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J. Phys. Chem. 1980, 84, 1982-1986
(17) J. Peeling, F. E. Hruska, and N. S.McIntyre, Can. J . Chem., 58, 1555 (1978). (18) R. E. Christoffersen, Adv. Quantum. Chem., 6,333 (1972). (19) R. E. Christoffersen, D. Spangler, G. M. Maggiora, and G. 0. Hall, J. Am. Chem. SOC.,95,8526 (1973). (20) J. L. Whltten, J . Chem. Phys., 44, 359 (1966). (21)J. L. Whitten, private communication. (22) G. G. Hall, f r o c . R. SOC.London, Ser. A , 205,541 (1951). (23) C. C. J. Roothaan, Rev. Mod. fhys., 23, 69 (1951). (24) J. L. Whitten and M. Hackmeyer, J. Chem. Phys., 51,5584 (1969).
(25) L. L. Shipman and R. E. Christoffersen, Chem. Phys. Lett., 15, 469 (1972). (26) E. Westhof, Y. Lion, and A. Van De Vorst, Int. J. Rad&. Bioi., 32, 499 (1977). (27)J. L. Whitten and M. Hackmeyer, J. Chem. fhys., 51,5584 (1969). (28) M. Hackmeyer and J. L. Whitten, J . Chem. fhys., 54, 3739 (1971). (29) I. L. Cooper, and R. McWeeny, J. Chem. Phys., 45, 226 (1966). (30) 8 . T. Sutcliffe, J. Chem. fhys., 45, 235 (1966). (31) M. Simonetta, E. Gianinetti, and I. Valdoni, J. Chem. Phys., 48, 1579 (1968).
CNDO Study of the Site I1 and the Site I11 in Faujasite-Type Zeolites W. J. Mortier” Centrum voor Opperviaktescheikunde en Colloidale Scheikunde, Ka tholieke Universiteit Leuven, De Croylaan 42, 8-3030 Leuven (Heveriee), Belgium
and P. Geerllngs Dlenst Algemene Chemie, Fakuiteit Wetenschappen, VriJe Universitelt Brussei, Plehlaan 2, B- 1050 Brussel, Belgium (Received November 27, 1979)
A CNDO study of the electronic structure of the faujasite-type framework and the cation-exchange sites of type I1 and type I11 was made on model molecules. Previous statements (site 1’) on charge delocalization upon isomorphous substitution of Al for Si and upon interaction with exchangeable cations are confirmed. The increase of the CO stretching frequency upon adsorption on Mg2+is rationalized via model calculations on Mg-CO and is in agreement with the observations. It was shown that influences of the structure (bond angles) on the OH stretching frequency for bridging hydroxyls (Si-OH-Al) are negligible as compared to the influence of the chemical composition and the environment.
