequencies at 1535 cm-' and at 1546 cm-' for 0, in the small and large cages of type I1 CH. While the latter value is close to the maximum of the peak at 1545.3 cm-' in Fig.2B(4, which was attributed to 0, in CH, no Raman signal could be detected at 1535 cm-'. Occupancy of only the large cages in 0,-CH can therefore be assumed. Note that the most substantial shift of the 0, molecular vibration is already observed after adsorption of 0, gas by ASW, and that crystallization of the ice matrix and enclathra- tion of the 0, molecules in the voids of the CH are less influ- ential. I thank the Forschungsforderungsfondsof Austria for instru- mental support. References 1 M. G. Sceats and S. A. Rice, in 'Water, a Comprehensive Treatise', ed. F. Franks, Plenum Press, London, 1982, vol. 7, ch. 2. 2 E. Mayer and R. Pletzer, J. Chem. Phys., 1984,80,2939. 3 E. Mayer and R. Pletzer, Nature (London), 1986,319,298. 4 B. Schmitt, J. Ocampo and J. Klinger, J. Phys. (Paris), 1987, C1, 519. 5 E. Mayer and A. Hallbrucker, J. Chem. Soc., Chem. Commun., 1989,749. 6 J. S. Tse, Y. P. Handa, C. I. Ratcliffe and B. M. Powell, J. Inclu-sion Phenom., 1986,4, 235. 7 A. Hallbrucker and E. Mayer, J. Chem. SOC., Faraday Trans., 1990,86,3785. 8 L. K. DeNoyer and J. G. Dodd, Am. Lab., 1991,23,24 D-H. 9 W. F. Kuhs, D. V. Bliss and J. L. Finney, J. Phys. (Paris), 1987, C1, 631. 10 J. Nakahara, Y. Shigesato, A. Higashi, T. Hondoh and C. C. Langway Jr., Philos. Mag., 1988,57,428. Paper 3/06345C; Received 21st September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000293

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 297-303 IR and NMR Studies of Hydrogen Bonding in Hexan-1 -01-Tetrabutylammonium Iodide Solutions in the Temperature Range 28-145 *C and in Tetrachloromethane Oleg N. Kalugin, Dmitry A. Nerukh, Ivan N. Vyunnik, Elena G. Otlejkina, Yurij N. Surov and Nikolaj S. Pivnenko Department of Inorganic Chemistry, Kharkov State University, 4 Freedom Sq.,310077 Kharkov, Ukraine IR spectra of (Bu,NI)-hexan-1-01 solutions at 25, 55, 85, 115 and 145°C in the OH stretching region have been investigated. The OH stretching spectra and the 'H NMR chemical shifts of the hexanol OH-group were obtained from Bu,NI-hexanol-CCI, solutions in the alcohol concentration range 3 x 10-3-7.8 mol dm-3. The relationship between absorbance and wavenumber is represented as the product of a Lorentzian and a Gaussian curve.Using this dependence deconvolution of the OH-band was carried out by the Simplex method. From these data, it was established that Bu,NI is a structure-breaker at moderate temperatures and/or low concentrations of CCI,. At higher temperatures or in very dilute solutions of Bu,NI-hexanol in CCI,, Bu,NI is observed to be a structu r e-maker . Based upon the statistical-mechanical theory of electrolyte solutions the square-mound potential d,/k, T for some 1 : 1 electrolytes in alcohol solutions was calculated in our pre- vious work.'V2 The perturbation energy d, could be approached as the Gurney cosphere overlap Gibbs energy, and is the sum of many effects relating to the relative energies of interaction of the free ions and ion pairs with the sur- rounding ~olvent.~.~ For most 1 : 1 electrolyte alcoholic solu- tions, especially those of higher alcohols, d,/k, T increases with temperature' and corresponds to the energy gain of an ion in alcohol medium as compared with an ion-pair (without considering the Coulombic part of the interionic potential). From a comparison of the temperature dependences of square-mound potentials and the Gibbs energy of intermo- lecular interaction calculated using the theory of dielectric liquids, the sign and values of the temperature change of dJk, T are determined by the orientational correlations of dipoles of the solvent molecules.In alcoholic solutions the most important role is played by chain association units.Starting from these conclusions, the purpose of the present paper may be formulated as follows: to investigate the influ- ence of electrolyte on alcohol association over a wide tem- perature range by a direct experimental method. For this purpose the IR spectra of Bu,NI-hexan-1-01 solutions at 25, 55, 85, 115 and 145 "C in the OH stretching region have been investigated. The OH stretching spectra and 'H NMR chemi- cal shifts of the hexan-1-01 (hexOH) OH-group were also obtained from Bu,NI-hexOH-CC1, solutions in the concen- tration range 3 x 1OP3-7.8 mol dm-3 at room temperature. In contrast to lower homologues the relatively high boiling temperature of hexan-1-01 (157.1 "C) allows one to carry out the investigation at an elevated temperature without special instrumentation.Furthermore, the temperature range is in accord with our previous studies.'V2 A literature overview has demonstrated that the majority of spectral investigations of alcohol solutions involve studies of lower alcohols at room or low temperatures.'-' ' The use of Bu,NI as an electrolyte enables us to simplify the interpretation of the spectra because of the absence of any influence of the Bu,N+ ion on the IR and NMR OH-group spectra.', The choice of this salt was dictated by its good solubility not only in hexOH, but also in the inert solvent CCl,, which was used as an analogue of the effects of tem- perature on the alcohol self-association. Experimental Hexan-1-01 was dried for 10 days over freshly made K,CO, , then fractionally distilled under a pressure of 100 Pa.The water content in the alcohol was determined by the Karl- Fisher method and did not exceed 0.01%. The Bu,NI used was of 'pure' grade and was recrystallizated six times from a benzene-hexane mixture. The final drying was performed at 55°C and 1 Pa over P,O,. CC1, was boiled with P,O, for 5 h and distilled. IR spectroscopic measurements were made on a double-beam Specord M80 spectrometer in NaCl, LiF and CaF, cells. CaF, cells were used at room temperature (28°C) for pure hexOH and Bu,NI-hexOH solutions, NaCl cells for measurements in CCl, medium. Our home-made LiF cell was used for multi-temperature investigations.Nuclear magnetic resonance studies were made on a Tesla BS-487-B 80 MHz spectrometer at room temperature. 'H chemical shifts were measured relative to hexamethyldisilane, which was added to samples in trace amounts. The following OH stretching spectra of hexOH were studied: (a) pure hexOH (I) and solutions of Bu,NI in hexOH (11) (molality 0.56 mol kg- ') at 28, 55, 85, 115 and 145 "C;(b)solutions of (I) in CCl, (111) and solutions of (11) in CCl, (IV) with a minimum concentration of alcohol of 4 x lop3 mol drn-,. All hexOH-Bu,NI-CC1, solutions were made by dilution of (11) in pure CCl,. This made it possible to keep the alcohol :electrolyte molar fraction constant (18 : 1) in all the solutions. NMR spectra were obtained from systems (111 and IV) at 28 'C.Results and Discussion Experimental spectra in the OH stretching region are shown in Fig. 1 and 2. Both for pure alcohol and for electrolyte solution the polymeric alcohol band shifted to higher wave- numbers and the intensity of the monomer band increased with temperature or CCl, concentration. In a qualitative sense these conclusions have long been known.'-'o* ',-' However, there are no quantitative descriptions of a simulta- neous influence of higher temperature and electrolyte on the OH band profile resulting from alcohol self-association by hydrogen bonding. J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 1 3 4 (4 I"r I I 80 -70 --Q) 60 C me g 50-A -40 -30 -20 -10 .., .. .. ...... ... ... ... .... .., , __ ........ ... . . . ....-.. ... .... . ......__. ... .. I I I .. I I 03700 3600 3500 3400 3300 3200 3100 wavenumber/cm -1 2 3 I I I 80 70 60 Q)0 5 50 %' 40 30 IV 2c 1c I.. .. . ..... . I I 1 IC I 3700 3600 3500 3400 3300 3200 3100 wavenumber/cm-l Fig. 1 Experimental (circles) and calculated (lines) IR spectra of hexOH-CC1, (111) and hexOH-Bu,NI-CCl, (IV) solutions at 28°C with alcohol concentrationsof (a) 3.5 and (b)0.015 mol dm-3. The numbers on the top of each figure correspond to calculated bands. We used an algorithm suggested by Symons6 to compute (4)the approximate band shape: In eqn. (1)-(4) Acalc is the calculated intensity, Ai is the (1) absorbance of each unit, B is the base line, &,, is the band height, Liand Ciare Lorentzian and Gaussian terms, respec-tively, vma, is the band position in abscissa units, a is the (2) half-bandwidth at half-height, rn is the number of bands producing the overall envelope, is a Gaussian contribution,1 v is the abscissa value at which the absorbance is to be calcu-(3) lated.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1 3I' 1 .o 0.8 Q) C 0.6 2 % 0.4 0.2 I I I0 3600 3500 3400 3300 3200 3100 wavenumber/cm-' 1 3 4 1.2 I I I 1 .o 0.8 Q) C e d 0.6 0.4 0.2 I I I I I 3600 3500 3400 3300 3200 3100 wavenumber/cm-' Fig. 2 Experimental (circles) and calculated (lines) IR spectra of pure hexOH (I) and hexOH-Bu,NI (11) solutions at (a) 55 and (b)115 "C Deconvolution of the experimental band shape was carried fitted parameters the diagonal elements of the covariation out by minimizing the sum of square deviation Q for k matrix were used: AXi = (COV Wii= J[s~(H-')~~] (6) s2 = -Q (7)m-n The individual band parameters (Ao,i,v,,,, i, ai) and the where H is the matrix of the second derivatives of Q with common parameters B and B were altered until a good fit respect to X and s is the approximation dispersion.with the experimental band was obtained. Eqn. (5)was mini- The results of data handling by the algorithm outlined mized by the Simplex method.I6 As a statistical evaluation of above are presented in Table 1 (for I11 and IV) and Table 2 J.CHEM. SOC. FARADAY TRANS., 1994,VOL. 90 Table 1 IR spectra of OH-groupvibration in hexOH-CCl, (111) and hexOH-CC1,-Bu,NI (IV) solutions at 28"C n" band height band width band position I11 c = 0.004,b/3 = 0.05, s = 0.017 1 154 f 2 11.7f0.2 3639(f9 x lop4) c = 0.02,/3 = 0.17,s = 0.005 1 145 f5 12.7f0.5 3639( k2 x lo-,)2 5.9f0.6 37 f5 3511( f1 x c = 0.04, /3 = 0.16,s = 0.025 1 140f5 13.1 f0.5 3639(+ 1 x 2 13.2f0.7 51 f5 352qf 1 x 3 4.9f0.5 70 f 11 3377(+5 x 10-6) c = 0.07, /3 = 0.29,s = 0.086 1 121 f 16 13 f 1 3639(f3 x lop3)2 15 f 3 49 f 6 3509(f9 x 3 10f 2 102 + 15 3366(f3 x c = 0.97,/3 = 0.53,s = 0.002 1 22 f 2 13 f 2 3636(+5 x lov6)3 241 f 21 119 f 8 3343(f2 x 4 17 f 58 69 f 16 3055(+2 x c = 3.42,/3 = 0.65,s = 0.003 1 6.0f0.5 15f2 363qf4 x 3 180f 12 120 f6 3338(f 1 x 4 15 f 14 76 f9 3068( f9 x c = 6.68,/3 = 0.68,s= 0.03 1 2.4f0.3 15f2 3637( f4 x 3 133 f 8 120 f 5 3335(f3 x 10-5) 4 12 f6 106 f 11 305qf 1 x 10-5) c = 7.98,/3 = 0.66,s = 0.005 3 144f7 119 f4 3331(f8 x 4 12 f4 126 f 16 3067(f 1 x IV c = 0.003/3 6,/3 = 0.14,s = 0.011 1 119 f 6 12.8f0.6 3639(+4 x lo-,)3 0.5 f 1 106 f 347 3422(f 1 x lo-') c = 0.01, = 0.19,s = 0.14 1 103 11 13 f 1 3639(+5 x 2 2.7f0.6 99 f 106 353qf 1 x 10-5) 3 3f2 49 f 21 3402(+3 x 10-5) c = 0.03,/3 = 0.21,s = 0.017 1 109f8 12.7f0.8 3639(+5 x 2 6.1f0.8 51 f9 353q+ 1 x 10-5)3 7fl 63 f9 3402(_+3x 10-5) c = 0.06,/3 = 0.29,s = 0.059 1 9991 f 11 13f 1 3639(f2 x 2 9fl 38 f 5 3508(f5 x 3 14f2 62 f 6 3392(+5 x c = 1.16,/3 = 1.00,s = O.ooOo6 1 17f 1 16 f 1 3636(f 1 x 3 192 _+ 16 119 f7 3361(_+3x lop6)4 18 f64 106 f23 3076(+ 1 x c = 3.71,/3 = 1.00,s = 0.001 1 5.7f0.2 18.8+_ 0.9 3635(+2 x s 3 155 f3 113 f 1 335qf3 x 10-5) 4 16f4 107 f7 3139(f2 x c = 7.45,/3 = 1.00, = 0.005 3 117 f 26 109 f 12 336qf5 x lop4)4 22 f 19 126 f29 3209(+ 1 x lop4) " Band number.Concentration in mol dm-3. (for I and 11). Typical resolutions of IR spectra for all of these er than those formed by terminal molecules (S,) in polymer systems is shown in Fig. 1 and 2. The resolved bands are or dimer units because of the reinforcing effect of the bond to labelled 1, 2 etc. from the high-frequency end. The band oxygen.We assume that absorption band 3 in the fundamen- labels correspond to the band number in Tables 1 and 2. tal (vOH) is due to doubly bonded hexOH molecules [see (ii)]. We now focus on the bands matched to the condition of Except for chain association units, alcohol molecules form the hexanol OH-group in (I-IV). Band 1, with v,,, at ca. branched or 'bush' (group) associated units where triply 3640 cm-l, is the typical band of the fundamental vibration bonded molecules (S,) are necessarily present [Scheme 1 of the monomeric alcohol 0H-gr0up.l~ At ambient tem-(iii)]. peratures its intensity is vanishingly small in samples contain- For these molecules the reinforcing effect will naturally be ing pure alcohol or in Bu,NI-hexOH solutions. Only at much more than for S,.Since the shift to low frequency is higher temperature, and particularly in dilute CCl, solutions, approximately proportional to the hydrogen-bond strength, is this band very much more pronounced (see Fig. 1 and 2 we conclude that band 4 corresponds to vibration of the OH-and Tables 1 and 2). groups of the S, molecules. Furthermore, following Symons' results for methanol,6 we It has been determined from dielectric measurements l7 suggest that band 2 at ca. 3570-3540 cm-' corresponds to that chain-type association is dominant for normal higher the fundamental vibration of the OH-group in terminal alcohols. This fact is responsible for the greater intensity of hexOH molecules in polymeric or dimeric association units. band 3 as compared with band 4.The results of dielectric It may be represented by equilibria (i) and (ii), where S, investigations" make it possible to establish that, in spite of denotes a solvent that forms n hydrogen bonds. the relatively long length of the hydrocarbon chain, the mean The wide low-frequency band in system (I), according to level of association of hexanol at 25 "Cis as great as 11 [i.e. our calculations, can be reconstructed with minimum of two n = 11 in eqn. (ii)-(iii)]. These two factors complicate the symmetrical bands, as indicated in Fig. 1 and 2, with v,,, at detection of terminal molecules in spectra of pure hexOH. In ca. 3330-3430 cm-' (labelled 3) and ca. 3045-3270 cm-' this case, the spectra are reconstructed with two (at low (labelled 4).It should be pointed out that the hydrogen bonds temperatures) or three bands, 1, 3 and 4. for average doubly bonded hexOH molecules (S,) are strong- Note that v,,, for bands 2-4 is dependent on the presence J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 30 1 Table 2 IR spectra of OH-group vibration in pure hexOH (I) and hexOH-Bu,NI (11) solutions at various temperatures ~~ ~~ ~~ nu band height band width band position t = 28,b fi = 0.66, s = 0.0052 3 144f7 119 + 4 3331(f9 x lo-') 4 12 f4 126 f16 3067(f2 x lo-') t = 55, #I = 0.78, s = O.OOO9 1 0.8 f0.1 11 f2 3638(f2 x lop6) 3 409 f4 122 f9 335qf9 x 10-6) 4 5 f10 134 + 29 304qf5 x 10-5) t = 85, = 1.00, s = 0.0009 1 1.3 f 0.2 11 f2 3640(+2 x 3 32 + 5 126 f10 3378(f2 x 4 5f 15 185 f11 3055( f6 x t = 115, #I = 1.00, s = 0.0013 1 2.4 f0.4 11 +_2 3641(+5 x 2 30 f7 119 f12 3425(f3 x lop5) 3 6f 19 157 f51 3228(f8 x lop6) t = 145, fi = 1.00, s = 0.0003 1 1.5 f0.9 16 + 6 3~3(f8 10-7) 3 7+3 109 & 51 3596(f8 x 4 4f4 133 f43 3270(+8 x 5 6 f74 97 f69 3084(+8 x I1 t = 28, p = 1.00, s = 0.0052 3 117 & 26 109 f12 336qf5 x lo-,) 4 22 f19 126 + 29 32oq+i x 10-4) t = 55, #I = 1.0, s = 0.001 1 1.4 +_ 0.1 27 3 3aq+_8x 10-7) 3 32 + 3 108 f3 3397(f2 x 4 7f7 153 f22 3209( +6 x t = 85, p = 1.0, s = O.OOO4 1 1.4 f0.1 27 f3 3~+810-7)x 3 32 f3 108 f3 3397(+2 x 4 7f7 153 f22 3209( f6 x lop6) t = 155, fi = 1.00, s = 0.0009 1 2.7 f0.1 15.6 f0.9 3639( +6 x lop6) 3 41 + 1 115 f1 3423(f2 x 10-5, 4 6f2 154 & 20 3164(f2 x r = 145, fi = 1.00, s = 0.001 1 4.0 & 0.2 12.1 f0.5 3639( +6 x 3 46.1 f0.7 113.7 f 0.5 3423(&2 x 4 8k1 147 k 12 3164(+2 x loW6) Band number.Temperature in "C. hex2\ -hex hex known empirical rule:1','4,18 0-H \\ li20-H---0-H AHH-bond = C(vmax, monomer -'ma,, polymer) SO Sl s, where C is a constant, i.e. the enthalpy change, AH, of H-bond formation depends upon the polymer band shift. Note that AHH-bond can result from (a) a change in the energy (strength) of a fixed number of hydrogen bonds; (b)a change in the extent of hydrogen bonding with a fixed energy. For this reason, in our opinion, it is more proper to discuss v,,,(ii) in terms of structure-making and structure-breaking effects. Taking into account eqn.(8), the position of polymer band 3 has been chosen as the quantitative characteristic of self- association of hexanol by hydrogen bonding . Our results (Fig. 3 and Tables 1 and 2) indicate that there is no need for the introduction of an additional band that corresponds to the OH-group H-bonded with I-ion in any of the spectra (large Bu,Nf ions are unlikely to form hydro- gen bonds with alcohol molecules and do not greatly modify ,O'H the total hydrogen bonding in alcohols6,' 2). The experimen- hex tal overlapping curve may be represented by as many bands as for pure hexOH without any essential difference in disper- sion (Table 1, 2). Moreover, as shown in Fig. 3, within the standard deviation, the width of band 3 does not depend on Scheme 1 the presence of electrolyte.That is, the bands 3 and 4 in systems with and without Bu,NI are the same. of Bu,NI, the CCL, concentration and the temperatures, Summarizing, we can contend that the influence of tem- unlike v,,, for monomeric band 1. This derives from the fact perature, inert solvent and electrolyte on the state (strength) that, in effect, these bands are the sum of individual closely of hydrogen bonds in liquid hexanol can be described with related bands due to the energetic states of the OH-group. the aid of a single qualitative characteristic, namely, the posi- That is, the influence of temperature and inert solvent leads tion of the polymer band 3 (vmax)(at least for an electrolyte to the distribution of the molecules amongst the possible con- such as Bu,NI).formations in the polymer, which is reflected in the intensities There is another important reason for this conclusion. As of the corresponding OH bands. As a result, the positions of follows from quantum-chemical calculations" of the the polymer bands change. This serves as a basis for the hydrogen-bonded complex of ethanol with Br-, the alcohol 302 150 150 r c I 120 E \k 120 * b" b" 90 90 60 L 60 -1.0 -0.5 0 0.5 1 .O 30 60 90 120 150 log c tl" c 400 300 300 7 9-4 50' I I-, I 8' I -1.04.5 0 0.5 1.030 60 90 120 150 log(c/mol dm-') t/T Fig. 3 Width of (a)polymer band 3 and (b) polymer band 4 of OH- group at various temperatures (on the right: 0, 11) and alcohol I; .,concentrations (on the left: 0,II; 0,IV) molecule is brought into the strong negative field of the anion.As this takes place, about 10% of the negative electro- static potential of Br- is retained at a range of three intervals between the hydrogen of the alcohol molecule and the Br- ion (1.0-1.5 nm). In the condensed phase we conclude that the magnitude of this field offers the possibility of a large influence of the anion on reformation of hydrogen bonds between alcohol molecules at long distances from the anion. This deduction correlates well with the appreciable change in v,,, of band 3 in spite of the low content of electrolyte mol- ecules (1 :18). For example, at 28 "C vmaX of band 3 for pure hexOH is 3331 cm-' and that for Bu,NI solution is 3360 cm-'; this corresponds to an increase of temperature of 50 "C for pure hexOH.The main results of this work may be represented by the dependence of polymer band position (vmax) us. concentration of an inert solvent [CCl, Fig. qa)] or temperature [Fig. qb)] for all solutions investigated (1-IV). Conclusions When the intrinsic structure of the solvent is clearly defined (low temperature, low inert solvent concentration) an electro- lyte, such as Bu,NI, plays a structure-making role. In contrast, if some conditions (higher temperature, near the boiling point, very dilute solutions in the inert solvent) led to significant destruction of the set of hydrogen-bonds in alcohol, the electrolyte (even one such as Bu,NI) could act as a structure-maker, that is it could lead to the reinforcement of hydrogen bonds in the solvent.This result is supported by the results of NMR investiga-tions (Table 3). It is known13 that hydrogen-bond formation leads to a downfield shift of the hydroxy proton chemical J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 3425 -3400 5--. Xm ,E 3375 3350 3325 I I 1 I -3 -2 -1 0 1 log(c/mol d~n-~) ,,? (b) 3475 3450 -c I5 3425 ---. x sz 3400 --3375 -3350 3325 I I I I I I I 20 40 60 80 100 120 140 160 t/"c Fig. 4 Polymer band 3 position us. concentration of alcohol (a) for hexOH-CC1, (11) and hexOH-Bu,NI-CC1, (IV) solutions and us. temperature (b) for pure hexOH (I)and hexOH-Bu,NI (11)solutions shift.The hydroxy proton peak for hexOH in Bu,NI solu-tions (IV) is upfield compared with systems without electro- lyte, i.e. S(II1) > 6(IV), whereas in dilute solutions G(II1) < 6(IV) (Table 3). This difference corresponds to an alcohol concentration of ca. 0.06 mol dm-3 (cf: 0.04 mol dm-3 from IR measurements). Table 3 'H Chemical shifts (6) of hexOH OH-group for hexOH-CC1, (111) and hexOH-CC1,-Bu,NI (IV) solutions at 28 "C as a function of alcohol concentration (c) c/mol dm-3 6 I11 0.005 0.93 1 0.009 1.288 0.018 2.000 0.048 3.188 0.094 3.863 0.191 4.344 0.492 4.813 0.741 5.044 1.Ooo 5.200 IV 0.005 1.013 0.010 1.388 0.018 2.150 0.05 1 3.250 0.096 3.848 0.187 4.225 0.483 4.650 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 We thank Dr. S. Yarmolenko and Dr. A. Doroshenko for their support in the performance of the experiment and helpful remarks. The work was financed by Ukrainian State Committee on Science and Technology and Fund of Funda-mental Investigations (section 3, ‘Chemistry’, project N3/89). References 1 0.N. Kalugin, S. M. Gubsky, I. N. Vyunnik, M. Grigo and R. Saendig, J. Chem. SOC., Faraday Trans., 1991,87,63. 2 0.N. Kalugin, Dissertation, Kharkov State University, 1987. 3 (a)J. C. Rasaiah and H. L. Friedman, J. Phys. Chem., 1968, 72, 3352; (b) J. C. Rasaiah, J. Phys. Chem., 1970,52,704. 4 H. L. Friedman and C. V. Krishnan, in Water: A Comprehensive Treatise, ed. F. Franks, Plenum Press, New York, 1973, vol.111, p. 17. 5 I. M. Strauss and M. C. R. Symons, J. Chem. SOC., Faraday Trans., 1978, 74, 2146. 6 H. L. Robinson and M. C. R. Symons, J. Chem. Soc., Faraday Trans., 1985,81, 2131. 7 I. M. Strauss and M. C. R. Symons, J. Chem. SOC., Faraday Trans., 1977, 73, 1796. 8 T. R. Grifliths and M. C. R. Symons, Mol. Phys., 1960,3,174. 9 I. M. Strauss and M. C. R. Symons, Chem. Phys. Lett., 1977, 45, 423. 10 S. E. Jackson, I. M. Strauss and M. C. R. Symons, J. Chem. SOC., Chem. Commun., 1977, 174. 11 Problems of Solution Chemistry. Ionic solvation, ed. G. A. Krestov, Nauka, Moscow, 1987. 12 B. S. Krumgalz, J. Chem. Soc., Faraday Trans., 1983,79,571. 13 G. C. Pimentel and A. L. McClellan, The Hydrogen Bond, W. H. Freeman, San Francisco and London, 1960. 14 L. J. Bellamy, The Infrared Spectra of Complex Molecules, Methuen, London, 1954. 15 V. I. Malyshev, Usp. Phys. Nauk, 1957,63, 323. 16 D. M. Himmelblau, Applied Nonlinear Programming, McGraw-Hill, New York, 1972. 17 M. I. Shahparonov, Mechanisms of Fast Processes in Liquids, Vyshaya shkola, Moscow, 1980. 18 L. V. Vilkov and Yu. A. Pentin, Physical Methods in Chemistry. Structure Methods and Optical Spectroscopy, Vyshaya Shkola, Moscow, 1987. Paper 3/03 1565 ;Received 3rd June, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000297

