J. Phys. Chem. 1082, 86, 4107-4112
power give way to C3H,+, C2H,+,CH,+, and H+ (point 2). Through a combination of ab initio calculations, photoelectron spectroscopy, and symmetry arguments, the tenphoton fragmentation scheme of Figure 7 was derived for norbornadiene in a laser pulse at 3550 A. The ionic products resulting from nonlinear photochemistry in quadricyclane are very similar to those observed in norbornadiene, presumably from the common intermediate C5H6+in the two systems.
Summary In this work, we have attempted to construct a coherent picture of laser-driven molecular photochemistry using assorted observations on widely different compounds. The end result is the classification scheme and list of characteristics outlined in Table 11. Though it is not yet clear
4107
as to how general (and useful) such a classification scheme will be, it is clear that there are at least three distinct types of nonlinear photochemical behavior. One also sees that the course of one-photon photoionization in the vacuum UV can be very different from that of multiphoton ionization in the visible/UV even though equivalent amounts of energy are deposited in the molecule in the two situations. The stepwise nature of the molecular MPI process is responsible for this difference. Though we observe only ionic end products, a certain amount of information can be gleaned from this concerning the nature of the photochemistry occurring among neutral species. Acknowledgment. We acknowledge helpful conversations with G. J. Fisanick, William Wadt, and T. Bowmer, and the computational aid of Tali Gedanken.
ARTICLES Infrared Spectroscopic Study of Hydroxyl Groups on Silica Surfaces Isao Tsuchlya Electrotechnlcal Laboratory, Umezono, Sakura-mura, Nllharlgun, Ibarakl, Japan 305 (Received: June 9, 198 7; I n Flnal Form: June 17, 1982)
Infrared spectra of Aerosil silica were measured for samples which were subjected to bakeout, deuterium exchangb, and diethyl ether adsorption. The absorbance difference spectrum between the infrared spectra obtained at different conditions showed several peaks and shoulders for each experiment. In consideration of a model of surface hydroxyls, it is concluded that infrared absorption of the silica degassed under vacuum below 500 "C contains a number of contiguous bands and that most of these bands are attributed to various types of hydroxyl species in the model. In an attempt of decomposition of the difference spectra by a curve resolver, these were respectively resolved into several components,which could be related to each other among the three experiments.
Introduction Many investigators have reported studies of hydroxyl groups on silica surfaces by infrared spectroscopy. It had been previously indicated that infrared absorption due to the OH stretching vibration of the surface hydroxyls consists of a sharp band and a very broad one, which had been assigned to isolated and hydrogen-bonded hydroxyls, respe~tively.'-~ Thereafter, the surface hydroxyls have been considered to be classified into more than two molecular species. Only the isolated hydroxyls exist on silica surfaces degassed at very high temperature. It was pointed out that the OH stretching band due to the isolated hydroxyls, when degassed above 800 "C, has a fine structure and is resolved into three component bands by a curve reso1ver,4v5though (1)L. H. Little, A. V. Kiselev, and V. I. Lygin, "Infrared Spectra of Adsorbed Species", Academic Press, London, 1966. (2) M. L. Hair, "Infrared Spectroscopy in Surface Chemistry",Marcel Dekker, New York, 1967. (3) A. V. Kiselev and V. I. Lygin, "Infrared Spectra of Surface Compounds", Wiley, New York, 1975, Israel Program for Scientific Translation, Jerusalem. 0022-365418212086-4107$0 1.2510
objections to this result were brought forward by other For silica treated at lower temperatures the infrared absorption should contain many more bands because of isolated and hydrogen-bonded hydroxyl^.^ The bands observed for silica gel baked out at 600 "C were assigned to four different types of surface hydroxyls.1° The infrared absorption of silica baked out at relatively low temperatures extends from 2.5 to about 3.2 pm and appears to have a smaoth and long tail on either side of the sharp band due to the isolated hydroxyls. But both of the tails are qbite different in shape and moreover the absorption intensities of the tails do not vary uniformly, when the sample is subjected to any treatment such as (4)M. L. Hair A d W. Hertl, J. Phys. Chem., 73,2372 (1969). ( 5 ) F. H. Van Cauwelaert, P. A. Jacobs, and J. B. Uytterhoeven, J. Phys. Chem., 76,1434 (1974). (6)J. A. Hockey, J. Phys. Chem., 74,2570 (1970). (7)B. A. Morrow and I. A. Cody, J . Phys. Chem., 77, 1465 (1973). (8)P.R. Ryason and B. G. Russell, J.Phys. Chem., 79,1276 (1975). (9)S.Kondo, M. Muroya, and K. Fujii, Bull. Chem. SOC.Jpn., 47,553 (1974). (10)A. J. Van Roosmalen and J. C. Mol, J . Phys. Chem., 82, 2748 (1978).
