J. Phys. Chem. 1996, 100, 18515-18523
18515
Carbonyl 13C Shielding Tensors and Heats of Adsorption of Acetone Adsorbed in Silicalite and the 1:1 Stoichiometric Complex in H-ZSM-5 Jelena Sˇ epa, C. Lee, R. J. Gorte, and David White* Departments of Chemistry and Chemical Engineering, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104
E. Kassab, E. M. Evleth, H. Jessri, and M. Allavena Laboratory for Theoretical Chemistry, Pierre and Marie Curie UniVersity, 4 Place Jussieu, 75252, Paris Cedex 05, France ReceiVed: June 19, 1996; In Final Form: September 17, 1996X
The principal components of the carbonyl carbon chemical shift tensor of the hydrogen-bonded 1:1 stoichiometric acetone-H-ZSM-5 adsorption complex have been determined from an analysis of 13C NMR spectra of static and magic angle sample spinning powder samples at 78 and 130 K, respectively. In a similar manner the principal elements of this tensor have been determined for physisorbed acetone in silicalite and the pure solid in order to separate changes due to hydrogen bonding of the acetone molecule in the zeolite complex from confinement effects defined as interactions of the adsorbed molecule with the siliceous cavity. The energetics associated with such changes have also been measured using microcalorimetry. The differential heats of adsorption of acetone adsorbed in H-ZSM-5 and silicalite over a wide range of surface coverage are reported. The results are compared with ab-initio calculations of the reaction of acetone with model zeolite structures to form a stoichiometric hydrogen-bonded cluster-molecule complex. At the Hartree-Fock level the agreement with respect to both energetics and isotropic shifts is good but only fair for the shielding anisotropy. The magnitudes of the chemical shifts due simply to confinement of the acetone molecule are of the same order of magnitude as those associated with hydrogen bonding.
I. Introduction The observation of a stoichiometric 1:1 adsorption complex from temperature-programmed desorption experiments (TPD)1-3 of both strong and weak bases adsorbed in zeolites has generated considerable interest for the determination of the properties of the hydroxyl site responsible for the catalytic activity of molecular sieves. Recent heat of adsorption4 and 13C NMR studies,5 as a function of surface coverage, have confirmed the existence of a discrete number of localized complexes associated with the Brønsted sites, along with demonstrating the near equivalence of such sites in many acidic zeolites.4 This consideration is extremely important in any spectroscopic or thermochemical characterization of the molecule-zeolite interactions at these sites, including both the local structure and dynamics. In the case of weak bases, such as small aldehydes and ketones, both infrared studies6 and 13C NMR isotropic chemical shifts7 suggest hydrogen bonding to the zeolite framework at the Brønsted site. While H-bonding or other interactions associated mainly with the acid proton of the zeolite, referred to as the “local” interactions, undoubtedly plays a significant role in the spatial localization, orientation, and electronic structure of the molecule, the “nonlocal” van der Waals and electrostatic interactions, often referred to as confinement, long-range electrostatic, or embedding effects that modulate the “local” interactions, may be of equal importance in the determination of factors that influence the reactivity of these complexes to nucleophilic attack. Yet very little is known of this interplay that creates the environment in which the adsorbed molecule finds itself: an environment that differentiates the local structure and dynamics of the adsorbed molecule * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, November 1, 1996.
S0022-3654(96)01817-5 CCC: $12.00
in the vicinity of the acid site as well as the orientation of the adsorbed molecule with respect to the framework in different zeolites. Theoretical modeling of the interactions of small molecules adsorbed in acidic zeolites usually employ small fragments of the zeolite crystal, or clusters, to simulate the acid site.8,9 The main focus of these ab-initio quantum calculations has been on questions relating to the importance of optimization methods, cluster size, size of basis sets, electron correlation, and “nonlocal” interactions in promoting proton transfer. Using the cluster approximation to investigate “local” interactions at a Brønsted acid site has the advantage of reducing computational effort enormously compared to modeling the entire zeolite unit cell. However, the necessity of artificially superimposing additional “nonlocal” interactions in order to account for experimentally observed spectra, proton transfer, or energetics often raises questions as to the physical significance of this approach as well as to the adjustable parameters defining these interactions.10,11 Recent advances in massively parallel computing, together with improved algorithms, have permitted the simulation of methanol in a sodalite cage12,13 and in the eightmembered-ring opening of the chabazite cage13 using firstprinciple, quantum-mechanical calculations that take into account the periodicity of these systems. Unlike cluster models which are not specific to any zeolite, these calculations demonstrate differences in behavior associated with particular zeolite structures. Unfortunately even for this hypothetical case, let alone the more practical cases involving much larger unit cells such as ZSM-5, such calculations are beyond the processing ability of computer facilities in most laboratories today. Nevertheless, the above case for methanol clearly demonstrates the importance of modeling the entire environment in determining the local structure and dynamics at the acid site. © 1996 American Chemical Society
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18516 J. Phys. Chem., Vol. 100, No. 47, 1996 Recent developments in NMR spectroscopy, as well as in computational chemistry, have led to an enhanced interest in the anisotropy of the chemical shielding and the changes due to environmental effects.14-19 While phase changes are reflected in isotropic chemical shifts, the magnitudes and orientations of the principal elements of the chemical shielding tensor have been shown to be much more sensitive measures of the nuclear environment. They also provide a more stringent test of theory and thus a better understanding of theoretical deficiencies. In this paper we explore the changes in the carbonyl carbon shielding tensor of acetone when this nucleus is embedded in a variety of different environments in order to obtain some insight into the effect of “local” and “nonlocal” interactions on the molecular charge density in the adsorbed phase. Measurements of changes due to taking the molecule from the gas to the solid phase and to the physisorbed and the chemisorbed phase in ZSM-5 are reported, with the chemisorbed