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 305-309 Effect of Glycerol on the Translational and Rotational Motions in Lipid Bilayers studied by Pulsed-field Gradient 'H NMR, EPR and Time-resolved Fluorescence Spectroscopy Greger Oradd, Goran Wikander, Goran Lindblom and Lennart B-A. Johansson" Department of Physical Chemistry, University of Umed, S-90187 Umed, Sweden Glycerol can replace water in both lipid vesicles and lyotropic liquid-crystalline phases. 1,2-Dioleoylsn- glycero(3)phosphocholine (DOPC) forms a lamellar (La)liquid-crystalline phase in arbitrary mixtures of glycerol and water (Biochim. Biophys. Acta, 1993,1149,285.). Monoolein (MO) forms La and also cubic liquidcrystalline phases in glycerol-water mixtures. The present study is focussed on characterizing the influence of glycerol on the molecular dynamics in the lipid bilayer.By EPR and time-resolved fluorescence spectroscopy we measure the rotat i onaI mo bi Ii ty of spin-l abe1led fatty acids [243carboxypropyl)-4,4-d i methy 1-24ridec y Ioxazol id i n-3-y lox y l (5-DS) and 2-(14carboxytetradecyl)-2-ethyl-4,4-dimethyloxazolidin-3-yloxyl (16-DS)] and a hydrophobic fluoro- phore, 2,5,8,1l-tetra-tert-butylperylene(TBPe), respectively. The translational diffusion of MO in the cubic phase is obtained by pulsed-field gradient 'H FT NMR experiments. The rotational rate of 16-DS and TBPe decreases continuously with increasing glycerol concentration, being a factor of 2-3 lower at 100% glycerol. A continuous decrease in the lipid translational diffusion coefficient, D, is also found with increasing glycerol content, so that D = 12.6 x 10-l2 m2 s-' at 0% and D = 1.9 x 10-l2 m2 s-' at 100% glycerol. The effects of glycerol on both the translational diffusion of the lipid in the bilayers and the rotational dynamics of the probe molecules residing in the interior of the hydrophobic regions are ascribed to changes of the viscosity in the interbilayer regions.It has been shown that lipid bilayers form not only with water, but also in other polar solvents such as glycerol, for- mamide and ethylene glycol. 1-5 In biological systems, gly- cerol is often used as a cryoprotectant and it substitutes very well for water.6-' Recently, we have found that 1,2-dioleoyl- sn-glycero(3)phosphocholine(DOPC) forms a lamellar liquid- crystalline phase (La) in arbitrary mixtures of glycerol and water.5 The phase was characterized by means of X-ray dif- fraction, 31P NMR spectroscopy and differential scanning calorimetry (DSC).Moreover, it was found that unilamellar vesicles could be prepared from a diluted suspension of the lamellar phase, as was shown by means of 31PNMR, EPR and fluorescence spectro~copy.~The last two methods revealed a strong dependence of the rotational relaxation rates of fluorescent and spin probes with glycerol concentra- tion, although these probes resided in the interior of the lipid bilayer. In the present paper we have investigated the molecular dynamics occurring in the La phases of DOPC and mono- olein (MO) and in the cubic liquid-crystalline phase of MO, which is built up of bilayer aggregate units.6 Cubic phases are convenient systems for studies of the motion of lipids and probes, since such phases are optically isotropic, which may simplify the interpretations of fluorescence depolarization and EPR experimental data.Furthermore, the conventional 'H NMR diffusion technique can be applied straightfor- wardly on cubic phases.' The rotational relaxation and molecular ordering of spin labels and fluorophores localized in the lipid matrix have been determined, as well as the lipid translational diffusion coefficients. Experimental DOPC was purchased from Avanti Polar Lipids (USA). The purity of the lipid was better than 99%, as checked by thin- layer chromatography at our laboratory.MO was purchased from Sigma (Missouri, USA) and was used without further purification, and glycerol was purchased from BDH (Canada). 5-DS[S-doxylstearic acid, 2-(3-~arboxypropy1)-4,4-dimethyl-2-tridecyloxazolidin-3-yloxyl]and 16-DS [16-doxyl-stearic acid, 2-(14-carboxytetradecyl)-2-ethyl-4,4-dimethyl-oxazolidin-3-yloxyl] (see Fig. 1) were purchased from Molec- ular Probes Inc. (USA) and were used directly without any further purification. 2,5,8,1l-Tetra-tert-butylperylene(TBPe, see Fig. 1) was synthesized by Friedel-Crafts alkylation of perylene.' Vesicles for EPR experiments were prepared by sonication according to the following procedure. Appropriate amounts of the dry lipid powder were dissolved in a mixture of chloroform-methanol (2 :1 v/v).A suitable amount of the spin label, dissolved in a chloroform-methanol mixture (2 :1 v/v), was added to the former solution. The label :lipid molar ratio for the vesicle samples was 1: 100 in order to prevent spin-spin interactions. The solvent was evaporated and dried at 320 K and 0.1 Torr for at least 2 h. Afterwards, 3 ml of a glycerol-H,O mixture were added and the suspension was sonicated eight times for 5 min each time. During the sonica- tion the sample was cooled to ca. 283 K. The sonicator was a Soniprep 150 (MSE Scientific Instruments, England) supple- mented with an exponential microprobe. The level of the amplitude used was 10-14 pm. Between the different sonica- tion steps the lipid-glycerol-water mixture was frozen with liquid nitrogen and thawed several times in order to obtain as large aggregates as possible.Glass capillary tubes were so-s1 5-DS,n= 3,m=12 16-DS,n=14,m= 1 Fig. 1 Structures of 5-DS, 16-DS and TBPe. The electronic dipoles of the So++ S, transitions are polarized in the plane of the molecule, as indicated by the arrows. filled with the optically clear solution and thereafter flame- sealed. EPR spectra of the vesicle solutions were recorded at 274 or 298 K. In the preparation of cubic phases of MO for EPR experi- ments, a suitable amount of the spin label dissolved in a chloroform-methanol mixture was transferred to glass vials. The solvent was pumped off under an N, atmosphere.Appropriate amounts of the amphiphile and a glycerol-water mixture were added to the thoroughly dried label. The ratio between MO and glycerol-H,O was always 70: 30 (wt.%). Thereafter, the glass vials were sealed. The samples were thoroughly mixed by repeated centrifuging at 313 K for several days. They were stored in darkness for equilibration for some days. Small amounts of the samples were sucked into glass capillary tubes, sealed and thereafter measured on the EPR spectrometer. The label :amphiphile ratio was always kept at 1 :600 on a molar basis for the MO samples. The samples were run in the temperature range 298-328 K with a temperature increment of 5 K between each point. Temperature regulation (within k0.5 K) was achieved by means of a Bruker ER 41 11 VT variable-temperature regula- tor.Each sample was checked between crossed polarizers before and after every temperature variation experiment. The samples never displayed any birefringence. This indicated that an optically isotropic phase remained during the variable-temperature experiment. The EPR spectra were recorded with a Bruker model ESP 300E X-band spec- trometer (9 GHz). The modulation frequency was 100 kHz and the modulation amplitude was always less than 0.5 times the linewidth of the central peak in each spectrum. For the NMR experiments the samples were made by weighing appropriate amounts of MO, glycerol and D20 into 7 mm glass tubes which were then flame-sealed. The samples were mixed by centrifugation and left for some days to equilibrate. The glass tubes were then opened and the samples transferred to 5 mm NMR tubes.The self-diffusion coefficient was measured with the Fourier-transform pulsed magnetic field gradient spin-echo technique.' The pulse sequence (RWn/2-z-n-z), was utilized in the diffusion experiment, where RD is the waiting time between repetitive scans, and N is the number of scans collected in each experi- ment. The magnetic field gradient pulses of width 6 and strength g are separated by a time interval A and placed at each side of the n pulse. In order to obtain a steady-state condition of residual gradient tails, three dummy gradient pulses were separated by A, and applied before each pulse sequence.l4 Two dummy scans were rejected at the beginning of each experiment.The attenuation of the signal measured through the peak height is described by eqn. (1) and the fitting of the experimental data to this equation was made on a personal computer by using the program Sigma Plot for curve fitting. A(t) = A(O)exp[ -y2g2a2A(A -6/3)]exp[ -2z/T] (1) The diffusion experiments were performed on a Bruker ACP-250 spectrometer equipped with an HR-50 high-resolution VT diffusion probe for 5 mm samples (Cryomagnet Systems Inc., Indianapolis). The magnetic field gradient pulses were generated with a home built gradient unit driven by a Kenwood PD35-20D power supply. The temperature was kept constant to within 1 K by a heated air stream around the sample and was measured by means of a thermocouple placed close to the sample.The lipid diffusion was monitored through the peak height of the chain terminal methyl groups. Several experiments were made with different (constant) values of RD, z, A and g while 6 was varied. Typical settings on these parameters for the investigation were, RD = 2 s, T = 200-400 ms, A = 200-400 ms, g = 0.12-J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.5 T m-I. The observed diffusion coefficients did not depend on a variation of these parameters. The steady-state fluorescence spectra and anisotropies were obtained using a Spex Fluorolog 112 instrument (Spex Ind., New Jersey), equipped with Glan-Thompson polarizers. The spectral bandwidths were 5.6 and 2.7 nm for the excitation and emission monochromators, respectively.The fluorescence spectra were corrected. The fluorimeter was calibrated by using a standard lamp from the Swedish National Testing and Research Institute, Borgs, Sweden. A PRA 3000 system (Photophysical Research Ass. Inc., Canada) was used for single-photon-counting measurements of the fluorescence decay. The excitation source is a thyratron-gated flash lamp (Model 510 C, PRA) filled with deuterium gas and operated at ca. 30 kHz. The excitation wavelengths were selected by interference filters (Omega/ Saven AB, Sweden) centred at 409.4 nm [half band width (HBW) = 13.0 nm]. The fluorescence emission was observed above 470 nm through a long-pass filter (Schott KV 470, Schott). The maximum absorbance of all samples was kept below 0.08 which corresponds to a total concentration of less than mol dm-3.The time-resolved polarized fluorescence decay curves were measured by repeated collection of photons during 200 s, for each setting of the polarizers. The emission polarizer was fixed and the excitation polarizer rotated periodically. In each experiment the decay curves Fll(t)and F,(t) were col- lected. The subscripts 11 and Irefer to an orientation of the emission polarizer parallel and perpendicular with respect to the excitation polarizer. From these a sum curve S(t) = FIl(t) + 2GF,(t) (2) and a difference curve 4)= FllW -GF,(t) (3) were calculated. The correction factor, G, was obtained by normalizing the total number of counts, FII and F, ,collected in Fl,(t)and F,(t), respectively, to the steady-state anisotropy, Is, as G = (1 -rJ(1 + 2rs)-1F,l(F,)-' (4) The data were analysed with a MINC-11/03 computer using the deconvolution software (DECAY V3.0 a, ATROPY, V1.0) developed by PRA.Linear dichroism (LD) spectra were recorded on a Jasco 5-720 supplemented with an Oxley device and the absorption spectra on a Cary 119 spectrophotometer. Details of studying macroscopically aligned lamellar liquid crystals and the inter- pretation of data are given elsewhere." Results DOPC vesicles EPR spectra of 5-DS or 16-DS solubilized in DOPC vesicles have been recorded at different ratios of glycerol and water. The lineshapes of 5-and 16-DS in DOPC vesicles at various glycerol-water mixtures, are typical for reorientational rates in the time domain of intermediate and slow motions.Under these conditions it is not possible to calculate unambiguously the rotational correlation time from the EPR spectrum. Instead, computer simulations must be performed. Such an analysis gives values of the order parameter, S, and the rota- tional correlation time, z,. The N-0 label is assumed to undergo Brownian reorientations with the rotational diffu- sion constants of RII and R, parallel and perpendicular to the N-0 bond, respectively. For this case the correlation time, z, is defined by z, = 6-' (RIIR,)-'j2. Details concerning these J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 Fig. 2 Experimental (-) and simulated (. .) EPR spectra of 5-DS solubilized in vesicles of DOPC at different mixtures of glycerol and water. Composition of the solvent mixture (percentage glycerol) : (a) 0, (b)50, (c) 70, (d) 90. 298 K, total scan range, 8 mT. theoretical models are given elsewhere.16 Fig. 2 shows the simulated (dotted lines) and experimental (solid lines) spectra of 5-DS in DOPC vesicles. Table 1 displays the best fit values of S and z,. Vesicles containing the hydrophobic fluorophore TBPe were studied by using steady-state and time-resolved fluores- cence spectroscopy. The rotational motions of TBPe in DOPC vesicles formed in water and in a glycerol-water mixture (91 : 9, wt.%) were compared. The values of the steady-state fluorescence anisotropies are 0.21 and 0.32 in the water and the solvent mixture, respectively.Since the fluores- cence lifetimes of TBPe in these systems are very similar (about 4.8 ns), the different values of the fluorescence aniso- tropy suggest that the rotational rates are considerably slower in the DOPC vesicles formed in the glycerol-water mixture. In these studies (as well as in the EPR experiments) note that the influence of rotational motion of the vesicles can safely be neglected. A direct proof for slower rotational motions, in the presence of glycerol, is obtained from the time-resolved fluorescence anisotropy. For both kinds of system the decay of the fluorescence anisotropy is biexponen- tial with the rotational correlation times about four times 307 smaller in the vesicles suspended in water, than in those in the glycerol-water mixture.On a nanosecond timescale, the actual anisotropy decays at 274 K are given by r(t) = 0.11 exp(-t/3.5) + 0.21 exp( -t/25) and r(t) = 0.10 exp(-t/ 10.8) + 0.27 exp( -t/124). Cubic Phases EPR experiments reveal that the rotational motions of the spin label (16-DS) occur in the intermediate time domain. Table 1 displays z, obtained from lineshape simulations for cubic phases of monoolein at different glycerol : water ratios. The correlation time can be interpreted as the reciprocal rate of rotational relaxation of the label, having a temperature dependence given by z, = A' exp(-EJRT) In eqn. (5), E, is the activation energy for the reorientation process, A' is a pre-exponential factor (of dimension frequency-'), R is the universal gas constant and T is the temperature.From eqn. (5) an estimate of the activation energy of reorientation of the spin label is obtained. The acti- vation energy is about 35 kJ mol-and changes only slightly with glycerol content (Table 1). The hydrophobic fluorophore, TBPe, was solubilized in the cubic phase at the same ratios of glycerol and water as were used in the EPR experiments. The fluorescence relaxation of TBPe is monoexponential (with the lifetime zfgiven in Table 2) as is expected, if all TBPe molecules are homogeneously distributed in the lipid bilayer. We find a small, but signifi- cant decrease of zf with increasing glycerol content, which means that the physico-chemical properties change in the vicinity of the TBPe molecules.The zfvalues observed in the absence of glycerol are very similar to those found in other liquid-crystalline phases composed of non-ionic or ionic detergents in water.'* From the time-resolved and steady- state fluorescence anisotropies we obtain the (second-rank) rotational correlation function of TBPe. This function is fitted to exponential functions, as was done for the vesicle systems. We find that the rotational correlation function does not fit to a single-exponential function, but does fit very well to a biexponential function. The two correlation times ob- tained are summarized in the Table 3. One of the correlation Table 1 crystals MO (b)in different mixtures of glycerol and water' (a) Vesicles of DOPC in mixtures of glycerol and water Results from lineshape analysis of EPR spectra obtained for 5-DS and 16-DS solubilized in vesicles of DOPC (a) and cubic liquid glycerol (%) glycerol (Oh) glycerol (%) content ?,Insb content t,/nsb content S r,/ns 16-DS at 298 K 16-DS at 274 K 5-DS at 298 K 90 1.32 90 2.56 90 0.44 4.18 70 1.11 70 2.08 70 0.42 2.41 50 1.04 50 1.85 50 0.36 1.60 30 0.98 30 1.75 0 0.34 1.39 10 0.93 10 1.67 0 0.83 0 1.59 ~ (b)Cubic phases of MO (70%)in glycerol and water (1 6-DS at 298 K) glycerol : water temperature ratio rr/nsb EJcJ mol-' range/K 1 :o 1.15 34.3 298-328 2:l 1.04 34.1 298-328 1 :2 0.88 37.8 298-328 0:l 0.56 35.5 298-328 The parameters S, 7, and E, denote the order parameter, rotational correlation time and the activation energy of rotation, respectively.The order parameter, S = (P,(cos B)), describes the average orientation (B) of the long axis of the probe molecule with respect to the local director of the bilayer. The errors in S and 7,are estimated to be within 5%. S = 0. Table 2 Fluorescence lifetime (zc)of TBPe in cubic phases of MO at various mixtures (given as molar ratios) of glycerol with water glycerol : water ratio %Ins xz 0: 1 4.7 1.15 2:1 4.7 1.05 1 :2 4.8 0.97 l:o 5.0 1.17 The MO content is kept constant at 70 wt.%. The parameter xz is a statistical test of the curve fit to experimental data.The accuracy of q is within kO.05 ns. times (41)is very long compared with tf. Consequently, cannot be determined with any accuracy and is therefore con- sidered to be equal to infinity on the timescale of fluores- cence. The second rotational correlation time, 42, increases by a factor of ca. 3, upon increasing the glycerol content from 0 to 100 mol% in the cubic phases. The second-rank order parameter (S) of TBPe was obtained from linear dichroism experiments, performed on a macroscopically aligned lamellar phase of MO and water. These lamellar phases contain glycerol and water of the same molar ratios, as were used for the cubic phases. The order parameters, shown in Table 3, are very small, which strongly indicates a nearly isotropic orientation of the TBPe mol- ecules with respect to the director of the lipid bilayers.More- over, these values are very similar to those found in lamellar phases composed of DOPC and other detergent^.'^.' Taken together, these data are compatible with a localization of TBPe in the hydrophobic interior of the monoolein aggre- gates, where their molecular tumbling is considerably ham- pered by increasing the glycerol content outside the lipid bilayer. The S values change sign, implying that the orienta- tional distribution of TBPe changes with the glycerol concen- tration. Hence, the lipid orientation and/or lipid packing are influenced by the glycerol content. The self-diffusion coefficient, D,,, of MO was determined from pulsed field gradient ‘H NMR spin-echo measurements. The values of D,, obtained at the different glycerol concen- trations and temperatures are summarized in Fig.3. The lipid translational diffusion increases by a factor of six as the gly- cerol content is decreased from 100 to 0% in the cubic phase built up of MO and a mixture of glycerol and water. From the temperature dependence of D,, the activation energy of the translational motion can be estimated. The activation J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 -23.5 -24.0 -24.5 -25.0 a C--25.5 -26.0 -26.5 -27.0 0.0030 0.0031 0.0032 0.0033 0.0034 K/T Fig. 3 Natural logarithms of the translational diffusion coefficient (DmJ of MO as a function of inverse temperature in cubic phases with different molar ratios of glycerol and water.The activation ener- gies (E&J mol-’) of translational motion are: (V) 2: 1 Gly :D,O, 38; (0) 1:2 Gly :D,O, 30.1:0 Gly :D,O, 41 ;(A)0 :1 Gly :D,O, 30; (0) Discussion In the cubic phase of MO, the dynamics of reporter mol- ecules (16-DS and TBPe) residing in the hydrophobic part of the lipid region is significantly reduced upon replacing water with glycerol. Similarly, a decrease in the translational diffu- sion of the lipid itself is observed with increasing glycerol content. The relative decreases in the rotational rates of 16-DS and the fluorescent label TBPe, at 0 and 100% glyc- erol in the cubic phase, are similar.The rotational correlation times of TBPe are slower than those for 16-NS, which is to be expected since the molecular volume of TBPe (ca. 500 A3) is much greater. energy decreases from about 40 kJ mol-’ to about 30 kJ In dispersions of unilamellar vesicles of DOPC, the rota- mol-upon decreasing the glycerol concentration from 100 tional motions of the probes are hampered with increasing to 0%. glycerol content. However, the relative decrease is slightly Table 3 Steady-state (T,) and time-resolved [r(t)] fluorescence anisotropy data of TBPe in the cubic phase of MO and various mixtures of glycerol with water ~~~ glycerol : water ratio rs a1 41 a2 42 xz S 0:l 0.256 0.08 cc 0.26 6.6 1.16 -0.02 2:l 0.214 0.06 00 0.28 6.4 1.51 -0.02 1.2 0.184 0.05 cc 0.27 4.8 1.45 0.02 l:o 0.126 0.08 co 0.14 2.3 1.20 0.04 The MO content is kept constant at 70 wt.%.The time-resolved fluorescence anisotropy was fitted to experimental data according to r(t) = a, exp(-t/4 + a, exp( -t/&) where #1+ tf,i.e. a, exp(-t/$l) is essentially constant on the timescale of the experiment. The parameter X’ is a statistical test of the curve fit to experimental data. Data were collected at 298 K. The order parameter (S) was calculated from linear dichroism and absorption spectra of TBPe in the L, phases of MO formed with different mixtures of glycerol and water. The MO content was kept constant at 90 wt.%. The L, phase was macroscopically aligned between quartz plates.The order parameter, S = (P,(cos /3)), describes the average orientation (/3) of the transition dipole moment of the probe molecule, with respect to the director of the bilayer. The errors in 4, and S are within 10 and 5%, respectively. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 larger for 5-DS than for 16-DS. The values of the order parameters of 5-DS (S = 0.4) and 16-DS (S = 0), estimated from the lineshape analysis, are compatible with 16-DS being located deeper in the interior of the lipid bilayer. Hence, the influence of glycerol on the dynamics decreases towards the bilayer interior. A comparison of the relative decrease in the dynamic parameters between 0 and 100% of glycerol in the cubic phase, shows that the lipid translational diffusion is much more reduced than the rotational motions of 16-DS and TBPe.Previously,” it has been concluded that inter- actions in the polar head groups of the lipids in the aggregate play a dominant role for lipid lateral diffusion. It seems rea- sonable, therefore, that the relative effect of glycerol on the dynamics should be larger on the lipid translation diffusion than on the rotational motions of probe molecules solubilized within the interior of the lipid bilayer. The most simple mechanistic explanation for the present observations is that the increase of the macroscopic viscosity, exerted by the addition of glycerol to the polar region of the liquid-crystalline phase, affects the dynamics of all molecules in the phase structure.The viscosity of pure glycerol is ca. 1500 times greater than that of water at 293 K. Thereby, the motional friction in the interface of the lipid aggregates increases and hampers both the acyl chain motions and the lipid translational diffusional motion. Note also that the vis- cosity of glycerol is more temperature dependent than that of water, which should influence the activation energy of the lipid translational diffusion. The present experimental results clearly demonstrate that glycerol influences the dynamics in the bilayer and, most likely, also the packing of the lipid molecules in the aggre- gates. In a previous st~dy,~ X-ray diffraction studies of DOPC show that the bilayer thickness decreases, while the polar head group area increases with increasing glycerol content in the L, phase.The order parameter of 5-DS in DOPC vesicles changes significantly with glycerol content, which also suggests an influence on lipid packing. Further- more, the order parameter of TBPe, solubilized in lamellar phases of MO, changes sign with increasing glycerol concen- tration. Taken together, these observations are all compatible with a change in the lipid packing caused by the replacement of water with glycerol. Thus, both the dynamics and the lipid packing are significantly influenced upon replacing water with glycerol in vesicles, and lamellar and cubic liquid crys- tals. We are grateful to Mrs. Eva Vikstrom for skilful technical assistance and to Mr. Stein-Tore Bogen for performing the light spectroscopic experiments.This work was supported by the Swedish Natural Research Council. References I R. V. McDaniel, T. J. McIntosh and S. A. Simon, Biochim. Biophys. Acta, 1983,731,97. 2 J. L. Green and C. A. Angell, J. Phys. Chem., 1988,93,2880. 3 M. A. El-Nokaly, L. D. Ford and S. E. Friberg, J. Colloid Inter- face Sci., 1981,84, 228. 4 B. A. Bergenstihl and P. Stenius, J. Phys. Chem., 1987,91,5944. 5 L. B-A. Johansson, B. Kalman, G. Wikander, A. Fransson, K. Fontell, B. Bergenstihl and G. Lindblom, Biochim. Biophys. Acta, 1993, 1149, 285. 6 G. Lindblom, K. Larsson, L. B-A. Johansson, K. Fontell and S. Forsen, J. Am. Chem. SOC., 1979, 101, 2204; G. Lindblom and L. Rilfors, Biochim. Biophys. Acta, 1989,998, 221.7 G. Lindblom and H. Wennerstrom, Biophys. Chem., 1977,6, 167. 8 R. J. Taylor, G. D. J. Adams, C. F. B. Boardman and R. G. Wallis, Cryobiology, 1974, 11,430. 9 P. Mazur, R. H. Miller and S. P. Leibo, J. Membrane Biof., 1974, 15, 137. 10 A. A. Newman, 1968, in Gfycerol,ed. A. A. Newman, CSC Press, Cleveland, p. 91. 11 B. Schobert, 2. Naturforsch., Teil C, 1979,34,699. 12 L. B-A. Johansson, J. G. Molotkovsky and L. D. Bergelson, J. Am. Chem. SOC., 1987,109,7374. 13 E. 0.Stejskal and J. E. Tanner, J. Chem Phys., 1965,42, 288. 14 S. J. Gibbs and C. S. Johnson Jr., J. Magn. Reson., 1991,93, 395. 15 B. Norden, I. Jonas and G. Lindblom, J. Phys. Chem., 1977,81, 2084; L. B-A. Johansson, A. Davidsson, G. Lindblom and B. Norden, J. Phys. Chem., 1978, 82, 2604; L. B-A. Johansson and A.Davidsson, J. Chem. SOC.,Faraday Trans. I, 1985,81, 1373. 16 D. J. Schneider and J. H. Freed, in Biological Magnetic Reson- ance: Spin Labelling Theory and Applications, 1989, ed. L. J. Ber-liner and J. Reuben Plenum Press, New York, vol. 8, ch. 1; G. Wikander P-0. Eriksson, E. E. Burnell and G. Lindblom, J. Phys. Chem., 1990, 94, 5964; Z. Liang, P-0.Westlund and G. Wikander J. Chem Phys., 1993,99, 7098. 17 B. Kalman, L. B-A. Johansson, M. Lindberg and S. Engstrom J. Phys. Chem., 1989, 93, 8371; B. Kalman, L. B-A. Johansson, J. Phys. Chem., 1992,%, 185. Paper 3/03708H; Received 29th June, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000305