0 1982 American Chemical Society
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The Journal of Physical Chemistry, Vol. 86, No. 21, 1982
deuterium exchange of the surface hydroxyls. It is, therefore, possible to expect many bands in the infrared absorption. In order to obtain information about detailed structure of the surface hydroxyls, an absorbance difference spectrum was taken between two infrared spectra measured at different conditions of a specimen subjected to a treatment of dehydroxylation, reaction, or hydrogenbond interaction with another molecule. These treatments were carried out by bakeout of the sample, deuterium exchange, and diethyl ether adsorption. Every difference spectrum showed peaks and shoulders unlike the infrared spectrum. The purpose of this paper is to discuss the characteristics of these peaks and shoulders in consideration of a surface hydroxyl model and to give evidence of various hydroxyl species, together with results of decomposition of the difference spectra obtained by a curve resolver.
Experimental Section Material and Infrared Measurement. The sample was silica powder @ioz: >99.8%) of Aerosil from Degussa, Ltd., with a specific surface area of 200 m2/g and an average diameter of the particles of 16 nm. The silica powder was pressed to a pellet of 13-mm diameter which contained 15-70 mg. The transmittance of the specimen was about 60% at 2.5 pm. The infrared absorption cell with NaCl windows was made of quartz and allowed thermal treatment, vapor introduction, and evacuation in situ.l' A prism-type infrared spectrometer was used for avoiding the influence of water vapor in the optical path.- The spectrometer (Shimadzu, Ltd.) was of a double-beam type, equipped with a LiF prism. The resolution was within 0.01 pm at 3 pm and the transmittance accuracy was less than 0.5%. The spectrometer was flushed with air dried by silica gel and molecular sieve. The wavelength scale was magnified so far as the spectrometer permitted. The slit widths and scanning rates were chosen so as to give minimum signal distortion. Repeated recordings showed that the noise of the absorption curve was about 0.2%, and the reproducibility was obtained within 0.3% on the transmittance scale. The spectra of heat-treated samples were recorded after the sample was cooled to room temperature. For physical adsorption of ether, the recordings were carried out after the temperature of the sample, which was warmed by radiation from the light source, became constant. Every recording was started from constant transmittance at 2.5 pm, where no absorption occurs for the silica, by regulating the reference beam of the spectrometer. Because of the limited reproducibility of the recorded curves, the error of spectral absorbance increases rapidly with decrease of the spectral transmittance. On the other hand, when the transmittances of both spectra are extremely high, the absorbance difference also has a large error. Thus, it is necessary to record two infrared spectra which give an optimum transmittance difference, for minimizing the error in the difference spectrum. The transmittance of a silica spectrum in the OH vibration region depends on the quantity of the sample and heattreatment temperature. As it is practically impossible to realize such an optimum transmittance difference, every experiment was performed with different quantities of several specimens, and at treatment temperatures higher (11) K. Kawasaki, K. Senzaki,and I. Tsuchiya, J. Colloid Sci., 19,144 (1964). (12) M. J. D. Low and N. Ramasubramanian,J.Phys. Chem., 70,2740 (1966).