phase corresponding to the 1:1 stoichiometric adsorption complex with the Brønsted sites. Paralleling these studies, we also report the changes in energy associated with the transformations from the gaseous state of acetone to the various condensed phases. And finally, with the aid of ab-initio calculations using several different cluster models to simulate the zeolite, we attempt to assess the effect of the hydrogen bonding on the properties of the adsorption complex, a property which has often been related to acidity. II. Experimental Section Two zeolite samples were used in this study. The H-ZSM-5 was prepared by Chemie Uetikon AG Zeocat-Pentasil-PZ-2/ 54N. It was ammonium ion exchanged three times and calcined to place it into the hydrogen form. X-ray diffraction showed that it was highly crystalline, and atomic absorption spectroscopy gave a bulk concentration of 630 µmol/g of Al. The pore volume, measured by n-hexane uptake at room temperature and 10 Torr, was 0.174 cm3/g, which can be compared to the ideal pore volume of 0.19 cm3/g for the ZSM-5 structure. The Brønsted acid site concentration was determined to be 500 µmol/g from the amount of isopropylamine which decomposed to propene and ammonia between 575 and 650 K in simultaneous TPD and thermogravimetric analysis (TGA) measurements.20 The silicalite, which has a framework identical to that of H-ZSM-5, was a commercial material (Linde S115) which was kept in the Na form for the adsorption experiments in this paper. TPD-TGA of the hydrogen-exchanged form was used to estimate the framework Al content, which was approximately 50 µmol/g, less than 10% of that of the H-ZSM-5 sample. The microcalorimeter has been described previously.4,21 It is a home-built, Calvet-type instrument which allowed the use of relatively large samples (∼0.5 g) spread into a very thin bed (∼1 mm thick) for rapid adsorption and rapid heat transfer to the thermopiles. Differential heats were measured as a function of coverage by admitting known quantities of acetone vapor to the sample. Because of concern that adsorption equilibrium would not be achieved (i.e., that molecules would not migrate to the strongest sites in the time frame of the experiment), heats of adsorption were measured at 360 K on H-ZSM-5 and at 350 K on silicalite, but the results were essentially identical at room temperature at the lower surface coverages. For NMR measurements, H-ZSM-5 and silicalite samples, approximately 200 mg in size, were degassed and weighed at 700 K in a Cahn microbalance at 10-6 Torr. The samples were then transferred to specially designed glass sample tubes and attached to a vacuum manifold, where they were again degassed at 700 K to 10-6 Torr.22 The degassed samples were exposed
to controlled amounts of gaseous C2, 99% 13C-enriched acetone (Cambridge Isotope Labs, Inc.) using a calibrated volume to permit the small volumes of adsorbate to be known to within 2%. To avoid bed effects in adsorption, the zeolite was spread along the length of a long7,22 evacuated, 1/2 in. diameter tube during exposure to the gaseous adsorbate and then poured into a smaller tube, without exposure to air. The smaller tube was sealed and inserted into a static NMR probe for spin-counting measurements, which in all cases agreed with the measured dosing volumes. For the magic angle spinning (MAS) spectra portions of the samples were transferred in an inert atmosphere to O-ring-sealed rotors. Comparison of the in-situ spectra at low surface coverages, obtained in both probes, indicated that there were no measurable changes on transferring of the samples from the sealed NMR tubes to the MAS rotors. The 13C NMR spectra of powdered samples of 13C in natural abundance or isotopically enriched were obtained at a field of 3.5 T (37.84 MHz for the 13C resonance) using a home-built spectrometer previously described.23 Two different probes were employed in these experiments, using the Libra pulse programmer and software provided by Tecmag.24 One was a Doty25 (DSI-574) probe for magic angle sample spinning of powdered samples at spinning speeds approaching 2 kHz at temperatures as low as 125 K. The second was a single coil, doubleresonance variation of the static probe of Pines, Gibby, and Waugh,26 in which the sample could be immersed in either liquid nitrogen or the cold gas contained in a small Dewar surrounding the coil. Line shapes and frequencies were determined from the observation of proton-decoupled, Bloch decays or Hahn echoes with and without MAS and cross polarization (CP). The echo sequences consisted of a series of 90° - τ -180° - τ pulses, in quadrature, with a time delay, τ, varying from 30 to 50 µs. Longtitudinal, T1, and transverse, T2, relaxation times were obtained using saturation recovery and Carr-Purcell pulse sequences.27 The experimental frequencies in the figures are referenced relative to room-temperature, liquid TMS, using the trace of the shielding tensor of one of the two features in the MAS spectrum of adamantane as a secondary standard.28 III. Results Microcalorimetry. The differential heats of adsorption, Q, for acetone on H-ZSM-5 and silicalite, measured at 360 K, are shown in Figure 1a. For H-ZSM-5, the differential heats are almost constant up to 400 µmol/g, a surface coverage approaching one acetone molecule per acid site. The average of the measurements from the lowest surface coverage up to one molecule per acid site is 130 ( 4 kJ/mol. (The uncertainty is the standard deviation of all points up to a coverage of 400 µmol/g, which is close to the Brønsted site density.) The analogous differential heats of adsorption in silicalite are constant over the entire range examined and equal 67 ( 2 kJ/ mol. The variation with surface coverage of the differential heats of adsorption of acetone on H-ZSM-5 is not typical of that commonly observed for other weak bases, such as diethyl ether, on the same two samples shown in Figure 1b. Beyond a surface concentration corresponding to one adsorbed molecule per acid site in the typical case, there is a sharp drop in the differential heat of adsorption due to the formation of a delocalized, physisorbed phase. However, for acetone adsorbed on H-ZSM-5, the differential heats at higher surface coverages, where physisorption occurs, fall only slightly. Previous NMR observations have shown that acetone reacts rapidly at high coverages, where delocalization and exchange take place, to form mesityl oxide and other condensation products. In contrast,
Acetone Adsorbed in Silicalite
J. Phys. Chem., Vol. 100, No. 47, 1996 18517
Figure 2. Proton decoupled carbonyl carbon 13C NMR spectra of the 1:1 stoichiometric acetone-H-ZSM-5 adsorption complex (0.8 acetone molecules per Brønsted site): upper left, static spectrum at 78 K; lower left, magic-angle sample-spinning spectrum at 130 K. The arrow indicates the isotropic shift. Upper and lower right: simulated spectra, parameters given in text.
