Theoretical Study of Microscopic Features of Gas-Phase Adsorption

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J. Phys. Chem. 1995,99, 15968-15972

Theoretical Study of Microscopic Features of Gas-Phase Adsorption onto a Few Adsorption Sites of Silica Gel Tetsuo Suzuki,* Makoto Hirano, Hajime Tamon, and Morio Okazaki Department of Chemical Engineering, Faculty of Engineering, Kyoto University, Kyoto 606-01,Japan Received: March 9, 1995; In Final Form: July 18, 1995@

Adsorption onto the surface of silica gel is studied by using the ab initio molecular orbital (MO) method. We have carried out MP2/6-3 lG(p,d)//HF/6-31G(p,d) level calculations to investigate gas-phase adsorptive interactions between adsorbates and silica gel. For adsorbates we have treated C2H4 and C6H6, which have -CWbonds, and for comparison NH3 has also been treated. In order to discuss preferable adsorption sites for these adsorbates, we treat three types of sites, which are constructed by an isolated =Si-0-H group, two =Si-0-H groups, and a =Si-O-Si= group. It is found that for C&, C6H6, and NH3 the former two types of sites, which are constructed by =Si-0-H groups, act as adsorption sites, but the site constructed by ‘Si-O-Si’ group does not. A preferable adsorption site depends on each adsorbate. For c6& the paired-type adsorption site, which is constructed by two -OH groups, is preferable. In the adsorption of NH3, the single-type adsorption site, which is constructed by one -OH group, is preferable. We can deduce that the distribution of -OH groups strongly affects the adsorption phenomena and the character of the adsorbent.

I. Introduction In this paper we report an ab initio molecular orbital (MO) study of gas-phase adsorption phenomena onto the surface of silica gel. Silica gel is a very popular material for adsorption. A lot of e~perimentall-~ and theoretical6-I6 studies have been reported for gas-phase adsorption phenomena and adsorptive interactions between adsorbates and the silica surface. In particular for more than 10 years many theoretical studies6-I6 have been done, and microscopic information has been obtained on the adsorption phenomena onto the silica surface, such as adsorptive interaction energies, orientation of adsorbates to adsorption sites or structures of adsorbate-adsorbent systems (adsorption geometries), and relations between structures of adsorption sites and adsorptive interaction energies. These studies illustrate that by executing ab initio MO calculations, we can get the knowledge that will help us develop a strategy for modification and design adsorbents having desirable characteristics. Hence, we are much interested in such theoretical studies, and the motivation of the present study is to obtain and accumulate more information for the adsorption phenomena. In the present study we discuss the adsorption of C2H4 and C&, which have -c=c- bonds, onto the surface of a silica gel, These molecules are often used in experimental studies of adsorption. However, ab initio MO studies concerning adsorption of C2& and C6H6 onto the surface of silica gel have not been reported as far as we know. Hence, it is interesting and useful to discuss the microscopic features of the adsorption for these adsorbates. Such work is done in this paper, and for comparison NH3 is also treated as an adsorbate. One main aim of the present study is to discuss preferable adsorption sites for these adsorbates. The silica surface is constructed by two kinds of functional groups, that is, the =Si-0-H group and the ESi-O-SiZ group. It is known that in most cases =Si-0-H groups act as adsorption sites. Therefore, we can expect that the interactions between the adsorbates and the =Si-0-H groups contribute much to @

Abstract published in AdYance ACS Abstracts, October 1, 1995.

0022-3654/95/2099-15968$09.00/0

adsorption. However, the investigation of the interaction between the adsorbates and the =Si-O-Si= group is also important because the surface area consisting of the -i-O-Si= group should be larger than that consisting of the =Si-0-H group; the number of ‘Si-0-H groups is, although it would depend on the type and the treatment temperature of silica gel, about 5 per 1 nm2.I5-l7 Hence, we discuss the role of the ISi-0-H and S S i - O - S i i groups in the adsorption of C&, c6&, and N H 3 onto silica gel. Moreover, for the sites constructed by the =Si-0-H group, there are several types of ~ i t e s , ~that , ~is,~ isolated, . ~ ~ geminal, triminal, and vicinal type sites. We are much interested in the difference of character of these adsorption sites. For example, which acts as a more preferable site, the isolated type site constructed by one =Si-O-H group or the vicinal type site constructed by two =Si-0-H groups? This paper is a preliminary study of these problems.