Tsuchiya Wave n u m b e r , cm-1 I
I
3500
3700
3900 h
I
I
1
i
,
3300 I
'
,
,
1
-
"
Y
Wave1 e n g t h , pin
Flgure 1. Infrared spectra of degassed Aerosil silica: after bakeout at (a) 400 and (b) 500 "C;(c) difference spectrum a-b.
than 300 "C, at which physically adsorbed water is removed from the silica surface. The error in the absorbance difference between two infrared spectra is considered to be 0.01 at most. Experimental Procedure. The infrared spectra were measured twice at different conditions for each sample, as described below. Bakeout of the sample was carried out under vacuum (1X torr) for several hours in the infrared cell at 400 and then 500 "C. After bakeout at the respective temperatures, the infrared spectrum was recorded. Deuterium exchange of surface hydroxyls was performed as follows: after the sample was pretreated under vacuum at 350 "C, heavy water vapor was introduced onto the adsorbent in the infrared cell for several minutes at room temperature and then baked out again at 350 "C. Infrared spectra were recorded before and after deuteration. Concerning ether adsorption, after the sample was degassed under vacuum at 350 "C in the infrared cell, diethyl ether vapor was introduced into the cell for several seconds from a flask connected with the cell. Infrared spectra were measured before and after adsorption. Difference Spectra. Three kinds of difference spectra were obtained from the infrared spectra of the sample baked out at 400 and 500 "C, those before and after deuteration, and those before and after ether adsorption. The absorbance difference curve was drawn by passing through the plots obtained from calculations at some wavelength intervals, using the transmittances of the two spectra. The plots were given at intervals of 0.002 pm for steep slopes of the infrared absorption curves. Decomposition of Difference Spectra. It was carried out by a Du Pont type 310 curve resolver for the three kinds of difference spectra. As the band shape, a Lorentzian function was used for the component due to the isolated hydroxyls or their deuterated form, and a Gaussian function for other components. The difference spectrum for ether adsorption (cf. curve c in Figure 4) is not resolved on the assumption of such a function for the longer-wavelength region. Because the difference curve crosses the abscissa at 2.73 pm, it is necessary to smooth out the curve around this wavelength to carry out the decomposition. For this purpose a small foot was added to the crossing of the curve at a little expense of accuracy in the longer-wavelength region. Every difference spectrum was resolved repeatedly by starting afresh every time. The error for the peak wavelength of the obtained component was less than 0.01 Mm. The relative intensities of the components had an error within 20%. Results Bakeout. Curves a and b in Figure 1 show infrared spectra of the sample baked out at 400 and 500 "C, re-
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982 4109
Hydroxyl Groups on Silica Surfaces
Wave n u m b e r , cm-l
Wave number, cm-l
3700
3500
3300
,000 I
3900 I
I
'
3800 I
3700 I l
l
Wavelength, p m
Figure 2. Infrared spectra of deuterated Aerosil silica in OH vibration region: (a) before and (b) after deuteration; (c) difference spectrum a-b. Wave n u m b e r , cm-'
2800 II
2700 I
2600 I
Wavelength, p m
Figure 4. Infrared spectra of Aerosil silica in adsorption experiment: (a) before and (b) after diethyl ether adsorption; (c) difference spectrum a-b. Wave n u m b e r , cm-'
3800
3700 3600
3500
3400
3300
Wavelength, p m
Figure 3. Infrared spectra of deuterated Aerosil silica in OD vibration region: (a) before and (b) after deuteration; (c) difference spectrum a-b.