Figure 1. Differential heats of adsorption as a function of surface coverage: (a) closed circles: acetone on H-ZSM-5 at 360 K; open circles: acetone on silicalite at 350 K; (b) closed circles: diethyl ether on H-ZSM-5 at 360 K; open circles: diethyl ether on silicalite at 350 K.
acetone is unreactive at low surface coverages, even at temperatures as high as 400 K.29,30 Since the standard heat of reaction of acetone to form mesityl oxide is -124 kJ/mol of acetone, this could account for the abnormally high differential heats of adsorption of acetone at high surface coverages where bimolecular processes take place. That heat from the bimolecular reaction does not contribute to the results on H-ZSM-5 at low coverages is suggested by two additional observations. One, the differential heats at low coverage were the same when adsorption was measured at low temperatures (i.e., 295 K). Two, the normal desorption temperature for acetone in TPD (470 K), when carried out in vacuum,20 is only slightly lower than the desorption temperature of ammonia (500 K), which has a differential heat of adsorption of 145 kJ/mol when carried out under the same conditions.3,4 Heats of adsorption should scale approximately with the desorption peak temperatures if identical conditions are used in TPD and readsorption contributes to the same extent for both molecules, although the complexity of the desorption process means that these arguments should only be used as a check for consistency.31 For silicalite, acetone is easily evacuated from the sample at room temperature. The differential heat for acetone in silicalite determined in this study is also similar to that reported for butane in silicalite, 55 kJ/ mol,32 a molecule similar in size to acetone. 13C NMR. The proton decoupled 13C NMR powder spectrum of a 99% 13C carbonyl enriched stoichiometric 1:1 acetoneH-ZSM-5 adsorption complex, at 78 K, is shown in Figure 2. In this spectrum the complex has been formed with nearly 80% of the available Brønsted sites, or a surface coverage corresponding to 0.8 acetone molecules per acid site. As in earlier experiments,5 this spectrum does not appear to change with surface coverage, providing the acetone concentration is less than one molecule per acid site. Further, except for a small decrease in broadening (∼1 ppm), this spectrum is identical to one obtained at 130 K, the temperature of the magic-angle, sample-spinning experiments shown below.
The proton decoupled MAS spectrum for this sample at 130 K and a spinning frequency of 1500 Hz is shown in the lower part of Figure 2. Several different spinning speeds were employed to identify the zero-order sideband or isotropic chemical shift of the adsorption complex, shown by the arrow. In the case of acetone adsorbed in silicalite, a sample containing approximately 250 µmol/g was employed. The 300 K static spectrum taken immediately after sample preparation indicates motional narrowing of the carbonyl 13C chemical shielding tensor due to diffusion. The line width of the Lorentzian line shape at half-maximum is approximately 22 ppm, and the band center, corresponding to the isotropic shift σ j , is 215 ppm. This is consistent with the earlier observation of an isotropic shift of 217 ppm for acetone adsorbed on highsilica H-ZSM-55 at very high loadings where most of the adsorbate is physisorbed. Both the static and MAS spectra at low temperatures are characteristic of a broadened, nearly axially symmetric, chemical shielding tensor similar to that of the stoichiometric complex in H-ZSM-5. In order to obtain the principal elements of the carbonyl 13C shielding tensors, σ11, σ22, and σ33, from the static spectra and the MAS data, we have used a method similar to that described by Maricq and Waugh33 and by Herzfeld and Berger,34 but with the inclusion of quadrupole effects to simulate the line shapes.19 The latter can be of importance in the case of the acetone adsorption complex when hydrogen bonding brings the aluminum atom of the zeolite in close proximity to the 13C atom. The parameters defining the carbonyl 13C chemical shielding of acetone in several different phases are presented in Table 1. These are given as shifts relative to the isotropic shift of lowpressure, gaseous TMS at room temperature, in order to permit a comparison with theoretical, isolated molecule calculations j TMS given in the next section. (σ j TMS (low pressure gas) - σ (liquid) is taken as -4 ppm.14) The line broading corresponding to FWHM is denoted by δ. The elements of the carbonyl 13C shielding tensor in solid acetone, derived from the MAS spectra at 130 K, are, within experimental error, identical to that previously reported by Pines et al.35 The simulation of the static spectrum of the 1:1, stoichiometric, acetone-H-ZSM-5 complex at 78 K is shown in Figure 2. It was obtained employing two different models, both of which lead to the same shielding parameters. In case (a), Table 1, it is assumed that the 13C-27Al interaction of the isolated pair is simply treated as inhomogeneous broadening
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18518 J. Phys. Chem., Vol. 100, No. 47, 1996 TABLE 1: Experimental Acetone 13C Carbonyl Carbon Shielding Tensors and Energy Changes on Adsorptiona acetone-H-ZSM-5 1:1 stoichiometric complex static at 78 K (a) static at 78 K (b) MAS at 130 K σ11 (ppm) σ22 (ppm) σ33 (ppm) σ j (ppm) δ (ppm) θ (deg) φ (deg) rC-Al (Å) Q (kJ/mol) a
318 ( 2 280 ( 2 83 ( 2 227 ( 1 16 ( 1
318 ( 2 280 ( 2 83 ( 2 227 ( 1 12 ( 1 30 - 60 30 - 90 4.0 ( 0.2 130 ( 5
321 ( 4 277 ( 4 86 ( 4 228 ( 1 5(1
acetone adsorbed on silicalite static at 130 K
solid acetone MAS at 130 K
298 ( 4 269 ( 4 88 ( 4 219 ( 1 6(1
283( 2 272 ( 2 84 ( 2 213 ( 1 1 ( 0.5
gaseous acetone 300 K
202 ( 1
67 ( 3
The elements of the shielding tensors and the isotropic chemical shift are relative to low-pressure gaseous TMS at 300 K.14
of the powder pattern. In case (b), the dipolar interaction specifically included the spin Hamiltonian, with θ and φ corresponding to the polar angles of the radius vector, rC-Al, separating the 13C atom from the aluminum atom in the frame of the principal axes of the 13C chemical shift tensor. In the latter case, the least-squares fit of the data leads to a large number of local minima with respect to the parameters rC-Al, θ, and φ, where the standard deviations are nearly identical. As a result, only the ranges for rC-Al, θ, and φ are given in Table 1. Comparing the simulations (a) and (b), it is interesting to note that, treated as inhomogeneous broadening, the dipolar interaction of an isolated C-Al pair contributes about 5 ppm to the line broadening of the various orientations represented in the powder pattern on the average. This magnitude is comparable to the dipolar coupling constant, γC γAl/r3 (∼3 ppm), at a distance of 4 Å. We will not attempt to quantitatively account for the observed broadening of either the static or the MAS spectra of adsorbed acetone in any of the samples, since it is complicated by bulk susceptibility effects associated with large changes in diamagnetic susceptibly across the boundary formed by the zeolite framework with the angstrom size cavity, within which the adsorbed molecule is located.36,37 This problem is in many ways similar to broadening described by Ailion