Introduction Recently, the changes in overall properties of zeolites could be related to their compositional variation (Si/A1 ratio, cation type, and exchange degree) irrespective of their structure type. When Sanderson’s electronegativity model was used, acidity, carboniogenic activity, and strength of interaction with transition metal ions1 were quantified. Accordingly, a CND0/2 study of the site I’ faujasite-type six-ring2showed that the charges are considerably delocalized. Isomorphous substitution of A1 for Si as well as the presence of exchangeable cations resulted in only minor variations of the framework oxygen charges. The charges of the tetrahedral silicon atoms were somewhat more sensitive for isomorphous substitution. These studies could suggest that structural variations are of minor importance for the charge distribution in the zeolite-type framework. However, studies of Tossel and Gibbs3 have clearly shown the influence of the bonding angles on the bonding distance. This was explained in terms of covalent bonding and related to the overlap population. These relations were also observed for different types of zeolite framework^.^-^ Geometrical variations can indeed have their influence on the charge distribution. In order to investigate the extent of these structural effects on the distribution of the charges, we made a CNDOI2 study of a faujasite-type six-ring at site I1 and the sequence of four-rings at site 111. Furthermore the different effects for monovalent (Na+)or divalent (Mg”) cations, the adsorption of CO, and the protonation of the bridging oxygens were considered. Model and Calculations The site 11- and IIT-type “molecules” were taken from the same idealized structure as before2 by using Si-0 = 1.61 A, A1-0 = 1.75 A, and 0-T-0 = 109.5’ (T = Si or Al). Drawings of these molecules are given in Figures 1 0022-3654/80/2084-1982$01 .OO/O
and 2. The open T-0 bonds were terminated by protons at 0.96 A in the direction of the next T atom in the framework. The site I1 six-ring is the six-ring of the cubooctahedron facing the large cavity, and the site I11 is a sequence of four-rings of TO4tetrahedra in the large cage, adjacent to the sites of type 11. For more details of the structure, we refer to ref 7. The molecules considered in our calculations are (I1 and I11 refer to the sites I1 or 111, A to an all-Si molecule and B to a 1:l Si/A1 isomorphous substitution):
I1 B I1 BNa I1 BMg I1 BMgCO I11 A I11 B, I11 BH4a I11 BH4P Full details of the calculation methods, CNDOIB, CNDOIBD, and the molecular electrostatic potential were given before.2 It should be emphasised that 3d orbitals were not included in the calculations because of convergence problems. The inclusion of the 3d orbitals in the basis set for CNDO-type calculations was considered in more detail by Tossel and GibbsEfor their application to bond-angle studies. The inclusion of 3d orbitals leads to erroneously large Si(3d) participation. They found that for their study of bridging-bond-angle variations in corner-sharing tetrahedra more accurate results were obtained by using the sp basis set. It should be kept in mind that the absolute values of the computed molecular properties such as atomic charges, force constants, and the molecular electrostatic potential 0 1980 American Chemlcal Society
CNDO Study of Faujasite-Type Zeolites
Figure 1. Drawing of the site 11-type six-ring (I1 B) in the faujasite-type structure seen from the side of the cubooctahedron. The 0(1) and O(3) oxygens have protons attached in the direction of the next neighboring T atom.
Figure 2. Drawing of the sequence of four-rings (site 111 and 111’) (111 B) in the faujasiite-type structure seen from the large cage side.
at a given point are not very trustworthy because of limitations inherent in the Hartree-Fock theory and its CNDO appralximateform. Consequently we will compare only properties calculated within the same quantum mechanical method, e.g., the charge of a given atom in one of the model molecules before and after complexation with CO, both values computed with the CNDO method. It should furthermore be stressed that the model must be chosen carefully, The atomic plositions obtained by X-ray structure determinations are average positions and deviate consliderably from the local configuration, especially when the positions of the exchangeable cations are only partially occupied as is most often the case of zeolite structures. ‘Phis can lead to contradictory results. Beran and Dubskyg used for similar calculations the atomic coordinates from a dehydrated Na-Y-type structurelO without further idealization of Si-0, A1-0, and Na-0 bond distances and of the tetrahedral bond angles. This model includes therefore T-0 distances ranging from 1.58-1.70 A, irrespective of the tetrahedral atom and tetrahedral bonding angles (0-T-O) in the range 104-117’. The details of the charge distribution and of the bonding parameters are therefore affected. The bonding parameters (charges (andbond orders) differed considerably among the site I (Na-0, 2.71 A), site I’ (Na-0, 2.44 A), and site I1 (Na-0, 2.33 A). The ideal Na-0 distance is 2.35 A (11) and was used for the present calculations. The site I1 results are therefore in closer agreement with our calculations.