spectively. Each spectrum consists of a sharp band at 2.67 pm and asymmetric long tails on both sides of the band, which had been assigned to the isolated and hydrogenbonded hydroxyls, respectively. These spectra are similar to those reported by other in~estigators.'-~The absorption of the tail on the long-wavelength side shows roughly a smooth curve, but during baking out from 400 to 500 "C the absorption intensity does not vary uniformly with the wavelength. On the other hand, the intensity of the tail on the short-wavelength side decreases only slightly during the bakeout. The absorbance difference in this region should have an extremely small value and large error, since the absorption curves a and b have high transmittances and the difference is very small. Accordingly, the absorbance difference on this side was excluded from the difference spectrum shown by curve c in Figure 1. I t is indicated that the difference spectrum does not have a contour similar to the infrared spectra in Figure 1;i.e., the former shows only weak intensity around 2.67 pm. And the difference spectrum has peaks and shoulders a t 2.67, 2.68, 2.71, 2.73, and 2.79 pm. In addition, bands are expected to exist on both sides of a somewhat loose crook around 2.90 pm. All specimens gave similar results. Deuterium Exchange Reaction. Figures 2 and 3 show infrared spectra in the OH and OD stretching vibration regions, respectively, for the sample pretreated at 350 "C and for the same specimen deuterated by D20vapor and subsequently evacuated a t the above temperature. The spectral change by deuteration in Figure 2 is somewhat different from the change shown by other investigators. The intensities around 2.68 pm and on the short-wavelength side of the 2.67-pm band decrease remarkably, and besides a weak but distinguishable band is found at 2.96 pm for any deuterated specimens. The difference spectra obtained in the OH and OD regions are also given in Figures 2 and 3, respectively. Curve
Wavelength,
pm
Figure 5. Decomposition of curve c in Figure 1: (soli line) Figure IC; (dotted and chain lines) components obtained by decomposition.
c in Figure 2 shows that the difference spectrum has a peak at 2.67 pm and shoulders at 2.62,2.71, 2.76, and 2.81 pm together with a crook around 2.97 pm. A peak or shoulder is not found at about 2.68 pm, in spite of the remarkable decrease of the absorption intensity at this wavelength. It is, however, possible to consider that a band exists here because of the asymmetric shape of the 2.67-pm peak. The difference spectrum in the OD region has a peak at 3.625 pm, a crook around 3.66 pm, a shoulder at 3.77 pm, and a long tail toward the long-wavelength region as given by curve c in Figure 3. The peak at 3.625 pm shows an asymmetric shape as the 2.67-pm peak. It is, therefore, pointed out that several bands exist also in the OD region. But no appreciable tail can be found on the short-wavelength side of the 3.625-pm peak, which is due to isotope shift of the 2.67-pm peak. Diethyl Ether Adsorption. Curves a and b in Figure 4 give infrared spectra of the sample baked out at 350 "C and of the same specimen on which diethyl ether was adsorbed at room temperature. The sharp band at 2.67 pm diminishes in intensity, and an extremely broad band appears which is based on the shift of the surface hydroxyl band by hydrogen bonding with ether. The transmittances decrease both around 2.68 pm and on the short-wavelength side of the 2.67-pm band. This resembles the result for deuteration. Curve c in Figure 4 is the absorbance difference between the two spectra. The curve is given only on the shortwavelength side of the intersection of the infrared spectra, as the surface hydroxyl bands on the longer-wavelength side are masked by the shifted bands due to the hydrogen bonding. In addition to the peak at 2.67 pm, the difference spectrum shows one more peak at 2.68 pm, a slight
4110
The Journal of phvsical Chemistry, Voi. 86, No. 2 1, 1982
Tsuchiya
TABLE I: Wavelengths (gm), Relative Intensities,u and Isotopic Shift Ratios in Frequency
( U O H / U O Dfor ) the
Components
Obtained from Decomposition of the Difference Spectra bakeout (400 deuteration
500 "C)
-+
[2.65Ib
OH OD
2.65 (10) 2.67 ( 3 ) 3.625 ( 3 ) 1.36 2.56 ( 5 ) 2.64 ( 1 6 ) 2.67 ( 7 )
VOH/'OD
ether adsorption (I
2.67 ( 2 )
In parentheses.