in imaging of bubbles37 and can be quite large, perhaps 5 ppm or greater in the present case. In the case of the stoichiometric H-ZSM-5 complex, this effect, together with the homogeneous broadening as determined from T1 and T2 measurements at 78 K (T2 ) 2.1 msec, T1 ) 1.4 s), can account for the observed broadening of the static spectra. The unusually large broadening of the zero-order sideband of the MAS spectrum of the complex is undoubtedly affected by the gradient in the bulk susceptibility described above, although some small fraction can be attributed to site inhomogeneity. It is clearly not a result of any slow-motion effect such as described by Suwelack, Rothwell, and Waugh,38 as evidenced by the independence of line width with sample spinning speed. Another possible source of broadening is the presence of the 27Al quadrupole nucleus.39 This, however, is ruled out by the unusually large broadening also observed in the MAS spectrum of acetone adsorbed in silicalite, which does not contain Brønsted sites. Furthermore, a simulation of the MAS spectrum for a 13C-17Al separation of 4 Å indicates a broadening of only ∼1 ppm or less, depending on the orientation of the principal axes of the electric field gradients (EFG) of 27Al relative to the principal axes of the 13C shielding tensor. Comparing the elements of the carbonyl 13C shielding tensor of solid acetone, of acetone adsorbed in silicalite, and of acetone in the stoichiometric complex reveals that the most significant change occurs in the least shielded element, σ11. The most shielded element, σ33, which for many carbonyls is found to be
perpendicular to the sp2 plane,38-41 is unaffected by the change in environment; this characteristic is also found in comparing different molecular species.40 While there is a large change in the isotropic shift when the gaseous molecule (σ j ) 202 ppm) is confined in the condensed phase (liquid, solid, or complex), the changes in isotropic shifts due to different condensed-phase environments are similar in magnitude. On the other hand, it is evident that the individual elements of the 13C shielding tensor are far more sensitive to the environmental changes which affect the electronic structure. The largest change occurs in σ11 of the stoichiometric complex whose axis, by comparison with results from crystallographic studies of a number of organic carbonyls,41-43 in all probability lies in the sp2 plane perpendicular to the CdO bond direction. IV. Ab-Initio Cluster Model Calculations In this paper, we simulate the zeolite by a small cluster, ZOH, consisting of a few atoms surrounding the zeolite hydroxyl bond. Two such clusters depicting the acid site, HOHAlH2OH and H3SiOHAlH2OSiH3, ZOH(1) and ZOH(2), respectively, employed in the quantum calculations, are shown in Figure 3. In both cases, the dangling bonds are saturated with hydrogen atoms. The basic assumption of such commonly adopted cluster models, which are not specific to any type of zeolite, is that the chemical interaction involved in the chemisorption process forming the stoichiometric complex is a strictly local phenomenon. The ab-initio calculations relating to the energetics of the various isolated species considered here were performed with the Gaussian 94 program44 at the Hartree-Fock (HF) level using 6-31G* basis set functions (HF//6-31G*). Optimization procedures were applied as follows: first, the acetone molecule (designated B) and the ZOH(i), i ) 1, 2, clusters are independently fully optimized. Second, a broad exploration of a section of the energy surface has been performed by following the reaction coordinate r(OH) in order to localize the regions of lowest energy. In these regions full optimization is performed in order to obtain true unconstrained minima and the change in energy, ∆Ec, for the reaction
ZOH(i) + B f ZOH(i) B
(1)
It is well-known that optimization leads to significant structural relaxation of the cluster. For example, calculations show a significant structural rearrangement in a cluster to accommodate the substitution of an aluminum atom.45 Since experimentally it has been demonstrated that structural changes occur for both aluminum substitution in silicalite,46 ion exchanged in zeolite RHO,47 and adsorption at low coverages in silicalite,48 we have explored this effect in our calculations. At the HF//6-31G* level the optimized ZOH(2) cluster leads to
Acetone Adsorbed in Silicalite
J. Phys. Chem., Vol. 100, No. 47, 1996 18519
Figure 3. Perspectives of ZOH(1)-acetone, ZOH(2)-acetone hydrogen-bonded complexes, and ZOH(2)-acetone addition compound. With the exception of the dangling bond protons, the atoms of the cluster lie in the plane of the paper. The bond distances and angles are drawn to scale (see Table 2), and the direction of the angles are given by the arrows.
Si-O-Al bond angles of 155° for the nonprotonated oxygen and 135° for the protonated oxygen. On the other hand, for the optimized ZOH(2)-acetone H-bonded complex structure (Figure 3) these angles are 166° and 132°, respectively. Since structural relaxation within the model zeolite clusters ZOH(1) and ZOH(2) is perhaps far more than is possible in the more rigid zeolite framework, calculations were also performed for the case where the geometry of the large cluster (ZOH(2) stoichiometry) was held fixed. Two “frozen” clusters were considered in the complexing of the large model cluster with acetone. In the first, the optimized structure of ZOH(2) was simply frozen. Thus, while structural relaxation in the model cluster occurs on substitution of an aluminum atom for a silicon, no additional relaxation is allowed on complexing. In the second, designated ZOH(F), the relaxation constraint is even greater. The bond distances and angles for this model cluster are those reported by Mortier et al.49 for faujasite. Here the Si-O-Al bond angles are 143° and 146° for the nonprotonated and protonated oxygens, respectively, with an O-Al-O angle of 109° (see Table 2). Admittedly, this represents a large change from the fully optimized case. However, recognizing that the Si-O-Al bond angles at the 12 T sites in H-ZSM-5 at which Al substitution can possibly take place vary from 140° to as large as 175°,46 this choice of parameters for ZOH(F) is not unreasonable. It represents an extreme where structural relaxation is highly constrained while the model cluster structure in the fully optimized ZOH(2)-acetone complex represents the other extreme where structural relaxation is uninhibited by any steric effects. Extended calculations were also done with these geometries at the MP-2 level (MP2//HF//6-31G*) to determine the influence of electron correlation. Two different programs were employed in the calculation of the 13C chemical shielding anisotropy: the TEXAS 90 program of Pulay50 at the HF level and the ACES II program of Bartlett.51 They both provide results at the HF//6-31G* level, but the ACES II program includes corrections for correlation effects by using MBPT perturbation procedure. The reported isotropic shifts,
Figure 4. Schematic representation of changes in binding energy of adsorption complex at unconstrained minima of energy along reaction coordinate, r(OH), for several model clusters. The values of r(OH) corresponding to the minima are given in each case.