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 311-314 31 1 Reactivity of some Square-planar Palladium(ii) Complexes in Aqueous Solution and in Heptane-AOT-Water Microemulsions F. Paolo Cavasino, Carmelo Sbriziolo," M. Liria Turco Liveri and Vincenzo Turco Liveri Dipartimento di Chimica Fisica, Universita di Palermo, 90123 Palermo, Italy The substitution reactions at square-planar palladium(ii) complexes { [PdX(en)12', where X = 2,2'-bipyridine or 4,4'-dimethyL2,2'-bipyridine, en = ethylenediamine) with ethylenediamine or N,N-dimethylethylenediamine have been studied at 25.0 "C in aqueous solution and in heptane-AOT-water microemulsions with various molar ratios, [H,O]/[AOT] = W, the AOT concentration being kept constant at 0.12 mol dm-3. The kinetic data, obtained by using both conventional and stopped-flow spectrophotometric methods, show that the reaction rates, either in bulk aqueous solution or in the microemulsions, decrease with increasing steric hindrance of both the entering nucleophile and the ligand coordinated to the metal.Moreover, the reaction rate in the microemulsions is accelerated compared with that in the bulk aqueous solution and exhibits a depen-dence on the molar ratio W. The microscopic AOT/water interface is suggested as the reaction site. Over the last few years microemulsions have been the subject of constantly increasing interest from both theoretical and experimental viewpoints because they may find applications in a variety of such as artificial photosynthesis and solar energy conversion, the production of semiconductor microcolloids, polymerization, separation and purification processes, the synthesis of microparticle catalysts, enzyme encapsulation and drug delivery.Microemulsions are optically transparent, thermodynami- cally stable isotropic liquid mixtures of surfactant, water and oil (non-polar solvent), with or without addition of a fourth component (co-surfactant). These systems are heterogeneous on a microscopic scale and consist of microscopic domains of water and oil separated and stabilized by a monolayer of sur- factant molecules. In most cases, depending on the experi- mental conditions, microemulsions can be regarded as dispersions of either water droplets in an oil continuous phase (w/o) or oil droplets in water (o/w), each droplet being surrounded by the surfactant monolayer. Among the amphiphiles capable of forming w/o micro- emulsions without the need of an added co-surfactant the most used is the surfactant sodium bis(2-ethylhexy1)sulfo- succinate (AOT)having two branched alkyl chains with nega- tively charged SO; head groups.The three-component AOT-stabilized microemulsions have been widely studied and characterized by various physical techniques. They exhibit a remarkable ability to solubilize large amounts of water forming spherical droplets (pools) having an almost mono- dispersive size distribution. The water content in such pools depends mainly on the molar concentration ratio, W, and can easily be varied over a wide range. This will result in droplets of different sizes having mean radius directly pro- portional to the W parameter.' From the viewpoint of chemical reactivity, owing to the existence of large internal interfaces and to the unusual state of water in the droplets, the w/o microemulsions (also termed reversed micelles) have attracted much interest because they can be used as substitutive novel media6-9 capable of altering significantly the rates of chemical and biochemical (e.g.enzymatic) reactions compared with their rates in the bulk water. In recent years we have studied the effect of various micelle-forming surfactants on the kinetics of a number of inorganic reactions"-' drawing our attention to, in particu- lar, the reaction mechanism and the role of the hydrophobic and electrostatic interactions involved in the solubilization process of the reactants in the micellar pseudo-phase.In the present work we have extended the reactivity studies in the organized assemblies to the w/o microemulsion systems by examining the kinetics of the substitution reaction (1) at square-planar palladium(r1) complexes in a ternary mixture of heptane, AOT and water. In reaction (1) X = 2,2'-bipyridine (bipy) or 4,4-dimethyl-2,2'-bipyridine (dmbipy), and Y = ethylenediamine (en) or N,N-dimethylethylenediamine (dmen). [Pd(en)X12' + Y + [Pd(en)Y-J*' + X Since the palladium(rr) complexes considered are positively charged ions and, according to previous calorimetric observa- tion~,~~both entering ligands (en and dmen) are distributed between the aqueous core and the palisade layer of the reverse micelle, the chosen substitution reaction (1) would take place only inside the water droplets, facilitating the interpretation of the kinetic data obtained.Moreover, the selected reactants enable us to examine the kinetic effects of steric hindrance and hydrophobicity brought about by the alkyl substituents of both the entering and leaving ligands. This kinetic study has been performed in heptane-AOT- water microemulsions at varying values of the W parameter (at fixed surfactant concentration). Rate determinations in water have also been carried out for comparison. Note that, as far as we know, no kinetic investigation of reactions involving square-planar metal complexes in micro- emulsions has so far been made.Experimental Ethylenediamine, N,N-dimethylethylenediamineand heptane (Fluka) were used as received. AOT was supplied by Sigma and dried under vacuum for several days to remove water. The palladium(r1) complexes in perchlorate form, prepared and characterized according to the standard procedure, were kindly provided by Prof. M. Cusumano from the University of Messina (Italy). Water doubly distilled from alkaline KMnO, was used. Standard solutions of AOT in heptane and of the palladium(I1) complexes and nucleophilic reactants (Y)in water were prepared by dissolving weighed amounts of the substances in the pure solvents. Fresh AOT solutions were made routinely to avoid ester hydrolysis. For kinetic experi- ments in the self-assembling systems, microemulsions con-taining a given complex or nucleophile at the desired W were obtained by adding appropriate aliquots of the aqueous (diluted, when necessary) solution of the pertinent reagent to J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 the organic solution of AOT. These microemulsions, pre- 15.( pared before use, were then mixed to follow the reaction rates. Kinetic runs were carried out with a Beckman DU-7 HS spectrophotometer or a HI-TECH SF-61 stopped-flow spec- trophotometer interfaced to a Tandon computer for all data collection and analysis. Both apparatuses were equipped with thermostatted compartments. The rates of disappearance of the palladium(r1) complexes were followed at 307-309 nm.In all cases, pseudo-first-order conditions were used with a large 10.1excess of nucleophile, the initial complex concentration being always 5 x mol dm-3. A 100-fold excess was used in r Iv)the kinetic runs at constant nucleophile concentration. The Cvalues of the observed pseudo-first-order rate constant, kobs, I were reproducible to better than f3%. 0 r --%Preliminary spectrophotometric measurements were per- 0 formed with both aqueous solutions and microemulsions containing each reactant to verify the stability of the solu- tions. For the substitution reactions studied in water no effect of 5.I ionic strength (supporting electrolyte NaClO,) on the rate was observed up to 0.10 rnol dm-3, as expected for reactions involving an uncharged reagent.This provides experimental support to the fact that the ionic strength, which is particu-larly high inside the water core of the reversed micelles at low W values, does not contribute to the observed kinetic effects in the microemulsion examined. The temperature of the experiments was 25.0 f0.1 “C. Unless otherwise indicated, all the concentration values re- ported in this work are expressed as mol dm-3 of total solu- tion. 0 2.o 4.0 6.O [Y]/10-2 mol dm-3 Fig. 1 Plot of kobs us. [Y] for the substitution reaction (1) in Results and Discussion aqueous solution, t = 250°C: (a) X = dmbipy, Y = en, n = 4; (b) X = bipy, Y =en, n = 3; (c) X = dmbipy, Y = dmen, n = 4; (d)Kinetics in Homogeneous Aqueous Solution X = bipy, Y = dmen, n = 3 In most cases substitution reactions at square-planar palla- dium and platinum complexes occur15*16 via an associative mechanism which may imply two parallel paths involving kinetic measurements at varying concentrations of the enter- direct attack at the metal by the nucleophile (Y) and the ing ligand (Y) in homogeneous aqueous solutions.Fig. 1 solvent, respectively. In the latter case the intermediate shows that straight lines with zero intercepts are obtained by solvato species formed is further converted by the nucleophile plotting kobs as a function of [Y] for all the reactions exam- into the substitution product. ined, suggesting that the observed pseudo-first-order rate In order to ascertain the rate equation valid for the substi- constant kobs is given by kT[Y] and, then, that only the tution reactions under study, we performed preliminary nucleophile direct pathway contributes significantly to the Table 1 Second-order rate constants ky (dm3 mol-’ s- ’) and k”;k; for the substitution reaction (1) in heptane-AOT-water microemulsions as a function of W ky k”;k: [Pd( bipyxen)] + [Pd(dm bipyxen)] + [Pd(bipy)(en)] + [Pd(dmbipyXen)] + W en“ dmenb ene dmend en“ dmenb enf dmend 4.0 66 3.10 21.0 1.02 47 1 238 724 340 4.5 58 2.49 14.0 0.74 414 192 483 247 5.0 41 2.32 11.7 0.65 293 178 403 217 6.0 32.0 1.68 9.2 0.49 229 129 317 163 7.0 29.0 1.40 7.5 0.40 207 108 259 133 8.0 20.0 1.00 4.5 0.310 143 77 155 103 9.0 15.0 0.90 3.24 0.287 107 69 112 96 12 8.6 0.83 2.63 0.217 61 64 91 72 15 8.4 0.53 2.19 0.145 60 41 76 48 20 4.2 0.44 1.29 0.120 30 34 44 40 25 3.5 0.320 0.91 0.101 25 25 31 34 30 2.8 0.308 0.64 0.069 20 24 22 23 [AOT] = 0.12 rnol dm-3, t = 250°C.The second-order rate constants for the substitution reaction (1) in water (ky) and microemulsions (ky) obtained by least-square fits (Fig. 1, 2) are: ” k; = 0.140, k‘; = 0.013, k‘; = 0.029, ky = 0.003; “ ky = 41 and ky = 2.32 (at W = 5); a ky = 2.8 and * ky = 0.308 (at W = 30). J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 overall substitution rate under the experimental conditions used.The estimated second-order rate constants ky (Table 1) show that the reactivity of the two palladium(r1) complexes under study depends markedly on the steric hindrance of both the entering nucleophile (Y) and the leaving ligand (X) coordinated to the metal. In particular we can observe that, for a given reactant complex, or for a given entering nucleo- phile, the substitution rate decreases significantly when two methyl groups are present in the incoming or in the leaving ligand, respectively. These findings provide support for the associative mechanism involved in the substitution reactions examined in the homogeneous aqueous solution. Kinetics in HeptaneAOT-Water Microemulsions The kinetic measurements in microemulsions were performed at a fixed AOT concentration (0.12 rnol dm-3)as a function of the composition parameter W (630).Preliminary rate experiments using the complex [Pd(en)bipy]*+ were carried out at W = 5 and 30 as a func- tion of the entering ligand (Y) concentration in order to verify whether the rate law observed in bulk water was also fol- lowed in the microheterogeneous media. In all cases investi- gated straight lines with zero intercepts are found (Fig. 2) by plotting kobs us. [Y], yielding the second-order rate constants 12.1 8.C -I v) CI 0 f* 4.O (d 0 I I I I 1 0 1.o 2.0 3 .O [Y]/10-2 mol dm-3 Fig. 2 Plot of kobs us. [Y] for the substitution reaction (1) with X = bipy in heptane-AOT-water microemulsions at [AOT] = 0.12 mol dm-3, t = 250°C: (a) W = 5, Y = en, n = 1; (b) W = 30, Y = dmen, n = 3; (c) W = 5, Y = dmen, n = 2; (6)W = 30, Y = en, n=l k;"t given in Table 1.Moreover, we can see that, at each molar ratio, W, examined, the reaction rate is notably lowered when two methyl groups are present in the entering nucleophile. These findings suggest that the substitution reac- tions (1) in microemulsions occur by the same overall rate law as found in homogeneous aqueous solution, only the direct attack at the palladium of the nucleophile being the operating pathway . Having suggested the mechanism of reaction (1) in both homogeneous solution and microemulsions, the kinetics in microemulsions, at constant [AOT] = 0.12 mol dm-3 varying W,will now be discussed.It must be pointed out that the metal complexes are closely associated with anionic AOT head groups over the W range studied and the diamines are partitioned between the micellar aqueous core and the pali- sade layer. Table 1 shows the second-order rate constant (ky) as a function of W. It can be seen that ky decreases, as for the same reactions in water, with increasing steric hindrance of the entering nucleophile (Y) as well as the X ligand coordi- nated to the central metal. Thus, for both complexes and incoming nuclephiles (Y) we can observe that at low W value the second-order rate constant decreases ca. 20-fold or four-fold when two methyl groups are added to the entering or leaving ligand, respectively. Table 1 indicates that the reaction rates in the micro- emulsions also decrease when the droplet size increases, ky appearing to approach the value of the second-order rate constant obtained for the same reaction in water (k:). This means that the decrease in the rate of reaction when the amount of water in the microemulsions is large, can be mainly ascribed to a decrease in the local concentration inside the water droplets.From a geometrical point of view, assuming a nearly constant surface area occupied by each surfactant molecule at the interface, the water content in the w/o microemulsions can increase only if the size of the water pool increases. Moreover, we can say that the rate of the process depends on the average distance between the two reacting species and is slowed because of the difficulty of the two reactants in approaching each other.The dependence of the second-order rate constants on water content is observed as a good linear correlaton between ky and l/W2. This relationship is shown, for all cases studied, in Fig. 3. It is well known' that water pool radius (r,) varies linearly with W, the water pool concentration being lowered with decreasing W. Moreover, as already proposed18 for reactions between hydrated electrons and various probes such as cyto- chrome c in AOT reversed micelle, it is possible to assume that the average distance between the two reactants is approximately equal to the water pool radius (r,). Hence, if the reaction takes place mainly at the interface region it seems logical to expect that the reaction rate varies with 1/w2.These findings, in accordance with the calorimetric concl~sion'~about the site of solubilization of the nucleo- philes, enable us once more to conclude that the replacement of X in the palladium(I1) complex by diamines proceeds at the microscopic surfactant/water interface region. Comparison of the reactivity data in homogeneous aqueous solution and in water pool microemulsions shows that the rate constant, ky, is notably larger than that (kp)for t ky has the same units [dm3 (of total solution) mol-' s-'I as the second-order rate constant ky. In fact it is known that," if reacting species are totally confined within the water pools and the slow process is the chemical reaction, the second order-rate constant obtained in homogeneous solution (k;) and that in w/o micro-emulsions (Icy)can be compared directly. 0 2 4 6 1/w Fig.3 Plot of ky us. l/Wzfor the substitution reaction (1)at [AOT] = 0.12mol dmP3, t = 25.0”C:(a)X = bipy, Y = en, n = -1, m = 4; (b) X = bipy, Y = dmen, n = 0, m = 3; (c) X = dmbipy, Y = en, n = -1, m = 2;(d)X = dmbipy, Y = dmen, n = 0, m = 1 the reaction in bulk aqueous solution. The extent of rate enhancement can be seen from the ratios k”,k; (Table 1). A maximum rate enhancement is observed when the process is carried out both at low W values and in the presence of ethylenediamine as entering ligand. For instance, at the lowest W value the rate in microemulsion is ca. 5 x 10’ faster than in water or 7 x lo2 times faster than in water when two methyl groups are present in the leaving ligand.In addition, at the highest W value, the second-order rate constant ky is CQ. 20-fold greater than ky. These results lead us to conclude that steric hindrance, as well as the hydrophobicity of both the incoming and leaving ligands, plays an important role in determining the rate of the reaction. A valid explanation of the observed rate acceleration can be provided in terms of the above-mentioned localised con- centration enhancements of the reacting species confined in the volume of the aqueous core droplet rather than an ionic strength effect. In fact, as also pointed out before, the ionic strength does not contribute to the observed kinetic effects in the systems examinea.UI course, a ainerenr situation exisis in Muiioz et al.’s study of anionic reactants in oil-AOT-water microemulsions’ where the results were interpreted on the basis of the high ionic strength inside the water pool. More- over, we have to bear in mind that the properties of the J. CHEM. SOC. FARADAY TRANS., 1994,VOL. 90 dispersed water (particularly at low W values) in the microemulsion systems are quite different from those of ordi- nary bulk water and that other factor^,'^ which are not present in conventional aqueous solution, might play a role in the reaction under study. In conclusion, these interesting kinetic results lead us to extend the present work to provide additional detail concern- ing the reactivity in these self-assembling systems.This work was supported by ‘Progetto Finalizzato, Chimica Fine’ CNR (Rome) and by Minister0 dell’universita e della Ricerca Scientifica e Tecnologica. References 1 J. H. Fendler, in Membrane Mimetic Chemistry. Characterization and Application of Micelles, Microemulsions, Monolayers, Bilayers, Vesicles, Host-Guest Systems and Polyions, Wiley, New York, 1982, and references therein; P. L. Luisi, M. Giomini, M. P. Pileni and B. H. Robinson, Biochim. Biophys. Acta, 1988, 947,209,and references therein. 2 M. A. Lijpez-Quintela and J. Rivas, in Structure, Dynamics and Equilibrium Properties of Colloidal Systems, ed. D.M. Bloor and E. Wyn-Jones, Kluwer, Dordrecht, 1990. 3 P. Lianos and J. K. Thomas, J. Colloid Interface Sci., 1987, 117, 505. 4 P. D.I. Fletcher and J. Parrott, J. Chem. SOC., Faraday Trans. 1, 1988,84,1131. 5 B. H.Robinson, Chem. BY., 1990,26,342. 6 M. L.Moyh, C. Izquierdo, J. Casado and A. Rodriguez, Znt. J. Chem. Kinet., 1992,24,19. 7 M. L. Moya, C. Izquierdo and J. Casado, J. Phys. Chem., 1991, 95,6001. 8 E. Muiioz, C. Gomez-Herrera, M. del Mar Graciani and M. L. Moya, J. Chem. SOC., Faraday Trans., 1991,87,129. 9 R. Schomacker, K. Stickdorn and W. Knoche, J. Chem. SOC., Faraday Trans., 1991, 87, 847; Yu. L. Khmelnitsky, A. V. Kabanov, N. L. Klyachko, A. V. Levashov and K. Martinek, in Structure and Reactivity in Reverse Micelles, ed.M. P. Pileni, Elsevier, Amsterdam, 1989,p. 230. 10 F. P. Cavasino, C. Sbriziolo, M. Cusumano and A. Giannetto, J. Chem. SOC., Faraday Trans. 1, 1989, 85, 4237; A. I. Carbone, F. P. Cavasino and C. Sbriziolo, J. Phys. Chem., 1987,91,4062. 11 M. Cusumano, A. Giannetto, F. P. Cavasino and C. Sbriziolo, Znorg. Chim. Acta., 1992,201,49. 12 G. Calvaruso, F. P. Cavasino and C. Sbriziolo, J. Chem. SOC., Faraday Trans., 1991,87,3033. 13 G. Calvaruso, F. P. Cavasino and C. Sbriziolo, J. Chem. SOC., Faraday Trans., 1992,88, 1669. 14 G. Pitarresi, C. Sbriziolo, M. L. Turco Liveri and V. Turco Liveri, J. Solution Chem., 1993,22, 279. 15 W. H.Baddley and F. Basolo, J. Am. Chem. SOC., 1966,88,2944; R. Roulet and H. B. Gray, Inorg. Chem., 1972,9, 2101;R. van Eldik, D. A. Palmer, R. Schmidt and H.Kelm, Znorg. Chim. Acta, 1981,50,131. 16 E.L.J. Breet and R. van Eldik, Znorg. Chem., 1984,23,1865. 17 P. D. I. Fletcher, A. M. Howe, B. H. Robinson and D. C. Steyt- ler, in Reverse Micelles. Biological and Technological Relevance of Amphiphilic Structures in Apolar Media, ed. P. L. Luisi and B. E. Straub, Plenum, New York, 1984,p. 73. 18 C. Petit, P. Brochette and M. P. Pileni, J. Phys. Chem., 1986,90, 65 17. 19 J. Nishimoto, E.Iwamoto, T. Fujiwara and T. Kumamaru, J. Chem. SOC., Faraday Trans., 1993,89, 535, and references cited therein. Paper 3/06076D; Received 1lth October, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000311

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 315-320 Spectrochernistry of Solutions Part 26.-tAlkaIi=metal and Alkaline-earth-metal Thiocyanates in Dimethylformamide and Acetonitrile Solutions :Hot Bands, Stability Constants and Thermicity for the Formation of Inner- and Outer-sphere Ion Pairs Peter Gans,* J. Bernard Gill* and Peter J. Longdon School of Chemistry, The University of Leeds, Leeds, UK LS2 9JT The IR spectra of solutions of the thiocyanates of (CH,),N+, (C2H5),N+, (C4HJ4N+, Li+, Na+, K+, Cs+, Mg2+, Ca2+, Sr2+ and Ba2+ in acetonitrile and dimethylformamide have been recorded at various temperatures in the range 278-348 K. Peaks ascribed to ion pairs shift with temperature. This shift is explained in terms of the existence of hot bands. Equilibrium constant calculations have been attempted using the IR data and where possible AH has been estimated.[SrNCS]+ forms both inner- and outer-sphere complexes with an additional thiocyanate ion in acetonitrile. The IR spectrum of the species ([SrNCSl'NCS-}, in which one thiocyanate is in the inner- and one thiocyanate is in the outercoordination sphere of the strontium cation, has been deduced with the aid of an equilibrium calculation. The separation of the absorbance maximum of this species from that of [SrNCS]+ is one sixth of the half-width of the bands. Experimental observed spectra of solutions at a range of total salt concen- IR spectra were measured on a Philips HP9545 Ratio trations by means of the Beer-Lambert law (l), Recording spectrophotometer controlled by a BBC micro- AA, s = L(&NCS, ACNCS-ls + &WNCS, ACMNCS1s) (I)computer.Each spectrum is the average of at least nine scans. CaF, windows were used for the 2050 cm-' region and ZnSe where the subscript 2 refers to a wavelength and the subscript windows for the 750 cm-' region. A medium band-pass s refers to a solution. The concentrations [NCS-] and multidielectric filter was placed between the cell and the IR [MNCS] were obtained at each total salt concentration by source to minimize heat gain in the sample. The cells were solving the equations of mass-balance with the calculated thermostatted to fO.l "C by means of water circulating value of the equilibrium constant. The equilibrium constant through the cell jacket.Spectra of the solutes were obtained calculation was deemed to have failed if the calculated by digital subtraction of a solvent spectrum, obtained at the spectra contained unacceptable features such as excessive relevant temperature, from a spectrum of the solution. The asymmetry or negative values. The free thiocyanate ion has solvent and solution spectra were recorded using the same vibrations at cu. 2055 and 735 cm-'. Ion-pair vibrations cell, with air as reference. The salt concentrations used are occur at higher wavenumbers with the exception of the given in Table 1. species Sr(NCS), and Ba(NCS), whose v(C-N) vibrations Equilibrium constant calculations were attempted by occur in the region 2030-2035 cm-'. means of the new program HYPERQUAD.2 The calcu- Resultslations were based on absorbance data at about 12 selected wavelengths along the whole spectrum.Effect of Changing Temperature Where possible AH was calculated by using the expression As the temperature is increased the v(C-N) band due to the In K x -AH/RT with the assumption that AH is indepen- ion pairs shifts to lower wavenumber, as illustrated in Fig. 1dent of temperature: the slope of the least-squares line of for LiNCS in dimethylformamide (DMF). In all our previous In K as a function of 1/T was taken as equal to -AH/R. work a shift of peak position has been taken to indicate the Molar absorbances at each wavelength, cNa, A and cMNCS,A, presence of bands, due to two or more species, whose inten- were calculated, by the method of least-squares, from the sity varies with changes in equilibrium composition; when the bands are close together this results in an apparent move- t Part 25: ref.1. ment of the peak. In this work the presence of many species Table 1 Concentrations (mmol dm-3) of solutions examined ~ ~~ 5 10 25 100 300 ~~ ~ 25 50 100 200 400 800 25 75 350 700 lo00 30 75 150 400 500 lo00 25 50 75 100 150 200 25 50 100 200 500 lo00 18.5 40 50 100 150 300 240 480 240 120 240 240 75 5 10 15 20 25 30 50 60 90 120 21 43 85 170 395 6 12 37.5 75 180 360 130 160 320 400 25 30 35 40 45 50 37.5 75 in equilibrium seemed unlikely. We will therefore first estab- lish that the shifts in peak position with temperature are not due to changes in chemical equilibria. The argument is a long one and has three parts.It is necessary to demonstrate that the phenomenon is independent of both the cation and of the solvent and to provide a plausible mechanism for it. The shift of the ion-pair v(C-N) band to lower wavenum- ber with increasing temperature is shown by the following ion pairs: LiNCS, NaNCS, KNCS, CsNCS, [MgNCS]', [CaNCS] +,[SrNCS] +,[BaNCS]', Sr(NCS), and Ba(NCS), but is not observed in spectra of solutions containing only the free thiocyanate ion such as solutions of R4N NCS (R = CH,, C,H,, C4H,). When bands due to both an ion pair and free thiocyanate can be seen separately resolved in the spectra (e.g.