2.68 ( 6 )
2.71 ( 7 )
2.73 ( 2 3 ) 2.77 ( 2 9 ) 2.83 ( 2 8 ) 2.96 ( 5 )
2.68 ( 1 1 ) 3.63 (8) 1.35 2.68 ( 7 0 )
2.71 ( 1 2 ) 2.76 ( 2 0 ) 2.84 ( 3 0 ) 2.94 (10) 3.06 ( 4 ) 3.65 (10) 3.68 ( 3 1 ) 3.76 ( 3 2 ) 3.84 ( 1 2 ) 3.94 ( 4 ) 1.35 1.33 1.32 1.31 1.29 2.71 ( 2 )
The component expected from spectra a and b in Figure 1. Wave n u m b e r , c m - l
3800
4000 1
'
1
'
1
3600 '
I
'
I
'
I
'
3400 I
'
'
I
3200 I 1
Wave n u m b e r , cm-I
-
4000 0 3
nl
*
I
3900
3800
3700
Ih
I
Wavelength, p m
Figure 8. Decomposition of curve c in Figure 2 (solid line) Figure 2c; (dotted and chain lines) components obtained by decomposition. 2800
Wave n u m b e r , cm-' 2700 2600 I
I
Wavelength, pm
Figure 8. Decomposition of curve c in Figure 4: (solM line) Figwe 4c; (dotted and chain lines) components obtained by decompositiin; (broken line and arrow) added foot and its position, respectively.
2500 1
Figure 7. Decomposition of curve c in Flgure 3: (solld Hne) Figure 3c; (dotted and chain lines) components obtained by decomposition.
shoulder at 2.71 pm, and a tail which is longer than that in deuteration. Decomposition of DifferenceSpectra. The results obtained by the curve resolver are shown in Figures 5-8 for the three experiments, and listed in Table I, which gives the wavelengths, relative intensities, and isotope shift ratios for the obtained components. Table I shows some relationships among the experiments, as enumerated below. The peak wavelengths of the components agree approximately with those expected from the profiles of the difference spectra. The number of components in the upper three rows is the same, except for the 2.65-km component in the OH region for deuteration. Table I and Figures 5-8 show that the relative intensities and widths of the corresponding components are considerably close to one another. The peak wavelength of each component for bakeout in the longer-wavelengthregion is shorter than that of the corresponding component for deuteration. This result is presumably based on differences in the treatment temperatures. The isotope shift ratio decreases regularly from 1.36 to 1.29 with increase of the component wavelength. A very broad component is obtained at 2.65 pm in the long tail of the OH region of each difference spectrum. This component for bakeout is bracketed in Table I, since the existence can be expected from the slight difference between the infrared spectra a and b in Figure 1,as described before. For adsorption it was necessary to
add one more component at 2.56 pm to resolve the tail. The components, which ought to be shifted from 2.56 or 2.65 pm to the OD region by deuteration, could not be obtained. This is similar to the result by Van Cauwelaert et al.5
Discussion Existence of Various Hydroxyl Species. Physically adsorbed water on Aerosil silica surfaces is already removed at the treatment temperatures in this study. Besides, the sample is perfectly amorphous silica, without containing any crystal of SiOz polymorphism. Therefore, the infrared absorption in the OH vibration region is related only to the hydroxyls distributed on the amorphous silica surface. The existence of several peaks and shoulders in every difference spectrum implies that the infrared absorption of the silica contains many bands. Of these the bands located at the wavelengths longer than 2.67 pm are shifted to the OD region by deuteration. These can be attributed respectively to a type of surface hydroxyl species. It is, therefore, concluded that various hydroxyl species exist on the silica surface. Three or four hydroxyl bands have been shown for silica treated a t relatively high t e m p e r a t u r e ~ . ~ ~ Various ~J~J~ assignments were given: isolated and hydrogen-bonded hydroxyls in single or geminal silanol in various states. As the Aerosil silica surface has no similar regularity to its crystal surface, it can be considered that the surface OH groups are distributed at random from the completely isolated state to the state more or less perturbed by the oxygen atoms of adjacent OH groups or silica network, in accordance with the 0-0distance. From the analogy with alcohol p~lymerization,'~ it is indicated that the distribution is divided into three parts, giking three infrared bands, except for the isolated hydroxyls. These hydroxyl types are schematically shown with respect to single silanol groups as follows: (13) S. Kondo, K. Tomoi, and C. Pak, Bull. Chem. SOC.Jpn., 52,2046 (1979). (14) U. Liddel and E. D. Becker, Spectrochim. Acta, 10, 70 (1957).