as well as the elements of the 13C tensor, are all relative to the calculated isotropic shielding of an isolated TMS molecule. At the HF//6-31G* level the absolute shielding is 201.7 ppm. At the HF + MBPT level, using the same basis set, this is 208.5 ppm. As indicated above, the energy minimum surface associated with reaction 1 has been explored in the planar section defined by the reaction coordinate r(OH) in order to locate the true minima at which full optimization was performed. Since the present studies do not focus on the vibrational modes of the adsorption complex, the normal modes at these minima are not presented here. The main features of these calculations are illustrated in Figure 4 where the variation of the HF//6-31G* optimized energies of the ZOH-acetone complex as a function of r(OH) are given. For both ZOH(1) and ZOH(2) one observes double minima. With increasing r(OH) the first minimum
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18520 J. Phys. Chem., Vol. 100, No. 47, 1996 corresponds to the hydrogen-bonded ZOH‚‚OdC (CH3)2 complexes shown in Figure 3. The second minimum corresponds to the neutral addition compound, ZO-C(CH3)2OH, from the proton transfer reaction in which the non protonated oxygen of the model cluster forms a C-O bond with the carbonyl carbon atom of acetone. The stability of such addition complexes has also been demonstrated in the ab-initio calculations of Evleth et al.52 for the reaction of several hydrocarbons with model clusters. That the addition compound may exist under some transient conditions is likely, given that it is very similar to the silyl-ether complex we have observed in the adsorption of tertbutyl alcohol by H-ZSM-5.53 An examination of the results shown in Figure 4 indicates two interesting features. First, the stability of the addition complex relative to the hydrogen-bonded complex is strongly dependent on cluster size. Second, in the case of ZOH(F), the addition complex is not stable. In fact, this is true of all of the frozen clusters explored and clearly points to the fact that the formation of addition compounds requires structural relaxation well beyond that observed in forming the H-bonded complexes. It should be noted that the inclusion of correlation corrections (MP2//HF) does not significantly modify these results. In the present paper we focus solely on the hydrogen-bonded complexes. More detailed results for the acetone-cluster addition compound, as well as more generally for addition complexes involving aldehydes and ketones, will be presented at a later date. Using quite similar procedures, Florian and Kubelkova6 have also explored the proton transfer energetics of acetone complexing with model clusters. They also confirmed the stability of H-bonded acetone-cluster complex but explored only the formation of an ion-pair in a process involving proton transfer, rather than the possibility of a neutral addition complex as was done in the present investigation. Although the ion-pair structure proved unstable, it may be important as a transition state, the saddle point of order 1 along the reaction coordinate rOH, in the reaction of the H-bonded complex to form the stable addition compound. Preliminary calculations for the small ZOH(1) cluster suggest that this may indeed be the case. Calculations for large model clusters are currently underway. In Tables 2-5, the parameters of the calculated structures, the energies, and the principal components of the 13C carbonyl carbon shielding tensor are given for the hydrogen-bonded acetone-cluster complex in each of the three cases. Except as noted, all of the results in the tables are based on calculations at the HF level, using a 6-31G* basis set. The structural parameters listed in the first column of Table 2 are defined in Figure 3 for the optimized geometries of the H-bonded ZOH(1) and ZOH(2) cluster complexes. In Figure 3, with the exception of protons used to saturate the dangling bonds, the atoms of the clusters along with the CdO bond of the acetone molecule lie nearly in the same plane. In ZOH(1) and ZOH(2), this plane bisects the sp2 plane of the complexed acetone molecule. Also noteworthy is the difference between the two Si-O-Al bond angles within the ZOH(2) cluster when compared with the nearly tetrahedral O-Al-O bond angle. In the case of the ZOH(F)acetone complex, both of these angles are nearly the same. This constraint results in two important changes in the structure of its acetone complex as compared to ZOH(2) shown in Figure 3. First, the acetone molecule is moved away from the plane containing the atoms of the zeolite cluster. (In the complete structure, this would correspond to moving the molecule into the framework cavity away from the Brønsted site.) Second, this leads to a shorter hydrogen bond (O‚‚‚H-O distance) and brings the carbonyl carbon closer to the Al atom of the cluster.
TABLE 2: Optimizeda Structures of the Hydrogen-Bonded Acetone-ZOH Complexesb parameterc
acetone-ZOH (1) acetone-ZOH (2) acetone-ZOH (F) H-bond complex H-bond complex H-bond complex
r(OAl) r(AlO) r(OH) r(H‚‚‚O) r(OC) r(CO) r(O‚‚‚H-O) ∠OAlO ∠AlOH ∠OHO ∠HOC ∠ΟCΟ [OAlO, AlOH]d [AlOH, OHO] [OHO, HOC]
1.746 1.987 0.968 1.790 1.204 3.008 2.70 97.4 111.8 163.4 122.6 90.8 18.8
1.733 1.972 0.964 1.830 1.200 4.109 2.75 96.2 111.8 166.6 149.3 58.6 0.0
1.742 1.746 0.994 1.656 1.207 3.535 2.65 109.3 102.3 171.1 131.8 70.5 23.7
57.4
180.0
132.0
274.2
180.0
259.9
a
Optimization performed at the HF level using 6-31G* (HF//631G*). b Parameters are restricted to description of psuedo ring OAlOH‚‚‚OdC‚‚‚ (see Figure 3). Other parameters are available on request. c The bond distances are given in angstroms, the angles in degrees. d Dihedral angles at which the planes in the brackets intersect.