Results and, Discussion The resultti of the CNDO atomic charges and the gross atomic populations following a Mulliken population
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1983
analysis after deorthogonalization (CND0/2D)12for the site I1 and site I11 molecules are given in Tables I and 11, respectively. On the basis of these results, the following comparisons can be made: (a) the influence of the configuration differences at site I’ and site I1 for the BMg-type six-rings, (b) a comparison between I1 BMg and I1 BMgCO for the study of the electronic interactions upon adsorption, (c) a comparison between the I1 BNa and I1 BMg six-ring for the analysis of the effect of monovalent and divalent cations, (d) the study of the influence of the configuration between sites I1 and I11 (I1 B vs. I11 B), (e) the influence of the isomorphous substitution (I11 A vs. I11 B), and (0 the effect of the protonation of the bridging oxygens on the atomic charges (I11 B vs. I11 BH4). a. Comparison between the Site I’ and Site 11 T y p e Six-Rings. Both rings have a similar configuration. The site I’ six-ring contains oxygens of the O(2) and O(3) type, the O(3) oxygen being coordinated to cations if they are present. The site I1 six-ring contains O(2) and O(4) oxygens, the O(2) oxygens pointing towards the center of the ring (see Figure 1). Crystallographically the atoms have a different point position such that coordination distances and bond angles differ somewhat: site 1’, Si-O(S)-Al129.5’ and Si-O(2)-A1 132.8’, vs. site 11, Si-O(2)-Al 133’ and Si-0(4)-A114l0. The Mg-O(3) distance used for the site I’ was 1.96 A, while the closest approach for site I1 cations to the O(2) oxygens is 2.11 A from the center of the ring. Despite these differences, the CNDO charges and the CNDOI2D gross atomic populations do not differ by more than 0.02 e, with the exception of Mg (0.04 e higher for the CNDO results and 0.06 e lower for CNDO/BD). The Mg-0(2) overlap population follows the longer bond distance and decreases by 0.01 e for the site 11. The Si-0 and A1-0 overlap populations follow the variation in TO-T angle; i.e., with a wider bond angle, a shorter T-O distance is predicted.13 The differences are 0.02 e for Si-0(2), 0.03 e for Si-0(4), 0.02 e for A1-0(2), and 0.03 e for A1-O(4). The molecular electrostatic potential14 (MEP)shows the same features as for site 1’: and the same conclusions hold for site 11: The MEP being very sensitive to residual charges, a bidentate adsorption for C02 is possible in the case of a local negative charge excess via the C+-0- local dipole, an O(4) framework oxygen and the exchangeable cations yielding carbonate species.15J6 The adsorption of CO occurs in monodentate complexes via the carbon lone pair as the “reactive site” of the adsorbing molecule, the polarity of CO being very low. The enhancement of the CO stretching frequency upon adsorption17is discussed further in the next section. b. Interaction with CO ( I I BMg us. II BMgCO). For the CNDO case virtually identical charges were obtained for all framework atoms. The differences are situated in the Mg-CO linkage. A shift of the negative charges from the CO entity to the six-ring is observed (0.21 e). A comparable situation arises in hydrogen bonds of the type X-A-He .B-Y where CNDO and ab initio calculations always indicate that an electron transfer takes place in the direction of the proton donor.18 [The finer details of the charge redistribution upon hydrogen bonding (increased electron density on A and B, decreased electron density on X, H, and Y) do not correspond to those found in the present “supermolecule” as, e.g., the Mg atom gains electrons.] But after deorthogonalization, a negative charge excess on the CO entity is found, and the Mg atom now becomes more positive. The charge of the O(2) atom is just as in CNDO practically unchanged after complexation.
