Vibrational Modes of Double Four-Member Rings of Oxygen-Bridged

[Mx]x+[H8Si8-xAlxO12]x-, where Si and Al occupy tetrahedral sites, 0e x e 4, and M ... the ring-opening vibrations as synchronized movement of the tet...
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J. Phys. Chem. B 2000, 104, 7299-7305

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Vibrational Modes of Double Four-Member Rings of Oxygen-Bridged Silicon and Aluminum Atoms: A DFT Study Ellie L. Uzunova and Georgi St. Nikolov* Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria ReceiVed: February 18, 2000; In Final Form: May 24, 2000

The vibrational modes of double four-member ring fragments (D4R) of the general composition [Mx]x+[H8Si8-xAlxO12]x-, where Si and Al occupy tetrahedral sites, 0e x e 4, and M+ is a monovalent chargecompensating cation, are assigned to the framework vibrations of Linde type A (LTA) zeolites. The normal vibrations that arise from configurations with different Si,Al arrangements are calculated by the B3LYP method. Displacement vectors are used to define the ring-opening vibrations as synchronized movement of the tetrahedral and oxygen atoms. The vectors are oriented radially to the ring center and parallel to the ring plane. The fragment model Na4H8Si4Al4O12 with D2d symmetry is selected for the interpretation of the infrared and Raman active fundamentals of zeolite 4A. Lower symmetry configurations are found to be appropriate models for the normal mode interpretation of Si-rich analogues of the LTA structure. The ring-opening vibrations of the cluster models are correlated to the LTA framework modes via the higher symmetry group D4h, which describes the entirely siliceous tetragonal prism, H8Si8O12. Synchronized oxygen atom displacements are part of all ring-opening modes.

Introduction Zeolites are built of corner-sharing tetrahedral TO4 as primary building units, where T is Si or Al. The T-O-T angle may vary in a wide range (120-180°), spanning a considerable number of different symmetries. The strong regularity of the zeolite framework is a consequence from the assembly of larger structural units; they are known as secondary building units,1 some of which are stable as individual electroneutral species (with only Si atoms at tetrahedral sites) or anions (with Si partly substituted by a lower-valence element, which assigns a negative charge to the structure). Cage-shaped hydrosilasesquioxanes are stable as individual molecules and Si/Al substitution in isolated cage molecules was achieved experimentally.2-4 They are of the general formula (HTO3/2)2n, where T) Si, Al in tetrahedral oxygen coordination; n ) 2, 3, etc.; those with n ) 4, 6, and 8 correspond to secondary building units of zeolites. Double fourmember rings (double rings with four tetrahedral atoms in the ring, D4R) are secondary building units in LTA zeolites, the mineral gismondine, and elsewhere.5 IR and Raman spectra of silicate molecules with n ) 4, 5 (H8Si8O12 and H10Si10O15) have been reported;6,7 the vibrations of smaller and larger doublering fragments of these groups were found to be correlated and they were discussed with emphasis on the ring-opening vibrations.6 The D4R fragments are secondary building units in zeolite 4A, together with single six-member and eight-member rings. The NaA zeolite unit cell is of the general composition Na48Si48Al48O192, the framework has space group Fm3hc, factor group Oh6. The whole structure can be regarded as built up of D4R fragments only, connected via oxygen bridges.8 Strict alternation of tetrahedrally coordinated Si and Al is obeyed. The reduced unit cell consists of 144 atoms (Si,Al)48O96 and the predicted number of vibrational modes is 11A1g(R) + 22Eg(R) + 25T2g(R) + 28T1u(IR).9 The vibrational spectra are a source of valuable information on molecular sieve materials. Systematic empirical IR studies