TABLE 3: Calculated Energies of Acetone, Model Clusters, and Acetone-Cluster Complexes species
HFa (au)
MP2//HFb (au)
acetone ZOH(1) ZOH(2) ZOH(F) ZOH(1)‚‚‚OdC (CH3)2 ZOH(2)‚‚‚OdC (CH3)2 ZOH(F)‚‚‚OdC (CH3)2 ZO(1)-C(CH3)2OH ZO(2)-C(CH3)2OH
-191.962 236 -394.587 474 -974.836 245 -974.796 140 -586.569 738 -1166.816 26 -1166.786 84 -586.568 35 -1166.789 34
-192.521 610 -395.012 592 -975.407 799 -975.367 431 -587.558 040 -1167.950 309 -1167.925 440 -587.564 36 -1167.938 81
a Geometry optimization and energy at HF level (HF//6-31G*). Geometry optimized at the HF level; energy calculated at the MP2 level (MP2//HF//6-31G*).
b
TABLE 4: ∆EcH-bond and ∆Ecadd for Various Complexed Model Clustersa ∆Ec (kJ/mol) H-bonded complex ZOH(1)‚‚‚OdC (CH3)2 ZOH(2)‚‚‚OdC (CH3)2 ZOH(F)‚‚‚OdC (CH3)2 addition compound ZO(1)-C(CH3)2OH ZO(2)-C(CH3)2OH b
HF
MP2//HF
-52.6 -46.7 -74.7
-62.6 -54.9 -95.5
-3.7 -70.6
+16.6 -30.2
a ∆EcH-bond ) E[H-bonded complex] - {E[ZOH(i)] + E[acetone]}. ∆Ecadd ) E[H-bonded complex] - E[addition complex].
The calculations of the energies shown in Tables 3 and 4 were performed at two different levels but using the same basis set, 6-31G*, in order to estimate the effect of electron correlation. These are the HF geometry optimized energies and MP2/ /HF energies corresponding to HF geometry optimization but energies at the MP2 level. The main purpose of these was to determine the heat evolved in reaction 1, ∆EcH-bond, to form the hydrogen-bonded complex and how this parameter is influenced by electron correlation. Here only differences not the absolute energies are significant. At the HF level these differences lead to values of ∆EcH-bond of -53, -47, and -75 kJ/mol for the complexes formed with ZOH(1), ZOH(2) and
Acetone Adsorbed in Silicalite
J. Phys. Chem., Vol. 100, No. 47, 1996 18521
TABLE 5: 13C Carbonyl Carbon Shielding Tensora and Corrected Energy Changeb on Formation of H-Bonded Acetone-Cluster Complexes
σ11 (ppm) σ22 (ppm) σ33 (ppm) σ j (ppm) θ (deg) φ (deg) rC-Al (Å) ∆EcH-bond(corr HF) (kJ/mol) ∆EcH-bond(corr MP2) (kJ/mol)
acetone-ZOH (1) H-bond complex
acetone-ZOH (2) H-bond complex
acetone-ZOH (F) H-bond complex
283 278 69 210 23 70 4.00 -41 -42
277 271 70 206 50 90 4.99 -36
297 277 68 214 64 50 4.08 -57
acetone-(isolated molecule) 279 226 72 192
a TEXAS Pulay program, HF//6-31 G*. Shielding tensor represented as shifts relative to isolated TMS molecule. b For reaction 1 calculated at HF//6-31 G* and MP2//HF//6-31G* levels. ∆EcH-bond(corr) takes into account basis set superposition errors and zero-point energy corrections.
ZOH(F) respectively. Taking into account electron correlation at the MP2//HF level changes, these quantities become -63, -55, and -95 kJ/mol. The calculated elements of the carbonyl-carbon shielding for the three cluster complexes are given in Table 5. The orientation of the principal axes of this shielding tensor with respect to the frame defining the acetone-ZOH complex is expressed by the polar angles (θ and φ) of the 13C-Al internuclear vector. With respect to the molecular frame of acetone, the calculations indicate that σ11 and σ22 lie in the sp2 plane of the molecule, the latter in the CdO bond direction, with σ33 perpendicular to this plane. This, as indicated earlier,40-42 is characteristic of organic carbonyls. The calculations also show that σ33, as in many carbonyls, is not sensitive to the environment in which the molecule finds itself.40 This is also borne out in the experiments summarized in Table 1. The calculations indicate that the most significant deshielding on complexing an isolated acetone molecule occurs in σ22 which results in a large change in the isotropic shift σ j . However, σ22 is not very sensitive to changes in environment associated with cluster size or the geometric constraint such as imposed in ZOH(F) which markedly effects the orientation of the molecule with respect to the cluster. The most significant change occurs in σ11 when the carbonyl carbon of the acetone molecule is twisted out of the plane containing the atoms of the model cluster simulating the zeolite. V. Discussion In comparing the differential heats of adsorption with the calculated energy changes associated with the formation of the cluster-molecule complex, it is helpful to partition the energy changes associated with the formation of the 1:1 stoichiometric complex into two contributions, ∆E1 and ∆E2, (CH3)2CO + H–ZSM–5 –Q
∆E1
(CH3)2CO[H–ZSM–5] ∆E2
(CH3)2CO • • • H–ZSM–5
where ∆E1 corresponds to energy change in simply confining the acetone molecule to the zeolite channels as the physisorbed phase, (CH3)2CO[H-ZSM-5], and ∆E2 the change in energy in transforming the physisorbed phase to the localized hydrogenbonded complex. Thus, the differential heat of adsorption Q(H-ZSM-5) equals -(∆E1 + ∆E2). If it is assumed that physisorbed acetone does not energetically differentiate between the predominantly siliceous environment of the zeolite and one containing an acid site, then ∆E1 can be identified with the differential heat of adsorption of acetone in silicalite, Q(ZSM5), the siliceous ZSM-5 structure containing no Brønsted sites.