11. Methodology Computational Method. Calculations were performed with the GAUSSIAN 9218 program. It seems that in previous studies6-I5 calculated results using basis sets of valence double-l; plus polarization functions succeeded, to some extent, in explaining the experimental results. Thus, the 6-3 1G(p,d)Ig basis set was adopted throughout because of computational expense. Structures of the adsorbates, those of the adsorption sites, and the adsorption geometries were determined based on the geometry optimization method with Hartree-Fock (HF) level calculation. Electronic correlation was taken into account based on second-order M01ler-Plesset perturbation theory” under frozen-core conditions. Hence, the calculation level of the present study can be denoted as MP2/6-3lG(p,d)//HF/631G(p,d),according to popular notation. Basis set superposition error (BSSE) was estimated by the counterpoise method.2’ Molecular Models Used in this Study. As we have described above, we discuss the interaction between adsorbates and the =Si-0-H groups and that between adsorbates and the =Si-O--Sis group. The molecular models of adsorption sites used in this study are discussed in the following. 0 1995 American Chemical Society

Microscopic Features of Gas-Phase Adsorption

J. Phys. Chem., Vol. 99,No. 43, 1995 15969 H

l,6x20'0 107.0

/

111'4 ~

H

3019

H @(HIOSiH21=60.2

H 0.941

WHICIOI- 111 7 UHIOSIHlI=Ml 6(HIOEIHII -500

I

/

H e(H3Si101)= 100.5 )(HISilOlSi2) = 180.0 O(H202Si101) = 102.5 q(H3SiIOlSi2) E 60.0 )(OZSilOlSi2) = -60.0

I

(tH202SiI011=698

H

HI HHlClOSiIl*500

Figure 2. Adsorption geometries for C2H4 where G represents the center of mass of C 2 b : (a) adsorption geometry for a single-type adsorption site (C, symmetry); (b) adsorption geometry for a pairedtype adsorption site (C2);(c) adsorption geometry for the ESi-O-Si=

H1

H

Olclmlsll)- 0 0

H1

H B(HlS110Sr2) = 0.0 q(HZSilOSi2) = 120.0

Figure 1. Optimized geometries for adsorbates and adsorption sites: (a) c2H4 (Dzk symmetry); (b) CsH6 (D6h); (C) NH3 (c3~); (4 single-

type adsorption site (Cs); (e) paired-type adsorption site (C2); (f) adsorption site constructed by +3i-O-Sil group ( C 2 J . In order to discuss the adsorptive interaction between adsorbates and =Si-O-H groups, we treat two cases: one case is that adsorbates interact with an isolated type site7-I5,l7and the other case is that adsorbates interact with a kind of vicinal type ~ i t e . ~For . ' ~the former case, we denote the site as singletype adsorption site in the present study. A lot of previous studies6-I6 treated this type of adsorption site. In those studies SiH30H or Si(OH)4 was used as the molecular model of the adsorption site. These two molecular models seem to be equivalent, judging from the study of Ugliengo et a1.I2and that of Pelmenschikov et al.I4 In these studies adsorptive interactions between water and silica gel were discussed. The absolute values of the results using basis sets of valence double-g plus polarization function are 25.79 and 25.9 kJ/moll4 at the HF level calculation and 31.96 l 2 and 30.4 kJ/moll4 at the M E level calculation. Hence, in the present study we chose SiH3OH as a molecular model of the single-type adsorption site for convenience. The structure of the molecular model is shown in Figure Id. The structure is fully optimized under the constraint of C,symmetry. Next, let us consider the case that adsorbates interact with a kind of vicinal type ~ i t e . ~We . ' ~ have introduced H2Si(OH)O-Si(OH)H* as a molecular model. The structure is shown in Figure le. Two =Si-0-H groups are located on opposite sides for the Si-0-Si surface, and the structure has C2 symmetry. In this paper we call this site a paired-type adsorption site. For the model it is expected that the Si-0-Si angle strongly affects the degree of interaction between adsorbates and the two =Si-0-H groups because the distance of the two =Si-0-H groups depends on the Si-0-Si angle. Therefore, in order to make the results of the present calculation more realistic, it is desirable that one uses the actual value of