[MgNCS]' and NCS- in DMF) only the band due to the ion pairs shifts with temperature. This establishes the fact that the phenomenon is independent of cation. A comparable temperature shift was found with solutions in acetonitrile (AN), DMF and tetrahydrofuran (THF). This shows that the phenomenon is independent of solvent. Since the temperature shift is independent of both cation and solvent and is observed only in bands assigned to ion pairs, it must be an intrinsic property of molecules containing the coordinated thiocyanate ion. We propose that the mechanism underlying the tem-perature shift involves the population of a low-frequency vibrational state of the ion pair [(solvent),M(NCS),]"' , giving rise to hot bands as previously identified in the cyanate ion: in that case for those molecules which are in the excited state where the low-frequency 6(NCO) vibration is populated the v(C-N) vibration occurs at lower wavenum- ber than that of the molecules in the ground state.3 The ratio of the number of molecules in an excited state, with quantum number n, to the number in the ground state (n= 0) is given by eqn.(2h4 N -= exp(-nV/0.6925T) (2)No where V is the wavenumber cm- and T is the temperature in K. It follows that the number of molecules in each excited state, expressed as a fraction of the total number of mol- ecules, is given by the expression (3). 1 -exp(-iJ/0,6925T) exp( -niJ/0.6925T);'= exp(-V/0.6925T) n = 0, 1, 2, ... (3) For the free thiocyanate ion the G(NCS) vibration at ca. 475 cm-' has, at 293 K, about 92% of the molecules in the ground state, (n = 0), giving rise to an absorption band at CQ.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2055 cm-' and 7% in the first excited state (n = l), giving rise to the hot band at ca. 2040 cm-': the hot band appears at lower wavenumber because of the effects of anharmonicity. As the temperature is raised, so the relative population of the excited state increases, as demonstrated in Fig. 2 by the increase in intensity at ca. 2040 cm-'. It may also be noted that the fundamental band broadens slightly with increasing temperature, as shown by the increasing intensity at ca. 2070 cm-': its maximum intensity decreases as a result of three effects; the broadening of the band, the reduced ground-state population and the thermal expansion of the solution.Now, if we postulate that the ion pair has a very low fre- quency vibration then many excited states are populated, as illustrated for V = 100 cm-l, in Fig. 3. Because of the effects of anharmonicity the first hot band is at lower ,wavenumber than the fundamental in the free thiocyanate ion. So, if a very low frequency vibrational state is populated there may be a progression of hot bands to ever lower wavenumbers. The calculated molar absorbances of LiNCS at 288 K and 338 K are shown in Fig. 4. The progression in the hot bands may be seen as resulting in an asymmetric band with a long tail on the low-frequency side.There are a number of possible candidates for the low- frequency vibration(s) that are responsible for the progression in hot bands. The Li-N stretching vibration has been found' to lie at 390 cm-', which is too high to account for the phenomenon in LiNCS, but with other metals the M-N 1.o 0.8 a) 6 0.6 f! 0.4 0.2 2080 2070 2060 2050 2040 2030 2020 wave n urnber/c m -' Fig. 2 IR spectra in the v(C-N) stretching region of a solution of (C2H5),N NCS in DMF (0.24 mol dm-') at temperatures between 278 and 348 K in steps of 10 K. The arrows indicate increasing tem- perature. h $ 30-v > C $ 20-0 ' 0 280 300 O 320 340 360I I 1 I 2090 2080 2070 2060 2050 2040 2030 wavenumber/cm-' TIK Fig. 1 IR spectra in the v(C-N) stretching region of a solution of Fig.3 Occupancy (Yo)of vibrational levels with quantum number n LiNCS in DMF (0.8 mol dm-3) at temperatures between 278 and of a vibration with fundamental wavenumber of 100 cm-'. (0) 348 K in steps of 10 K. The arrows indicate increasing temperature. n = O;(A)n= l;(+)n= 2;( x)n= 3;(0)n = 4. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 wavenumber/crn-' Fig. 4 Molar absorbances calculated from spectra of solutions of LiNCS in DMF. (-) NCS-at 288 K; (-.-.-) NCS-at 338 K; (---) LiNCS at 288 K; (-. ...) LiNCS at 338 K. stretching frequency is much lower (estimates vary in the range 100-200 cm-'). The most likely vibration in the (solvent),LiNCS case is the M-(NCS) wagging mode, S(MNC). This has been found in the region 98-105 cm-' in a number of octahedral thiocyanato complexes6 though a normal coordinate analysis performed on [Zn(NCS)J2 -led to the suggestion that G(MNC) is thoroughly mixed with S(NMN) which occurs at lower frequencies.' Excitation of either G(MNC) and/or v(M-N) may be expected to weaken the metal-thiocyanate interaction and therefore give rise to absorption maxima at lower wavenumbers, an effect to be added to the effects of anharmonicity.We have searched the region from ca. 2800 cm-' to ca. 1600 cm-' for both sum and difference bands of v(C-N) with the low-frequency vibration, without success, so that the actual value(s) remains unknown. The intensity of the transition u, -+u,+ will be propor- tional to the population of the u, level and will show the same tendencies as the populations illustrated in Fig.3. The effect of increasing temperature is to reduce the population of the ground state whilst the populations of the excited states increase. The decrease in ground-state population is more rapid than the increase of any single excited state. Thus, assuming that the fundamental and hot bands are extensively overlapped, the spectrum will show a decrease in intensity at the high-frequency side of the absorption band, and a general increase in intensity at the positions of all the hot bands. This will result in a band which appears to be shifted to lower frequency, is broader and has a more pronounced tail on the low-frequency side.The calculated spectrum at 338 K shown in Fig. 4 displays precisely these characteristics. Note, the small change in intensity at ca. 2040 cm-' is due to the pro- portion of the hot band of the free thiocyanate ion increasing with temperature. Having established that band shifts with temperature are not due to changes in equilibria it becomes possible to analyse the effect of changing temperature on the ion-pairing equilibria. In other words, the band intensities can be used to estimate equilibrium constants notwithstanding the fact that the peak positions change with temperature. DMF Solutions Lithium Thiocyanate There are only two species in equilibrium, the free ion and the contact ion pair. The stability constants for the 1 : 1 ion Table 2 Stability constants for LiNCS in DMF at various tem- peratures 278 0.94 (6) 288 0.97 (6) 298 0.93 (6) 308 0.94 (6) 316 0.79 (8) 328 0.80 (9) 338 0.70 (9) pair are given in Table 2 and calculated molar absorbances are shown in Fig.4.Log /3 appears to decrease slightly with temperature which indicates that the formation of this ion pair is weakly exothermic: AH = -7 & 2 kJ mol-'. In the v(C-S) region there is a band due to DMF coordinated to Li which overlies the coordinated thiocyanate band, making + the latter difficult to observe. Sodium Thiocyanate Representative spectra obtained in the region of the v(C-N) bands are shown in Fig. 5. There is extensive overlap between the bands due to the contact ion pair and the free ion.Because of this, the spectrum shows a single asymmetric peak which appears to narrow and move to higher wavenumber with increasing temperature, a trend which is difficult to interpret unambiguously. In the v(C-S) region, Fig. 6, the band at 765 cm- ',which has been assigned to the N-bonded ion pair NaNCS,* shows evidence of being a multiplet and although the peak 1.4 1.2 $ 1.0 C 0.8' 0.60, 0.4 0.2 I 1 2090 2080 2070 2060 2050 2040 2030 wavenurnber/crn-' Fig. 5 IR spectra in the v(C-N) stretching region of a solution of NaNCS in DMF (0.4 mol dm-3) at temperatures between 278 and 348 K in steps of 10 K. The arrows indicate increasing temperature. 1.2 Q) g 1.0 (0 0.84 0.6 0.4 0.2 I 800 790 780 770 760 750 740 730 720 710 wavenurnber/cm -' Fig.6 IR spectra in the v(C-S) stretching region of a solution of NaNCS in DMF (2.0 mol dm-3) at temperatures between 278 and 338 K in steps of 10 K. The arrows indicate increasing temperature. maximum does not shift with temperature the band shape changes. At the same time the band due to the free ion moves to slightly lower wavenumber, and broadens considerably. It is noteworthy that, in contrast with the v(C-N) region, there is a general decrease in intensity with increasing temperature. Potassium Thiocyanate Representative spectra in the v(C-S) region are shown in Fig. 7. The potassium and sodium thiocyanate systems are qualitatively similar with the intensities suggesting that pot- assium forms a weaker ion-pair than does sodium.Caesium T hiocyanate There is evidence for ion pairing, but the bands due to the two species are extensively overlapped. Magnesium Thiocyanate Representative spectra are shown in Fig. 8. This salt has limited solubility in DMF but nevertheless an approximate stability constant of log fi = 1.7 at 293 K was obtained. The intensity of the ion-pair band increases with temperature showing that the formation of this species is endothermic. Calcium Thiocyanate Representative spectra are shown in Fig. 9. Log fi for this ion pair is ca. 0.9. The small decrease in maximum intensity of the ion-pair band, taken together with the fact that the inten- sity of the free ion band usually decreases indicates that the ion-pairing process is approximately athermic.".s 1.2'11 .o 0.8' I1 4 0.6 0.4 o.20.0I- I 760 750 740 730 720 710 700 wavenumber/crn-' Fig. 7 IR spectra in the v(C-S) stretching region of a solution of KNCS in DMF (1.0 mol dm-3) at temperatures between 278 and 338 K in steps of 10 K. The arrows indicate increasing temperature. I I 2120 2100 2080 2060 2040 2020 waven umber/crn -' Fig. 8 IR spectra in the v(C-N) stretching region of a solution of Mg(NCS), in DMF (0.09 mol dm-3) at temperatures between 298 and 348 K in steps of 10 K. The arrows indicate increasing tem- perature. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1.4 1.21 Q, 1 .o 0.8e2 0.6 rn 0.4 o.2I 2090 2080 2070 2060 2050 2040 2030 waven urn ber/cm -' Fig.9 IR spectra in the v(C-N) stretching region of a solution of Ca(NCS), in DMF (0.395 mol dm-3) at temperatures between 278 and 338 K in steps of 10 K. The arrows indicate increasing tem- perature. Strontium Thiocyanate and Barium Thiocyanate The spectra of solutions of these salts are qualitatively similar to those of Ca(NCS), but with more extensive overlap between the free ion and ion-pair bands. AN Solutions Lithium Thiocyanate We have reported previously details concerning solutions of LiNCS in AN at 293 K.' We postulated the existence of both inner-and outer-sphere ion pairs. Representative spectra showing the effects of changing temperature are given in Fig.10. Since the band due to the inner-sphere ion pair decreases in intensity whilst the composite band representing the free ion and the outer-sphere ion pair remains approximately constant we conclude that the ion-pairing process is exother- mic, and that the proportion of inner- and outer-sphere coni- plexes remains constant. Unfortunately all attempts to compute the equilibrium constants using HYPERQUAD failed. Sodium Thiocyanate Representative spectra are shown in Fig. 11. Extensive band overlap precludes an unambiguous interpretation. Potassium Thiocyanate The spectra of solutions of this salt are similar to those of sodium thiocyanate solutions, but band overlap is more extensive. 0.181 0.161 Q, 0.14 0.12 e 0.10 0.08 g.06 0.04 0.02 2100 2090 2080 2070 2060 2050 2040 wavenumber/crn-' Fig.10 1R spectra in the v(C-N) stretching region of a solution of LiNCS in AN (0.010 mol dm-3) at temperatures between 288 and 338 K in steps of 10 K. The arrows indicate increasing temperature. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 0.35 r A/ I 0.30 0.25 0.20 0.15 0.10 0.05 2090 2080 2070 2060 2050 2040 2030 wavenumber/crn-Fig. 11 IR spectra in the v(C-N) stretching region of a solution of NaNCS in acetonitrile (0.025 mol dm-3) at temperatures between 288 and 338 K in steps of 10 K. The arrows indicate increasing tem- perature. Strontium Thiocyanate Representative spectra are shown in Fig. 12. The equilibria appear to be as follows : [Sr2+] + 2NCS-GSr(NCS), [Sr2'] + 2NCS-e([SrNCS'lNCS-) (1) The formula {[SrNCS'INCS-} represents a 1 : 1 outer-sphere ion pair between [SrNCS] and NCS- ; it could also + be called an inner-and-outer-sphere complex.In these spectra there is no evidence for the presence of free thiocyanate. This implies that log is so large that the free thiocyanate con- centration is undetectable, i.e. log PI > ca. 5. The calculated equilibrium constants for the reaction (11) are given in Table 3. [SrNCS'] + NCS-eSr(NCS), + {[SrNCS']NCS-} (11) The decrease with temperature appears to be within the experimental errors, but if this fact is ignored the value of AH z -3.6 f0.5 kJ mol-' may be obtained, showing that the reaction may be weakly exothermic. Calculated molar absorbances are shown in Fig.13. The two bands belonging 1.4 1.2 $ 1.0 C 0.8 02 0.6 m 0.4 0.2 2080 2070 2060 2050 2040 2030 2020 wavenurnber/crn-' Fig. 12 IR spectra in the v(C-N) stretching region of a solution of Sr(NCS), in AN (0.36 mol dm-3) at temperatures between 278 and 328 K in steps of 10 K. The arrows indicate increasing temperature. Table 3 Stepwise formation constants for the formation of Sr(NCS), from [SrNCS]+ and NCS- in AN at various temperatures T/K log K 278 0.88 (6) 288 0.85 (6) 298 0.84 (6) 308 0.81 (6) 319 0 21 40 2100 2060 2020 ' waven u rnber/crn-Fig. 13 Molar absorbances calculated from spectra of solutions of Sr(NCS), in acetonitrile. (-) {[SrNCS'INCS-1 (ca.2065 cm-') and Sr(NCS), (ca. 2035 cm-'); (---) [SrNCS]+. to the species of 1 : 2 stoichiometry are well separated; the inner-and-outer-sphere complex band is on the high wave- number side of the 1 : 1 ion-pair band, whilst the inner-sphere complex has a band at much lower wavenumber. It should be noted that both bands shift with temperature. Barium Thiocyanate The spectra shown in Fig. 14 resemble those of the strontium system. Log K z 0.7 at 298 K. This reaction is approximately athermic. THF Solutions We have confirmed the temperature shift of LiNCS in THF illustrated in Fig. 1 of the paper by Goralski and Chabanel." Discussion For the most part we agree with the assignments of the vibra- tional bands given in the literature, except that a triple ion has been suggested for magnesium and calcium thiocyanates in DMF," for which we find no evidence.No data for stron- tium and barium thiocyanates in AN have been previously published. The strontium thiocyanate-AN system has provided the first instance in which the spectrum of an outer-sphere complex has been resolved from the experimental data. Nor- mally the band due to an outer-sphere complex is very close to the band due to the free ion, and its presence has been I 2080 2070 2060 2050 2040 2030 2020 waven u mber/crn-' Fig. 14 IR spectra of the v(C-N) stretching regon of a solution of Ba(NCS), in AN (0.045 mol dm-3) at temperatures between 278 and 338 K in steps of 10 K. The arrows indicate increasing temperature.3 20 inferred from the asymmetry of the second derivative of the latter.12 In this case the bands are 'resolved' by means of an equilibrium constant calculation. It can be seen in Fig. 13 that the outer-sphere complex of [SrNCS]+ has a band at slightly higher wavenumber than that of the [SrNCS] + ion itself: the separation is approximately one sixth of the half- width which means that the two bands could not be separat- ed either by curve resolution' or numerical differentiati~n.'~ It is also interesting to note that [SrNCS]+ forms both inner- and outer-sphere complexes with thiocyanate in AN that appear to have comparable stability in so far as they are both present in equilibrium with the core ion. This behaviour is an exact parallel to what we have proposed, on the basis of less direct evidence, for the Li + ~ation.~ The enthalpy of the ion-pairing interaction can, in prin- ciple, be calculated from the variation of the equilibrium con- stant with temperature.However it is well known that this method requires equilibrium constants to be known with great precision and we have not yet been able to achieve a precision high enough to enable enthalpies to be calculated in all cases. Nevertheless, ion-pairing processes can be classed as exothermic, athermic or endothermic on the basis of the qualitative changes in spectra with changes of temperature. A summary of the available information is shown in Table 4. The formation of a contact ion pair is a complex process involving the desolvation of both cation and anion as well as the formation of a cation-anion link and reorganization of the solvent.It must therefore be seen as fortuitous and so many of the reactions studied have proved to be athermic, endothermic or, at best, weakly ex other mi^.'^ However, if ion-pairing is athermic or endothermic (or weakly exothermic with a large equilibrium constant) it follows that the entropy change that accompanies the process must be positive, that is, Table 4 Thermicity of ion-pair information in DMF and AN ion-pair DMF AN LiNCS exothermic exothermic MgNCS endothermic -[CaNCS] athermic -+ Sr(NCS), -athermic" Ba(NCS) -athermic Both inner- and outer-sphere complexes are formed. J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 that there must be an overall increase in disorder in the solu- tion. This may appear to be paradoxical as the formation of the ion pair is accompanied by a loss of some of the trans- lational entropy of the separated ions. The paradox is re- solved by noting that the entropy change is determined by the same set of factors as the enthalpy change. In addition, when two solvated ions come together to form a contact ion pair one or more solvent molecules will presumably be liberated to the bulk solvent with a concomitant gain of translational entropy. It may be noted that the electrostatic potential of the monopolar ions in solution operates over a much longer range than that of the dipolar ion pairs.The increase in dis- order can be interpreted as due to a loss of local order around the cations and anions. Whether this amounts to evidence for a pseudo-lattice structure in solutions of fully dissociated salts may well be a matter for debate. References 1 P. Gans, J. B. Gill and L. H. Johnson, J. Chem. SOC., Dalton Trans., 1993, 345. 2 A. Sabatini, A. Vacca and P. Gans, Coord. Chem. Rev., 1992, 120, 389. 3 J. Rannou and M. Chabanel, J. Chim. Phys., 1980,77,201. 4 P. Gans, Vibrating Molecules, Chapman and Hall, London, 1971. 5 D. Paoli, M. Luqon and M. Chabanel, Spectrochim. Acta, Part A, 1978,34, 1087. 6 R. J. H. Clark and A. D. J. Goodwin, Spectrochim. Acta, Part A, 1970,26, 323. 7 D. Forster and W. D. Horrocks, lnorg. Chem., 1967,6,339. 8 D. Paoli, M. Luqon and M. Chabanel, Spectrochim. Acta, Part A, 1978,34,1087. 9 P. Gans, J. B. Gill and P. J. Longdon, J. Chem. SOC.,Faraday Trans., 1989,85, 1835. 10 P. Goralski and M. Chabanel, Znorg. Chem., 1987,26,2169. 11 I. S. Perelygin, V. S. Osipov and S. I. Gryaznoz, Russ. J. Phys. Chem., 1985,59, 1462. 12 P. Gans, J. B. Gill and J. N. Towning, Z. Phys. Chem., 1982, 133, 159. 13 P. Gans and J. B. Gill, Anal. Chem., 1980,52,351. 14 P. Gans and J. B. Gill, Appl. Specrrosc., 1983,37, 515. 15 D. D. K. Chingakule, P. Gans and J. B. Gill, Monatsh. Chem., 1992,123,521. Paper 3/05194C; Received 31st August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000315

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 321-325 32 1 Electrochemical Synthesis of Soluble Poly(9-hexylfluorene) and Poly(1-hexylindene) Junzo Matsuda, Kunitsugu Aramaki and Hiroshi Nishihara" Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223, Japan Electrochemical oxidation of 9-hexylfluorene and 1-hexylindene in 0.1 mol dm-, Bu,NBF,-nitromethane affords n-conjugated polymers, PHF and PHI, respectively, which are fusible and soluble in common organic solvents. Electrode materials affect the formation of the polymers significantly; the molecular weight of PHF increases in the order, glassy carbon (GC) < indium-tin oxide (ITO) < SnO,, and that for PHI, SnO, % GC < ITO. 'H NMR and IR spectra of the polymers indicate that PHF and PHI comprise mainly linkage of 1,4-fluorenylene and 2,4-indenylene units, respectively.PHF undergoes a reversible oxidation reaction in acetonitrile, whereas electro- chemical oxidation of PHI is irreversible. The electrical conductivities of PHF and PHI doped with SO, are and S cm-', respectively, at room temperature. Recent advances in the processing of linearly n-conjugated conducting polymers have enlarged their application to areas such as composite materials, devices, sensors (for some recent reviews, see ref. 1). One method to improve the solubility of these polymers in common organic solvents is by the intro- duction of long alkyl chains to the polymer backbones., We have recently reported an electrochemical method to prepare soluble p~ly(n-alkylphenylene)s,~and their utilization as n-acid ligands for the synthesis of n-conjugated organometallic polymer^.^.' Our greatest interest in such n-conjugated organometallic polymers centres around the modification of the physical and chemical properties of the n-conjugated systems by electronic interactions with d elec- trons and/or vacant d orbitals of the transition metal centres.The present study deals with soluble polymers of 9-hexylfluorene and 1-hexylindene, PHF and PHI, respectively, which could act as either 9'-or q6-n-coordinating ligands,6 and might also be of use in homogeneous synthetic reactions. Poly(9-alkylfluorene) and poly(9,9-dialkylfluorene) have been prepared by a chemical method using FeC1,,7 and their properties such as electroluminescence has been studied by Yoshino and co-workers.8 We here employ an electrochemical method for synthesis of PHF and PHI, whose structures are then determined based on FTIR and 'H NMR spectra.The redox and electrical properties of the polymers are also reported. Experimental Chemicals and Equipment Anhydrous solvents were obtained from Kanto Chemicals Co., Inc. Other reagents were guaranteed reagent-grade chemicals and used as received. IT0 and Sn0,-coated glasses with resistance <30 R were purchased from Nippon Sheet Glass Co. Ltd. Tokai Carbon GC-20 glassy carbon plates were used as electrodes. Bulk electrolysis was carried out with a Toho Technical Research 2001 potentiostat/ galvanostat and a 3320 coulometer.Infrared, UV-VIS and 'H NMR spectra were recorded with Shimadzu FTIR8100M, MPS-2000 and JEOL GX400 spectrometers, respectively. The molecular weights of the polymers were determined by gel permeation chromatography (GPC) with a Shimadzu LC-5A system based on polystyrene standards. Spin coating was carried out with a Kyowa K-359s-1 spinner, and the film thickness was measured with a Tokyo Seimitsu Surfcom 900A surface profilometer. Cyclic voltammetry was carried out in a standard one-compartment cell equipped with a Pt wire counter-electrode and an Ag/Ag+ [lo mmol dm-, AgClO, in 0.1 mol dm-, Bu,NBF,-MeCN, Eo (ferrocenium/ ferrocene) = 0.197 V us. Ag/Ag+] reference electrode with a Toho Technical Research PS-07 polarization unit and a Riken Denshi F-35 X-Y plotter.Spectroelectrochemical mea- surements were carried out in the same manner as has been reported previo~sly.~.~ The electrical conductivity of the polymer films was measured by the two-probe method under vacuum using a Toho Technical Research 2020 potentlostat/ galvanos tat. Preparation of 9-Hexylfluorene and 1-Hexylindene Introduction of hexyl groups to fluorene and indene was carried out by deprotonation with Bu"Li in THF followed by reaction with hexyl bromide according to the method reported by Cedheim and Eberson," and the product was purified by distillation under reduced pressure. 