The Journal of Physical Chemistty, Vol. 86, No. 21, 1982 4111
Hydroxyl Groups on Silica Surfaces
Type A is a completely isolated silanol group, B is a group perturbed at the oxygen atom, C at the hydrogen atom, and D a t both atoms. The frequencies of the four bands should decrease the sequence A B C D. The three bands due to types B-D should be very broad, because of hydrogen bonding and the distribution, and appear at contiguous wavelengths. Four similar types can also be considered for geminal silanol groups. At the degassing temperatures in this study both kinds of silanol groups may coexist on the Aerosil silica surface. Consequently eight infrared bands should be observed concerning the surface hydroxyls. However, the frequencies of these bands should be so close to one another that it is difficult to observe all these bands separately. In the difference spectra of Figure IC and 2c, about seven bands due to surface hydroxyls can be expected. It is, therefore, considered that these bands are roughly separated by obtaining the difference spectrum. The infrared bands at 2.67 and 3.625 pm have already been known as the type A hydroxyl and its deuterated form, respectively. In the difference spectra these bands are observed as a slight shoulder for bakeout, and as evident peaks for deuteration and adsorption. This fact proves that the type A silanol is difficult to dehydroxylate but is sensitive to deuteration or adsorption, since the hydroxyl is apart from adjacent hydroxyls or silica network 0 atoms. The infrared band of the type B hydroxyl should be located near the band due to A and would be sensitive to deuteration and adsorption like the type A, because of the free state of the hydrogen atom. Synder and Ward16 have pointed out the existence of “reactive hydroxyls” besides the isolated and hydrogen-bonded hydroxyls on silica surfaces. From fast intensity decrease of the band at 3720 cm-l by deuteration, Van Roosmalen and Molloexplained the band as due to the type B hydroxyl, by reason of enhanced dipole strength of the O-H bond. In this study the absorption intensity around 2.68 pm decreased markedly in deuteration and adsorption, and consequently a peak is separated at this wavelength from the 2.67-pm peak, as shown by curve c in Figure 4. On the other hand, the type B silanol should be readily dehydroxylated with the neighboring hydroxyl. In bakeout, an evident shoulder is observed at 2.68 pm. From these facts it is concluded that the band due to type B is located at this wavelength. In deuteration the existence of this type can be inferred from the asymmetric shapes of the peak tops at 2.67 and 3.625 pm, as shown by the curves in Figures 2c and 3c. The hydrogen atoms of both types C and D silanols are in hydrogen-bonded states. Accordingly, these hydroxyls should be liable to be dehydroxylated with the neighboring hydroxyl, and be less sensitive to deuteration or adsorption than the A and B hydroxyls. Both of the bands due to the C and D hydroxyls should be broader on account of the hydrogen-bonding structure and the distribution of the two types of hydroxyls on the surface. The difference spectra show large absorbance from 2.68 pm toward the longwavelength region in bakeout, and considerably less absorbance in this range than at 2.67 and 2.68 pm in deu-
--
(15)L. R. Snyder and 3. W. Ward, J.Phys. Chem., 70,3941 (1966).
