J. Phys. Chem. 1987, 91, 3325-3327 short time intervals. These results are interpreted to mean that pyrene in ClOOl8Cis localized near the crown ether ring due to the interaction between pyrene and the crown ether ring of the surfactant. The localization increases the effective pyrene concentration needed for excimer formation and therefore increases the probability of pyrene excimer formation. Temperture and pressure effects on I J I m of pyrene in C10018Cwith and without KCI are consistent with expected changes in micellar aggregation numbers under different temperature and pressure conditions.
3325
Acknowledgment. We thank the Army Office of Research and National Science Foundation for their generous support of this research. We are grateful to Dr. C. V. Kumar and Dr. I. R. Gould for their excellent technical assistance in the measurement of time-resolved emission of pyrene excimer formation and to Dr. K. C. Waterman and Dr. P. Chandar for helpful discussions. Registry No. ClOOl8C, 60742-60-1; C,PhE,,, 27942-26-3; pyrene, 129-00-0.
Effects of Structural and Chemical Characteristics of Zeolites on the Properties of Their Bridging Hydroxyl Groups A. G. Pelmenshchikov, V. I. Pavlov, G. M. Zhidomirov, Institute of Catalysis, USSR Academy of Sciences, 630090 Novosibirsk 90, USSR
and S. Beran* The J . Heyrovsk? Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Mcichova 7. 121 38 Prague 2, Czechoslovakia (Received: December 8, 1986)
The nonempirical SCF method with a STO-3G basis set is used to study the effects of the geometry (Si0 and A10 bond lengths and SiOAl angles) and chemical (the Si:A1 ratio) characteristics of zeolites on the vibrational frequencies of their OH groups modeled by the H3SiOHAIH3, H3SiOSi(OH)3, and H2A10Si(OH), molecules. It is shown that the values of the vibrational frequencies are particularly affected by the structural characteristics (e.g., changes in the Si-0 and A1-0 m and 20°, respectively, result in shifts of the stretching frequency uOH bond lengths and the SiOAl angle of 0.04 X of up to 40 cm-I), while the influence of the chemical composition of zeolites is negligibly small (e.g., a substitution of AI for Si in the third coordination sphere of OH leads to a change of about 1 cm-').
Introduction This paper deals with the influence of the geometrical characteristics of chemically identical fragments, particularly bridging OH groups, >SiOHAlf, in zeolites, on the vibrational frequencies vOH. This factor has not so far been taken into account in the interpretation of IR data for various zeolites. Moreover, some authors,'V2 refering to the results of CNDO/2 calc~lations,~ concluded that the effect of the structural characteristics of the 3 S i O H A l f fragment on the stretching vibrational frequencies vOH is negligibly small. Because of other factors which simultaneously affect the uOH frequency (see below) it is practically impossible to confirm or refute this conclusion on the basis of the experimental data available on zeolites. On the other hand, there is a whole series of analogous examples in IR spectroscopy of molecules which seem to contradict the conclusion in ref 3. For example, variations in the CCC angle in the 3CCH2Cf fragment in cyclobutane (LCCC = 60°), cyclopentane (LCCC = 90°), and cyclohexane (LCCC = 108') lead to the following values of the or symmetric, vibrational frequencies: uEH2 asymmetric, vEHH,, = 3081, 2981, and 2952 cm-' or $g2 = 3013, 2870, and 2866 cm-I, re~pectively.~The SiOAl angle in the similar 3SiOHAlf fragment also varies by about 40°, depending on its position in the zeolite framework. The difference in the geometry of both the + C C H 2 C f fragment in a series of molecules and the 3SiOHAlf fragment in different sites of the zeolite framework originates from the steric deformations. Reviewing the influence of the geometrical factor on the vibrational frequencies of the chemically identical fragments in various organic molecules,
Bellamy showed4that the variation in the frequencies is connected with the changes in the hybridization of the atomic orbitals on the C atom. Theoretical discussion of the effect of the hybridization of the atomic orbitals on the 0 atom on the properties of its bonds is given in ref 5 and 6 . For the reasons mentioned above, the role of the structural factors will be discussed in this article. However, to complete the picture of this phenomenon, it should be noted that the presence of different frequencies of the O H groups observed for various oxides has so far been assumed to be connected with a chemical factor; Le., with changes in the chemical composition of the fragments to which the OH group is bonded. In this case, it is important to known the manner in which the vibrational frequency, vOH, depends on the mutual arrangement of the O H group and the position in which a substitution of an atom takes place. Tsyganenko showed7 that the value of the vibrational frequency of OH groups in M,O, oxides is associated with the number and type of M atoms bonded to this group; Le., it depends only on the chemical properties of the atoms situated in the first coordination sphere of the O H group. As the coordination number of an O H group does not exceed 3, Knozinger explained the higher number of the vibrational frequencies of OH groups on alumina by taking into account the coordination numbers of the A1 atoms bonded to the O H group; Le., the second coordination sphere of the OH group was considered.8 With zeolites, the variation of the vibrational frequencies of the bridging O H groups was accounted for by the different distribution of the T = Si or A1 atoms in the (>TO)3SiOHAlf fragments; Le., by the different composition of the third coordination ~ p h e r e . ~
(1) Jacobs, P. A.; Mortier, W. J. Zeolites 1982, 2, 227. (2) Mortier, W. J.; Sauer, J.; Lercher, J. A.; Noller, H. J . Phys. Chem. 1984, 88, 905. (3) Mortier, W. J.; Geerlings, P. J . Phys. Chem. 1982, 84, 1980. (4) Bellamy, L. J. The Infra-Red Spectra of Complex Molecules, 2nd ed; Methuen: London. 1958.