have been carried out for a large group of synthetic and natural zeolites.10 Compared with the large number of atoms in a zeolite unit cell, the IR spectra exhibit a relatively small number of bands, due to the highly symmetric structure. Assignments to individual AlO4-tetrahedra are not possible, however, since the peak positions are affected by the average Si/Al ratio. Flanigen10 has classified the skeletal vibrations into two main groups. The first group comprises internal vibrations of the tetrahedral TO4 units that may be found in the spectra of all silicates; they are relatively structure independent. The vibrations of larger secondary structural subunits that are responsible for the molecular sieve topology are classified as structure-sensitive ones. The bands, arising from the second type of IR- and Raman-active vibrations may be used to recognize some specific structural elements and for structure determination. A number of theoretical studies examine the relation between the structural subunits and the vibrational spectra.6-16 Although the correct trends in the frequency shifts with variable Si/Al ratio could be traced in the calculated spectra,12 the effect of local Si,Al ordering on the manifestation of the vibrational modes has received less attention. Aluminosilicate fragments [Nax]x+[H8Si8-xAlxO12]x-, where 0 e x e 4 and Na+ is an extraframework cation, are selected as cluster models of the double four-member rings that are secondary building units (SBU) in zeolite LTA frameworks. The models are with variable Si,Al ordering, but they are all constructed so as to obey Lo¨wenstein’s rule for avoidance of Al-O-Al links.17 Symmetry point groups are assigned to the optimized geometries, according to the number and distribution of the framework’s negative charges in the fragment and the positions of the charge-compensating cations. The vibrations of the fragments are analyzed, with emphasis on ring-opening modes. The latter can be used to discern the underlying structural elements from the spectra and to evaluate the fluctuations in zeolite-cavity and pore aperture dimensions.18

10.1021/jp0006680 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/15/2000

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Figure 1. Equilibrium geometries of D4R. Atoms in decreasing size are O, Na+, tetrahedral Al and Si. Oxygen atoms are gray, Na+ are light gray, Al are white small circles, Si are black small circles. The sequence of T atoms linking is denoted below each model.

TABLE 1: Geometry Parameters of B3LYP-Calculated D4R Structures, Pictured in Figure 1 symmetry prism model

RSi-O (Å)

D4h; H8Si8O12 D2d; Na4H8Si4Al4O12 C2h; Na2H8Si6Al2O12 C2V; Na2H8Si6Al2O12 C2; Na2H8Si6Al2O12

1.64 1.65 1.65 1.64 1.65

RAl-O (Å) 1.79 1.78 1.79 1.79

∠Al-O-Si (deg) 137; 152 144 138 135

Methods Geometry optimizations and harmonic frequency calculations were performed in Cartesian coordinates by the B3LYP method, which includes local and nonlocal terms as implemented in the Gaussian 98 package.19-22 The standard 6-31G(d) basis set was employed. The absence of negative frequencies for the calculated normal modes proves that the geometry obtained is a minimum on the potential energy surface.23 DFT calculations are known to be in general basis set independent; for some flexible molecules such as H4SiO424 and disiloxanes,25 however, the calculated vibrational frequencies and the optimized geometry were found to depend on the basis set, to a larger extent at the Hartree-Fock level, than using DFT.26 To assess the basis-set dependence of our B3LYP results with the 6-31G(d) basis, we performed geometry optimization and frequency calculation of the Na4H8Si4Al4O12 fragment using the extended 6-311G(d) basis set. We expect that the differences in the calculated frequencies with the two basis sets may reflect the error incurred by the smaller basis set. The calculated vibrational frequencies differ by about 2-3 cm-1. The largest differences were between the lowest vibrational numbers and they did not exceed 8 cm-1. Vibrational Analysis. Five cluster models with different composition and symmetry point groups were selected to represent the tetragonal prism of the Linde type A structure, see Figure 1. The selection is based on the following arguments. The entirely siliceous fragment is with D4h symmetry, slightly distorted from Oh along one of the C4 axes. The model with Si/Al ) 1 ratio and four Na+ ions centered against Si2Al2O4 ring faces at positions analogous to Na3 in LTA is with D2d symmetry. Three structural isomers exist for the Si/Al ) 3 ratio, depending on the Si,Al ordering and the positions of the

∠O-Al-O (deg) 103; 115 109 108 109

∠Si-O-Si (deg)

∠O-Si-O (deg)

RNa-O (Å)

