J. Phys. Chem. 1988, 92, 5569-5571 less mobile on the electrode surface and perhaps cannot orient for most effective electron and energy transfer between the species. The rates of the reactions leading to R*, eq 1-6, may also differ in the monolayer. For example, production of the critical intermediate, CO;, may be more effective in the monolayer, if the direct oxidation at the electrode surface of it and C2042-are effectively blocked by the monolayer. In this case any C02- a t the monolayer will produce either R* directly via eq 6, or indirectly via (4) and (5); other processes leading to loss of this very reducing species, e.g., reduction of protons or dimerization, must also be occurring in both monolayer and solution reactions. While a fuller elucidation of the processes described above requires further studies, the availability of an excitable monolayer assembly on conductive substrates should allow a number of novel experiments to be carried out. For example, the study on the effect of electrode material and potential on the quenching of R* following photoexcitation should be possible. ECL has been used as a highly sensitive analytical m e t h ~ dthe ; ~ monolayer approach
5569
may provide for even higher sensitivity, with a lesser amount of material concentrated on an electrode surface. It may also be useful in the fabrication of a fixed probe for ECL. When combined with the techniques of S T M or scanning electrochemical micro~copy,'~ with video detection and imaging,14 it should be possible to detect optically the distribution of small numbers of molecules or the arrival of suitable reactant molecules at the monolayer/solution interface. Experiments of this type are under way in our laboratory.
Acknowledgment. The support of this research by the Robert A. Welch Foundation (F-079) and the Army Research Office (DAAG29-85-K-0104) is gratefully acknowledged. (13) Bard, A. J.; Fan, F.-R. F.; Kwak, J.; Lev, 0. J. Am. Chem. SOC., submitted fot publication. (14) See, for example: (a) Bilhorn, R. B.; Sweedler, J. V.; Epperson, P. M.; Denton, M. B. Appl. Spectrosc. 1987,42, 1 1 14; (b) 1987,41, 1125. (c) Hiraoka, Y.; Sedat, J. W.; Agard, D. A. Science 1987, 238, 36.
Silicon-29 Cross-Polarization/Magic Angle Spinning NMR Evidence for Geminal Siianois on Vacuum-Activated Aerosil Silica B. A. Morrow* Department of Chemistry, University of Ottawa, Ottawa, Canada K I N 6N5
and Ian D. Gay* Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A IS6 (Received: January 21, 1988; In Final Form: June 16, 1988)
Silicon-29 CPMAS NMR spectroscopy has been used to study aerosil silica which has been activated under vacuum up to 1000 OC and where spectra have been recorded with samples still under vacuum. For all temperatures up to 800 OC activation, a signal due to geminal [Si(OH),] silanol groups was detected, thus clearly demonstrating for the first time that these species exist on aerosil silicas which have been vacuum activated in this temperature range. Line broadening occurs for 1000 OC activation, and the geminal signal cannot be distinctly resolved.
Part of the infrared spectrum of an aerosil silica which has been activated at 450,600,800, or 1000 OC is shown in Figure 1. This spectral region is associated with the OH stretching vibrations of surface silanol groups, and the sharp peak near 3747 cm-I was attributed years ago' to this vibrational mode of isolated (that is, not hydrogen-bonded) single SiOH groups in which the surface Si atom is attached to three other Si atoms via SiOSi siloxane bonds. Several g r o ~ p s have ~ - ~ attempted to show that this sharp band, which is also in the spectra of silica gels, could be resolved into more than one component and that the various components could be assigned to single and geminal, Si(OH)*, silanol groups, where in the latter case, the surface Si atom would have two SiOSi bonds. In most cases,it was shown6,' that the apparent resolution could be attributed to improper balancing of the two beams of a double-beam infrared spectrometer due to the presence of water vapor which is strongly absorbing in this spectral region. Exceptionally, Fink and Plotzki5 reported a shoulder near 3742 cm-' for an aerosil activated at 650 O C which they attributed to geminal silanols, but this finding cannot be duplicated by
Theoretical c a l c ~ l a t i o n s have ~ ~ . ~shown ~ that the OH stretching frequencies of isolated single and geminal silanols should be very close, having a 1-2-cm-' separation, and for this reason one rationalizes the failure of infrared spectroscopy to "resolve" distinct components. However, Hoffmann and Knozinger* were the first to show that, following activation near 900 O C , the peak becomes relatively symmetrical and its maximum shifts to 3747 cm-', as is shown in Figure 1 (a lower resolution was used to obtain our spectra). They speculated that the peak at 3749 cm-' was due to single SiOH species and that part of the intensity at 3747 cm-' was due to geminal silanol species. However, other interpretations could be advanced for these experimental observations because the surface of silica changes considerably as dehydroxylation p ~ - o c e e d s ~(see ~ J ~Discussion). Therefore, infrared spectroscopy has not provided conclusive evidence that geminal silanol groups exist on vacuum-activated aerosil silica and this controversy remains unresolved. By contrast, silicon-29 MAS and CPMAS N M R have been very successfully used to study the SiOH structure of silica
(1) Kiselev, A. V.; Lygin, V. I. Infrared Spectra of Surface Compounds; Wiley: New York, 1975. (2) Hair, M. L.;Hertl, W. J . Phys. Chem. 1969, 73, 2372. (3) Van Cauwelaert, F. H.; Jacobs, P. A,; Uytterhoven, J. P. J. Phys. Chem. 1972, 76, 1434. (4) van Roosmalen, A. J.; Mol, J. C. J . Phys. Chem. 1978, 82, 2748. (5) Fink, P.; Plotzki, I. Wiss.Z . Friedrich-Schiller-Uniu.Jena, Math.Naturwiss. 1980, 29, 809. (6) Morrow, B. A.; Cody, I. A. J . Phys. Chem. 1973, 77, 1465. (7) Hockey, J. A. J. Phys. Chem. 1970, 74,2570. (8) Hoffmann, P.; Knozinger, E. Surf. Sci. 1987, 188, 181.