9-Hexylfluorene, v,,,/cm -: 3065s, 304Om, 301 7m, 2928s, 2857s, 1913w, 1908w, 1802w, 1607w, 1582w, 1478m, 1449s, 1377w, 1321w, 1296w, 1102w, 1030w, 936w, 739s, 662m, 621m, 426w and 415w (neat); 6, (CD,Cl,) 7.7-7.1 [8 H, m, H(1)-(8) of ring], 3.89 [l H, t, JHH 5.9 Hz, H(9) of ring], 1.96 (2 H, m, ring-CH,), 1.16 (8 H, m, CH,C,H,CH3), 0.76 (3 H, t, JHH = 5.7 Hz, CH3).1-Hexylindene, v,,Jcm- ': 3065m, 2928s, 2857s, 1609w, 1460s, 1449s, 1377m, 1364m, 1069w, 1019m, 936m, 774s, 741m, 712m, 436w and 405w (neat); 6, (CDCl,) 7.2-6.9 [4 H, m, H(4)-(7) of ring], 6-61 [1 H, dd, HH(I)H(3) Hz, JH(2)H(3) 5.7 Hz, H(3) of ring], 6.36 [1 H, dd, JH(,)H(,) 1.8 Hz, H(2) of ring], 3.27 (2 H, m, ring-CH,), 1.12 (8 H, m, CH2C4H8CH3), 0.69 (3 H, t, JHH = 5.8 Hz, CH3). Preparation of Soluble PHF and PHI A typical procedure for the synthesis of PHF is as follows.Controlled potential electrolysis of 2.7 mmol 9-hexylfluorene was carried out at an Sn0,-coated glass electrode (electrode area: 5.60 cm') in 0.1 mol dmP3 Bu,NBF4-nitromethane (150 cm3) at 2.5 V us. Ag/Ag+ for 30 h (1378 C of electricity were passed) in a standard H-type two-compartment cell, fol- lowed by electrochemical dedoping at -1.0 V until the cathodic current decreased to the background level. The elec- trode surface was covered with some fragile film, but most of the products were dissolved in solution. The contents of the cell were collected using dichloromethane and the solution thus obtained was concentrated into a volume of 10 cm3. Addition of a hydrazine-MeOH mixture to this solution yielded dark-brown precipitates, which were collected by fil- tration and washed with distilled water and MeOH.After drying, the solid products were extracted with dichloro-methane for 2 days. Solvents were evaporated from the extract to give a dark-brown solid, PHF, in a yield of 0.81 g (23% based on the current passed). Elemental analysis: (Found: C, 90.75; H, 7.64%. C19,,HZOn requires C, 91.95: H, 8.05%). As for PHI, similar procedures were used to obtain soluble products. Elemental analysis of PHI prepared at ITO: (Found: C, 87.09; H, 8.85, N, 1.52%. ClSnHIBnrequires C, 90.92; H, 9.08%). Results and Discussion Electrochemical Synthesis of PHI and PHF We have employed Bu,NBF,-nitromethane as the electrolyte solution for the polymerization of 9-hexylfluorene and 1-hexylindene, since solvents with low donor numbers have been known to be effective for the electro-oxidative poly- merization of benzene and its derivative^.^.' Cyclic voltam- mograms of hexylfluorene at GC, SnO, and ITO, fluorene at IT0 and hexylindene at IT0 are displayed in Fig.1. In the first cycle, the anodic current for the oxidation of hexylfluo- rene increases from ca. 1.2 V us. Ag/Ag+ at every electrode, and its magnitude at GC is larger than that at SnO, or ITO. During the backward scan, a cathodic wave appears at 0.8 V, and the corresponding anodic wave is seen at 1.2 V in the second cycle. This suggests the formation of an electroactive polymer," whereas growth of these waves with the number of cyclic scans is considerably less significant than that for unsubstituted fluorene [cf.Fig. l(c) and (41. This is due to the higher solubility of the hexyl-substituted polyfluorene than the unsubstituted one. It has been observed visually that the dark-brown products formed from hexylfluorene at the electrode surface diffuse into the solution. As shown in Fig. l(e), oxidation of hexylindene takes place at a potential cu. 1.5 V us. Ag/Ag+ more positive than that of hexylfluorene. Coloration of the solution around the elec- trode is also observed, as in the case of hexylfluorene, but no redox waves appear in the cyclic voltammograms in the fol- lowing potential scans. This indicates that the electrochemi- cally formed polymer, PHI, in the doped (oxidized) form, is electrochemically inactive in the potential range, 0-2.5 V.Controlled-potential electrolysis of hexylfluorene and hexy- lindene was carried out for the bulk syntheses of PHF and PHI, respectively. The concentration of the monomers was 0.2 mol dm-3 and the applied potential was 2.5 V us. Ag/Ag+. Soluble components in the products were dedoped electrochemically and then chemically with hydrazine, and extracted with dichloromethane as described in the Experi- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1 I I I I I 0 0.5 1.0 1.5 2.0 2.5 EN vs. Ag/Ag + Fig. 1 Cyclic voltammograms of 9-hexylfluorene at GC (a), SnO, (b) and IT0 (c), fluorene at IT0 (6)and l-hexylindene at IT0 (e) in 0.1 mol dm-3 Bu,NBF,-nitromethane at 0.1 V s-'.S = 5, 2, 2, 5 and 1 mA for (a), (b), (c), (d) and (e), respectively. Numbers in the figure refer to those of the cyclic scans. Electrode area, 1.4 cm2. mental. Both the PHF and PHI thus obtained are soluble in organic solvents of low polarity such as chloroform, benzene or tetrahydrofuran, but insoluble in polar solvents such as acetonitrile or propylene carbonate. Results of the synthesis for the three kinds of electrode materials are given in Table 1. In the electrolysis of hexylfluorene, the initial current was high but the current decay with time was rapid at GC, resulting in a smaller total amount of electricity passed during the electrolysis compared with that for IT0 and SnO, . The electrode material also influences the physical properties of the soluble products: the average molecular weight estimated based on polystyrene standards and the Table 1 Dependence of yield and physical properties of PHF and PHI on the electrochemical conditions" form at polymer electrode electricity/(= yield (%)b room temperature Mw PHF GC 1210 14 solid 1250 PHF SnO, 1330 13 solid 2730 PHF IT0 1380 23 solid 2540 PHI GC 1220 13 solid 2680 PHI PHI IROSnO, 1340 1440 20 3 solidliquid 4560 < lo00 ~~ Electrolysis was carried out at a 5.6 cmz area plate electrode in 0.1 mol dmV3 Bu,NBF,-nitromethane monomer at 2.5 V us.Ag/Ag+ for 30 h. Based on electricity passed during electrolysis. mp/"C E,,IeV E,zIeV 130 3.31 3.26 160 2.97 2.85 160 3.06 2.83 120 3.06 - - - - 160 2.99 - ~~ (150 cm-3) containing 2.7 mmol of the J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 melting point are higher for the products at SnO, and IT0 than those at GC. The effects of the electrode materials on the preparation and properties of soluble PHI are different from those for PHF. The soluble product obtained at SnO, is oily and of low molecular weight, whereas the product at IT0 is a film- processable solid with a molecular weight of 4560. The glassy carbon electrode affords a solid product with a lower molecu- lar weight and lower melting point than that obtained at ITO. Such significant effects of the electrode materials on the electrosynthesis of n-conjugated polymers have been reported for several cases, but the reasons have not yet been wholly clarified.12*13 One likely rationale might be that the regiosel- ectivity of the polymerization (coupling) reaction depends on the electrode material.Even a slight formation of non-conjugated products in the coupling reaction would decrease the conductivity of the product seriously, resulting in termi- nation of the polymerization. We speculate that the differ- ences in the effects of electrode materials for PHF and PHI are related to the discrepancies in their structures, since both the effect of electrode material and the structure for PHF are similar to those for poly(hexylpheny1ene) but different from those for PHI (vide infr~).~ Characterization of PHF and PHI The chemical structures of PHF and PHI were characterized by their infrared and 'H NMR spectra, which are shown in Fig. 2 and 3.Absorption peaks due to the out-of-plane CH deformation appear at 818, 768 and 737 cm-I in the infrared spectrum of PHF. This indicates the existence of benzene rings with four adjacent and two adjacent free hydrogen atom^,'^ and thus connection of the fluorene rings at the 1,4- positions in the polymerization, as shown in Scheme 1, can be elucidated. This structure is inconsistent with that of poly(9-alkylfluorene) reported by Fukuda et aE., where fluor- ene moieties are connected at the 2,7-positions.' This might be attributed to the different oxidation method used for the alkylfluorene. The chemical oxidation with FeCl, employed by Fukuda et al.may proceed by way of charge-transfer com- I I I I I I0 2000 1500 1000 50( wavenumber/cm-I Fig. 2 Infrared spectra of PHI (a)and PHF (b) 323 87654321 I I I I I I I 1 87654321 6 Fig. 3 'H NMR spectra of PHF (a) and PHI (b)in CD,CI, plexes with the aromatic rings, leading to a different type of monomer coupling from that of the electrochemical oxidation carried out in this study. The 'H NMR spectrum of PHF in Fig. 3(a) shows a peak due to the proton in the five-membered ring at S = 3.9-4.2 ppm, but gives insufficient information on the linkage structure of the fluorenylene moi- eties because of the broadness of the peaks. The infrared spectrum of PHI shows a broad and strong band around 740 cm- ' due to the out-of-plane CH deforma- tion, indicative of benzene rings with three or four adjacent free hydrogen atoms.The former case denotes that the coup- ling of the indene proceeds at either the 4-or 7-position in the six-membered ring and a position in the five-membered ring. The latter indicates coupling at two positions in the five- membered ring. In the 'H NMR spectrum of PHI, peaks due to the proton at the 3-position around 6.8 ppm exist but those due to the proton at the 2-position around 6.5 ppm disappear.' This indicates that the coupling occurs at the 2-position, and thus the possible positions for polymerization are limited to the 2,4- or 2,7-positions. Between these two possibilities, only poly(2,4-indenylene) is n-conjugated.As the polymer formed is semiconducting as described below, it is concluded that poly(2,4-indenylene) is the most likely struc- ture from the spectroscopic data. + 2.5 V W. AglA# c Bu4NBF4-nitromethane PHF Bu4NBF4-n#ranethane+ 2.5 V vs. Ag/Ag+ -6 C6H13 C6H13 PHI Scheme 1 324 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1.01 240 300 400 500 600 700 wavelength/nm Fig. 4 UV-VIS absorption spectra of the polymers prepared at ITO: (a) PHF in CH,Cl, (9.8 mg drn-,), (b) PHF coated on quartz, (c) PHI in CH,Cl, (6.1 mg drn-,) and (d)PHI coated on quartz Physical Properties The UV-VIS spectra of PHF and PHI prepared at ITO, dis- solved in dichloromethane and in the film form, are displayed in Fig.4. The peak positions of the films are similar to those of the solutions. The band gap values were estimated from the absorption edge using an equation for inorganic semicon- ductors,16 and the results for the polymers prepared at three electrodes are listed as E,, in Table 1. Note that the higher molecular weight polymer has lower band gap, probably owing to the increase in the conjugation length. The values obtained, 2.9-3.3 eV, are similar to those reported for poly(fluorene) derivatives by Fukuda et aL7 Cyclic voltammograms of PHF spin-coated on ITO-coated glass electrodes are shown in Fig. 5(a) where oxidation re- I I I I 0 0.5 1 .o 1.6 E/V vs. AglAg+ Fig. 5 Cyclic voltammograms of PHF (a) and PHI (b) spin-coatedon IT0 in 0.1 mol dm-3 Bu4NBF4-MeCN.The film thickness was 0.5 pm for PHF and 0.1 pm for PHI. Numbers indicate the scan rates in V. waveleng th/nm Fig. 6 Visible absorption spectra of PHF/ITO (film thickness 0.5 pm) at the potentials: (a) 0.2, (b) 0.8, (c) 1.0, (d)1.1, (e) 1.2 and (f) 1.4 V vs. Ag/Ag+ reduction waves appear around 1.0 V us. Ag/Ag+. Fig. 6 dis-plays changes in the visible spectrum of PHF with potential shift in the positive direction from 0.2 V us. Ag/Ag+. An increase in absorbance above 460 nm and a decrease at 400 nm owing to the formation of the polaronic state are seen at potentials more positive than 1.0 V, and the change saturates at ca. 1.2 V. The band gap energy was estimated from the isosbestic point appearing in the UV-VIS spectra with poten- 71 ** m O (b) m 0 m a 0 (d) 0 00 0 no O 0 no 0 0 0 I n 00 0 0 103KIT Fig.7 Arrhenius plots of electrical conductivity for undoped PHF (a),SO,-doped PHF(b), undoped PHI(c)SO,-doped PHI(d) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 tial shift and the results are listed as Eg2 in Table 1. The discrepancy between E,, and E,,, is within 0.23 eV, and is most likely derived from the considerable distribution in con- jugation length and structural disorder in PHF. The cyclic voltammogram of PHI in acetonitrile shows only an irreversible oxidation wave at 1.3 V us. Ag/Ag+ as displayed in Fig.5(b). In the course of this cyclic voltam- metry, the PHI film was not dissolved in acetonitrile nor detached from the electrode, and thus this electrochemical irreversibility should be caused by chemical changes in PHI. The irreversibility is in accordance with the cyclic voltam- mogram for the formation of PHI given in Fig. 1; i.e. the oxidized form is not electroactive in the potential region 0-1.6 V us. Ag/Agf. This difference between the electro- chemical behaviour of PHI and that of PHF or poly(he~y1phenylene)~might be related to the dissimilarity in the structure as described above. Note that the undoped PHI was obtained by electrochemical and chemical reduction of PHI in the doped form generated under electro-polymerization conditions. Since the electrochemical dedop- ing is almost ineffective as described above, reduction with hydrazine is the important process for making undoped PHI.The usefulness of the hydrazine treatment to give a perfectly undoped form of polyaniline has also been reported.17 Arrhenius plots of the electrical conductivity of PHF and PHI undoped and doped with SO, measured by the two- probe method under vacuum are shown in Fig. 7. The con- ductivity of the undoped form is S cm-' at 60°C, and increases greatly upon SO,-doping for both polymers. This indicates that PHI can be doped chemically, and therefore PHI should have a n-conjugated structure as noted above. Doping mehods for obtaining higher conductivities for PHI and PHF are currently under investigation.Conclusion Electrochemical oxidation of 9-hexylfluorene in Bu,NBF,-nitromethane gives poly(9-hexyl-1,4-fluorenylene), which is electroactive and soluble in common organic sol- vents. Electrochemical oxidation of 1-hexylindene affords soluble poly( l-hexyl-2,4-indenylene),of which the electro-chemical oxidation is irreversible. The physical properties of the polymers such as the molecular weight or the melting point depend markedly on the electrode materials used for the preparation. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan (no. 056406059), the Ogasawara Foundation for the Promotion of Science and Engineering, and the General Sekiyu Research and Development Encouragement and Assistance Foundation.References 1 Handbook of Conducting Polymers, ed. T. A. Skotheim, Marcel Dekker, New York, 1986; A. 0.Patil, A. J. Heeger and F. Wudl, Chem. Rev., 1988, 88, 183; J. L. Bredas and G. B. Street, Acc. Chem. Res., 1985, 18, 309; H. Kumany, M. Mehring and S. Roth, Electronic Properties of Conjugated Polymers, Springer, Heidelberg, 3rd edn, 1989; S. Roth, Synth. Met., 1989, 34, 617; A. J. Heeger, Faraday Discuss. Chem. SOC., 1989, 88, 203; S. Roth, H. Bleier and W. Pukacki, Faraday Discuss. Chem. SOC., 1989, 88, 223; A. G. MacDiarmid and A. Epstein, Faraday Discuss. Chem. SOC., 1989, 88, 317; A. J. Heeger, Synth. Met., 1993,57,347 1. 2 M. Sato, S. Tanaka and K. Kaeriyama, J.Chem. SOC., Chem. Commun., 1986, 873; R. L. Elsenbaumer, K. U. Jen and R. Oboode, Synth. Met., 1986, 15, 169; R. Sugimoto, S. Takeda, H. B. Gu and K. Yoshino, Chem. Express, 1986, 1, 635; M. R. Bryce, A. Chissel, P. Kathirgamanathan, D. Parker and N. R. M. Smith, J. Chem. SOC., Chem. Commun., 1987,466. 3 T. Shimura, H. Funaki, H. Nishihara, K. Aramaki, T. Ohsawa and K. Yoshino, Chem. Lett., 1992,457. 4 H. Hunaki, K. Aramaki and H. Nishihara, Chem. Lett., 1992, 2065. 5 H. Nishihara, H. Funaki, T. Shimura and K. Aramaki, Synth. Met., 1993,55,942. 6 W. E. Watts, in Comprehensive Organometallic Chemistry, ed. G. Wilkinson, F. G. A. Stone and E. W. Abel, Pergamon, Oxford, 1982, vol. 8, p. 1013. 7 M. Fukuda, K. Sawada and K. Yoshino, Jpn. J. Appl. Phys., 1989,28, L1433. 8 Y. Ohmori, M. Uchida, K. Muro and K. Yoshino, Jpn. J. Appl. Phys., 1991, 30,L1941; M. Uchida, Y. Phmori, C. Morishima and K. Yoshino, Synth. Met., 1993,57,4168. 9 H. Nishihara, M. Noguchi and K. Aramaki, Inorg. Chem., 1987, 26,2862. 10 L. Cedheim and L. Eberson, Synthesis, 1973, 159. 11 T. Ohsawa, H. Nishihara, K. Aramaki, S. Takeda and K. Yoshino, Polym. Commun., 1987,28, 628. 12 H. Nishihara, H. Harada, K. Ohashi and K. Aramaki, J. Chem. SOC.,Faraday Trans., 1991,87, 1187. 13 H. Nishihara, M. Tateishi, K. Aramaki, T. Ohsawa and 0. Kimura, Chem. Lett., 1987, 1064. 14 L. J. Bellamy, The Infrared Spectra of Complex Molecules, Chapman and Hall, London, 1975. 15 E. Pretsch, J. Seibl, W. Simon and T. Clerc, Tabellen zur Strukturaujklarung Organischer Verbindungen mit Spektroskopis- chen Methoden, Springer, Berlin, 1981. 16 I. Kudmar and T. Seidel, J. Appl. Phys., 1962,33, 771. 17 T. Ohsawa, H. Nishihara, K. Aramaki and K. Yoshino, Chem. Lett., 1991, 1707. Paper 3/04614A; Received 2nd August, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000321

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 327-332 Structural Studies on Paracyanogen and Paraisocyanogen Leonardus W. Jenneskens," Jan W. G. Mahyt and Edward J. Vlietstra Debye Institute, Department of Physical Organic Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands Simon J. Goede and Friedrich Bickelhaupt Scheikundig Laboratorium, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands Polymers derived from the isomeric C,N, monomers cyanogen (NCCN) and isocyanogen (CNCN) are investi- gated. Although both paracyanogen [poly(NCCN)] and paraisocyanogen [poly(CNCN)] consist of carbon and nitrogen in a close to 1 : 1 ratio, thermogravimetric (TG) and spectroscopic analyses (DRIFT, EPR, UV-VIS-NIR and XPS) reveal that their molecular structures are markedly different.Despite the occurrence of n-conjugation, no regular ladder structures are found. In line with the spectroscopic data, conductivity measurements show that pristine poly(CNCN) and poly(NCCN) are an insulator and a semiconductor, respectively. The theoretical prediction (extended Huckel bandstructure calculations) by Whangbo et ul.,' which was corroborated by valence effective Hamiltonian (VEH) bandstructure calcu- latioIw2 that paracyanogen [poly(NCCN)] with the regular ladder structures shown in Scheme 1 possesses metallic behaviour, has initiated a quest for this potentially intrinsic organic metal. However, despite the fact that poly(NCCN) was already prepared in 1815 by Gay Lussac by heat treat- ment of mercury(r1) cyanide, its intractable nature hampered its structural el~cidation.~ Only in the last decades has the synthesis of paracyanogen from different starting materials and its characterization received attenti~n.~ Nevertheless, there is still a paucity of experimental data and little is known about its structure and properties, especially with respect to its anticipated intrinsic metallic behaviour.In 1988 the synthesis of another C,N, isomer prepared by flash vacuum thermolysis (FVT, 773 K at lo-' Torr) of nor- bornadienone azine in 60% yield was rep~rted.~ Although initially the C2N2 isomer was identified as diisocyanogen (CNNC) since only one signal typical for is~cyanides'*~ was observed in its 13C and 14N NMR spectra at 173 K, the use of other precursors' and 15N labelling* in combination with spectroscopic analysis (high-resolution IR, microwave and '5N NMR spectroscopy), unequivocally revealed that another C,N, isomer, i.e. isocyanogen (CNCN), is the primary low-molecular-weight product.Apparently, at some stage during the thermolysis of the norbornadienone azine, an efficient isonitrile-nitrile rearrangement takes place (Scheme 2). Analogous rearrangements have been invoked previously to rationalize the behaviour of related compounds under thermolysis conditions.' In this respect, it is note-worthy that careful spectroscopic analysis (I3C and "N NMR, 173 K) of the norbornadienone azine pyrolysate showed that, besides the major product isocyanogen, addi- tional minor side products, such as hydrogen cyanide (2%), cyanogen (8%) and ethyne (2%), are present as impurities even after low-temperature distillation (temperature range 143-173 K).' The identification of these side products pro- vides evidence that other fragmentation pathways, such as retro-Diels-Alder cleavage,' are also operative in the ther- molysis of the norbornadienone azine.In line with theoretical predictions,' O isocyanogen (CNCN) was found to be considerably less stable than its isomer cyanogen (NCCN): It already polymerizes in solution at 193 K! At this temperature typical isonitrile reactions do not have a competitive rate." Hitherto, only two distinct low- molecular-weight reactions of isocyanogen have been report- ed, i.e.formation of N-cyanodibromoformaldimine by reaction with bromine' and its chromium pentacarbonyl complex.l2 Combustion analysis of pristine paraisocyanogen [poly(CNCN)] confirmed qualitatively the theoretically expected C :N ratio of 1 : 1 [experimental (YO)C: 50.15, H: 2.12, N: 41.37, (CN), calculated (%) C: 46.14, N: 53.86; experimental C : N ratio = 1.4 : 11.' The discrepancy between the theoretical and experimental data can be attributed to the incorporation of low-molecular-weight side-products in the polymer (vide infra). For paracyanogen prepared from several cyanogen precursors combustion analysis gave C and N values in the range of 32-34% and 32-44%, re~pectively.~" Although also for paracyanogen the theoretical and experi- mental results are at variance, it should be stipulated that it is well documented that its combustion analysis presents con- siderable experimental dific~lties.~Nevertheless, the data show that both paracyanogen and paraisocyanogen are pri- marily composed of carbon and nitrogen and may be looked upon as representative of heteroatom-substituted carbon-aceous materials. Hence, we were prompted to study in more detail the molecular structure and properties of paracyano- gen and paraisocyanogen.NC: cis trans FVT -46H6 -&Scheme 1 Proposed ladder structure for para~yanogen'*~ t Present address: Akzo Research Laboratories Arnhem, P.O. Box Scheme 2 Formation of isocyanogen by FVT of norbornadienone 9300,6800 SB Amhem, The Netherlands. azine5v8 Here we report the results of an investigation of both paracyanogen and paraisocyanogen with polarization microscopy, wide-angle X-ray powder diffraction (WAXD), thermogravimetry (TG), and diffuse reflectance Fourier-transform infrared (DRIFT), solid-state optical (UV-VIS- NIR, diffuse reflection technique), electron paramagnetic resonance (EPR) and X-ray photoelectron (XP) spectro-scopies.In addition, the electrical conductivity of pristine paraisocyanogen is determined and compared with that of paracyanogen. Experimental Synthesis and Polymerization of Isocyanogen Isocyanogen was obtained by FVT of norbornadienone azine at 773 K and Torr as described in detail elsewhere.8 Paraisocyanogen was prepared via two routes. (1) In a high- vacuum system isocyanogen (52 mg, 1 mmol) was dissolved in diethyl ether (5 ml) at 173 K and slowly warmed to ambient temperature during which paraisocyanogen precipi- tated (yield 60%).(2) In a high-vacuum system isocyanogen (52 mg, 1 mmol) was sublimed into the gas phase in a reac- tion vessel (volume 50 ml) at 253 K and warmed to ambient temperature during which pariasocyanogen precipitated on the glass surface of the vessel (yield 10-50%). Spectroscopic analysis revealed that both routes lead to identical paraisocy- anogen. Solubility experiments showed that paraisocyanogen, like paracyan~gen,~-~?