148

110 107; 116 109 113 109÷113

2.3; 2.55 2.3; 2.5 2.40 2.30÷2.55

136 135 137

extraframework charges. The highest symmetry one (C2h) is with maximum separation of Al atoms and Na+ ions coordinated to the Si3AlO4 faces. The symmetry is lowered for the fragments with one Al-rich Si2Al2O4 ring. Depending on the position of the Na+ cations, the symmetry is either C2V, when one Na+ ion is centered over a Si2Al2O4 ring and the other is centered over the opposite Si4O4-ring, or C2, when both Na+ ions are positioned on Si3AlO4 faces. The relative stability of the Na2H8Si6Al2O12 isomeric fragments decreases in the order C2h > C2 > C2V. A more detailed discussion of the electronic structure and energetics of H8T8O12 clusters is presented elsewhere.27 The calculated vibrations of D4R with Si and Al at tetrahedral framework sites of hydrosilasesquioxanes fit into Flanigen’s general scheme for the zeolite group frequencies.10 The vibrations of the models from Figure 1 are interpreted in terms of the symmetry point groups for each molecule, using the B3LYP-optimized geometries, see Table 1. The IR and Raman frequencies, calculated with these geometries, are summarized in Tables 2 and 3. In terms of group theory, the vibrational degrees of freedom (Γ) of a siliceous D4R with Oh symmetry span the following irreducible representations (IR ) infrared active, R ) Raman active, in ) inactive vibrations). For Oh (H8Si8O12)

Γ ) 3A1g(R) + A2g(in) + 4Eg(R) + 3T1g(in) + 6T2g(R) + 3A2u(in) + 3Eu(in) + 6T1u(IR) + 4T2u(in) A slight distortion of the Oh fragment along one of the C4 axes produces a tetragonal prism with D4h symmetry. This symmetry is more realistic with respect to D4R representation of the LTA structure than the Oh symmetry point group, because it allows

Vibrational Modes of D4R of Si and Al Atoms

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TABLE 2: IR Active Vibrations of Tetragonal Prisms:a B3LYP-Calculated Frequencies (in cm-1) and Symmetry Species (in parentheses) in the 250-600 cm-1 Rangeb D4h

393 (A2u)RO 395 (Eu) 450 (A2u)RO 454 (Eu) 558 (A2u)

D2d

C2h

C2V

C2

321 (E) 334 (B2) 385 (E) 405 (B2)RO 451 (E) 475 (B2)RO 488 (E) 498 (B2)RO 549 (E)

245 (Bu) 376 (Bu) 393 (Au) 395 (Bu) 425 (Au) 488 (Bu) 493 (Au) 509 (Bu) 542 (Bu)

327 (B2) 363 (B2) 379 (A1)RO 392 (B2) 393 (B1) 403 (A1)RO 425 (B1) 495 (A1)RO 499 (B2) 502 (B1) 540 (A1)

379 (A) 384 (B) 396 (B) 402 (A)RO 429 (B) 436 (A) 483 (B) 492 (A)RO 498 (B) 546 (B) 585 (A)

559 (Eu)

584 (Au)

zeolite 4A;c,e

zeolite N-A;d,e

H8Si8O12f

378m

393m

399ms

464m

475m

465m

550ms

581ms

566w

a

Vibrations, dominated by attached H atoms displacement and vibrations with IR intensity lower than 0.5% of the most intense line are omitted. All calculated vibrations and their intensities in the 0-1300 cm-1 range are listed as Supporting Information in Table 2S and Table 3S. c Si/Al ratio 0.96.10 d Si/Al ratio 3.0.10 e s)strong, m)medium, w)weak. f Ref 7. Superscript RO ) ring-opening vibrations.

b

TABLE 3: Raman-Active Vibrations of Tetragonal Prisms:a B3LYP-Calculated Frequencies (in cm-1) and Symmetry Species (in parentheses) in the 250-600 cm-1 Rangeb D4h

D2d

(A1)RO

325 (Eg)

395 (Eg) 397 (B1g) 414 (A1g)RO 415 (B2g)

267 276 (E) 321 (E) 334 (B2) 350 (E) 384 (A1)RO 385 (E) 398 (B1) 405 (B2)RO 419 (B2)RO 422 (A1)RO 451 (E)