( 9 ) Ryason, P. R.; Russell, B. G. J . Phys. Chem. 1975, 79, 1276. (10) Ghiotti, G.; Gamone, E.; Morterra, C.; Boccuzzi, F. J . Phys. Chem. 1979,83, 2863. (1 1) We have used six different aerosils, from Cabot Corp. and from Degussa, and after recording about 100 spectra under varying conditions, we have been unable to duplicate the published spectra in ref 5. (12) Sauer, J.; Schrder, K. P. Z . Phys. Chem. (Leipzig) 1985, 266, 379. (13) Sauer, J. J . Phys. Chem. 1987, 91, 2315. (14) Morrow, B. A.; Cody, I. A. J . Phys. Chem. 1976, 80, 1995, 1998. (15) Morrow, B. A,; Cody, I. A.; Lee, L. S. M. J . Phys. Chem. 1976,80, 2761.
0022-3654/88/2092-5569%01.50/0 0 1 9 8 8 American Chemical Society
Letters
5570 The Journal of Physical Chemistry, Vol. 92, No. 20, 1988 A
Figure 1. Infrared spectra of an aerosil disk (10 mg/cm2) which has been activated under vacuum a t the indicated temperatures.
and gels produced via the sol-gel process,2°s21and the evidence shows that for unactivated gels at 25 OC about 15% of the OH groups might be geminal. [Infrared spectroscopy cannot easily be used to study unactivated silica because of the broad band in the OH region due to H-bonded silanols.'] Such gels, which can have surface areas up to 750 m2/g, are usually microporous and are expected to contain geminal and single OH groups. Although studies of dehydrated gels have been reported,18*20,21 the samples were, as far as can be determined from published experimental procedures, placed in conventional rotors (presumably in a dry box) and capped. To our knowledge, there is no published account of an MAS or CPMAS N M R study of an aerosil silica. In this letter we report a CPMAS study of an aerosil silica which has been activated under vacuum up to 1000 OC and where spectra have been recorded with samples still under vacuum or after backfilling with dry oxygen. The results show that geminal silanols persist up to at least 800 OC activation.
Experimental Section Silicon-29 and proton spectra were measured on a 1.4-T home-built instrument. Rf field strengths of 4&50 kHz were used for cross-polarization and decoupling. A cross-polarization contact time of 4 ms was used, and about (1-5) X lo5 scans were accumulated. Samples were spun a t 1 kHz in a spinner previously described,22which permits spinning of vacuum-sealed samples. For the 450 and 600 O C samples a study of the signal intensity vs spinning rate was carried out. The signal decreased linearly by 25% as the spinning speed was increased from 600 to 2500 Hz. This shows that there was no strong variation in C P efficiency with spinning rate for these samples, such as can be inferred for Because of the adamantane from the results of Stejskal et low proton content for the samples activated at higher tempera(16) Maciel, G.E.; Sindorf, D. W. J . Am. Chem. Soc. 1980, 102,7607. (17) Sindorf, D. W.; Maciel, G. E. J. Phys. Chem. 1982, 86, 5208; Ibid. 1983,87, 5516. (18) Sindorf, D. W.; Maciel, G.E. J . Am. Chem. SOC.1983, f05, 1487. (19) Fyfe, C. A.; Gobbi, G. C.; Kennedy, G. J. J. Phys. Chem. 1985.89, 277. (20) Brinker, C. J.; Kirkpatrick, R. J.; Tallant, D. R.; Bunker, B. C.; Montez, B. J . Non-Cryst. Solids 1988, 99, 418. (21) Grimmer, A. R.; Rosenberger, H.; Burger, H.; Vogel, W. J. NonCryst. Solids 1988, 99, 371. (22) Gay, I. D. J. Magn. Reson. 1984, 58, 413. (23) Stejskal, E. 0.;Schaefer, J.; Waugh, J. S. J . Magn. Reson. 1977, 28, 105.