*an intractable material insoluble in is common organic solvents and concentrated sulfuric acid.Synthesis and Polymerization of Cyanogen Cyanogen was prepared from mercury(I1) cyanide and poly- merized folowing reported procedures ;4a mercury(I1) cyanide (4 g, 14.8 mmol) was heated at 713 K in a sealed ampoule (Pyrex, volume 20 ml) for 24 h. After cooling to ambient tem- perature the ampoule was opened in a nitrogen atmosphere and paracyanogen was separated from mercury(0). Pristine paracyanogen was additionally heated at 473 K for 8 h in uucuo to remove traces of mercury(0) (yield 95%). The absence of mercury(0) in paracyanogen was established with XPS (vide infra). Characterization of Paraisocyanogen and Paracyanogen For the TG experiments a Perkin-Elmer TGS-2 equipped with an autobalance AR-2 was used (temperature program 323-1073 K, heating rate 20 K min-I).DRIFT spectra were recorded on either a Bio Rad FTS-7 or a Mattson Galaxy Series FT-IR 5000 spectrophotometer using a diffuse reflec- tance accessory; the samples were diluted with optically pure potassium bromide. Powder EPR spectra were recorded on a Bruker ESP 300 X-band spectrometer operating at 9.6 GHz. The g values of the peaks and the EPR spectrometer fre- quency were calibrated against solid diphenylpicrylhydrazyl radical (DPPH) standard assuming g = 2.0036.13 The EPR samples were sealed in high-purity quartz capillaries. Solid- state UV-VIS-NIR spectra were measured on a Varian Cary 5 dispersive spectrometer using a diffuse reflectance acces- sory (Praying Mantis); the samples were diluted with opti- cally pure potassium bromide.XP spectra were measured on a VG Escalab MkII spectrometer with non-monochromated Mg-Ka X-rays. The powder samples were mounted on a standard holder using two-sided tape. The absence of inter- fering signals from the tape was monitored and ascertained J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 by the observation of the spectral region in which Si from silicone present in the tape is expected. Data handling was performed using the VGS 5250 EDP software package. Results and Discussion Polarization Microscopy and X-Ray Powder DifFracton Polarization microscopy (crossed polarizers, magnification 400x ) indicates that both materials are amorphous solids. This is supported by wide-angle X-ray powder diffraction measurements ; no reflections are discernible ! This shows unequivocally that neither polymer possesses a three-dimensional ordering with regular repeat distances in the range of 2-10 A.14 The absence of reflections typical for graphite-or poly(acene)-like stacking, anticipated if both paracyanogen and paraisocyanogen possess a regular ladder- type structure as previously proposed for the former by Whangbo et a/.' and Bredas et al.,' suggests that the poly- mers are composed of small ordered arrays at most.Note that the latter may well generate diffuse scattered intensity at wide Bragg angles. Thermal Stability Although paracyanogen has been prepared before and has been the subject of several investigations, few experimental details concerning its thermal stability are a~ailable.~,~ To our knowledge only paracyanogen prepared via electro-polymerization of cyanogen has been subjected to controlled heat treatements with the objective to assess its conversion into carbon fibres.I5 To gain insight into the thermal stability of paracyanogen and paraisocyanogen both polymers were studied with TG under inert (N2) and thermo-oxidative (air) conditions.After loss of water of hydration (2 wt.%) at 373 K, the TG (N2) curve of paracyanogen shows no weight loss up to 673 K. Above 673 K, gradual thermal degradation and volatilization of the polymer takes place and is complete at 1123 K; no residue is found. In contrast, for pristine parai- socyanogen, which contained ca.6 wt.% water of hydration, loss of weight sets in already at 423 K followed by thermal degradation and volatilization yielding a residue of 18.5 wt.% at 1123 K. Under thermo-oxidative conditons the onset tem- perature for weight loss decreases for both polymers (paracyanogen: 573 K and paraisocyanogen: 373 K), fol-lowed by gradual thermal degradation and volatilization of both polymers leaving no residue at 1073 and 873 K, respec-tively. The differences in thermal behaviour indicate that the structures of pristine paracyanogen and paraisocyanogen are markedly different. However, it should be remembered that paracyanogen, in contrast to paraisocyanogen, is prepared by heat treatment of mercury(r1) cyanide in a sealed ampoule.Therefore, we have also studied the effect of a similar heat treatment of pristine paraisocyanogen at 713 K both in a sealed ampoule and by isothermal TG (N2). The isothermal TG (N2) curve showed a weight loss of ca. 50% within the first 30-40 min of the experiment after which the weight of the sample remained constant. TG (N2) analysis of heat- treated paraisocyanogen from the isothermal TG and the ampoule experiment gave identical TG curves. The heat- treated polymer is stable up to 773 K. Above this tem-perature thermal degradation occurs yielding a residue of 16.8 wt.% at 1123 K. Note that the total weight loss found with TG (N2) after heat treatment in either the isothermal TG or in the ampoule experiment agrees with the total loss of weight determined with TG (N2)for pristine paraisocyano- J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 gen. These results suggest strongly that the molecular struc- ture of pristine paraisocyanogen is altered by the heat treatment (cf.next section). Molecular Structure of Paracyanogen and Paraisocyanogen DRIFTS To gain insight into the molecular structure of both polymers DRIFT spectra were measured. The DRIFT spectrum of paracyanogen contains only broad absorption bands C2170- 2230 cm-' (w), 1750-1000 cm-' (s) and 800-700 cm-' (w), Fig. l(a)]. The absorption band centred at 2200 cm-' pro-vides evidence for the presence of pendent isonitrile (-NC) and nitrile (-CN) groups and the strong broad absorption band centred at 1500 cm- ' suggests that paracyanogen also contains imine (-C=N-) and alkene (-C=C-) type structural units.These data are in excellent agreement with those previously reported for paracyanogen prepared from a variety of cyanogen precursor^.^ For pristine paraisocyano- gen, a markedly different IR spectrum is obtained. Besides broad absorption bands at 2170-2240 cm-' (w) and 1750- lo00 cm-' (s) indicating the presence of pendent isonitrile (-NC) and nitrile (-CN) groups, and imine (-C=N-)- and alkene (-C-C-) groups, respectively, an additional strong, broad, featureless absorption band is found in the region 2500-3500 cm- ' (s) [Fig. l(b)]. Its shape suggests the I I I I I I I 3500 3000 2500 2000 1500 1000 500 ' wavenum ber/cm -Q) c-2 0,-n 0 3560 3000 2500 2000 ti00 1000 500 wavenumber/cm-' Fig.1 DRIFT spectra of (a)paracyanogen, (b)(-) pristine para-isocyanogen and (---) paraisocyanogen after heat treatment at 713 K presence of a wide variety of amine (-NHR) and carbon- hydrogen (-CH, and =CH,) groups in considerably differ- ent molecular environments. Apparently during polymerization of isocyanogen, the hydrogen-containing minor side-products, hydrogen cyanide and ethyne, which still contaminate isocyanogen isolated from the crude pyroly- sate by low-temperature distillation are efficiently incorpor- ated in the This is supported by the combustion data of pristine paraisocyanogen (experimental C : N ratio = 1.4 : 1) which deviate from the theoretically expected ratio = 1 : 1 (uide supra).Heat treatment of pristine parai- socyanogen at 713 K either in a sealed ampoule or with iso- thermal TG (NJ, leads to a reduction of the absorption bands at 2500-3500 cm-' and 2170-2240 cm-' and an increase in the 1750-1000 cm-' band [cf. Fig. l(b): the inten- sity ratio of 3250 and 1750 cm-' for pristine paraisocyano- gen is 0.52 and for paraisocyanogen after heat treatment is 0.221. Our data are in agreement with reported results on the effect of a heat treatment of poly(acrylonitrile).' Upon heat treatment, a decrease in intensity of the nitrile (-CN) absorption band concomitant with an increase of the broad absorption band at the position for imine (-C-N-) and alkene (-C=C-) type units was observed, which was attributed to nitrile cyclization.In the case of paraisocyano- gen, heat treatment undoubtedly will also lead to isonitrile- nitrile rearrangement^.^*'* ' Presumably, the nitrile substituents will subsequently cyclize under the high-temperature conditions.' Note, however, that even after heat treatment DRIFTS indicates that the structure of heat-treated paraisocyanogen differs from that of paracyanogen (Fig. 1). Nevertheless, DRIFTS suggests that both paracyano- gen and paraisocyanogen are amorphous networks derived from n-conjugated chains of different length consisting of coupled imine (-C=N-) and alkene (-C=C-) building blocks with, especially in the case of paraisocyanogen, a varying amount of pendent nitrile, isonitrile and amine-type substituents. Moreover, our DRIFT analysis provides evi- dence that heat treatment of pristine paraisocyanogen leads to an increase in unsaturation in combination with the for- mation of heteroatom-substituted cyclic n-conjugated struc- tures due to nitrile cyclization.EPR Powder EPR spectroscopy of pristine paraisocyanogen and paracyanogen showed the presence of a single broad reson- ance with g values of 2.0019 and 2.0017, respectively (reference DPPH, g value 2.0036) with a moderate peak to peak linewidth of 6 G at ambient temperature. The g values indicate the radicals to be carbon ~entred.'~ The absence of hyperfine splitting can be attributed to line broadening in the solid-state samples.In addition, the EPR signal for paracya- nogen is unsymmetrical, which suggests that the EPR spec- trum of paracyanogen is derived from different, but structurally related radical centres. In contrast to the powder EPR spectrum of paracyanogen, the EPR spectrum of pris- tine paraisocyanogen changed upon exposure of the sample to air. Besides a considerable decrease in intensity, a change in g value to 2.0035 (peak to peak linewidth 8 G at ambient temperature) was observed. This g value suggests the forma- tion of iminoxyl or nitrogen radical centres upon exposure to air.13 Unfortunately, owing to the small amounts of polymer available, we were hitherto unable to obtain reliable esti- mates of the spin density of paracyanogen and paraisocyano- gen before and after exposure to air.Nevertheless, the peak to peak linewidths found in the EPR spectra for both pristine polymers are indicative of the occurrence of delocalization of the radical centres and suggest, in agreement with the DRIFT analysis, that they are composed of n-conjugated structural units.'6a In passing, we would like to remark that as a conse- quence of the presence of unpaired electrons in both materials solid-state 3C (CP) MAS NMR measurements were thwarted. Electronic Absorption Spectra Based on the structure of both C2N2monomers and sup- ported by the DRIFT and EPR analysis, it is expected that paracyanogen and paraisocyanogen consist of n-conjugated chains. An estimate of the amount of n-electron conjugation in paracyanogen and pristine paraisocyanogen was obtained from solid-state diffuse reflectance UV-VIS-NIR measure-ments.The spectra show that the level of z-electron delocal- ization is markedly different for both polymers. Optical band gaps (Eg,cut-off energies) of ca. 0.85 and 1.45 eV are derived from the optical spectra for paracyanogen and pristine para- isocyanogen, respectively [Fig. 2(a) and (b), solid line].' In accord with the TG and DRIFT results, heat treatment of pristine paraisocyanogen leads to considerable changes of the solid-state UV-VIS-NIR spectrum. For the heat-treated paraisocyanogen samples prepared either with isothermal TG (N2, 713 K) or in a sealed ampoule (713 K) an increase in n-electron conjugation is found; similar UV-VIS-NIR spectra are obtained with an optical band gap of ca.0.96 eV [Fig. 2(b), broken line]. Hence in agreement with our DRIFT data, solid-state UV-VIS-NIR spectroscopy supports the conclusion that heat treatment of pristine paraisocyanogen leads to an increase of n-electron delocalization as a conse- quence of an increase in unsaturation (extrusion of hydrogen) in conjunction with nitrile cyclization reactions. In addition, the difference in optical band gap between heat treated parai- socyanogen (Eg 0.96 eV) and paracyanogen (Eg 0.85 eV) sup- ports our contention that even after heat treatment the structure of paraisocyanogen differs from that of paracyano- gen. XPS and Electrical Conductivity Measurements The conclusions derived from DRIFT and solid-state UV- VIS-NIR spectroscopy are corroborated by XPS measure-Fig.2 ments. In Table 1, the surface element concentrations (atom Yo)derived from XPS wide-scan spectra and the results of the decomposition of the C 1s peak shapes (XPS narrow-scan spectra) are presented. Although 0 is detected besides C and N, the C : N ratio for both paracyanogen and pristine para- isocyanogen are in satisfactory agreement with the bulk data obtained by combustion analysis (XPS: paraisocyanogen, C : 50.7, N: 37.4 and 0: 11.9, C :N ratio = 1.4 : 1; paracyano-gen, C: 46.8, N: 46.3 and 0:6.9, C :N ratio = 1 : 1).Based on the observed 0 1s binding energy (0Is, 534 ev) in com-bination with the 0 1s peak shape, the oxygen present can be attributed to physisorbed water (cf.also Thermal Stability section).I8 A superposition of the C 1s peak shape as well as the N 1s peak shape for paracyanogen and paraisocyanogen, respectively, shows directly that, in line with the DRIFT results, the bonding situation in both polymers is consider- ably different (Fig. 3). Analysis of the C 1s and N 1s peak J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 500 1500 2500 A/nm Afnm Solid-state UV-VIS-NIR spectra of (a) paracyanogen, (-) pristine paraisocyanogen and (---) heat treatment at 713 K shapes reveals that pristine paraisocyanogen contains carbon atoms mutiply bonded to nitrogen (C Is, 289 eV, 68%)and carbon atoms bonded directly to carbon (C Is, 285 eV, 32%).For paracyanogen the related values are 87% and 13Y0, respectively. Moreover, it should be stipulated that for pris- tine paraisocyanogen, both the carbon-to-carbon and carbon-to-nitrogen contribution to the C 1s peak shape is symmetrical. In contrast, paracyanogen possesses a distinct asymmetrical C 1s peak shape; a broad shoulder on the higher-binding-energy side, i.e. an energy-loss feature, is observed in the spectrum. The energy-loss feature can be attributed to interband transitions involving n states which are excited by some of the photoemitted electron^.'^ Similar observations also apply to the N 1s peak shape of para- Table 1 Surface elemental concentration (XPS wide scan) and relative functional group concentrations derived from XPS (narrow scan) C 1s peak-shape decomposition sample paracy anogen' paraisocyanogen (CNL C 1s binding energy 285 eV.18 surface concentration (atom YO) C N 46.8 46.3 50.7 37.4 50.0 50.0 C 1s binding energy 289 eV.'* relative concentration (Yo) 0 c=c/c-C" -CN/-NC/-C-N-' 6.9 13 87 11.9 32 68 -No mercury(0) could be detected by XPS.Theoretical composition. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 30 mz 25 X Y 20 38 15 10 I I I I I I I 280 285 290 295 300 305 EbP c XI I I 1I I 1 I 1 390 395 400 405 410 41 5 420 EbP Fig. 3 (a) C 1s and (b) N 1s peak shapes obtained with XPS of (---) paracyanogen and (-) pristine paraisocyanogen cyanogen with respect to that of pristine paraisocyanogen (Fig.3). The results strongly support that n-electron conjuga- tion is much more pronounced in the case of paracyanogen. The presence/absence of an energy-loss feature in the XPS spectra of paracyanogen and pristine paraisocyanogen, respectively, is indicative of the former being a semiconductor and the latter an ins~lator.'~ In agreement with this interpre- tation, electrical conductivity measurements (four-point-probe method)2o show that pristine paraisocyanogen and paracyanogen are an insulator and semiconductor, respec- tively (paraisocyanogen, ts = 1.0 x S cm-' and paracy- anogen, Q = 2.9 x S cm-' 'I). To assess if the experimental Q values for paraisocyanogen and paracyanogen are in line with anticipated intrinsic Q values the following crude calculations were made.Using the standard equa- tion for an intrinsic semiconductor ts = nep with n = no exp(-E$2k, T) and E, = 1.45 eV for paraisocyanogen and 0.85 eV for paracyanogen, under the assumptions that no, the number of electrons in the conduction band, is cm3 and p, the sum mobility of electrons and holes, is lod5 cm V-' s-1,22.23intrinsic Q values of 8.8 x lo-'' S cm-I and 1.0 x lop9S cm-', respectively, are calculated. Clearly, for both polymers the estimates are orders of magnitude lower than the experimental ts value and suggest that extrin- sic doping either by unintended impurities incorporated during polymerization or by oxidation in air has occurred. In the case of pristine paraisocyanogen doping with iodine vapour gave a substantial increase in electrical conductivity (298 K, cr = 1.0 x S cm-') which showithat the addi- tional extrinsic conductivity is p-t~pe.'~ Conciusions The results presented show that polymers derived from iso- cyanogen and cyanogen, respectively, i.e.paraisocyanogen and paracyanogen, possess different molecular structures and properties. Spectroscopic analysis shows that n-electron delo- calization is considerably less in the case of pristine para- isocyanogen. However, a heat treatment of pristine 33 1 paraisocyanogen leads to an increase in n-electron delocal- ization presumably due to hydrogen extrusion (increase in unsaturation) in conjunction with nitrile cyclization reactions Despite the presence of n-conjugated structures, regular ladder structures as initially proposed for paracyanogen by Whangbo et al.are not found.',' Hence, both polymers can be represented as amorphous heteroatom-substituted carbon- aceous materials. Experimental contributions of Dr. E. E. Havinga (Philips Research Laboratories, Eindhoven, The Netherlands), A. Schouten (Utrecht University, The Netherlands) and E. Neven (Vrije Universiteit, Amsterdam, The Netherlands) are gratefully acknowledged. References 1 M. H. Whangbo, R. Hoffman and R. B. Woodward, Proc. R. SOC.,London A, 1979,366,23. 2 J. L. Bredas, B. Tremans and J. M. Andre, J. Chem. Phys., 1983, 78, 6137. 3 L. J. Gay Lussac, Ann. Chim. (Paris), 1815,95, 175.4 (a)L. L. Bircumshaw, F. M. Tayler and D. H. Whiffen, J. Chem. SOC.,1954, 931; (b) J. Peska, M. J. Benes and 0.Wichterle, Coll. Czech. Chem. Commun., 1966, 31, 243; (c) M. M. Labes, Mol. Cryst. Liq. Cryst., 1989, 171, 243 and references therein; (d) J. J. Kampa and R. J. Lagow, Chem. Muter., 1993,5,427. 5 T. van der Does and F. Bickelhaupt, Angew. Chem., 1988, 100, 998. 6 0.Grabandt, C. A. De Lange, R. A. Mooyman, T. van der Does and F. Bickelhaupt, Chem. Phys. Lett., 1989,155,221. 7 F. Stroh and M. Winnewisser, Chem. Phys. Lett., 1989, 155, 21; F. Stroh, B. P. Winnewisser, M. Winnewisser, H. P. Reisenauer, G. Maier, S. J. Goede and F. Bickelhaupt, Chem. Phys. Lett., 1989, 160, 105; M. K. Scheller, H.G. Weikert and L. S. Ceder-baum, J.Electron Spectrosc., 1990, 51, 75; K. M. T. Yamada, M. W. Markus, G. Winnewisser, W. Joentgen, R. Kock, E. Vogel and H-J. Altenbach, Chem. Phys. Lett., 1989, 160, 113; G. Maier, H. P. Reisenauer, J. Eckwert, C. Sierakowski and T. Stumpf, Angew. Chem., 1992,104,1287. 8 S. J. Goede, F. J. J. De Kanter and F. Bickelhaupt, J. Am. Chem. SOC.,1991,113,6104. 9 R. W. Hoffmann, A. Riemann and B. Mayer, Chem. Ber., 1985, 118, 2433; A. Riemann and R. W. Hoffmann, Chem. Ber., 1985, 118,2544. 10 M. Sana and 0. Leroy, J. Mol. Struct., 1981, 76, 259; M. T. Nguyen, Chem. Phys. Lett., 1989, 157, 430; K. K. Sunil, J. H. Yates and K. D. Jordan, Chem. Phys. Lett., 1990, 171, 185; P. Botschwina and J. Flugge, Chem. Phys. Lett., 1991, 180, 589; W. B. De Almeida and A.Hinchliffe, J. Mol. Struct. (THEOCHEM), 1990, 65, 77; P. Botschwina and P. Sebald, Chem. Phys., 1990, 141, 311; F. M. Bickelhaupt, N. M. M. Nib-bering, E. M. van Wezenbeek and E. J. Baerends, J. Phys. Chem., 1992,%, 4864. 11 T. Saegusa and Y. Ito, Zsonitrile Chemistry, ed. Y. Ugi, Academic Press, New York and London, 1971, ch. 3 and 4. 12 G. Christian, H. Stolzenberg and W. P. Fehlhammer, J. Chem. SOC., Chem. Commun., 1982, 184; G. C. Schoemaker, D. J. Stuf- kens, S. J. Goede, T. van der Does and F. Bickelhaupt, J. Organomet. Chem., 1990,390, C1. 13 C. Wentrup, Reactive Molecules, Wiley, New York, 1984, ch. 2. 14 JCPDS Powder Diffraction File, International Centre for Dif- fraction Data, Swarthmore, PA, 1989; M. G. Northolt, L.H. Veldhuizen and H. Jansen, Carbon, 1991,29,1267. 15 J. H. Chen and M. M. Labes, Macromolecules, 1985, 18, 827; J. H. Chen and M. M. Labes, Macromolecules, 1986, 19, 1528 and references therein. 16 (a) A. Bhuiyan and S. V. Bhoraskar, J. Mater. Sci., 1989, 24, 3091; (b) T. Usami, T. Itoh, H. Ohtani and S. Tsuge, Macro-molecules, 1990, 23, 2460 and references therein ;(c) Encyclopedia of Polymer Science and Engineering, ed. J. I. Kroschwitz, Wiley, New York, 2nd edn., 1990, index vol., pp. 170-175. 17 C-J. Yang and S. A. Jenekhe, Chem. Mater., 1991,3,878. 18 K. Siegbahn, C. Nordling, G. Johanson, J. Hedmann, P. F. Heden, K. Hamrin, U. Gelius, T. Bergmark, L. 0. Werme, R. Manne and Y. Baer, ESCA Applied to Free Molecules, North-Holland, Amsterdam, New York, 1969, ch. 5. 332 19 F. R. McFeely, S. P. Kowalczyk, L. Ley, R. G. Cavell, R. A. Pollak and D. A. Shirley, Phys. Rev. B, 1974, 9, 5268 and refer- ences therein; W. M. Tong, D. A. A. Ohlberg, H. K. You, R. Stanley Williams, S. J. Anz, M. M. Alvarez, R. L. Whetten, Y. Rubin and F. N. Diederich, J. Phys. Chem., 1991,95,4709. 20 L. J. Van der Pauw, Philips Res. Rept., 1958, 13, 1. 21 Ya. M. Panshkin, T. P. Vishnyakova, A. F. Lunin and S. A. Nizova, Organic Polymeric Semiconductors, Wiley, New York, 1974,ch. 1. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 22 N. W. Ashcroft and N. D. Mermin, Solid State Physics, Interna- tional Edition, W. B. Saunders, Philadelphia, 1976. 23 E. E. Havinga, W. ten Hoeve and H. Wynberg, Synth. Met., 1993,55-57,299. 24 J. M. G. Cowie, Polymers: Chemistry & Physics of Modern Materials, Chapman and Hall, New York, 1991, ch. 17. Paper 3/05809C; Received 27th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000327

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