441 (A1g)RO

581 (A1g)RO 596 (B1g) 597 (Eg)

475 (B2)RO 488 (E) 498 (B2)RO 511 (A1) 517 (E) 549 (E) 553 (A1)RO 587 (B1)

C2h 265 (Bg) 297 (Ag) 344 (Ag) 364 (Bg) 383 (Ag)RO 411 (Ag) 421 (Bg)

425 (Ag)RO 438 (Bg)

516 (Bg) 527 (Ag)RO 546 (Ag)RO 578 (Ag) 609 (Bg) 648 (Ag)

C2V

C2

273 (A1) 320 (A1) 327 (B2) 329 (A2) 355 (B1) 363 (B2) 379 (A1)RO 392 (B2) 393 (B1) 403 (A1)RO 424 (A2) 425 (B1) 429 (A1)RO 429 (B2) 440 (B1) 495 (A1)RO 499 (B2) 502 (B1) 528 (A1)RO 538 (B2) 539 (A2) 540 (A1) 560 (B1) 575 (A1)

251 (A) 267 (B) 281 (A) 300 (B) 334 (B) 350 (A) 362 (B) 379 (A) 384 (B) 396 (B) 402 (A)RO 408 (A)RO 408 (B) 419 (A) 422 (A)RO 429 (B) 436 (A) 483 (B) 492 (A)RO 498 (B) 517 (A) 522 (B) 546 (B) 562 (B) 564 (A)RO 585 (A)

zeolite 4A,c

zeolite ZK4,d

H8Si8O12,e

280 343

335

403

412

465 490

414 423

456 496

580

a Vibrations, dominated by attached hydrogen atoms displacement are omitted. b Vibrations in the 0-1300 cm-1 range are listed as Supporting Information in Table 3S. c Ref 6. d Refs 28, 29; Si/Al ratio 2.7. e Ref 7. Superscript RO ) ring-opening vibrations.

O(1) and O(3) positions to become discernible. The following irreducible representations are valid for D4h (H8Si8O12):

Γ ) 7A1g(R) + 3A2g(in) + 5B1g(R) + 6B2g(R) + 9Eg(R) + 3A1u(in) + 6A2u(IR) + 6B1u(in) + 4B2u(in) + 10Eu(IR) Vibrations of structural subunits larger than the TO4, such as rings and double rings, deserve special attention. Synchronized vibrations of the rings are observed, when all T-O-T bending and T-O stretching modes are effected in-phase. To study in detail the collective vibrations of the D4R, we have introduced an appropriate set of coordinates. The displacement vectors of each T and O atom that represent the ring opening motion: p for Si, q for Al, r for O(1), and s for O(3) in the D4R fragment of zeolite 4A. Indexes denote the atom number in the prism as noted in Scheme 1. The O(1) type atoms bear the numbers 1, 2, 3, and 4 and the two TO(3)4 rings, building the D4R unit are above and below the plane of the O(1) atoms, respectively. The irreducible representations, spanned by the

SCHEME 1: Numbering of the Atoms in D4R. Arrows Depict the Vectors Representing O(1), O(3), Si and Al Atom Motions for One Atom of Each Type, Chosen to Illustrate the Displacements.

vibrations in the planes of the rings for the D4h symmetry group are

Γin-plane ) 3A1g(R) + B1g(R) + 2B2g(R) + 2Eg(R) + 2A2u(IR) + B1u(in) + B2u(in) + 3Eu(IR) The following external symmetry coordinates of the ringopening motions were constructed by projection techniques in

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Figure 3. Symmetry descent correlation diagram for the T4O4 ring vibrations in D4R (H8T8O12).