(5) Newton, M. D.; Gibbs, G. V. J . Phys. Chem. Miner. 1980, 6, 221. (6) Newton, M. D. Structure and Bonding in Crystals, Vol. 1, O'Keefe, M., Navrotsky, A,, Eds.; Academic: New York, 1981. (7) Tsyganenko, A. A,; Felimov, V. N . J . Mol. Struct. 1973, 19, 579. (8) Knazinger, H.; Ratnasamy, P. Catal Reu. Sci. Eng. 1978, 17, 31. (9) Von Balmoos, R.; Meier, W. H. Nature (London) 1981, 289, 782.
0022-3654187 12091-3325SO1SO10
0 1987 American Chemical Societv
Pelmenshchikov et al.
3326 The Journal of Physical Chemistry, Vol. 91, No. 12, 1987
TABLE I: Geometry, r (1O-Io m), Energy, AE (kcal/mol), and Vibrational, u (cm-I), Characteristics Calculated for H$iOHAIH3 Molecules with Different Steric Deformations Aa and AR (lo-'' m)
r(Si0) r(Al0
?(OH) r(SiH') r(SiH) r(AIH')
r(AIH) &OH
LSiOAl AE "OH "SOH
equil geometry
d a = -10" AR = -0.02
1.698 1.867 0.973 1.422 1.422 1.484 1.484 114.4 129.9 0.0 4276 1351
1.689 1.850 0.971 1.416 1.417 1.471 1.486 117.3 120.2 1.2 4304 1304
Finally, the shifts in the frequency of the O H groups of zeolites with changes in their chemical composition are often explainedlO~ll by using Sanderson's concepti2of the average electronegativity. This approach, however, neglects the effect of the position (of the coordination sphere) in which substitution of the atoms occurs. In contrast, IR data on the molecules indicate4 that the vOH frequency, for instance in the R3-R2-Rl-OH fragment, is determined primarily by the properties of the R , atom and is practically independent of the properties of the R3 atom. For example, a change in the electronegativity ( A x = 0.9) of the X atom in the XH2C-C0,-OH molecule, corresponding to the substitution of Cl for H, leads to no change in the f r e q ~ e n c y , ~ vOH = 3584 cm-I. Similarly, a change in the electronegativity ( A x = 0.3) of the T atom, resulting from the substitution of Si for A1 in the ( +TO),Si-OH fragment (representing the terminal O H groups of various silicates, aluminosilicates, or zeolites) does not affect the vibrational frequency13 vOH, which always equals 3745 cm-I. These examples reflect the local character of the chemical interactions which corresponds to the model of the inductive influence of substituents proposed by Del ReI4 and later by Gasteiger and M a r ~ i 1 i . l ~From this model it follows that, if fragment, a change in the charge on R l in the X-R,-R2-R resulting from a change of the electronegativity of X, has a value of dq,, then changes in the charge on R2 attains a value of dq2 = pdq, and on R, dq, = p2dq,, where p SiOHAlf group we used its molecular analogue the H,SiOHAIH, molecule and the nonempirical quantum chemical method with the minimum STO-3G basis set. Examples of exploitation of this approach, as well as discussion of its adequacy, are given in ref 2 and 21. As shown by Gibbs et al.,**the average statistical (19) Dubsky, J.; Beran, S.; BosPEek, V . J . Mol. Card. 1979, 6 , 321. (20) Zhidomorov, G . M.; Chuvylkin, N. D. Usp. Khim. 1986, 55, 353. (21) Datka, J.; Geerlings, P.; Mortier, W. J.; Jacobs, P. A. J. Phys. Chem. 1985,89, 3483. (22) Gibbs, G. V . Am. Mineral. 1982, 67, 421.