teration. Therefore, the bands due to C and D would correspond to the peaks and shoulders observed in this range. It is difficult to deduce whether each peak or shoulder is ascribed to the single or geminal silanol. Two sharp peaks of the type A and two broad ones of B should be observed in the region from 2.67 to about 2.68 pm of the difference spectrum. Van Roosmalen and Molloindicated that the silica gel shows three absorption bands in this region, which were assigned to the type A single and geminal silanols, and the type B single silanol, respectively. In this study only two peaks or shoulders are observed in the region. Two peaks due to the type A hydroxyls might not be resolved because of low resolution of the prism spectrometer or the two peaks having equal frequencies. Concerning the type B bands, the peak or shoulder at 2.68 pm corresponds presumably to overlap of the two hydroxyl bands due to the single and geminal silanols by the same reason as described above. But it may not be impossible to regard the shoulder at 2.71 pm in every difference spectrum as another one of the type B. An infrared band is found at 2.96 pm after deuteration, as shown in Figure 2. This band becomes more notable with progress of deuteration. But after methoxylation of the hydroxyls such a band is not confirmed in this region, as will be shown in the following paper. All specimens gave the same results. It is improbable that a new 0-H band is formed on the surface by deuteration or that the hydroxyl band found after deuteration is not observed after methoxylation. It is, therefore, reasonable to consider that appearance of the 2.96-pm band results from decrease of the O-H absorption in this region, and the absorption decrease for methoxylation is much smaller than for deuteration. The infrared absorption of pressed silica contains absorption due to interaction between the hydroxyls on the surfaces of different particles? Tyler et al. showed that for pressed silica the band at 3650 cm-’ remaining after treatment with D20or BC13 corresponds to the hydroxyl interaction by interparticle contact.16 In the author’s experiment the 2.96-pm band remains together with the above band, as shown in Figure 2. Accordingly, this band is considered to be due to the same origin as mentioned above. Appearance of the band at relatively long wavelength in the OH region may imply that the band is attributed to hydroxyls perturbed strongly by surrounding particles, such as the type C or D hydroxyl. For methoxylation it is difficult for methyl groups to attack the hydroxyls between the particles, as compared with deuterium atoms. The long tail on the short-wavelength side of 2.67 pm, which is found in the difference spectra for deuteration and adsorption, is not shifted to the OD region, as described before. But a shoulder can be observed around 2.62 pm. Therefore, the tail is inferred to correspond to a very broad infrared band. It is not concluded at present that this band is attributed to a type of hydroxyl species. In adsorption the tail is extremely long and somewhat different in shape from the tail in deuteration, as shown by the curves in Figures 2c and 4c. This fact implies that a broad band, which is sensitive only to adsorption, appears around 2.55 pm. It has been pointed out that a band is observed at 3850 cm-’ on dehydroxylation of porous glass.12 A similar band has been found at 3870 cm-’ for silica gel? The nature of the species related to these bands has not yet been ascertained. (16)A.J. Tyler, F. H. Hambleton, and J. A. Hockey, J. Catal., 13,35 (1969).
J. Phys. Chem. 1B02,86,4112-4119
4112
Correctness of Results in Decomposition. Spectral anomalies peculiar to pressed sampled7 should not be observed in the infrared spectra shown in this paper, since the particle size of Aerosil silica is far smaller than the wavelength of infrared radiation, and the pressed pellets are made only from a single sample. The spectrometer used is of a wavelength-linear type. The decomposition should be performed for a spectrum obtained with a wavenumber-linear-type spectrometer by using functions such as Lorentzian or Gaussian. It is, however, considered that the decomposition of wavelength-linear spectra exerted little influence on the number, positions, and intensities of the obtained components, since the difference spectra were resolved by the aid of the positions of the peaks, shoulders, and crooks of the spectra. The investigators, who resolved the infrared absorption mainly due to isolated hydroxyls on silica surface, used Lorentzian functions or various types of LorentzianGaussian mixed functions for the band shape.45,' It is more difficult to select reasonable functions for bands due to hydrogen-bonded hydroxyls. It was far from satisfactory to fit the difference spectrum with the curve in the resolver by using a Lorentzian function for all the components. Accordingly, a Gaussian function was used for the components due to the hydrogen-bonded hydroxyls, i.e., the (17) C. N. R. Rao, 'Chemical Applications of Infrared Spectroscopy", Academic Press, New York, 1963.