group (CJ. the Si-0-Si angle. For silicates it is known that the Si-0Si angle is in between about 130" and 150°,' and the mean value is 140°.22,23 For amorphous silica the Si-0-Si angle is randomly centered around 14O0.I5 However, from the case of silicates, probably the angle tends to be distributed much nearer to 140" or at least is distributed comparably near 140' compared to other angle values. Hence, as a preliminary study we have fixed this angle at 141.1".24 Four H-Si-0 angles and two 0-Si-0 angles will not differ much from 109.5", and the subtle difference will hardly affect the interaction between adsorbates and adsorbents. Thus, we have fixed these angles at 109.5". The other parameters, that is, bond lengths, H-0-Si angles, and dihedral angles of H-0-Si-0, have been determined by optimization. For the molecular model of the site constructed by the ESi-O-SiE group, we have introduced the molecular model shown in Figure If. In this case the Si-0-Si angle itself will not affect the interaction with the adsorbates much.25 Hence, the structure was determined by geometry optimization under the constraint of C2" symmetry, and the Si-0-Si angle became 171.1". Finally, the method of determination of adsorption geometry is described in the following. In all cases symmetries of adsorbates and adsorption sites were kept as their original symmetries, and orientations of adsorbates to the adsorption sites were fixed under the constraint of proper symmetries. For interaction between adsorbates and the single-type adsorption site, adsorption geometries were determined by optimizing the distance between adsorbates and the adsorption site and the deformation of them under the constraint of C,symmetry. For the adsorption geometries for a paired-type adsorption site, our treatment of the adsorbates and the adsorption site is semirigid. Because the present model mimics a part of the bulk silica su$ace, we kept the Si-0-Si angle fixed for searching adsorption geometries. We searched adsorption geometries mainly by varying the distance between the adsorbates and the adsorption sites and the two dihedral angles of H-0-Si-0 under the constraint of the proper symmetries and without considering relaxation of the adsorbates. The adsorption geometries for the S3i-O-SiE group were determined by optimization under the constraint of proper symmetries. The adsorbates and the adsorption site were treated as rigid. Values of parameters that were treated by the search of adsorption geometries are shown in Figures 2-4. 111. Results and Discussion

In this section we discuss the results of the calculation. Total energies of adsorbates (El), those of adsorption sites (E2), and those for adsorption geometries (E3) are listed in Tables 1-3.

15970 J. Phys. Chem., Vol. 99, No. 43, 1995

Suzuki et al.

TABLE 4: Interaction Energies AE (kJ/molyl

H

" I Y

adsorbate

H

9

H,

C2H4 H

C& ~

H* ' I 9

Hi

3m

NH3

H

-

Figure 3. Adsorption geometries for C6H6 where G represents the center of mass of C6H6: (a) adsorption geometry for a single-type adsorption site (C, symmetry); (b) adsorption geometry for a pairedtype adsorption site (C2); (c) adsorption geometry for the ISi-O-Si" group (CZ,).

Figure 4. Adsorption geometries for NH3: (a) adsorption geometry for a single-type adsorption site (C, symmetry); (b) adsorption geometry for a paired-type adsorption site (CI);(c) adsorption geometry for the =Si-O-Sil group (CJ.

TABLE 1: Total Energies E1 of Adsorbates (au) adsorbate

HF

MP2

czH4

-78.038 841 -230.713 860 -56.195 545

-78.316 810 -231.504 583 -56.382 846

CbH6 NH3

TABLE 2: Total Energies E2 of Adsorption Sites (au) silica gel single-type paired-type ISi-O-si"

HF

M E

-366.141 921 -806.107 715 -656.267 749

-366.425 977 -806.835 080 -656.639 423

TABLE 3: Total Energies E3 for Adsorption Geometries (au)

C2H4 C6H6 NH3

AEHF

AEMR

AEcou~l AEdim

-1 1.63(3.50) - 16.30(5.48) -3.37 -2.69 -9.99(4.82) - 16.21(7.78) -2.28 -3.26 - 1.18(2.O4) -4.92(3.50) -12.33(3.46) -20.65(7.01) -7.54 -4.77 -19.39(7.77) -31.35(12.64) -8.11 -7.09 -1.52(2.38) -6.41(4.10) -35.34(3.21) -45.4X7.88) -18.43 -5.44 -28.98(5.63) -38.45(12.83) - 1 1.07 -2.27 -0.60(1.58) -3.78(3.12)

Values in parentheses are the corresponding BSSE estimated by the counterpoise method.