The difference, then, between the heat of adsorption of acetone in H-ZSM-5 at surface coverages corresponding to the formation of a stoichiometric adsorption complex and that of acetone adsorbed in silicalite under similar conditions, Q(H-ZSM-5) - Q(ZSM-5) ) 63 ( 5 kJ/mol, is a measure of ∆E2. This is the energy required to localize and hydrogen bond the physisorbed species to the acid site in H-ZSM-5 due to what we have referred to earlier as the “local” interactions that, in principle, are simulated in the ab-initio calculations involving cluster models. It can also be viewed as the difference in energy between a “gaseous” species confined to a narrow channel and one which is hydrogen bonded to the wall of the channel at specific sites. Since the ab-initio calculated ∆Ec’s (see Table 4) correspond to energies required to form the hydrogen-bonded cluster complex from the isolated species, then except for an error due to neglect of the zero-point energy of the physisorbed phase, ∆E2 and ∆EcH-bond, corrected for basis set superposition errors (BSSE) and zero-point effects (ZPE), can be identified with the same process. A comparison of the experimental difference with the calculated values for the three different model clusters (see Table 5) indicates that when a structural constraint such as in ZOH(F) is included, there is good agreement between theory and experiment. That this artificial constraint reflects the physical situation associated with confinement in the three-dimensional cavity in zeolites is highly questionable. It is to be viewed simply as a parameter that reflects the rigidity of the framework of the zeolite. In a series of calculations at the HF//6-31G* level of the ZOH(F) type, where O-Al-O tetrahedral angle and bond distances are fixed, but the two Si-O-Al angles are varied over the narrow range 140-149°, it is found that ∆EcH-bond falls in the range 75 ( 2 kJ/mol. On the other hand, when the structure of the optimized ZOH(2) cluster is fixed, a constraint, relative to full optimization, far less than in the case of ZOH(F), then the calculated ∆EcH-bond, while within 1-2 kJ/mol of the fully optimized case, differs significantly from ZOH(F) cluster complex. One can only conclude while the introduction of constraints reducing the structural relaxation of the model cluster does give better agreement, given the assumptions involved, the good agreement between theory and experiment in the case of ZOH(F) is somewhat fortuitous. Besides the approximations involved in the model we have used to simulate the confinement effect in zeolites, inherent in the equating of -∆E1 to Q(ZSM5), is the assumption that structural relaxation on adsorption is the same in both H-ZSM-5 and silicalite. The latter neglects the unique changes in the framework structure associated with specific molecule-zeolite interactions. The model does however exhibit some expected features associated with hydrogen bonding. The correlation of hydrogen bonding energy with O-O bond distances by Hibbert and Emsley54 for a large
18522 J. Phys. Chem., Vol. 100, No. 47, 1996 number of cases reported in the literature indicates a shorter bond distance with increasing energy. The expected bond distance for the energies reported here based on this correlation is from 2.4 to 2.5 Å. The H-bonded ZOH(F)-cluster complex does lead to a shorter hydrogen bond, (rO‚‚‚H-O ∼2.6 Å) than in the case of ZOH(2) (see Table 2), but not sufficiently to account for the calculated energy change. It is of interest to compare the heat of adsorption of acetone in silicalite (67 kJ/mol) where the interactions with the walls of the framework cavity lead to the confinement, with the heat of vaporization of acetone, 32 kJ/mol, at the boiling point (329 K). In the latter case the confinement of the molecule to the “liquid cage” is a result of intermolecular forces. The large difference between heat of adsorption and vaporization is principally due to two factors. The first is the extent of localization. This is considerably greater in the case of acetone adsorbed in silicalite as indicated by the 13C NMR line widths. In the liquid at room temperature the entire anisotropy of the carbonyl carbon shielding tensor is motionally averaged. In silicalite it is only partially averaged, the line width of 22 ppm indicating significant localization at least on the NMR time scale. The second, and perhaps the most important, contribution is the much deeper potential well representing the interactions between the polarizable framework atoms of the zeolite and the non bonded CH3 groups of the acetone molecule. The 13C chemical shifts of the components of the carbonyl carbon shielding tensor of adsorbed acetone given in Table 1 follow a pattern similar to that described by the energetics. In comparing theory with experiment, only the differences between the shifts in the gas phase and that of the different H-bonded complexes are significant. In both the theoretical and experimental results, one sees the importance of both “local” and “nonlocal” interactions in determining not only the changes in the isotropic chemical shifts but also the principal components of the carbonyl 13C shielding tensor. The large difference of 14-22 ppm in the calculated isotropic shifts between the isolated acetone molecule and acetone H-bonded to the model clusters is also observed in experiment irrespective of whether the gas is confined in the solid, silicalite, or the hydrogen-bonded complex in H-ZSM-5. In the cluster model, this change is entirely due to what has been referred to as the “local” interaction, i.e., the hydrogen-bonding and van der Waals interactions of acetone with the small cluster. However, experimentally, the large difference between the isotropic chemical shift of the gas and the 1:1 stoichiometric complex in H-ZSM-5, observed to be ∼25 ppm, is clearly a result of both “local”, as represented by a cluster model, and “nonlocal” interactions associated with stabilization of the H-bonded complex in a highly polarizable environment when confined to the channels of the zeolite framework. If one again assumes that these are separable and the “local” effects can be obtained from the difference between the isotropic shifts of acetone adsorbed in H-ZSM-5 and acetone adsorbed in silicalite, then one would attribute only 8-9 ppm to hydrogen bonding. Two facts emerge from this comparison of the calculated changes in isotropic shifts with experiment. First, while the “local” and “nonlocal” interactions both result in a deshielding, the magnitude of the former is overemphasized in the cluster model calculations. On the other hand, as we have pointed out before, the artificial separation of “local” and “nonlocal” interactions in experiment neglects the influence of structural relaxation as well as other factors which affects the H-bond strength. For example, we have previously shown that cavity size changes the isotropic chemical shift,7 and thus the “nonlocal” interactions clearly affect the molecule charge