types B-D hydroxyls. It may be reasonable to consider that these components are assumed to show Gaussian curves, since the hydroxyls, which are subjected to various degrees of hydrogen-bond strength, distribute on the silica surface. Decomposition of the difference spectrum consisting of many contiguous bands would inevitably have arbitrariness to some extent, for the reason that fitting of the difference spectrum with the curve in the resolver is judged by human eyes. Especially the arbitrariness would be based on extreme broadness of the bands due to the type C and D hydroxyls. Nevertheless, the repeated operations of the decomposition showed the same result within the error described before. Besides the values and regularity of the isotope shift ratios are reasonable in consideration of the structures of the types A-D silanols. The components at 2.68 pm and its isotope-shifted one at 3.63 pm have much higher intensities than the components at 2.67 and 3.625 pm, respectively, particularly for adsorption. This result agrees with the conclusion mentioned before. It is, therefore, suggested that decomposition of the difference spectrum due to the surface hydroxyls would serve as a good reference for information about the structure and phenomena related to the surface hydroxyls. Acknowledgment. I thank Dr. K. Kurosaki and the members Of the Research Laboratories, Fuji Photo Co. Ltd. for the spectrum analysis by the curve resolver.
Ab Initio and Semiempiricai Studies of the Equilibrium Geometry and In-Piane Vibrational Spectra of Tetracyanoethyiene and the Tricyanovinyi Alcoholate Anion K. W. Hipps' and R. D. Poshusta Department of Chemistry and Chemical Physlcs Program, Washlngton State Unlvers& Pullman, Washlngton 99 164 (Recelved: March 29, 1982; I n Flnal Form: June 15, 1982)
Assignments for the experimental vibrational spectrum of tricyanovinyl alcoholate anion (TVA) are made by using a synthesis of ab initio SCF calculations and experimental data. The method is an extension of that developed by Blom and Altona, which has been shown to reliably reproduce previously assigned vibrational spectra. Partial calibration of the parameters employed is accomplished by analysis of the (previouslyassigned) spectrum of tetracyanoethylene (TCNE). Judging from the limited evidence presented here, the method provides useful information when the spectrum of only one isotopic species is available and can provide nearly complete assignments if spectra from two isotopic isomers (isotopomers) are available. Introduction This paper is about the assignment of a vibrational spectrum. It follows neither the semiempirical route from purely experimental data nor the purely theoretical deduction of the spectrum from ab initio quantum mechanics. Rather it is an eclectic combination of the two routes. The theoretical component of this work takes its inspiration from the work of several authorsl-10and follows most closely Altona et al.1-3The experimental component is based on the venerable technique of isotopic substitution. The interface at which the two routes merge is the valence force potential (VFP) model. In order to assign a spectrum, one must find a correspondence between the actual atomic motions and the observed transition energies. In cases where the motions
* Alfred P. Sloan Fellow. 0022-3654/82/2086-411280 1.2510
are highly localized, this correspondence can easily be made by the purely experimental observation of isotopic shifts. For those motions which include a large fraction of the atoms of the molecule, solutions of the mechanical (or (1) Blom, C. E.; Altona, C. Mol. Phys. 1976, 31, 1377. (2) Blom, C. E.; Otto, L. P.; Altona, C . Mol. Phys. 1976, 32, 1137. (3) Blom, C. E.; Altona, C. Mol. Phys. 1977,33, 875. (4) Hehre, W. J.; Stewart, R. F.; Pople, J. A. J. Chem. Phys. 1969,51, 2657. (5) Newton, M. D.; Lathan, W. A,; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1970,52, 4064. (6) Radom, L.; Lathan, W. A.; Hehre, W. J.; Pople, J. A. J. Am. Chem. SOC. 1971, 93, 5339. (7) Swanson, B. I.; Amold, T. H.; Dewar, M. J. S.; Rafalko,J. J.; Rzepa, H. S.; Yamoguchi, Y. J. Am. Chem. SOC. 1978,100, 771. (8) Raflko, J. J.; Rzepa, H. S.;Swanson, B. I. J.Mol. Spectrosc. 1979, 75, 363. (9) Swanson, B. I.; Arnold, T. H.; Yamagouchi, Y. J. Mol. Struct. 1979, 78, 125, 139. (10) Arnold, T. H.; Swanson, B. I.; Yamagouchi, Y.; Nelson, D. J. J . Mol. Struct. 1979, 78, 267.
0 1982 American Chemical Society