WIOHII =WO UHIOS~HZ).M.I uCIGOSI) 0.0

adsorbate

silica gel single-type paired-type ESi-O-SiE single-type paired-type ISi-O-si' single-type paired-type rSi-O-SiE

silica gel single-type paired-type =si-O-sil single-type paired-type Esi-O-siE single-type paired-type ESi-O-si=

HF

-444.185 195 -884.150 365 -734.307 039 -596.860 483 - 1036.828 965 -886.982 188 -422.350 933 -862.314 305 -712.463 524

MP2 -444.749 -885.158 -734.958 -597.938 -1038.351 -888.146 -422.826 -863.232 -713.023

OOO 073 108 432 631 446 144

603 711

By use of these results, the interaction energy AE is estimated by the following equation:

A E = E3 - E, - E2 AE can be determined by both the result of HF level calculation and that of M E level calculation. In this paper these two kinds of AE are denoted as AEw and AEM~z, respectively. Those values are listed with the corresponding BSSE values estimated by the counterpoise method in Table 4. As we described above, geometries of adsorbates, those of adsorption sites of silica gel,

and adsorption geometries for adsorbates of c2&, C6H6, and NH3 are shown in Figures 1-4. As a general feature, it is found that the two dihedral angles of H-0-Si-0 change much by adsorption and the two -OH groups rotate toward the inner side for the paired-type adsorption site. The angles vary from 102.5' to 69.8" by the adsorption of C2H4 and vary to 71.3" and 70.4" for C6H6 and NH3, respectively. Hence, the two =Si-0-H groups can interact more effectively with the adsorbates. The deformation plays a crucial role for adsorption. BSSE and Comparison between Present Calculated Results and Experimental Results. For adsorption geometries of C 2 b and C6H6, estimated BSSE values of single-type and paired-type adsorption sites are 28-48% of UHF and 34-48% of U M P 2 . For adsorption geometries of NH3, the estimated BSSE value is 9% of AEHFfor the single-type adsorption site, which is rather small and agrees with the results of the same calculation of ref 11. Other than that, the BSSE is 17-33%. The BSSE for adsorption geometries of the ISi-O-SiE group is more than 64%. As seen from these results, the 6-31G(d,p) basis set is not large enough for high quantitative accuracy of calculation for the present case. Although the BSSE value estimated by the counterpoise method is an important criterion of calculation accuracy, it is known that counterpoise-corrected energy is not more reliable than uncorrected en erg^.^^,^' Therefore, it is helpful to estimate the accuracy of the calculation by referring to the results of similar calculations and correspondingexperimental results. For the calculated results of adsorption of CO,Io NH3," and H20I2 onto a single-type adsorption site using basis sets of valence double-f; plus polarization functions, the uncorrected interaction energy at the MP2 level calculation is 118-128% of the corresponding experimental value. For the corresponding experimental results, the interaction energies for C2H4, C6H6, and NH3 are between -30 and -20 k . I / m ~ l ,between ~ * ~ ~ -50 and -40 kI/mo1,2-28and between -60 and -40 k.I/mol,'-28 respectively. It is found that for C2H4 and C6H6 the absolute values of the calculated results underestimate those of experimental results. However, the maximum absolute values of u M P 2 for c 2H4 and C6H6 are about 80% of the corresponding absolute values of the interaction energies obtained from experiments. From these results we can consider that for the present case the agreement of the calculated results using the 6-31G(d,p) basis set with the experimental results is not so bad. Moreover, microscopic features of adsorption discussed in the succeeding sections are not changed whether one uses counterpoise-corrected interaction energies or uncorrected ones. Hence, we use A&~z for discussions in the following sections, except for the discussion on the calculation of dispersion energies. Adsorption of CZ&. For C2H4 the interaction energy U ~ p 2 for the adsorption onto the single-type adsorption site is -16.30

