Vibrational Modes of Double Four-Member Rings of Oxygen-Bridged Silicon and Aluminum Atoms:  A DFT Study

Abstract

The vibrational modes of double four-member ring fragments (D4R) of the general composition [M x ] x +[H8Si8 - x Al x O12] x -, where Si and Al occupy tetrahedral sites, 0≤ x ≤ 4, and M+ is a monovalent charge-compensating 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 D 4 h , which describes the entirely siliceous tetragonal prism, H8Si8O12. Synchronized oxygen atom displacements are part of all ring-opening modes.

Infrared Investigation of Fluoride Occluded in Double Four-Member Rings in Zeolites

“…Concerning model CG, these values are in good agreement with previous theoretical calculations performed at a similar level of accuracy. 16 The corresponding force constant matrixes have been calculated analytically for the optimal geometries and then diagonalized to obtain the normal vibration modes and frequencies. The characterization of the modes have been performed with use of the symmetry and atomic component analysis implemented in the GAUSSIAN-94 program.…”

Local anisotropy in single crystals of zeotypes with the MFI framework structure evidenced by polarised Raman spectroscopy

“…Infrared spectroscopy is an established method for quantitative studies of chemical reactions occurring inside the pores of zeolites, but vibrational spectroscopy in general (including Raman spectroscopy, infrared spectroscopy and inelastic neutron scattering) has been sparingly used to characterise the framework structure in zeolites. [7][8][9][10][11][12][13][14][15][16] Infrared spectroscopy does not provide sufficiently high spectral resolution to clearly distinguish different zeolite structures through their characteristic vibrational modes, 9,11 while the use of Raman spectroscopy has been limited due to problems of luminescence (in particular in zeolites containing Fe atoms), a low scattering cross section, and many times minor spectroscopic changes upon atomic substitution (e.g. Si -Al).…”

“…17 To compensate for these experimental limitations, complementary theoretical calculations have been performed with the aim to elucidate distinct spectral features arising from specific and isolated structural sub-units (such as double 4-membered rings or double 6-membered rings, etc.). 10,12,14,15,18 Group theory has also been invoked to break down the complexity in vibrational spectra of zeolites, which although representing an important contribution also showed that many zeolites of technological and scientific importance are in fact poor candidates for fulllattice dynamics analysis due to the large number of atoms in the Bravais unit cell, and the low space group and site group symmetry. 7 Maroni anticipated already in 1988 7,19 the potential usefulness of polarised Raman spectroscopy on single crystals of zeolites for an otherwise impossible peak assignment.…”

Theoretical and experimental study of ion-exchange process on zeolites from 5-1 structural group

“…These vibrations can be both, stretching and bending in nature. As mentioned, in the case of analyzed unit, due to a different number of tetrahedral atoms in 5-membered ring and distinct ring deformation it is hard to talk about typical RO vibrations, however, the characteristic vibrations, which to a large extent fulfilled the criteria of ring vibrations [8,12], of the whole unit can be identified at 524 (Fig. 4a) and 560 cm -1 (Fig.…”

“…4, 5). Ideal ring vibrations cause simultaneous and radial (with respect to the ring centre) displacements of atoms from their starting positions without changing the ring symmetry [8,12]. These vibrations can be both, stretching and bending in nature.…”