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Mortier and Geerlings
TABLE I: CNDO and CNDOl2D Atomic Charges and Overlap Populations for the Framework Six-Ring at Site I1 I1 B
I1 BMg
I1 BNa
I1 BMgCO
Atomic Charges, CNDOICNDOIBD 1.3911.77 -0.591-0.76 - 0.70j- 0.87 -0.561-0.72 - 0.731- 0.91 0.05/(0.07-0.08) 1,2711.80 -0.611-0.76 - 0.591-0.73 0.01/(0.01-0.02)
1.4511.79 -0.601-0.75 -0.721-0.92' -0.551-0.72 - 0.7 11- 0.90 0.08/(0.11-0.12) 1.2811.75 - 0.601- 0.75 -0.581-0.73 0.03/( 0.04-0.06)
1.4811.85 -0,561-0.74 -0.751-0.99' - 0.551- 0.7 2 - 0.701- 0.89 (0.11-0.12)/(0.14-0.16) 1.2611.77 - 0.581- 0.14 -0.581-0.72 0.06/(0.03-0.08) 0.9011.17
1.4811.76 -0.561-0.74 --0.751-0.99' 0.551- 0.72
- 0.711-
0.90 0.1 1/(0.14-0.16) 1.2611.84 - 0.5Sl- 0.74 - 0.581- 0.72 0.06/(0.07-0.08) 0.7811.33
0.3410.71 0.18/0.08 0.031- 0.14
Overlap Populations 0.26 0.41 0.24 0.48 0.49-0.52 0.33 0.33 0.31 0.38 0.51
0.31 0.43' 0.29 0.45 0.52-0.54 0.35 0.31' 0.34 0.32 0.50-0.52 0.11 0.04
0.37 0.40' 0.32 0.43 0.53-0.55 0.39 0.25' 0.35 0.28 0.52-0.53 0.23 0.04
a Terminal oxygens, carrying a proton in the direction of the next neighboring T atom. the exchangeable cation.
The Si and A1 atoms unchanged in CNDO now become less and more positive, respectively. We think that this "instability" of the results might be ascribed to the essentially incorrect way in which the polarity of the charge distribution of CO is described with the CNDO method and more generally with wave functions not including electron correlation. Extensively correlated wave functions have to be used in order to obtain a molecular dipole moment in the experimental direction C+O-.19,20 A (MG. .CO)2t complex was also studied separately. The C-0 interatomic distance was varied in order to derive the potential energy curve. Since at the minimum this curve can be satisfactorily approached by a parabola, i.e., U = 1/2 h(r - rJ2 (ref 21) (where U is the potential energy, lz the force constant, r the interatomic distance, and re the equilibrium distance), the force constant is easily derived. The calculated value for isolated CO (4229 N m-l) is more than two times the experimental value (1901.9 N m-1).22 This large overestimation of a diagonal force constant is typical for the Hartree-Fock method and is enhanced by application of the CNDOI2 approximation^.^^^^^ However, as relative magnitudes might be reproduced quite well even within the CNDO approximate scheme, the calculated increase of the force constant when passing from isolated CO to the Mg-CO complex (74 N m-l) is significant despite the enormous deviation between theoretical and experimental values for the initial force constant (isolated CO). More elaborate calculations will undoubtedly also lead to a force constant increase upon complexation. Moreover, the present findings can easily be reconciled with the experimentally observed increase in the CO stretching frequency. Indeed, Angel1 and Schaffer17obtained vibration frequencies of 2213 cm-l for Mg-Y and 2205 cm-l for Mg-X, both values being appreciably larger than the CO stretching frequency of the isolated gas-phase molecule
0.36 0.41' 0.32 0.43 0.53-0.55 0.39 0.26' 0.35 0.28 0.52-0.53 0.22 0.04 -0.23 - 0.02 1.04
' Bridging oxygen in contact with