333J. CHEM. SOC. FARADAY TRANS.. 1994. m21. 333-33s Photochromism, Thermochromism and Solvatochromism of Some Spiro[indolinoxazine]-photomerocyanine Systems:Effects of Structure and Solvent G. Favaro,* F. Masetti, U. Mazzucato and G. Ottavi Dipartimento di Chimica, Universita di Perugia . 06122 Perugia , Italy P. Allegrini and V. Malatesta" EniChem Synthesis, 20097 San Donato Milanese Itat./~ Three spiro[indoline-naphthoxazines] and a spiro[ind~iine-phenanthroxazine], which exhibit photochromic and thermochromic properties, have been investigated. So vent and structure effects on the absorption spectra of the merocyanines produced under UV irradiation and kine:ic parameters for the ring-closure and ring-opening reac- tions were studied. Positive solvatochromism was fcund, indicating that the opened form is a weakly polar species.Equilibrium constants and rate constants for the forward and back reactions spiroxazine emerocya-nine increase with increasing the solvent polarity and with electron-donating groups in the oxazine moiety. The reaction is endothermic by 10-20 kJ mol-' and almost isoentropic. The activation entropy is generally negative, while the activation Gibbs energy is approximately independent of solvent and structure. Photochromism involving changes in the visible absorption pi wnd. I ,3,3-trimethylspiro[indoline-2.3'[3H]naphth[2,1 -h] spectrum has attracted much attention in the last decades [I .J]oxazine] 1, the 6'-piperidine substituted compound 2. because of the variety of practical applications of photo- the 5-Br compound 3 and the unsubstituted spiro[indoline-2. chromic systems.' The photochromic properties of the spir o-2' [?H]phenanthr[9,10-b][ 1,4)oxazine) 4. A11 these molecules compounds are well known. Spiropyrans have been supplied by EniChem Research Sp.4. The solvents were wIw extensively studied2 and, more recently. spiroxazines hac t' re igent grade Carlo Erba products. been the subject of many investigation^.^ Is Interest in these compounds is justified by their high durability with respect to photoexci tation. The photochromism of these molecules is due to photo- cleavage of the spirobond under UV irradiation to give 'in open merocyanine structure (photomerocyanine) which absorbs in the visible region.'YGeneral agreement can be found in the literature about W some aspects of the mechanistic behaviour of these photo- 4chromic systems. It is well known, from experiments uirh 1 2 3 SH H Brpicosecond time resolution, that the C-0 bond breakage in Y H piperidine H the excited state occurs on the picosecond timescale.-' Experimental evidence has been reported for the production of several (at least two') merocyanine isomers in a transoid Equipment structure.' Thermal bleaching of the coloured form is knoun Almrption spectra were recorded on Perkin-Elmer Lambda to be a relatively slow process (rate constant: 0.01 1Cl 5 ind Lambda 16 spectrophotometers. For absorption mea-1 1.'L.5.13 sus'ements at varying temperatures. a cryostat (Oxford However, there are some doubts about the nature of the In~rument)was used, equipped with a temperature control- primary photoproduct 'X', whether it is a non-planar cisoid ler operating between 77 K (if liquid nitrogen was used for structure,-or a transoid form undergoing very fast thermd co ,ling) and 500 K.A 250 W medium-pressure mercury lamp equilibration.' Conflicting results have been reported on the fihered by a CS 7-54Corning filter.which transmits 240-400 quantum efficiency of the photocolouration reaction, which nn light. was used for producing the coloured form in some cases differs, for the same molecule, by more than 100°/0.4.5.'3Large discrepancies can also be found in the MIdar Absorption Coefficientsmolar absorption coefficients of the open forms determined by different method^.^.' 3*1J Thr determination of the molar absorption coefficients of the In this paper we report the effect of the solvent and strub- op:n forms was carried out at 223 K in order to minimize the ture on the thermal equilibrium between coloured and efihct of thermal bleaching. The room-temperature concentra- colourless forms, on the kinetics of thermal bleaching of the tio 1s of the solutions ( lo-' mol dm-3) were corrected photomerocyanines and on their absorption properties.for the volume contraction upon cooling. In toluene. owing These measurements. which give an overall view of the ener- to the sharp decrease of solubility of the spiro compounds getics of these systems, can also give mechanistic information with increasing temperature.a further correction was neces- about the thermal breaking and reforming of the spirobond. sar v to account for partial solute precipitation, even when the corcentration was kept below 5 x mol dm- '. The solu- Experimental tio IS were irradiated for 10 min. sufficient to obtain a con- Materials sta it absorption of the coloured form. In order to avoid The photochromic molecules under study were three diflusion effects. the whole surface of the sample cell was spiro[indoline-naphthoxazines] : the unsubstit uted com-hoiwgeneously irradiated. Equilibrium Constants The constants of the thermal equilibrium K = [merocy-anine]/[spiroxazine], were determined in ethanol and toluene by measuring the visible absorbance of the open form in cor- respondence to the maximum absorption (570-615 nm) where the spiro form does not absorb.The determinations were carried out in the temperature range 280-335 K, using sample concentrations on the order of lo-' mol dm-'. The accuracy can be considered within 20%. Kinetics of Thermal Bleaching The kinetics of ring-closure reaction were studied following the disappearance of the coloured form at the wavelength of maximum absorbance. The solutions (ca. lo-' mol dm-3) were previously irradiated with the filtered light of the mercury lamp for 5 min and analysed immediately. The mea- surements were performed in the temperature range 266-300 K, ca. 30 min after having set the temperature control in order to allow the solution to reach thermal equilibrium. First-order rate constants were obtained from linear log A us.time plots. Results and Discussion Absorption Spectra The absorption spectra of the colourless and coloured forms of the four molecules in ethanol are illustrated in Fig. 1. 1.2 1 0.6 ,,, , 0.0 0.2 Q) C $n 0.0 (D 1.O - , I ' I , , 0.5 0.0 4 0.2 0.0250 350 450 550 650 750 L/nm Fig. 1 Absorption spectra of the colourless (--) and coloured (---) forms of the 1,2,3and 4 in ethanol at 223 K J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Absorption characteristics of the spiro[indolinoxazines] in ethanol solution indoline moiety oxazine moiety A,,/nm Emax /dm3 mol-' an-' Amax /nm Emax /dm3 mol-' cm-' 1 233 36000 318 4400 2 249 24000 363 loo00 3 231 50900 310 8OOO 4 252 43600 340 6OOo The absorption spectra of the colourless forms consist of localised n-transitions in the UV region, belonging to the two orthogonal halves of the molecule.The band maxima and molar absorption coefficients of the indoline moiety and those of the naphthoxazine or phenanthroxazine moieties are reported in Table 1. The effect of the electron-donating sub- stituent and electron-withdrawing substituent on the naphtho-derivatives is to shift the absorption band to lower and higher energies, respectively. For 4, the phenanthrenic structure was observed. The spectra of the photomerocyanines, which were produc- ed at 223 K in order to avoid thermal bleaching, are charac- terized by an intense band in the visible region (A,,, rz 600 nm); thus the solutions are deeply coloured.The spectral characteristics of the coloured forms in two solvents are given and compared with literature data in Table 2. The agreement is not completely satisfactory for both A,,, and molar absorption coefficients. This is particularly evident in the case of 1 for which E values in ethanol range from 51 x lo3 dm3 mol-' cm-' '' to 73 x lo3 dm3 mol-' ~m-'.~The ques- tion of the poor correspondence between molar absorption coefficients determined in different laboratories has been raised by Wilkinson et ~1.'~in a recent paper which appeared when this work was already in progress.In that paper a method is described for measuring the molar absorption coef- ficient of photochromic compounds which seems completely convincing since it requires a minimum of assumptions. However, the E values determined are generally smaller than those previously reported (as well as those obtained in this work). These discrepancies are underlined by Wilkinson but no reason is proposed for them. Our determinations were carried out under conditions similar to those used by Chu4 and Kh~lmanskii,'~ that is at a temperature low enough to inhibit thermal bleaching. Thus, possible sources of error could arise from photochemical ring-closure or spectral changes which occur upon cooling. The first source can be excluded for several reasons.First, the quantum efficiency of the back reaction, Ob,was found to be Table 2 Absorption characteristics of the photomerocyanines (223K) ethanol toluene Amax Emax Amax Lax /nm /dm3 mol-' cm-' /nm /dm3 mol- ' cm-1 614 73600 601 58700 (612)" (73000)" (594)b (31000)c (607)b (51000)' (592)' (52000)' (594)e 2 593 77000 574 62400 (5W (32000)'(565)'3 615 68100 600 57000 4600 63300 588 68300 (592)J From ref. 4; from ref. 5; from ref. 13; 'from ref. 14; from ref. 15; from ref. 6. J. CHEM. SOC. FARADAY TRANS.. 1994. VOL. 90 very small compared with that of the forward reaction. a+. (U+ + Ob2 Of);'3 secondly, the absorbance of the coloured form in the exciting wavelength range is also very small: finally, not taking photobleaching into account should led to smaller, rather than higher, E values.Thus, we believe that the main reason for the discrepancies lies in some spectral evolution occurring upon cooling. The best agreement. in fact, is found with Chu's data,4 not only in the E value. but also in the spectral position of the absorption maximum of 1. which is shifted to lower energies with respect to that mea-sured at room temperature. It is possible that the tern-perature difference (ca. 70 K) between different kinds of measurements is critical to establish equilibrium among dif- ferent merocyanine isomers. To verify this hypothesis, absorption spectra of merocya- nines (complete conversion) were recorded over a range of temperatures (165-225 K) low enough to ensure exclusion of thermal bleaching.As an example, the behaviour of 2 in ethanol is shown in Fig. 2. It can be observed that lowering the temperature enhances the visible absorption intensit) without appreciably affecting the UV region. A slight, but sig- nificant, red shift of the coloured band was also observed. This behaviour is qualitatively similar for all the molecules under study. It is worthwhile noting that the absorbance rs temperature plot is linear (Fig. 2, inset). at least in the tem- perature range explored. The increase in E with decreasing temperature is ca. 0.3-0.4°/0 dm3 mol-' cm-' K-' in ethanol. Toluene solutions could not be investigated due to the poor solubility of the substrates in this solvent at 10% temperature.However, similar spectral modifications were also found in a non-polar solvent (methylcyclohexane). Assuming that linearity also holds approaching room teni-perature. E values at 298 K could be extrapolated. Solvatochromism The absorption band of the open forms exhibited a batho- chromic shift with increasing solvent polarity. An example is given in Fig. 3. To correlate the transition energies in differ-ent solvents. the empirical solvent parameter ET(30) of DimrothI6 was used, which accounts for hydrogen-bond interactions as well as dipolarity effects. Moreover, it is based on a reference compound (a pyridinium-N-phenolate-betaine) which is similar in structure to the merocyanines investigated here. The following correlations were obtained: 1, F/cm-' =: 17880 -27.1 ET(30); 2, F/cm-' = 20 390 -63.8 €,(30): 4.f;cm -= 18 600 -34.1 ET(30); and are illustrated in Fig. 4. 250 350 450 550 650 753 i.nm Fig. 2 Spectra of the colourless (---I and coloured (--) forms of 2 in ethanol at 215, 205. 195, 185, 175 and 165 K (in direction of the arrow). Inset : plot of absorbance rs. temperature. i. nm Fig. 3 Solvatochromic effect on the absorption spectrum of 4 at 298 K in (u)methylcyclohexane, (b)ethyl acetate, (c)acetonitrile and (d) etl anol l9 30 35 40 45 50 55 60 ET(30) Fig 4 Correlation of the transition energy of the photomerocya- nin :s with the solvent polarity parameter €,(30): 1 (0); 4 (A)2 (0); 'The bathochromic shift observed (positive solvato-chi omism)' ' is indicative of an increased dipole moment up+In electronic excitation.Therefore, the ground-state we tkly polar molecule should approach the configuration of the quinoid form. This hypothesis was previously put forward for 1 by Kellmann et a!. using the Brooker's empirical param- ete.s for the correlation,5 as well as by Lenoble and Becker for the photomerocyanine of an indolinespirobenzopyran. In contrast. in the case of some merocyanines derived from the spironitropyrans. which are also closely related in struc-tur.: to the molecules investigated here, a negative solvatoch- roniism was found.lg Consequently, the substituent effect on sol.:atochromism was opposite to that found here, that is, the sen iitivity to solvent polarity increased with electron-dorlating groups.The results from the solvatochromic study alsc, seem to conflict with recent theoretical calculations on the..e molecules20 which predict a prevalent zwitterionic stri cture for the ground state. However, since calculations consider the system in the absence of solute-solvent inter-actions. the disagreement is not surprising. Tht,rmosquilibriumand Thermochrornism Concentrated solutions of 1, 2 and 4 (c 4c lop3mol dm-3) showed a low intensity absorption band in the visible region at 'oom temperature, denoting that a thermal equilibrium was established between the open and closed forms. The abs xption intensity. which was weaker in a non-polar J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 Thermodynamic parameters of the reaction spiroxazine +photomerocyanine (298 K) 1 2 3 4 ethanol toluene ethanol 1.1 0.12 58K 298/1o4 AH"/kJ mol-' 20.9 21.7 15.0 (1 7.9)" AG"/kJ mol -' 22.6 28.0 12.8 ASo/J mol -' K --5.6 -21 7.4 a From ref. 4. solvent than in a polar one, increased as the temperature increased, that is, these molecules exhibited thermochromic properties. For 3, the coloured band was hardly detectable, even in a saturated solution of a polar solvent at 320 K (absorbance at 600 nm less than 0.005). In principle, the absorbance in the visible region of non- irradiated concentrated solutions can be used to calculate the equilibrium constant of the spiroxazine merocyanine system, if the molar absorption coefficient of the open form is known.To our knowledge, such a calculation has never been reported before, probably because of the difficulty of obtain- ing reliable E values for the merocyanines. The equilibrium constants determined in ethanol and toluene are reported in Table 3. It can be observed that the K29, values are spread over a large interval ranging from to lo-'. The equi- librium constant increases with the solvent polarity and with electron-donating groups, while it decreases to an unmeasur- able value (K29, < in the presence of the electron- withdrawing Br substituent. Variations introduced in K values by both solvent and structure are so large that the uncertainty of the E values used for their calculations should not invalidate the relative comparison.The enthalpy of reaction could be evaluated by measuring the absorbance of the coloured form at several temperatures, according to the van't Hoff equation, d In K/d(l/ T)= d In A/d(l/T) = -AH/R. Plots of In A us. 1/T are shown in Fig. 5. AHo values are reported in Table 3 along with AGO and AF calculated by means of the thermodynamic relationships. Variations in AHo from ethanol to toluene for the same molecule are within the experimental error, while the decrease of AHo with electron-donating groups and the increase with the Br electron-withdrawing substituent are sig- nificant. The reaction entropy is generally small. Considering that is was estimated by calculating a small difference between two large and close numbers, the estimated values are almost of the same order of magnitude of the experimen- -0.5 I I I I -1.5 -C -2.5 -3.5 2.9 3.1 3.3 3.5 3.7 103 KIT Fig.5 Data for thermal equilibrium treated according to the van't Hoff equation in ethanol (empty symbols) and in toluene (filled symbols): l(0);2 (A);4 (0) toluene ethanol/toluene ethanol toluene 6.0 < 110 51 18.0 > 34 10.9 9.6 18.4 >34 11.2 13.1 -1.3 - -1.0 -11.7 tal error. Negligible ASo values indicate that the positive con- tribution to entropy due to increased torsional freedom in the open structure is approximately compensated by the solvent reorganization around the more polar merocyanine form.However, less negative (positive for 2) ASo values in ethanol than in toluene are responsible for the increased K in the polar solvent. Kinetics of Thermal Bleaching The kinetics of thermal bleaching of the photomerocyanines were investigated in ethanol and toluene following the fading of colour at the maximum absorption. The ring-closure reac- tion was found to be strictly first order in the temperature range explored for all molecules in both solvents. Typical first-order kinetic plots (p > 0.99) are shown in Fig. 6. The rate constants are given in Table 4, which also includes liter- ature data. The agreement with the latter can be considered satisfactory. However, we did not find any sign of the bi- exponential decay reported by Kellmann et d.for 1 in a non- polar solvent.' This is probably due to the different time-resolutions of the detection systems used. An increase in the reaction rate with increased solvent polarity is evident from the data reported in Table 4.Such a trend was also noted previously.' In addition, interesting structure effects are observed, such as the low rate for 2 in toluene and the high one for 4 in ethanol, while a close simi- larity is observed between 1 and 3. As can be seen from Table 4, the activation energy is more structure dependent in ethanol (41-81 kJ mol-') than in toluene (47-66 kJ mol- '). The corresponding Arrhenius plots are shown in Fig. 7. The thermodynamic activation parameters, calculated from the kinetic data, are also reported in Table 4.It can be seen that AG' is scarcely influenced by the solvent and structure (AG' = 70-80 kJ mol-'). This is due to a compensation 0 -1 -2 -? 5 -3 -4 (e )I,IIIIII 0 25 50 75 100 125 150 time/s Fig. 6 First-order kinetics for the thermal bleaching of merocyanine 4 in toluene. (a) 281, (6)286, (c) 291, (d)296 and (e) 301 K. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 Kinetic parameters for the reaction of thermal bleaching (298 K) 1 2 3 4 ethanol toluene ethanol toluene ethanol toluene ethanol toluene ils -0.23 0.23 0.58 0.035 0.55 0.18 3.2 0.14 (0.21)o (0.27)* (0.044)b (5.0)c (0.l)d(0.54)e (0.04)d (0.07)' (0.4)d (0.03)J (0.28) E,/kJ mol-l 81.1 47.3 41.1 64.4 73.5 58.0 65.6 66.5 (82.4)" (72.0)cAH#/kJ mol-' 78.6 44.8 38.5 AG#/kJ mol-I 76.5 76.7 74.2 ASf/J mol-' K-' 6.7 -107 -120 From ref.4; from ref. 13; from ref. 6; from ref. 15; from ref. 5; 1992,89, 897. between energetic (AH') and entropic (AS') factors. The activation entropies, in fact, range from values close to zero when AHf is large (1 and 3 in ethanol) to fairly negative values in the cases where AHf is small (2 in ethanol). Combining the kinetic parameters of the ring-closure reac- tion and the thermodynamic data obtained from the study of thermal equilibrium, the rate constants and kinetic activation parameters for the thermal breakage of the spiro bond were calculated and are reported in Table 5. Despite the slowness of the forward r_eaction (k = 10-6-10-2 s-'), the large differ- ence between k and & allows thermal equilibration to be attained in a few seconds.It can be observed that the rate parameters of the forward reaction vary over a much larger range than those of the back reaction, thus determining the large differences in the equilibrium constant and thermochro- mic behaviour. Concluding Remarks These results point to a similar reaction mechanism in toluene for all the molecules investigated. This mechanism passes through an activated state where entropy is lost, prob- ably because of the partial charge separation (negative on the oxygen and positive on the indoline nitrogen). The dipolar transition state interacts with the solvent much more than the closed and open forms, both being weakly polar molecules, as confirmed for the last species by the positive solvatochromic effect.In ethanol, the kinetic behaviour is more structure depen- dent. Even though AG' is similar for all the molecules (AG' = 70-76 kJ mol-' for the back reaction and AG' = 81-99 kJ mol- for the forward reaction), marked differences were observed in the contributions of AHf and AS'. The entropic factor dominates for 2, which was indicated by the solvatochromic effect to be the least polar in the ground state. The activation energy of the thermal reaction is lower owing to solvent stabilization of the dipolar transition state, while entropy is lost because of the solvent reorganization around it.For 1 and 3 (only back reaction explored), very 61.9 71.0 55.5 63.1 64.0 81.1 74.5 77.2 69.9 77.7 -64.8 -11.6 -72.9 -23.0 -46.4 from D. Eloy, P. Escaffre, R. Gautron and P. Jardon, J. Chim.Phys., -0.5 -1.5 -2.5 -3.5 4.5 -5.5 3.4 3.6 3.8 4.0 103 KIT -1.5 -2.5 -3.5 4.5 3.3 3.5 3.7 3.9 103 KIT Fig. 7 Arrhenius plots for the thermal-bleaching reaction (a) in ethanol and (b) in toluene of 1 (0,M), 2 (A, A),3 (a,*)and 4 (0, small AS' values point to a transition state quite similar to the starting molecule, while 4 represents an intermediate situ- ation. From this study, even if the number of molecules investi- gated is limited, some aspects emerge which can be gener- alized. Increasing the solvent polarity always increases both the equilibrium constant and the rate constants.The relative .) Table 5 Kinetic parameters of the forward reaction (298 K) 1 2 3 4 ethanol toluene ethanol toluene ethanolltoluene ethanol toluene 1;/104 S-EfiJ mol-' 0.25 102 0.028 69.0 33.6 56.0 0.2 1 82.4 <0.005 > 100 352 76.5 7.1 76.1 AH#/~Jmol-' AGf/kJ mol-' AS#/J mol-' K-' 99.5 99.2 1 66.5 104.7 -128 53.5 86.9 -112 79.9 99.6 -66 >loo 74.0 81.2 -24 73.6 90.9 -58 338 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 contributions of enthalpy and entropy are mainly determined by the dipolar nature of the normal and activated state. An electron-donating substituent such as piperidine decreases the electron-accepting capacity of the oxygen atom, thus leading to a lower rate parameter for the ring-closure reaction.The rate parameters of the forward reaction are dramatically affected by both solvent and structure effects, thus leading to 5 6 7 8 9 A. Kellman, F. Tfibel, R. Dubest, P. Levoir, J. Aubard, E. Pottier and R. Guglielmetti, J. Photochem. Photobiol., A, 1989,49, 63. U. W. Grummt, M. Reichenbacher and R. Paetzold, Tetra-hedron Lett., 1981,22, 3945. S. Schneider, 2. Phys. Chem., Neue Folge, 1987,154,91. S. Schneider, A. Mindl, G. Elfinger and M. Melzig, Ber. Bun-senges. Phys. Chem., 1987,91, 1222. S. Aramaki and G. H. Atkinson, Chem. Phys. Lett., 1990, 170, marked changes in the thermochromic behaviour. 10 181. E. Pottier, A. Samat, R. Guglielmetti, D. Siri and G.Pepe, Bull. This work was undertaken under the National Programme of Research for Chemistry sponsored by the Italian ‘Minister0 per 1’Universita e la Ricerca Scientifica e Tecnologica’ (Tema lO-Consorzio R.C.E.). Financial contributions by the Italian 11 12 SOC. Chim. Belg., 1992, 101,207. C. Bohne, M. G. Fan, Z-J. Li, J. Lusztyk and J. C. Scaiano, J. Chem. SOC., Chem. Commun., 1990, 571. C. Bohne, M. G. Fan, Z-J. Li, Y. C. Liang, J. Lusztyk and J. C. Scaiano, J. Photochem. Photobiol., A, 1992,66, 79. Consiglio Nazionale delle Richerche are also gratefully acknowledged. 13 14 F. Wilkinson, J. Hobley and M. Naftaly, J. Chem. SOC., Faraday Trans., 1992,88, 151 1. A. S. Kholmanskii and K. M. Dyrmaev, Dokl. Akad. Nauk SSSR, 1988,303,1189 References 15 D. Eloy, P. Escaffre, R.Gautron and P. Jardon, Bull. SOC. Chim. Belg., 1992, 101, 779. 1 R. Guglielmetti, in Photochromism, ed. H. Durr and H. Bouas- Laurent, Elsevier, Amsterdam, 1990, p. 855; N. Y. C. Chu, p. 879; K. Ichimura, p. 903. 2 See e.g. R. Guglielmetti, in Photochromism, ed. H. Durr and H. Bouas-Laurent, Elsevier, Amsterdam, 1990, p. 3 14. 16 17 18 19 K. Dimroth, C. Reichardt, T. Siepmann and F. Bohlmann, Liebigs Ann. Chem., 1963,661, 1. P. Jacques, J. Phys. Chem., 1986,90,5535. C. Lenoble and R.S. Becker, J. Photochem., 1986,34,83. S. R. Keum, M. S. Hur, P. M. Kazmaier and E. Buncel, Can. J. 3 N. Y. C. Chu, in Photochromism, ed. H. Durr and H. Bouas- Chem., 1991,69, 1940. Laurent, Elsevier, Amsterdam, 1990, p. 493. 20 V. Malatesta, G. Ranghino, U. Romano and P. Allegrini, Int. J. 4 N. Y. C. Chu, Can. J. Chem., 1983,61,300. Quantum Chem., 1992,42,879. Paper 3/03115B; Receiued 1st June, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000333