HO
OH
OH
0 -100 -200 Figure 2. 29Si CPMAS NMR spectra of aerosil silica activated under vacuum at the indicated temperatures ("C). The spectra were scaled to a fixed height so that relative intensities are not comparable. The scale is in ppm relative to TMS.
tures, it is not practical to carry out similar measurements with these samples. Although samples were sealed in glass, no silicon-29 signal was observed from an empty tube since cross-polarization is only effective when there is a nearby proton. The silica used was Cab-0-Si1 HS5 having a BET surface area of 332 m2/g, and it was pressed into disks in a stainless steel die at about lo7 Pa prior to use. [A similar procedure is used for most infrared studies.] Fragments of the disk were placed in a 5-mm Pyrex N M R tube to a depth of 2 cm, and they were heated overnight under vacuum to a chosen temperature. For the experiments at 600-1000 OC the sample was heated in a quartz sidearm and then transferred under vacuum to the N M R tube. For the spectra shown, 0.5 atm of carefully dried oxygen was added to the activated sample before sealing. The purpose of the oxygen was to shorten the proton T I which was otherwise inconveniently long, leading to poorer signal-to-noise. The 150 OC sample was run both with and without 02,and there was no differences between the spectra. The infrared spectra shown in Figure 1 were recorded by using a Bomem DA3.02 FTIR instrument at a resolution of 4 cm-'.
Results and Discussion The relative proton concentrations were measured by proton N M R using 90' pulses. At 450, 600, 800, and 1000 O C , respectively, the proton concentrations were about 42,28, 16, and 6% of that at 150 O C , which is in excellent general agreement with literature results.24 Figure 2 shows the 29SiCPMAS spectra of aerosil activated at temperatures between 150 and 1000 O C . Following c o n v e n t i ~ n , ' ~the - ~ -91 ~ * ~ppm ~ peak can be assigned to geminal Si(OH)2 groups, the -101 ppm peak to single SiOH, and the shoulder at about -109 ppm to siloxane silicons which are devoid of O H groups [these peaks are sometimes referred to respectively as 4 2 , 4 3 , and 4 4 p e a k ~ ~ ~Although , ~ ~ ] . the signal-to-noise ratio is not ideal, we believe that the results clearly show that geminal hydroxyls are still pqesent after activation at 400-800 O C . Because of the considerable broadening of the overall spectrum for 1000 OC activation, it is difficult to ascertain whether they are still present at this temperature. Similar broadening was observed by Sindorf and Macieli8 for heating silica gel in the 417-650 OC range and by Brinker et aLzoand Grimmer et a1.21 (24) Zhuravlev, L. T. Langmuir 1987, 3, 316. (25) Harris, R. K.; Kennedy, J. D.; McFarlane, W. NMR and rhe Periodic Table; Harris, R. K., Mann, B. E., Eds.; Academic: New York, 1978.
J . Phys. Chem. 1988,92, 5571-5575 for heating sol-gel-produced silicas in the 600-1 100 OC range, and no clear explanation is forthcoming. Sol-gel-produced silicas are quite different from aerosil, and Brinker et a1.%have speculated that part of the apparent broadening for samples activated near 600 OC may be due to the formation of so-called D2 defect sites [as detected by Raman spectroscopy] which were attributed to the condensation of single silanols to yield slightly strained sixmembered siloxane rings, (SiO)3. We have previously reportedI4J5 that a highly reactive surface site, characterized by a pair of infrared bands at 908 and 888 cm-', was formed when silica was heated above 450 OC. The number of sites increased rapidly above 1000 OC activation but never reached a concentration greater than about 0.15/nm2. The site facilitated the dissociative chemisorption of NH3, H 2 0 , and C H 3 0 H a t 20 OC, or the reversible coordination of pyridine or trimethylamine, and we postulated that the simplest model for the site was a highly strained siloxane bridge in which one of the silicon atoms might be electron deficient. Michalske and Bunker26 and Brinker et ale2' have suggested that this site might be a very strained four-membered siloxane ring, as follows:
5571
Further, weI4J5and othersl*28have shown that the silica surface becomes increasingly hydrophobic after activation at elevated temperatures, a view which has been reinforced recently by Lochmuller and K e r ~ e for y ~ silicas ~ activated above about 800 OC. Whatever the exact nature of the site, it is clear that the surface of silica is considerably modified as the temperature of dehydroxylation in vacuum is increased, and it may well be that the broadening of the 29Siresonance might be associated with these changes in the environment of the silicon atoms. Further speculation is unwarranted. It is interesting to note that the intensity of the 4 2 (geminal) peak relative to that of the 4 3 peak appears to increase as the temperature of activation increases from 150 to 800 OC. Qualitatively, similar features have been observed by others for other types of silica.'820p21Because cross-polarization was used, it would be premature to speculate whether this is a real effect without carrying out further experimentation. The important point here is that the N M R spectra show that geminal silanols do persist on aerosil silicas even after evacuation to 800 OC in spite of there being no direct infrared evidence to support this. In agreement with theoretical calculations,I2 the geminal and single O H stretching frequencies must have no more than about a 1-cm-' shift for this to be so.