(2169.2 cm-1).22If in a first approximation coupling effects in the potential and kinetic energy are neglected between the Mg. -C and the CO bonds, the ratio of the wavenumbers (e) of the CO vibrational frequency can be relaked to the force constants as n 2 / q = (k2/lz1)1/2,where indices 1and 2 refer to the isolated and complexed molecules. The calculated ratio of the wavenumbers is then 1.009, the observed ratio being 1.021 for Mg-Y and 1.017 for Mg-X. The increase in CO bond strength is also seen by the small decrease computed for the CO distance (6.10-3A). These results parallel the more qualitative predictions based upon MEP calculations2 in the foregoing paper. They are also in accordance with experimental26and theoretical findi n g on ~ the ~ CN ~ ~stretching ~ ~ frequency increase and distance decrease upon H-bond formation. Indeed, in these cases H bonding occurs via the N lone pair electrons and not via the a electrons of the CN triple bond (which would lead to a decrease in bond strength). In the present complex, the carbon lone pair region is responsible for the complex formation. 8
c. Comparison between the Influence of Na+ and Mg2' (II B, 11 BNa, and 11 BMg). As was also observed for the site I' six-ring, the charges are considerably delocalized. The introduction of a positive charge would therefore result in a decrease of the negative charges on the oxygens and an increase of the positive charges on the protons and the T atoms. This is generally true for the CNDO results with the exception of the O(2) oxygens which are directly coordinated to the Na and Mg ions. This is the same as for the site 1'. The charge on the Si atoms increases by 0.09 e units while the A1 atoms show only a small change, the charge on A1 for I1 BNa being 0.02 e higher than I1 B and 0.01 e higher for I1 BMg. The more important variation of the charge on Si is in agreement with the previously discussed2higher sensitivity of the Si KP emission
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1985
TABLE 11: CNDO and CNDO/2 Gross Atomic Charges and Overlap Populations on the Site I11 Atoms (in Electrons) .
I11 A
I11 BH,a
I11 B
I11 BH,p
Atomic Charges, CNDO/CNDO/BD 1.5812.05 - 0.741- 0.95 -0.731-0.93 -0.561-0.73 - 0.551- 0.73 1.5511.99 - 0.741- 0.95 -0.551-0.72 - 0.761- 0.96 -0.731-0.94
protons on bridging oxygens terminal protons: A1-OH Si-OH Si"-O(l) -0(2) -0(3)' -0(4)' Si"'-O( 1) -O(2)' -0(3\ -0i4j AlI'-O( 1) -0(2\ -0i39
(0.12-0.14)/( 0.16-0.18) 0.46 0.43 0.36 0.42 0.44 0.38 0.40 0.45
-0i2jQ -0(3) -0(4) bridging hydroxyls
1.3411.71 0.721- 0.89 -0.731-0.90 -0.561-0.73 - 0.591- 0.7 5 1.3511.70 - 0.7 71- 0.9 5 -0.581-0.75 -0.761-0.94 - 0.741- 0.92 1.2311.74 - 0.601- 0.74 - 0.591-0.75 1.2311.74 -0.591-0.74
1.4811.89 0.541- 0.671- 0.83 - 0.521- 0.68 - 0.551- 0.72 1.4011.76 - 0.541- 0.82 -0.521-0.68 -0.701- 0.87 -0.731- 0.91 1.3011.85 -0.561-0.71 -0.571-0.72 1.2811.81 - 0.541- 0.69 0.1610.15
-
-
O.Ol/O.O O.OSl(O.05-0.06)
(0.07-0.09)/(0.10-0.12) (0.08-0.15)/(0.17-0.19)
Overlap Populations 0.46 0.16b 0.44 0.42 0.21 0.34 0.25 0.43 0.44 0.12b 0.21 0.36 0.41. 0.40 0.43 0.47 0.45 0.12b 0.33 0.28 0.29 0.39 0.30 0.42 0.32 0.09b 0.29 0.40 0.37 0.33 0.39 0.40
1.5111.93 -0.711-0.88 -0.53/-0.82b -0.541-0.70 -0.541-0.71 1.4011.76 - 0.731- 0.90 -0.531-0.69 -0.701-0.87 -0.54/-0.82b 1.3011.84 -0.571-0.72 - 0.5 8I- 0.7 2 1.3111.83 - 0.541- 0.70 (0.14-0.15)/(0.15-0.18) (0.07-0.09)/(0.10-0.12) (0.14-0.16)/0.18 0.49 0.21b 0.37 0.40 0.49 0.38 0.39 O.llb
0.38 0.10b 0.39 0.41 0.34 0.42 0.33 0.14b
0.56
0.5 7-0.58
0.52-0.54 0.53-0.5 6
0.53-0.54 0.55-0.56
terminal
hydroxyls, on: A1 Si
0.5 3-0.56
0.48-0.49 0.50-0.51
Bridging oxygen, carrying a proa Terminal oxygen, carrying a proton in the direction of the next neighboring T atom. ton in the plane of the T-0-T angle at 0.096 nm. ' 11,111: the number of "bridging oxygens" in the tetrahedron.