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 339-344 Modified Interpenetration Function accounting for the Excluded-volume Effects in Ternary Polymer Systems Rosa Garcia, Clara M. Gomez, lolanda Porcar, Vicente Soria and Agusth Campos' Departament de Qulinica Fisica, Facultat de Quimica, Universitat de Valencia, E-46100 Burjassot, Valencia,Spain The excluded-volume effect in polymer-mixed-solvent systems has been investigated in the light of the two- parameter theory. These multicomponent polymer systems exhibit some specific features quite different from polymer-single-solvent systems, involving ternary interactions and the synergic action of the mixed solvents at a given composition on polymer dimensions. Both effects are taken into account in order to derive a new expression for the interpenetration ternary function, YT, and the expansion factor, a,, both being quantitatively evaluated for ternary systems based on polystyrene and poly(methy1 methacrylate).In addition, a complemen- tary treatment of the excluded-volume effects, proposed by Wolf, based on the evaluation of the intra- and inter-molecular interactions has been carried out. The effects of excluded-volume interactions upon the trans- port and equilibrium properties of a random-coil polymer chain in solution have received considerable attention in the literat~re.'-~ One of the major reasons is that the practical use of polymer materials demands knowledge of their physi- cal properties, which are strongly dependent on intra- and inter-macromolecular repulsive interaction, closely related to the so-called excluded-volume effect^.^ Up to date, much effort has been devoted to the prediction of these effects, the two most commonly used treatments being the so-called two- parameter2 and the renormalization-group (RG) theorie~,~ together with scaling concepts to simplify the calculation^.^ We briefly summarize how the excluded volume can affect the macromolecular dimensions.In a dilute solution of a flex- ible polymer with degree of polymerization, N, the chain expands by the excluded-volume effect and the mean-square end-to-end distance, (r2), is proportional to N2" where v is YKY,considering the intra- and inter-molecular interactions, according to Wolf's suggestions, l6 is included. Theoretical Background We proceed to show briefly the formalism of the two-parameter theory adapted to TPS.Thus, for convenience, we begin by reproducing some of the previously derived equa- tions, mainly concerning the intrinsic viscosity and second virial coefficient, that are required for the discussion. Polymer-mixed-solvent systems possess an 'excess viscosity ' defined as : where [qli3 and [qIT are the intrinsic viscosities of polymerthe Flory excluded-volume exponent.' The scaling the~ry~.~ (3) in a pure solvent i (i = 1, 2) and in a binary solvent describes the concentration dependence of (r2) in the semi- dilute regime, considering the unit of so-called 'blob' within which the excluded volume effect is exerted and outside which the effect is screened, thus (r2) oc g2"/(N/g) K Nc(~"-')'('-~"),where c is the polymer concentration and g is the number of monomeric units in the 'blob'.The main purpose of this work is the investigation of the excluded-volume effects for dilute polymer solutions in the frame of the two-parameter theory to give an explanation to many experimental findings in ternary polymer systems (TPS). The basic assumption in a two-parameter theory is the smallness of the excluded-volume interaction yielding univer- sal functions for flexible polymer chains. However, it deserves to be mentioned that there are some aspects absent from the current theory :7 (i) the assumption of two-body interactions instead of three-body ones which become important near to the @-state for polymer-polymer-solvent8~9and for polymer- mixed-solvent ternary systems ; (ii) the polydispersity of the polymer sample is and (iii) the chain stiffness and chain-ends effects, as has been very recently described by Yamakawa and co-workers,' '-I3 must be considered in order to explain accurately the experimental findings that are inconsistent with the two-parameter theory predictions.We present here an application of the two-parameter theory extended to TPS near to and far from @-conditions using a modified Kurata-Yamakawa interpenetration func- tion, YKY,including three-body interactions evaluated from ternary solution data, following the recently reported meth- ~dology.'~.'~In addition, a complementary treatment of mixture, respectively and #i is the volume fraction of solvent i.Since [qIE comes from the transport property, it is possible to establish a relationship between this property and ther- modynamic behaviour of the solvent mixture, usually denoted by the binary interaction parameter g12 in TPS or by a more fundamental thermodynamic magnitude, such as the excess Gibbs energy GE [g124142x GE/RT; see eqn.(4) in ref. 151. Then: where the factor C was determined to be 0.51;17 m0 is the Flory viscosity constant (2.5 x mol-' when [q] is in ml g-'); U3 the partial specific volume of the polymer; M the weight-average molar mass of polymer; NA the Avogadro constant and V, the molar volume of solvent 1. By combination of eqn.(1) and (2) and after some rearrangement, the following equation is obtained :l4 in which a plot of the left-hand side vs. g124' allows us to obtain [q]23 from the intercept and the (1 -2~4,) constant from the slope. Note that [qIz3 values used to be found from experimental measurements; however, the extrapolation pro- cedure proposed here is advantageous when the polymer, 3, is not soluble in the liquid, 2 (i.e. polymer-precipitant system). 340 On the other hand, the second virial coefficient for TPS in the framework of the Flory-Huggins polymer lattice solution theory modified by Pouchly18 can be expressed as: u;A ---2v1 x [41-k s42 -2x13 41 -2sx23 42 + 2(1 -2a,)g12 41421 (4) where s = V1/V2 the ratio of molar volumes of solvents and xi3 (i = 1, 2) the polymer-solvent interaction parameter.For the sake of simplicity and denoting the left-hand side of eqn. (5)by X, eqn. (4)can also be written as:18 = -sx23 + -2a,)g1241 (5) Plotting X against gI2 41, x23 and (1 -2a,) can be obtained from the intercept and slope, respectively. The A, for TPS expressed in eqn. (4) can be easily trans- formed into A, for binary polymer systems (BPS) when boundary conditions for solvent composition, 4i -,1, are applied. Since in polymer-mixed solvents there are two second virial coefficients, it is convenient to denote them by A2,i3, and hence A2,i3 = V2,/V1(1/2-xi3). In addition, this binary second virial coefficient can be defined, in the frame- work of the two-parameter theory, as A2,i3 = K[qIi3 Yzy, where K = 1.516NJQokf and Yky the classical Kurata- Yamakawa interpenetration function accounting for the excluded-volume effects.From these A2,i3 expressions, there can easily be found a correlation between xi3 and [qli3, and therefore the insertion of these two expressions into eqn. (4) holds : In general, Y can be evaluated experimentally from light scattering and viscosity measurements because Y FZ A2[q]-Assuming the nomenclature introduced in previouspaper^'^,'^ for a TPS, the second virial coefficient has the following functionality: A, = KY[q]. The selection of appro- priate values for both Y and [q] is decisive in order to obtain reliable A, data. Often, in the past, a binary interpenetration function, Yky, has been used for both BPS and TPS in com- bination with the experimental intrinsic viscosity, thus : A2 = K%YCZ?l* (8) however, as has been recently proposed,14 a new functional- ity for the interpenetration function, namely YFy,works better.Thus, one can write: (9) Consequently, the YFY values used throughout the paper have been obtained by means of the equation:14 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 To sum up, eqn. (3), (5) and (7) allow us to calculate (1 -2a,) from experimental [& data and g12 values at a given composition for each TPS. Results and Discussion The above formalism has been applied to the following ternary systems : ethyl acetate (EA)-cyclohexane (Ch)- polystyrene (PS);" EA-Ch-PS;20 Ch-dimethyl ketone (DMK)-PS2 and 1-chlorobutane (C1Bu)-acetonitrile (AcN)- poly(methy1 methacrylate) (PMMA),22 selected from the liter- ature due to the availability of [q]T and g1223 experimental values.As has been stated in the above section, the values of these magnitudes are the input data necessary to test the validity for TPS of the proposed approach. YTEvaluation including Ternary Interactions Fig. 1 depicts, as an example, the plots of (a)eqn. (3), (b)eqn. (5)and (c) eqn. (7) for the EA-Ch-PS-106 system.20 As can be seen, good linear fits are obtained in all cases, allowing accu- rate determination of [q]23 and (1 -2a,) values from the intercepts and slopes. The same trend and behaviour is observed for the remaining systems studied (not shown here).These values are gathered in Table 1, where it seems to be reasonable to adopt the (1 -2a,) values from eqn. (3) as ref- erence values because they have been directly determined experimentally. In general, there is good agreement between the three sets of c~7-J~~and (1 -2a,) values for each system, except for Ch-DMK-PS where the corresponding values for the higher polymer molar masses cannot be evaluated because the plots of eqn. (3), (5) and (7) yielded poor linear-fit correlations. 60 , Y W+v 030tI X -0.41 -0.43 +2+ m 250 -200 ?L 150 ZE6 50'I + "00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 glzh Fig. 1 Plot of (a) eqn. (3), (b)eqn. (5) and (c)eqn. (7) for PS-106 in EA-Ch mixed solvent (data taken from ref.20) J. CHEM. SOC. FARADAY TRANS., 1994, VOL 90 34 1 Table 1 Values of and (1 -2u,) obtained from eqn. 3). (5) and (7) for different ternary polymer systems [V123/dl g-' 1 -2a, solvent mixture polymer" eqn. (3) eyn. (5) eqn. (7) eqn. (3) eqn. (5) eqn. (7) EA-Ch' PS-35 15.8 15.8 15.8 0.111 0.1 11 0.1 11 PS-113 28.6 28.4 28.4 0.120 0.1 19 0.1 19 PS-177 36.3 36.6 36.6 0.120 0.119 0.120 PS-379 53.9 53.8 53.9 0.109 0.108 0.108 PS-6 19 67.5 67.5 67.4 0.107 0.106 0.107 EA--Ch' PS- 106 28.7 28.5 28.8 0.120 0.120 0.1 19 PS-294 49.1 49.1 49.2 0.112 0.1 12 0.112 PS-420 58.8 59.0 58.8 0.108 0.107 0.108 PS-640 73.6 72.7 73.3 0.101 0.101 0.100 PS-960 90.9 90.3 90.8 0.094 0.094 0.094 Ch-DMKd PS-50.8 12.8 12.9 12.9 0.079 0.079 0.079 PS- 140 20.3 20.4 20.3 0.065 0.064 0.064 PS-270 27.4 27.5 27.3 0.079 0.079 0.079 --__ ---PS-626 -__ --PS-870 CIBU-AcN' PMMA-73.4 14.1 12.9 12.9 0.105 0.104 0.104 PM MA-124 16.3 16.0 16.0 0.111 0.1 10 0.1 10 PMMA- 189 19.2 18.9 18.8 0.102 0.101 0.102 PMMA-232 21.3 20.6 20.7 0.099 0.098 0.099 'The number indicates the weight-average molar mass in kg.* Ref. 19; ref. 10: ref. 21; ref. 22. The availability of A, experimental data for TPS is rare; in:pection of the (1 -2a,) values we observed in all cases however, [qIT experimental values have often been reported thiit they are lower than those compiled in Table 1, so that all for these polymeric systems. This apparent drawback to the tht: values related to them will be underestimated.The eluci- application of our formalism can be easily obviated because dation of the most appropriate way to calculate (1 -2a,) or the evaluation of A, starting from [qIT is workable, as has A; values. to be introduced within the proposed formalism, been previously demon~trated'~ for some TPS using eqn. (8) wi 1 be further presented. or (9). However, as can be seen from the preceding section, In order to visualize the differences between both 'Piyand these equations differ only in the expression of the ikerpene- Y,, quantitatively, we can plot both functions us. the-cubed tration function and therefore the calculated A, values will radius expansion factor, x:, according to the classical plots also be different. As an example, Fig. 2 depicts the A, values de-ived from the two-parameter theory.Thus, in Fig. 3 Yiy evaluated theoretically using eqn. (8) and (9), 1's. for the (sclid line) and YT(symbols) have been plotted for various 41 tetraline (TET)-Ch-PS-294 ternary showing a va ues, as a function of a: for the systems (a) TET-Ch-PS24 strong discrepancy between both sets of values. The same anl (b)EA-Ch-PS2' It appears that, at a given r:, the YT plots (not shown here) have been performed for the other va ues are always higher than the corresponding binary ones, TPS listed in Table 1. In order to corroborate that YT is a an 1 this deviation between them increases with increasing r:. more appropriate function than Yiy,the A, values calcu- lated from eqn. (8) have been used to recalculate (1 -2u,) and indirectly [v],~ from ~23.These values are compiled in Table 2 Values of [q]23and (1 -2a,) obtained from eqn.(5) using Table 2 for the same systems reported in Table 1. From the .4, from eqn. (8) for different ternary polymer systems sc lvent mixture polymer" Crll23ldl g -1 -2a,-EA-Chb PS-35 16.5 0.081 PS-113 32.2 0.080 PS- 177 41.5 0.078 PS-379 62.8 0.069 "3 PS-619 84.7 0.067 0, EA-Ch' PS- 106 29.6 0.082 PS-294 52.8 0.072 PS-420 64.6 0.068 PS-640 80.0 0.063 PS-960 99.6 0.059 Jh-DMKd PS-50.8 4.8 0.058 PS-140 6.2 0.045 PS-270 14.7 0.053 PS-626 0lL-A PS-870 0.0 0.2 0.4 0.6 0.8 1 .o 'IBu-AcN' PMMA-73.4 13.5 0.08 8 41 PMMA- 124 15.8 0.106 Fig. 2 Comparison between the second virial coefficient calculated PM M A-1 89 16.8 0.078 eqn. (8) and (0)eqn.(9) as a function of the solvent com-PMMA-232 18.1 0.099with (0) -position for the PS-294 polymer sample in the TET-Ch mixed-solvent system " 4s Table 1. 342 0.4 0.3 -0.2 -0.1 -> Ym % v 8 (b1 $- - 0.3 -0.2 -0.1 0.0Y 0 1 2 3 4 a,” Fig. 3 Y, (symbols) and (solid line) plotted against a: for PS in: (a)TET-Ch and (b) EA-Ch. 4l = (a)0.80,(0)0.60, (H)0.40 and (0)0.20. This particular behaviour of the (Y,, a,”) pairs of values is generally attributed to the co-solvent feature of ternary polymer systems, as has been justified previ~usly’~ when g,, > 0. We believe that the thermodynamic behaviour of the solvent mixture, represented by g12 or its equivalent GE, is the main factor accounting for the discrepancies between Y, and YEyobserved in Fig.3. This speculation can be validated by Fig. 4 where it is assumed that the term on gI2in eqn. (10) is cancelled out, and consequently a universal plot or agree- ment between Yr and YEy values is obtained, independent of solvent composition. As can be seen in Fig. 3 the Y, values are always higher than the YE’ ones, even when using [SIT in the expression for YE’, because the variation of this magnitude with a: is unique whichever viscosity data are used. Consequently, the corresponding A, values calculated from eqn. (9) (with Yr) 0.3 0.2 Ym* z $-0.1 0.0 0 1 2 3 4 5 a,” Fig. 4 Y,(symbols) and YEy(solid line) plotted against a,” for PS in TET-Ch solvent mixture considered as a single liquid (gI2= 0).Symbols as in Fig. 3. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 are also higher than those obtained from eqn. (8) (with Y;’) as observed in Fig. 2. On the other hand, when YEy is used instead of YT,the A, values obtained from eqn. (8) do not agree with the experi- mental ones. This fact can be corroborated quantitatively from the comparison of [v],~ and (1 -2a,) values for PS and PMMA reported in Tables 1 and 2. From inspection of both sets of data, a clear disagreement is evidenced, supporting the argument that eqn. (9) and (10) are the most suitable, in the frame of the two-parameter theory, to predict the excluded- volume effects in TPS. Moreover, in contrast to the erron- eous data of Table 2, the corresponding ones compiled in Table 1 can be used to make right the interconversion of [q]T in A, or oice uersa by means of eqn.(9) when Y, is known. In this connection, it is interesting to note that the inclusion into eqn. (3) of an additional term has been suggested, namely AKO, to account for the deviation from ideality of [q],-. However, we believe that this correction can be neglected, at least in the systems studied here, because its contribution does not substantially affect the (1 -2a,) and [v],~sets of values, according to the same behaviour exhibited by other TPSs and recently reported by our group.” Y, Evaluation including Intra- and Inter-molecular Interactions We shall now analyse the excluded-volume effect in the light of the intra- and inter-molecular interactions clearly defined by Wolf.16 In this context, both types of interactions can be related to macroscopic properties of polymer solutions, as will be pointed out below.According to the above-mentioned formalism, the required transformation for TPSs has been recently p~blished,’~ and it follows that: A -A: + b~(0.5-4 (11)2-where A, and A? are the second virial coeficients of a polymer in a given solvent at finite and infinite molar mass, respectively; b and a are constants at a given 41.Both A; and b, are not accessible experimentally, but can be easily related to the intra- and inter-molecular interaction param- eters, namely xi3and xS3, following the original nomencla- ture reported by Wolf16 (the subscript ‘M3’, exclusively affecting this section, refers to the polymer, 3, immersed in a binary solvent mixture), as: A?= (1--xh3 +xi3)5 2 2 Vm and b= (xe, x”) ;;; where V, = V,V,/(41 V, + 4, Vl), the molar volume of solvent mixture and KO and K are the viscosity constants near to and far from 8 conditions.The following values have been used for PS and PMMA polymer samples: for the first system, U3 = 0.923 (ml g-1);23 KO = 76 x loW3(ml g-3/2)25 and K values depending on solvent composition are reported in Table 1 from ref. 20. For the second one, U3 = 0.805 (ml g-1),23KO = 62 x (ml molli2 g-3/2)25 and K values are reproduced from Fig. 6 in ref. 22. In Fig. 5, eqn.(11) has been plotted for EA-Ch-PS,’ covering the same range of molar masses as in Table 1. The A, data used have been calculated using eqn. (9) inserting the values of the viscometric exponent, a, given in Table 1 of ref. 20. A very good linear fit has been attained, which sup- ports the validity of the proposed extension of eqn. (11) to .I.CHEM. SOC. FARADAY TRANS.. 1994. VOL. 90 d... A N i0, 2.8 A ,1 0 A -/-E ' .'0 2.45 z--. 2 .o 1.6 0.10 0.12 0.14 0.16 0.18 0.20 - M(0.5 a) Fig. 5 Plot of eqn. (1 1) for PS at different compositions of EA--Ch solvent mixtures. 4, = (A)0.60.(0)0.65, (.I 0.70.(C)0.75 and (*I 0.80. TPSs, allowing accurate evaluation of A; and tt values from the intercept and slope.Because all the magnitudes involved in eqn. (12) and 13) are known. we can now evaluate the intra- and the inter- molecular interactions affecting a polymer coil in a binar) solvent mixture, namely xi3and xk3. Both zi3 and x',? values are plotted in Fig. 6 where the intra-cs. intcr-molecular interaction parameters are depicted for binar) systems" (solid line) and for polymer-mixed-solvent systems (symbols). These last results are clearly in conflict with the universal character exhibited in BPS because, as can be easil! seen, very good correlation is associated with BPS data whereas for TPS no correlation is observed, and most of the data are positioned to the right of the solid line. In order to give a theoretical explanation of this behaviour.we have assumed that the intramolecular interactions of a macro-molecular chain under the conditions reported here, are the same for BPS and TPS, that is x0 = xi3,and no dependence on solvent composition is considered. This assumption. strictly speaking, is suitable for uncharged homopolymers in solvents of very low polarity. In this way, Fig. 7(a)shows the solvent dependence of the difference -xi3which is a measure of the deviation between binary and ternary data. for the same systems as in Fig. 6. Convex upward curves are clearly evidenced in Ch-DMK-PS and CIBu-AcN-PMM A systems; however, a smoothed curve is obtained for the system EA-Ch- PS. A similar but more continuous function- ality, in the purely thermodynamic sense, corresponds to the solvent mixture composition dependence of GE:'RT.In order 0.8 3n= 0.4 I 0.2s 0.35 0.45 0.55 0.65 XI' or j(h3 0.05 I c \ 0.0 0 0.2 0.4 0.6 0.8 1 d1 Fit:.7 Variation of (a) -& and (h) C;" RT with the solkent mi iture composition.Symbols as in Fig. 6. to compare both functions. this last dependence has been pltltted in Fig. 7(h)for the same TPSs. where a certain coin- cicence on the placement of the maximum of the respective cu .\es can be observed, which could support the idea that the sh,ft from ideality of the solvent mixture is mostly responsible foi the discrepancies between z)' and zL3 values. The connec- tion between both magnitudes can be established in the framework of the Flory-Huggins theory of polymer solutions co ipled to TPS at infinite molar mass.In this regard, the second virial coeflicient A; can be defined as A; = (1 2 -l&)i;S, Ck where zG3 is the polymer-solvent mixture inter- aci ion parameter at infinite molar mass. The relationship be* ween the phenomenological interaction parameter &.3 anat the xC3 and xE3 ones can be described by using zh3 = [x:,.~+ %E3]!2. On the other hand. z,& has also been defined as It' wh;.re the last term of the right-hand side of eqn. (14) involves thc binary parameter gI2proportional to G'IRT [see eqn. (4) fro n ref. 15). Consequently, the magnitudes on the ordinates of =ig. 7 are closely related and hence a maximum in GE-.'R7' deliotes the same extreme condition for y12, so that xh3 acc.uires a minimum value.Following our argument, when z-is a minimum xL3 will also be a minimum and the differ- ente (XI' -~5,)will be a maximum. This explanation is sup- po'ted by the dependence of both molecular interaction pal ameters and excess Gibbs energy on solvent composition, as +hewn in Fig. 7. I 'inally, we expand somewhat the previous analysis14*' on Y, in order to show explicitly the contributions of intra- and intl:r-molecular excluded-volume interactions to the interpen- etr.ttion function as well as its dependence on solvent com- po!ition, dl. For this purpose, it is necessary to derive an exrression relating YTand the lI3 and lk3 parameters, by coribination of eqn. (15) and (16) of our preceding paper." Fig.6 Dependence of the intramolecular interaction parameters 2'' yie ding: and xG3 on the intermolecular interaction parameters z'' or ~6,~. Solid line BPS; (A) EA-Ch-PS: (0)Ch-DMK-PS: (CI CIBu-AcN-PMMA. 0.35 1 0, .-.--0--0--0--,$-0.25 --9.6 '0 0.20 -***** o.15+'1"'1'1'-Note that we have written for convenience a normalized interpenetration function for the ratio YT/"iy because YEy is a constant at a given a:, and hence the functionality of YT will remain unchanged. Fig. 8 depicts the plot of YT, from eqn. (lo), us. 41 for various a: for the EA-Ch-PS2' system. As can be seen, there is no linear dependence of YT on +1 and the curves connecting the data points are convex in all cases. However, it is increasingly difficult to select the 41 value where the maximum takes place as a: decreases, approaching the unperturbed state (a: = 1).This observed YTdependence on is not considered in the original two- parameter theory or in the RG theory, where a single com- posite curve is postulated, therefore we believe that it is an important goal to seek an explanation of this behaviour of the excluded volume in TPS in the framework of the two- parameter treatment. With this aim, focussing our attention on eqn. (l5), it seems that the difference (xi,-xb,) as a func- tion of solvent composition exhibits a maximum by analogy with that shown in Fig. 7(a). In contrast, VJV, obviously depends on c$~ but does not show any extrema condition (maximum or minimum) at least for the TPSs tested here; and the remaining parameters of eqn.(15) are not dependent on solvent composition. Thus, it may be concluded that the observed behaviour of YTin Fig. 8 cannot be well explained quantitatively using the extended Wolf formalism for TPS. In order to clarify this unsolved problem, a similar study is now in progress starting with the Y and a dependence on a modi- fied scaled excluded-volume parameter, z, especially for polymer-mixed-solvent systems. Much experimental and theoretical work still needs to be done. Conclusions A recently proposed formalism dealing with the excluded- volume effects in polymer-mixed-solvent systems has been tested using solution data reported by other author^'^-^^ for PS and PMMA.The new functionality proposed for the Kurata-Yamakawa interpenetration function has been shown to be better than the classical one for reproducing the experimental intrinsic viscosities and second virial coefficients for polymers in a solvent mixture. The importance of the ternary interactions contributions is noted and the a, parameter has been exhaustively and quan- titatively evaluated for PS and PMMA, covering a whole range of molar mass and solvent composition. Moreover, because one component of the solvent mixture behaves as a non-solvent (precipitant), a virtual intrinsic viscosity, [q]23, for the above samples has also been calculated by means of eqn. (3), (5) and (7) (see Table l), showing good agreement between the values which supports the proposed formalism.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 The values of YTfor the systems studied are always greater than the corresponding ones evaluated using the classical interpenetration function performed for binary polymer solu- tions, Yi'. In addition, the YTvalues are strongly dependent on solvent composition, in contrast with the universal char- acter claimed by the original two-parameter theory (see Fig. 3 and 8). In the light of eqn. (10) it is plausible to believe that the g12 term, closely related to the excess Gibbs energy, GE,is responsible for the observed differences between both interpenetration functions. In fact, when the single liquid approximation is done, gI2= 0, a universal function is found (see Fig.4). An alternative analysis of the excluded volume in TPS has been carried out by means of an extension of the formalism developed by Wolf16 for BPS. The intra- and inter-molecular parameters have been evaluated; however, we believe that this formalism does not give a good quantitative explanation of the YTbehaviour. This work has been partially funded by the Direccion General de Investigacion Cientifica y Tecnica (Ministerio de Educacion y Ciencia, Spain) Grant No. PB91-0808. One of us (I.P.) is indebted to Ministerio de Educacion y Ciencia (Spain) for a predoctoral long-term fellowship. References 1 P. J. Flory, in Principles of Polymer Chemistry, Cornell Uni- versity Press, Ithaca, New York, 1953.2 H. Yamakawa, in Modern Theory of Polymer Solutions, Harper and Row, New York, 1971. 3 P. G. De Gennes, in Scaling Concepts in Polymer Physics, Cornell University Press, Ithaca, New York, 1979. 4 K. F. Freed, in Normalization Group of Macromolecules, Wiley, New York, 1987. 5 K. F. Freed, Acc. Chem. Res., 1985, 18,38. 6 M. Daoud and G. Jannink, J. Phys. (Paris), 1976,37,973. 7 Z. Y. Chen and J. Noolandy, Macromolecules, 1992,2S, 4978. 8 L. Schafer and C. Kappeler, Colloid. Polym. Sci., 1990,268,995. 9 Y. Guan, T. H. Lilley and T. E. Treffry, Macromolecules, 1993, 26, 3971. 10 C. H. Kang and S. I. Sandler, Macromolecules, 1988,21, 3088. 11 F. Abe, Y. Einaga, T. Yoshizaki and H. Yamakawa, Macro-molecules, 1993, 26, 1884. 12 F. Abe, Y. Einaga and H. Yamakawa, Macromolecules, 1993, 26, 1821. 13 H. Yamakawa, F. Abe and Y. Einaga, Macromolecules, 1993,26, 1898. 14 V. Soria, C. M. Gomez, R. Garcia and A. Campos, J. Chem. Soc., Faraday Trans., 1992,88, 1555. 15 C. M. Gomez, V. Soria, J. E. Figueruelo and A. Campos, J. Chem. SOC., Faraday Trans., 1993,89, 1765. 16 B. A. Wolf, Macromolecules, 1985, 18,2474. 17 W. H. Stockmayer and M. J. Fixman, J. Polym. Sci., Part C, 1963, 1, 137. 18 J. Pouchly and A. Zivny, Makromol. Chem., 1983,184,2081. 19 P. Munk, M. T. Abijaoude and M. E. Halbrook, J. Polym. Sci., Polym. Phys., 1978, 16, 105. 20 A-A. A. Abdel Azim, S. S. Moustafa, M. M. El Dessouky, F. A- Rehim and S. A. Hassan, Polymer, 1986,27, 1406. 21 H. Maillols, L. Bardet and S. Grom, Eur. Polym. J., 1978, 14, 1015. 22 M. G. Prolongo, R. M. Masegosa, I. Hernandez-Fuentes and A. Horta, Macromolecules, 1981, 14, 1526. 23 A. Campos, R. Gavara, R. Tejero, C. Gomez and B. Celda, J. Polym. Sci., Polym. Phys., 1989, 27, 1599. 24 A-A. A. Abdel Azim and M. B. Hugh, Polymer, 1983, 24, 1429. 25 Polymer Handbook, ed. J. Bandrup and E. H. Immergut, Wiley Interscience, New York, 1975. 26 A. Campos, B. Celda, J. Mora and J. E. Figueruelo, Polymer, 1984,25, 1479. Paper 31054205;Received 9th September, 1993

ISSN:0956-5000    

DOI:10.1039/FT9949000339

出版商:RSC    

年代:1994

数据来源: RSC