Sˇ epa et al. distribution. The second is that it is simply not possible to relate the difference in isotropic shift between the stoichiometric hydrogen-bonded complex of the organic base in the zeolite and the base in the pure liquid, solid, or solution with the extent of proton transfer in the complex as is frequently done in the literature. Such changes provide some insight, but no quantitative basis, for their relation to such concepts as zeolite acidity. Both the experimental data and the ab-initio calculations of the elements of the carbonyl 13C shielding tensor provide some useful insights into the origin of the deshielding when the molecule acetone forms the stoichiometric complex in H-ZSM5. Unfortunately, the elements of the tensor are not known for the gaseous phase. If one accepts the calculated elements of the tensor of the isolated acetone molecule (Table 5) as representative of the gas phase, then correction for the reference that makes both the calculated and experimental gas phase isotropic shifts identical the components are σ11 ) 289, σ22 ) 236, and σ33 ) 82. Comparing this result with acetone in the condensed state (solid, adsorbed, cluster complex), one finds the principal contribution to the deshielding arises from σ22, the component parallel to the CdO bond. This has often been correlated with red shifts of λmax for the n f π* optical transition of such localized orbitals40,55,56 due to environmental effects. On the other hand, when one compares the changes within the various condensed phases, it is obvious that such generalization relating to environmental effects are not necessarily valid.17 Experimentally, the largest change occurs in the element σ11, the component perpendicular to the CdO bond in the sp2 plane of the molecule. The ab-initio calculations also indicate that the most significant changes when comparing different cluster models occur in σ11, in particular when the acetone molecule is twisted away from the plane containing cluster atoms. This is the case for ZOH(F) and other models not described here with constraints that shorten the rC-Al distance in the molecule cluster complex. Such an effect has also been noted by Wu et al. in benzaldehyde,17 where both σ11 and σ22 become more deshielded on twisting the formyl group out of plane of the benzene ring. Unfortunately, it is not possible to determine the orientation of the acetone molecule with respect to the H-ZSM-5 framework from the present studies, other than to note that the values of θ, φ, and rC-Al estimated from experiment (the latter on the assumption that the additional broadening is due to the dipolar interaction) are within the range calculated for the frozen cluster ZOH(F). On the other hand, it is not unrealistic to expect steric effects associated with confinement in the small channels to produce such changes in the orientation of the hydrogen-bonded complex in order to reduce repulsion. In the present study the experimental measurements have focused on results which could be more properly compared with ab-initio calculations based on model clusters representing the “local” interactions in zeolites. The comparison is based on the assumption that molecular interactions in zeolites are the sum of three separable contributions, namely, (a) the site interactions (such as a Brønsted site for example), modeled by a small cluster representing the zeolite which we have called the “local” interactions, (b) the confinement effect, due to the structure of the zeolite cavity and the interactions of polarizable framework atoms with atoms of the adsorbed molecule and (c) long-range adsorbed molecule electrostatic interactions with atoms beyond the framework of the cavity. Both (b) and (c) represent the “nonlocal” interactions. Admittedly, the calculation of the “local” interactions depends on factors such as cluster size and size of basis sets, etc., however, since in many cases we are dealing with differences there is some cancellation of
Acetone Adsorbed in Silicalite such effects. In spite of these limitations, the comparisons between theory and experiment presented here do provide a better understanding of the important factors in determining the energetics, charge density, and structural relaxation that leads to stable adsorption complexes and reaction products. Clearly, a more rigorous comparison between theory and experiment requires a more complete and realistic simulation of the zeolite framework such as is now beginning to emerge with the development of improved algorithms and increased computational facilities.12,13 Acknowledgment. The authors thank Drs. Bryan Suits, David Olson, and C. Giessener-Prettre for many helpful discussions during the course of this work and Dr. P. Pulay for providing his program used in many of the calculations. This work was supported in part by the National Science Foundation Grant CTS 94-03909. We also thank the Institut du Development des Resources en Informatique Scientifique (IDRIS) du CNRS for their support of the computing resources made available to this research. References and Notes (1) Aronson, M. T.; Gorte, R. J; Farneth, W. E. J. Catal. 1986, 98, 434. (2) Gricus, T. J.; Gorte, R. J.; Kokotailo, G. T.; Farneth, W. E. J. Catal. 1989, 115, 265. (3) Parrillo, D. J.; Adamo, A. T.; Kokotailo, G. T.; Gorte, R. J. Appl. Catal. 1990, 67, 107. (4) Parrillo, D. J.; Lee, C.; Gorte, R. J. Appl. Catal. A 1994, 110, 67. Parrillo, D. J.; Gorte, R. J. J. Phys. Chem. 1993, 97, 8786. (5) Biaglow, A. I.; Gorte, R. J.; White, D. J. Phys. Chem. 1993, 97, 7135. (6) See for example: Florian, J.; Kubelkova, L. J. Phys. Chem. 1994, 98, 8734 and references therein. (7) Biaglow, A. I.; Gorte, R. J.; Kokotailo, G. T.; White, D. J. Catal. 1994, 148, 779. (8) Sauer, J.; Ugliengo, P.; Garrone, E.; Sanders, V. R. Chem. ReV. 1994, 94, 2095 and references therein. (9) Van Santen, R. A.; Kramer, G. J. Chem. ReV. 1995, 95, 637 and references therein. (10) Allavena, M.; Seiti, K.; Kassab, E.; Ferencsy, Gy.; Angyan, J. G. Chem. Phys. Lett. 1990, 168, 461. Kassab, E.; Seiti, K.; Allavena, M. J. Phys. Chem. 1991, 95, 9425. Kassab, E.; Fouquet, J; Allavena, M. J. Phys. Chem. 1993, 97, 9034 (11) Teunissen, E. H.; Jansen, A. P. J.; Van Santen, R. A. J. Chem. Phys. 1994, 101, 5865. (12) Nusterer, E.; Blochl, P. E.; Swartz, K. Angew. Chem. 1996, 35, 175. (13) Shah, R.; Payne, M. C.; Lee, M. H.; Gale, J. D. Science 1996, 271, 1395. (14) Jameson, C. J. Nuclear Magnetic Resonance a specialist periodical report; Webb, G. A., Ed.; Royal Society of Chemistry: Cambridge, 1994; Vol. 23, p 47, and earlier papers in this series. Jameson, C. J.; Jameson, A. K.; Oppusunggu, D.; Wille, S. J. Chem. Phys. 1982, 76, 152. (15) Nuclear Magnetic Shieldings and Molecular Structure; Tossell, J., Ed.; NATO ASI Ser. C, Vol. 386; Kluiver Academic: Dordrecht, The Netherlands, 1993. (16) Duncan, T. M. A. Compliation of Chemical Shift Anisotropies; The Farragut Press: Chicago, 1990. (17) Wu, G.; Lumsden, M. D.; Ossenkamp, G. C.; Eichele, K.; Washylishen, R. E. J. Phys. Chem. 1995, 99, 15806. (18) Sˇ epa, J.; Gorte, R. J.; Suits, B. H.; White, D. Chem. Phys. Lett. 1996, 252, 281. (19) Suits, B. H.; Sˇ epa, J.; White, D. J. Mag. Reson., Ser. A 1996, 129, 88.
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