energies vs. the A1 KP emission energies.12 The same trends but less pronounced were found for the deorthogonalized wave functions, i.e., delocalization of the positive charges, an increased negative charge on the O(2) oxygens, and the somewhat irregular charge variation on Al. d . Influence of the Configuration, Le., II B us. III B. This is an important comparison because, for the site I1 six-ring, Si and AI tetrahedra are linked to only two neighboring 13 and .A1 atoms, while, for the site I11 model, tetrahedra with two and three neighboring T atoms are present. This can give an indication of the size of the cluster which is required. The CNDO results are not so much different, and only a small range of charges is observed. The range calculated for oxygens linked to tetrahedra having only two neighbors (type 11) is -0.71 to -0.73 and for oxygens shared by tetrahedra having three neighbors each (type 111) between -0.74 and -0.76. This difference cannot be considered significant as, e.g., O(1) oxygens bridging between type I1 and type I11 tetrahedra have a charge of -0.77. Oxygens carrying a proton show charges varying between -0.55 and -0.59 for Si tetrahedra and between -0.58 and -0.60 if on A1 tetrahedral. The differences between type I1 and type I11 tetrahedra are again insignificant. It can be concluded
here that much smaller models are equally suitable for the study of the aluminosilicate framework and its influences. The CNDOI2D results prove the same statements. e. Influence of the Isomorphous Substitution (III A us. III B). The results for the site I11 are very similar to the data calculated for the site i.e., minor changes in the oxygen charges but a considerable variation of the charges on Si (0.2 e), the effect being about equal for the tetrahedra of type I1 and of type 111. f . Influence of the Protonation (III B us. III BHJ. As expected, the overall effect of the addition of four positive charges is an increase of the charges on Si, Al, and H and a deErease of the negative charges on the.oxygens (I11 B VS. I11 BHJ. The CNDO results show the surprising result that the bridging hydroxyls behave similarly to the terminal OH groups on the Si tetrahedra, i.e., similar charges on oxygens and protons. There is no real difference between protons attached to 0(1),0(2), or O(4) oxygens. A more detailed picture is obtained after deorthogonalization. The gross charges on oxygen for a bridging OH group (-0.82) is intermediate between the charge for bridging oxygens (-0.88 on the average) and the oxygens of terminal hydroxyls (-0.70 on the average). The T-OH
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The Journal of Physical Chemistry, Vol. 84, No. 75, 7980
overlap population decreases with about 0.2-0.3 e as compared to the bridging T-0 and terminal T-OH bonds which have a similar overlap population. The significant decrease of the overlap population and the concomitant predicted increase in bond length are in agreement with the increase in bond length for T-0 bonds with increased Pauling bond strength received by the anion as extensively studied by Baur28and discussed in terms of quantum mechanical bonding parameters by Louisnathan and G i b b ~ .Pauling30 ~~ defined the bond strength received by an anion X from a cation i as si = zi/CNi, z being the nominal valence of the cation and CN its coordination number. If an anion is also coordinated to other cations, its total electrostatic bond strength is p x = Cisi. Pauling postulated that in stable ionic structures the formal charge of each anion changed in sign is exactly or nearly equal to its total electrostatic bond strength px. For the bridging oxygens px is 1.75 (formal valence Si4+,AP+), and for a bridging hydroxyl, 2.75. For a formal valence of the oxygen 2-, a considerable charge unbalance is created after addition of a proton. This charge unbalance is energetically partially eased by a lengthening of the T-0 distances. The observed effect, decrease