OH Bridging Group Effect on Zeolites
The Journal of Physical Chemistry, Vol. 91, No. 12, 1987 3321
TABLE III: Geometry Parameters, r (lo-’’ m), and Charges on Atoms, 4,Calculated for HqSiOSi(OH)?and H2AIOSi(OH)2Molecules“
H3Si’O’Si(OH)3 H2AIO’Si(OH)3
r(Si0’)
r(Si0)
r(0H)
&OH, deg
1.590 1.587
1.659 1.660
0.983 0.983
110.1 110.3
40, -0.665 -0.692
4% 1.357 1.328
“ I n the calculations the following parameters were kept constant: LTO’Si = 140’, LHAlH = 120’, LHSiH = 109.5’ parameters are r(Si’H) = 1.425, r(Si’0’) = 1.635, r(A1H) = 1.472, and r(Al0’) = 1.643.
geometry of the 9SiOHAlf fragment in zeolites roughly corresponds to its equilibrium geometry determined by internal interactions within the fragment alone. Changes in the equilibrium geometry of this fragment originating from its steric deformations and leading to different geometries of individual fragments are then modeled by a shift of the terminal H atoms in the H3SiOHAIH3 molecule from their equilibrium positions. Then the terminal atoms are fixed in these nonequilibrium positions, while the rest of the fragment (molecule) is optimized. Hence, if Rsio’, RAIo0,and LSiOAlO are the equilibrium geometrical parameters (cf. Figure l), then the shifted positions of the terminal H atoms corresponded to the following changes in these parameters: hRslo = MAI0 = f0.02 X 1O-Io m and ASiOA1 = 10’. The remaining internal parameters were constant. It should be noted that similar differences in the T-O bonds and TOT angles were found to occur between the structurally different 0 atoms in a zeolite. The effect of the chemical factor is then estimated from a comparison of the vOH values calculated for the H3SiOSi(OH)3and H2A10Si(OH), molecules (cf. Figure 2 ) . In calculating the stretching vOH and deformation vSIOH vibrational frequencies the harmonic approximation was employed based on three points on the potential curve corresponding to the equilibrium RoHodistance and RoHof 0.01 X m or to the equilibrium SiOHO angle and S O H o f O.O1/RoHo rad. Calculations were carried out by using the GAUSSIAN-80program with the Murtangh-Sargent optimization procedure.
Results and Discussion The results of calculations on the H3SiOHAIH3molecules listed in Table I indicate that the differences in the total energy of the individual structures studied, AE, are less than 1 kcal/mol. It followed from nonempirical Gibbs calculations2*that energy values of the same order of magnitude correspond to the energies of the steric deformations acting on the 9 S i O H A l f fragment in the zeolite lattice. Similarly, the variations in the T-0 bond lengths (AR = 0.02 X m), as well as in the SiOAl angles (Aa = 10’) also roughly correspond to the differences in these parameters between the individual sites in the zeolite lattice. Finally, steric deformations of the 9 S i O H A l f fragments employed lead to
90
9H
-0.497 -0.501
1.144 1.142
The remaining optimized
differences in the stretching vibrational frequency vOH of up to 40 cm-I. Thus the difference is of the same order as that observed for different types of bridging OH groups in zeolites. In agreement with the theoretical analysis carried out in ref 6, an increase in the valence bond angle on the 0 atom (here, the SiOH angle) results in an increase in the percentage s character of the hybrid atomic orbitals forming the bonds (here, the Si-0 and 0 - H bonds). As a consequence, the length of the 0-H bond decreases and, consequently, the vOH frequency increases. Analogical trends in changes in the bond lengths, bond angles, and vOH frequencies are observed for the XOH fragment in a series of molecules (cf. Table 11). Simultaneously, an increase in the stretching vibrational frequency vOH is associated with a decrease in the deformational frequency vSiOH in accordance with the classification of frequencies given in ref 23. Comparison of the results of calculations on the H3SiOSi(OH)3 and H2A10Si(OH), molecules reveals that the substitution of Si for AI (in the third coordination sphere of the O H group) results in a negligibly small change in the frequency, vOH = 4164 and 4165 cm-I, respectively. The calculated geometry parameters and charges on atoms of the -OSi(OH), group in these molecules (cf. Table 111) illustrate the above discussed local character of the effects of a substituent. Summarizing, the results show that the structural factor plays a decisive role in the determination of the vibrational frequencies of the bridging OH groups of zeolites. Jacobs demonstrated’O that there is a correlation between the vOH frequency of the bridging O H groups and their Si:Al ratio. If we take into account the above results, this correlation can be accounted for by the changes in the steric deformations of zeolites connected with the substitution of A1 for Si. The results of the calculations also indicate that models of the O H groups which neglect the effect of the structural factor (e.g., models7,*of alumina) are not quite correct. Registry No. H,SiOHAlH,, 88337-13-7; H,SiOSi(OH),, 10794093-2; H,AIOSi(OH),, 107940-94-3. (23) Kazansky, V. B. Kinet. Katal. 1982, 23, 1334. (24) Kfasnov, K. S. Molekuiyarnie postoyanie neorganicheskikhsoedinenii; Khimiya: Moskwa, 1979.