Microscopic Features of Gas-Phase Adsorption kJ/mol and that onto the paired-type adsorption site is -16.21 kJ/m01.~~ Hence, the two sites will contribute comparably to the adsorption of C2H4. By comparison to this, AEMRfor the adsorption onto the ?3i-O-Si= group is -4.92 kJ/mol. From the result we can conclude that the +3i-O-S+ group does not become an adsorption site for C2H4. Adsorption of C a b . In the adsorption of C6H6, h E ~ p for 2 the paired-type adsorption site is bigger than that for the singletype adsorption site. Those values are -31.35 and -20.65 kJ/ mol for the paired-type adsorption site and the single-type adsorption site, respectively. Hence, the paired-type adsorption site is favorable for the adsorption of C6H6. This differs from the results for C2H4. Such difference is caused by the fact that the orbitals of n-electrons of C6H6 extend wider than those of C 2 b and interact effectively with the two -OH groups of the paired-type adsorption site. For the adsorption site that consists of the ESi-O-SiE group, A E M Pis~ -6.41 H/mol. Thus, for the adsorption of C6H6, we can conclude that the Esi-o-siE group also does not become an adsorption site for C6H6. Adsorption of NH3. It seems that NH3 can act as both a proton acceptor and a proton donor for adsorption, but it is preferable that NH3 acts as a proton acceptor for the singletype adsorption site." Hence, in the present study we treat NH3 as a proton acceptor. The AEMPZ for single-type and pairedtype adsorption sites are -45.45 and -38.45 kJ/mol, respectively. The result for a single-type adsorption site is in good accordance with the result from the same calculation of ref 11 with the A basis set, which gave -43.50 kJ/mol. It is found that the single-type adsorption site is more favorable for the adsorption of NH3, and this is different from the results for C 2 b and C6H6. This is because the extent of lone-pair orbital of N atom is not so wide, and the orbital interact with one -OH group of single-type adsorption site more effectively than with two -OH groups of paired-type adsorption site. As for =Si-0-Si= group, A E M Pis~ -3.78 kJ/mol. After all, we can conclude that ESi-O-SiE group does not contribute to the adsorption of the three adsorbates treated in the present study. Contribution of AEcOul and AEdisp to Interaction Energy. It is interesting and important to know how each kind of interaction contributes to whole adsorptive interactions. In order to discuss the problem, we have carried out a kind of energy decompo~ition~~ for AE on single-type and paired-type adsorption sites. The Coulombic interaction energy AEcoul and dispersion interaction energy &isp have been estimated. AEcOul is estimated by using the Mulliken population determined by MP2 density. h E d i s p is estimated by the difference between AE",, and hEcHF,where hEcMp2and hEcHFare counterpoisecorrected values of h E ~ p 2and AEHF:~O

Table 4 shows the values of AEcOuland A E d i s p . Although the method of estimation of AEcoUland U d i s p is simple, the results are very suggestive. The result for NH3 shows that AEcOul contributes to adsorption more than U d i s p , which seems reasonable. For the adsorption of C2& AEcOulcontributes comparably compared to m d i s p . In the adsorption of C.5H.5 to a paired-type adsorption site, hEcOul and h E d i s p are also comparable. &??cOulcontributes more to the adsorption of C6H6 onto a single-type adsorption site. These results for C2& and C6H6 seem somewhat unexpected because they are nonpolar molecules. Maybe the result is exaggerated because the basis set is not large enough and the dispersion interaction is not so well estimated in the present study. Although we take this point into account, we can still deduce that even for the adsorption

J. Phys. Chem., Vol. 99, No. 43, 1995 15971 of C 2 b and C6H6 the contribution of hEcOulis not negligible compared to h E d l s p .

IV. Conclusion We have studied the adsorption of CzH4, C6H6, and NH3 onto the surface of silica gel by using the ab initio MO method. Microscopic features of adsorption, such as adsorption geometries and adsorptive interaction energies, have been discussed for a few adsorption sites. We have found that the ESi-O-SiE group does not construct adsorption sites for C2H4, C6H6, and NH3. For adsorption sites constructed by the =Si-0-H group, two types of adsorption sites, that is, single-type and pairedtype, are treated. The shape of adsorbates d e t e d n e s which adsorption site interacts with them more strongly. Therefore, we can consider that the distribution of -OH groups on the silica gel surface strongly affects the adsorption phenomena and character of adsorbent. The present study demonstrates the usefulness of the ab initio MO method to study microscopic features of adsorption phenomena. This study is a preliminary one, and the obtained knowledge is limited to qualitative features of adsorption. Extensive studies of more varied adsorption sites using more accurate calculations are needed to explain the experimental results more quantitatively. However, it seems promising that such elaborate studies will be helpful to develop a strategy for the modification and design of adsorbents.

Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research (C), No. 06650880 (1994) from the Ministry of Education, Science, and Culture of Japan, for which we express our gratitude. The authors also acknowledge the computer center of the Institute for Molecular Science in Okazaki National Research, the supercomputer laboratory of the Institute for Chemical Research in Kyoto University, and the data processing center of Kyoto University for their generous permission to use their computers. References and Notes (1) Hertl, W.; Hair, M. L. J. Phys. Chem. 1968, 72, 4676. (2) Shen, J.; Smith, J. M. Ind. Eng. Chem. Fundam. 1968, 7, 100. (3) Glass, R. W.; Ross, R. A. Can. J. Chem. 1972, 50, 1241. (4) Iler, R. K. The Chemistry ofbilica; Wiley Interscience: New York, 1979. (5) Valenzuela, D. P.; Myers, A. L. Adsorption Equilibrium Data Handbook; Prentice Hall: NJ, 1989. (6) Sauer, J.; Schroder, K.-P. Chem. Phys. Lert. 1984, 107, 530. (7) Sauer, J.; Zahradnik, R. Inr. J. Quantum Chem. 1984, 26, 793. (8) Sauer, J.; Morgeneyer, C.; Schroder, K.-P. J. Phys. Chem. 1984, 88, 6375. (9) Chakoumakos, B. C.; Gibbs, G. V. J. Phys. Chem. 1986, 90, 996. (10) Ugliengo, P.; Saunders, V. R.; Gamone, E. J. Phys. Chem. 1989, 93, 5210. (11) Ugliengo, P.; Saunders, V. R.; Gamone, E. Surf: Sci. 1989, 224, 498. (12) Ugliengo, P.; Saunders, V. R.; Gamone, E. J. Phys. Chem. 1990, 94, 2260. (13) Pelmenschikov, A. G.;Morosi, G.; Gamba, A. J. Phys. Chem. 1992, 96, 224 1. (14) Pelmenschikov, A. G.; Morosi, G.; Gamba, A. J. Phys. Chem. 1992, 96, 7422. (15) Sauer, J.; Ugliengo, P.; Gamone, E.; Saunders, V. R. Chem. Rev. 1994, 94, 2095. (16) Suzuki, T.; Tamon, H.; Okazaki,M. Chem. Lett. 1994, 2151. (17) Scott, R. P. W. Silica Gel and Bonded Phases; John Wiley & Sons: Chichester, England, 1993. (18) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A,; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L:; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. GAUSSIAN 92; Gaussian Inc.: Pittsburgh, PA, 1992. (19) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972.56, 2257. Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.

15972 J. Phys. Chem., Vol. 99,No. 43, I995 Gordon, M. S. Chem. Phys. Lett. 1980, 76, 163. Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80,3265. (20) M~ller,C.; Plesset, M. S. Phys. Rev. 1934, 46, 618. (21) Boys, S. F.; Bemardi, F. Mol. Phys. 1970, 19, 553. (22) Liebau, F. Z. Natulforsch. 1960, 15b, 468; Acta Crystallogr. 1961, 14, 1103. (23) Boem, H. P. Adv. Catal. 1966, 16, 179. (24) Takahashi, K. Mem. Fac. Eng., Kyoto Univ. 1986, 48, 396. (25) We have calculated the interaction energy between C6H6 and the ESi-O-Sis group with the Si-0-Si angle fixed at 141.1'. The result is that A€HF and AEM~L are -2.58 and -6.67 kJ/mol, respectively. These values are not very different from those listed in Table 4. Moreover, the discussion described below is same for both geometries of the ISi-O-SiI group. Thus, in the present study we used the optimized geometry of the SSi-O-SiE group. (26) Schwenke, D. W.; Truhlar, D. G. J. Chem. Phys. 1985, 82, 2418.

Suzuki et al. (27) Frisch, M. J.; Del Bene, J. E.; Binkley, J. S.; Schaefer, H. F., 111. J. Chem. Phys. 1986, 84, 2279. (28) Our unpublished experimental result. (29) We have calculated another adsorption geometry (C2 symmetry); for the geometry the 0 1 G line (refer to Figure 2b) is on the molecular plane of C2H4 and crosses vertically to C-C axis. The dihedral angle ~ C l G O l S i 2 )is 113.3'. The values of AEHFand AEMP~ are -9.16 and -18.09 kJ/mol, respectively. The difference in corresponding values for the adsorption geometry shown in Figure 2b is very subtle. In the present case geometry optimization is done at the HF level. Hence, we chose the more stable geometry at the HF level shown in Figure 2b. The discussion is not changed if one chooses another adsorption geometry. (30) Tsuzuki S.; Tanabe, K. J. Phys. Chem. 1992, 96, 10804.

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