of the overlap population by 0.2-0.3 e, is an order of magnitude larger than the effect of the T-0-T bond angles. In a previous study3’ the 0-H stretching frequencies for protons attached to the different framework oxygens of faujasite-type zeolites were classified according to the variation of the T-0-T bond angle. In order to obtain more insight into the influence of this angle, we studied a Si(OH),-OH-Al(OH), molecule in more detail. Without a proton on the bridging oxygen, [Si(OH)3-O-A1(OH)3]1-, the Si-0 and A1-0 overlap populations decreased by 0.04 and 0.03 e, respectively, with a decrease of the Si-0-A1 angle from 160 to 140O. This fully agrees with the previous results of Gibbs et al.,13predicting a bond lengthening for smaller bond angles. After a proton is attached to the bridging oxygen, however, the Si-0 and A1-0 overlap populations decrease by 0.2-0.3 e as also found for the I11 BH4 molecules. However the T-0 overlap populations show a different behavior, with respect to the unprotonated molecule, i.e., a decrease of 0.02 and 0.01 e for the Si-0 and A1-0 bonds with the wider bond angle. The 0-H overlap population decreases by 0.008 e with increased bond angle. The force constants derived from the OH stretching potential energy curve are, for a T-0-T angle of 140°, 1537 N m-l and, for 160°, 1542 N m-l. The difference of 5 N m-l is certainly not significant. The experimental OH stretching frequency for Y-type zeolites is about 3650 cm-l for the high-frequency band [0(1)-H] and about 3550 cm-l for the protons attached to the oxygens O(2)-O(4). A classification of the frequencies on the basis of the bond angle variation as stated before31is therefore not possible considering the minor influence of the bond angle. Two major effects have to be considered, i.e., the chemical composition and the direct interaction with the environment. It was proved for dehydrated H-chabazite5 that the vibration of a proton into an eight-ring framework shows a decrease of 30 cm-l in stretching frequency. The difference in stretching frequency between protons vibrating in the large cavity of Y-type zeolites and the small cages (i.e., protons associated with framework six-rings) is 100 cm-l. The effect of the chemical composition was quantified before using Sanderson’s electronegativityscale. The latter effect is not surprising considering the delocalization of the charges in aluminosilicate frameworks. This is also
Mortier
and Geerlings
a basic assumption for the evaluation of the “average electronegativity”. The structural influences (bond angles) being small and the effect of the environment being negligible for protons vibrating in the large cages (larger than eight-rings), the OH stretching frequency for this type of proton followed the electronegativity variation irrespective of the framework type.
Conclusion As a conclusion we can state that the present results show that the previously discussed data for the site I’ type six-ring can be extended to the sites I1 and 111. Further studies need not involve such elaborate models, and smaller model molecules can be used, allowing more detailed studies in terms of accuracy (e.g., ab initio calculations) and catalytically pertinent systems (e.g., proton transfer). The increase of the CO stretching vibration upon adsorption on Mg2+was rationalized via model calculations on Mg-CO and is in agreement with the observations. It was shown that influences of the structure (bond angles) on the OH stretching frequency are negligible as compared to the influence of the chemical composition and the immediate environment. Acknowledgment. W.J.M. thanks the “Belgisch Nationaal Fonds voor Wetenschappelijk Onderzoek” for a research position as “Bevoegdverklaard Navorser”. Financial support from the Belgian Government (Dienst voor Programmatie van het Wetenschapsbeleid) to the Katholieke Universiteit Leuven is gratefully acknowledged.
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