3. The 414 cm-1 A1g Raman-active ring-opening vibration and the 325 cm-1 Eg δ(O-T-O) vibration are mainly due to oxygen anion displacement and the corresponding peak positions should not depend on the type of the T atoms. The calculated frequencies are in agreement with the experimental spectra of LTA zeolites and the synthetic H8Si8O12 molecule.6,7,10,28,29 Comparison with previous Hartree-Fock calculations11,16 reveals that our B3LYP calculated frequencies better match the experimental data. The unit Na4H8Si4Al4O12 is closely related to the D4R structure in zeolite 4A1a,8 and is appropriate as a model for studying the vibrations of related structures with Si/Al ) 1. In our hypothetical prism, the compensating positive charges are all added at positions that correspond approximately to Na(3) in NaA.8 The symmetry point group becomes in this way D2d and the active modes are B2, E (both IR and Raman active), and A1, B1 (Raman active). The following symmetry modes are obtained for D2d (Na4H8Si4Al4O12): Figure 2. Approximate representation of the four-member ring-opening displacements involved in the vibrations (3A1g and 2A2u) of D4R with D4h symmetry. Si atoms are dark small circles, oxygen atoms are gray.

Γ ) 14A1(R) + 9A2(in) + 9B1(R) + 14B2(IR,R) + 22E(IR,R)

terms of the vector-displacement coordinates. Collective vibrations, leading to in-phase ring-opening vibrations, are with A1g and A2u symmetry only, as seen from the sum of displacement vectors:

T-O stretching: Γ(T-O) ) 4A1(R) + 2A2(in) + 2B1(R) +

P (A1g) ) (1/x8) (p5+p6+p7+p8+p13+p14+p15+p16) R (A1g) ) (1/x4) (r1+r2+r3+r4) S (A1g) ) (1/x8) (s9+s10+s11+s12+s17+s18+s19+s20) P (A2u) ) (1/x8) (p5+p6+p7+p8-p13-p14-p15-p16) R (A2u) ) 0 S (A2u) ) (1/x8) (s9+s10+s11+s12-s17-s18-s19-s20) The ring-opening vibrations of H8Si8O12 are represented in Figure 2. The 441 cm-1 A1g vibration represents a ring-breathing mode. The 414 and 450 cm-1 vibrations involve oxygen atom movements. In the 414 cm-1 vibration, the displacement vectors of O(1) and O(3) atoms are in opposite orientations. In the 581 cm-1 A1g mode, silicon and oxygen atoms move in opposite directions. In all A2u modes the displacements of the two (front and rear) T4O4 rings have opposite directions. Due to the very small distortion from Oh symmetry, the splitting of the IR-active vibrations T1u w (Eu, A2u) and the Raman-active vibrations T2g w (B2g, Eg) are in most cases very small, see Tables 2 and

4B2(IR,R) + 6E(IR,R) T-O-T bending: Γ(T-O-T) ) 2A1(R) + A2(in) + B1(R) + 2B2(IR,R) + 3E(IR,R) O-T-O bending: Γ(O-T-O) ) 4A1(R) + 2A2(in) + 2B1(R) + 4B2(IR,R) + 6E(IR,R) Γin-plane ) 4A1(R) + A2(in) + B1(R) + 4B2(IR,R) + 5E(IR,R) The collective vibrations of the rings are correlated to the vibrational modes of the unit cell group factor Oh via D4h as shown in Figure 3. The B2 modes in D2d, which are both IR and Raman active, correspond to the vibrational modes of the framework T2g(R) and T1u(IR) in Oh. The E modes in D2d are correlated to T2g(R) through Eg modes in D4h, and, through Eu modes, to T1u(IR) and T2u(inactive) modes. The totally symmetric “breathing” double-ring (DR)-opening modes A1 in D2d are manifested in the Raman spectra of the LTA framework, while the B2 symmetry DR vibration should be detected in both the Raman and IR spectral ranges. Detailed analysis of the symmetry coordinates in D2d and the atoms involved show that the in-phase collective vibrations of the rings are those of A1 and B2 mode:

Vibrational Modes of D4R of Si and Al Atoms

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P (A1) ) (1/2) (p5+p7+p14+p16) Q (A1) ) (1/2) (q6+q8+q13+q15) R (A1) ) (1/2) (r1+r2+r3+r4) S (A1) ) (1/x8) (s9+s10+s11+s12+s17+s18+s19+s20) P (B2) ) (1/2) (p5+p7-p14-p16) Q (B2) ) (1/2) (q6+q8-q13-q15) R (B2) ) (1/2) (r1+r3-r2-r4) S (B2) ) (1/x8) (s9+s10+s11+s12-s17-s18-s19-s20) Approximate representations of the four-member ring-opening displacements involved in the vibrations of D4R with D2d symmetry are depicted in Figure 4. The 267 cm-1 A1 vibration is a result of antisymmetric Si and Al atom displacements and O(1)4 ring opening. The 384 cm-1 totally symmetric A1 mode represents a ring-breathing mode: both T4O(3)4 rings open jointly with the O(1)4 bridging ring. Conversely, in the 553 cm-1 A1 mode, the displacements of oxygen atoms and T atoms are in opposite directions. Oxygen atom vibrations dominate the 422 cm-1 A1 mode: the displacements of oxygen atoms from the front and rear T4O(3)4 rings have the same orientation, but they are in opposite direction compared with the displacements of the O(1)4 ring atoms. The B2(IR,R) ring-opening modes are correlated to the T1u(IR) and T2g(R) LTA framework modes; they are composed of antisymmetric displacements of the atoms in the two T4O(3)4 rings. The 405 cm-1 vibration is dominated by O(3) atom displacements. In the 475 cm-1 vibration, O(3) atoms from the two rings move in opposite directions; the O(3) displacement vectors in each T4O(3)4 ring also have opposite direction to the T atom displacement vectors. The 419 and 498 cm-1 vibrations are more complicated: the first one involves only Al and O(3) atom displacements in a ring-opening motion, which is coupled to a T-O-T bending mode. In the 498 cm-1 vibration, Si-atom displacement in a ring-opening motion is coupled with a T-O-T angle deformation. Lowering the symmetry from D4h to D2d increases the number of IR- and Raman-active bands. The B3LYP-calculated IR-active modes agree in principle with the experimental spectrum of zeolite 4A, in which considerable band broadening is observed.10 Although extraframework cations were generally not included in the vibrational calculations in previous Hartree-Fock studies, it was evident that the calculated frequencies of fragments with Si/Al ) 1 could be interpreted only qualitatively.11 The Raman spectrum is also well reproduced, see Table 3. The 334 cm-1 B2-vibration, both IR and Raman active, is a O-T-O angle bending mode dominated by oxygen atom displacements and it is not directly affected by the Si/Al ratio. The lower symmetry configurations (C2h, C2V, and C2) correspond to a Si/Al ) 3 ratio, and they differ as to the ordering of the Si and Al atoms at T sites and to the extraframework cation coordination. The following irreducible representations were obtained for each model. For C2h (Na2H8Si6Al2O12) (sequence Al-O-Si-O-Si-O-Al):

Figure 4. Approximate representation of the four-member ring-opening displacements involved in the 4A1 and 4B2 vibrations of D4Rs with D2d symmetry. Si atoms are dark small circles, Al are white small circles, oxygen atoms are gray, Na+ are light gray circles.

Γ ) 27A1(IR,R) + 15A2(R) + 21B1(IR,R) + 21B2(IR,R) For C2 (Na2H8Si6Al2O12) (sequence Al-O-Si-O-Al):

Γ ) 43A(IR,R) + 41B(IR,R) The calculated vibrational frequencies of the C2h-symmetry cluster reproduce the IR spectrum of zeolite N-A and the Raman spectrum of ZK-4, see Tables 2 and 3. This fragment is of the highest stability among the Na2H8Si6Al2O12 isomers and represents a Si,Al ordering with maximum negative charge separation. The following irreducible representations are obtained for the coupled vibrations of the atoms in the ring planes:

Γ ) 24Ag(R) + 18Bg(R) + 18Au(IR) + 24Bu(IR)

Γin-plane ) 7Ag(R) + 3Bg(R) + 4Au(IR) + 6Bu(IR)

For C2v (Na2H8Si6Al2O12) (sequence Al-O-Si-O-Al), 1Na centered against the Al-rich face:

Collective ring-opening vibrations are of Ag symmetry and they are Raman active only. The unit cell factor group, Oh6, is

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Figure 6. Approximate representation of the four-member ring-opening displacements involved in 5A vibrations of D4R with C2 symmetry. Si atoms are small dark circles, Al are white small circles, oxygen atoms are gray, Na+ are light gray circles. Figure 5. Approximate representation of the four-member-ring opening displacements involved in 5A1 vibrations of D4R with C2V symmetry. Si atoms are dark small circles, Al are white small circles, oxygen atoms are gray, Na+ are light gray circles.

also centrosymmetric and ring-opening modes of the C2h fragment can contribute to the Raman spectra of LTA frameworks. The C2V- and C2-symmetry clusters represent Na2H8Si6Al2O12 configurations with the negative charges, located in one of the T4O4 rings and with the presence of Al-O-Si-O-Al links, respectively. The two isomers have different locations of the extraframework cations, see Figure 1. Band splitting is observed by lowering the symmetry point group, but for most of the frequencies and in the experimental spectra discussed, it is small and should bring about only band broadening. The ring-opening vibrations for the C2V and C2 models are A1(IR,R) and A(IR,R), respectively. They can contribute to both the IR- and Ramanactive LTA framework modes, see Figure 3. The vibrations with major contributions of ring-opening modes for the lower symmetry cluster models are depicted in Figure 5 (C2V model) and Figure 6 (C2 model). The 402, 408, and 422 cm-1 A modes of the C2 double-ring configuration are mainly due to oxygen atom displacements and they are not sensitive to the type and ordering of T atoms. Oxygen atom displacement also dominates the 334 cm-1 B mode, which is a δ(O-T-O) vibration. In the C2h and C2V configurations, the 344 cm-1 Ag and 327 cm-1 B2 modes, respectively, involve mainly oxygen atom displacements. Conclusions The vibrational spectra of LTA zeolites were interpreted on the basis of D4R fragment models with descending symmetry, Si and Al atoms taking T sites and extraframework cations compensating the framework’s charge. Ring-opening vibrations of D4R with D4h, D2d, C2V, and C2 symmetry were found to be

correlated to the Oh6 IR- and R-active LTA framework modes. A C2h symmetry model with maximum separation of the Al atoms reproduces best the vibrations of Si-rich LTA zeolites (N-A and ZK-4). Lower symmetry configurations (C2V and C2) generate significant band broadening in the IR and Raman spectra of zeolites. The ring-opening vibrations are identified in the 250-600 cm-1 range. Oxygen atom displacements contribute to all ring-opening modes. The totally symmetric ringopening modes of the D4h, D2d, and C2h fragments can contribute only to the Raman spectra of LTA zeolites, while for C2V and C2 fragments they should be active in both the IR and Raman spectra. Ring-opening modes in the 450-600 cm-1 range are found to be particularly sensitive to the T atom type and orderings. Acknowledgment. The authors gratefully acknowledge CPU time at the Computer Center, Technical University, Vienna, where most of the Gaussian 98 calculations were performed, as well as the help of Dr. Hans Mikosch from the same university in mastering the complex computations. Supporting Information Available: Tables with the calculated IR and Raman frequencies and IR intensities of the cluster models in the 0-1300 cm-1 range are available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Meier, W. M.; Olson D. H. Atlas of Zeolite Structure Types, 2nd ed.; Butterworth: London, 1987. (b) Newsam, J. M. In Solid State Chemistry: Compounds, Cheetham, A. K., Day, P. Eds.; Oxford University Press: Oxford, 1992; vol. 2, p 234. (2) Auf der Heyde, T.; Bu¨rgi, H.-B.; Bu¨rgi, H.; Tornroos, K. Chimia 1991, 45, 38. (3) Bu¨rgi, H.-B.; Bu¨rgi, H.; Calzaferri, G.; Tornroos, K. Inorg. Chem. 1993, 32, 4914. (4) Montero, M.; Voigt, A.; Teichert, M.; Uson, I.; Roesky, H. Angew. Chem., Int. Ed. Engl. 1995, 34, 2504.

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