Acknowledgment. We are grateful to NSERC of Canada for financial support. (26) Michalske. T. A.: Bunker. B. C. J. A D D ~Phvs. . 1984. 56. 2686. (27) Brinker, C.'J.; Taliant, D. R.;Roth, E. P.';*'Ashlei, C. S. J.'No;-Cryst. Solids 1986, 82, 117.
(28) Young, G. J. J. Colloid Interface Sci. 1958, 13, 67. (29) Lochmuller, C.H.; Kersey, M. T. Langmuir 1988, 4, 572.
Alternatlve Formalism for the Calculation of Atomic Polar Tensors and Atomic Axial Tensors R. D.Amos,* Department of Theoretical Chemistry, Cambridge University, Cambridge, CB2 1 E W, United Kingdom
K. J. Jalkanen, and P. J. Stephens* Department of Chemistry, University of Southern California. Los Angeles. California 90089-0482 (Received: May 26, 1988)
A new formalism for the calculation of atomic polar tensors is implemented at the ab initio SCF level using analytical derivative methods based on coupled Hartree-Fock perturbation theory. Results for NH3 obtained with a range of basis sets are compared to those given by the standard formalism at the SCF level using the same basis sets. The utility of this alternative methodology in the calculation of atomic axial tensors is also discussed.
The calculation of atomic polar tensors' is fundamental to the calculation of vibrational absorption intensities.2 Both atomic polar tensors and atomic axial tensors3 are required in calculating vibrational circular dichroism i n t e n ~ i t i e s . ~ -A~ variety of procedures exists for the ab initio calculation of atomic polar tensorse2 Currently, at the S C F level, analytical derivative methods based on coupled-Hartree-Fock (CHF) perturbation theory are most widely ~ s e d . ~ * ~Analytical ,' derivative methods using C H F perturbation theory have very recently been applied also to the (1) Person, W. B.; Newton, J. H. J . Chem. Phys. 1974, 61, 1040. (2) Amos, R. D. Adv. Chem. Phys. 1987, 67, 99. (3) Stephens, P. J. J. Phys. Chem. 1987, 91, 1712. (4) Stephens, P. J. J. Phys. Chem. 1985,89, 748.
(5) Stephens, P. J.; Lowe, M. A. Annu. Rev. Phys. Chem. 1985,36,213. (6) Amos, R. D. Chem. Phys. Lett. 1984, 108, 185. (7) Yamaguchi, Y.; Frisch, M.; Gaw, J.; Schaeffer, H. F.; Binkley, J. S, J. Chem. Phys. 1986, 84, 2262.
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calculation of atomic axial tensors a t the S C F level.*-I5 In this Letter we discuss an alternative formula for atomic polar tensors. This expression has been implemented at the SCF level, (8) Amos, R. D.; Handy, N. C.; Jalkanen, K. J.; Stephens, P. J. Chem. Phys. Lett. 1987, 133, 21. (9) Jalkanen, K. J.; Stephens, P. J.; Amos, R. D.; Handy, N. C. J . Am. Chem. Soc. 1987, 109, 7193. (10) Jalkanen, K. J.; Stephens, P. J.; Amos, R. D.; Handy, N. C. Chem. Phys. Lett. 1987, 142, 153. (1 1) Jalkanen, K. J.; Stephens, P. J.; Amos, R. D.; Handy, N. C. J. Phys. Chem. 1988, 92, 1781. (12) Jalkanen, K. J.; Stephens, P. J.; Amos, R. D.; Handy, N . C. J. Am. Chem. SOC.1988, 110, 2012. (13) Kawiecki, R. W.; Devlin, F.; Stephens, P. J.; Amos, R. D.; Handy, N. C. Chem. Phys. Lett. 1988, 145, 411. (14) Stephens, P. J.; Jalkanen, K. J.; Amos, R. D.; Lazzeretti, P.; Zanasi, R., to be submitted for publication. (15) Jalkanen, K. J.; Kawiecki, R. W.; Stephens, P. J.; Amos, R. D, to be submitted for publication.
0 1988 American Chemical Society
