THEORETICAL AND INELASTIC NEUTRON-SCATTERING STUDIES OF TETRAETHYLAMMONIUM CATION AS A MOLECULAR-SIEVE TEMPLATE

The low-frequency vibrations of tetraethylammonium (TEA) cation have been investigated by complementary theoretical and experimental approaches for the purpose of exploring the role of variations in template conformation on the templating of molecular sieve structures. Ab initio molecular orbital theory has been used to calculate equilibrium structures, normal-mode vibration spectra, and torsional potential energy surfaces of TEA using the STO-3G, 3-21G, and 6-31G* basis sets. Inelastic neutron scattering (INS) has been used to measure the vibrational spectra of TEA in the iodide salt, zeolite beta, and SAPO-34 molecular sieve in the range 0-400 cm(-1). In the theoretical calculations, four local minima corresponding to D-2d, S-4, C-1, and C-2 structures were located in the potential energy surface of the free TEA cation. Their relative energies at the HF/6-31G* level were 0.0, 0.8, 3.5, and 6.8 kcal/mol, respectively. A search of the potential energy surface for the lowest energy paths between these conformers via ethyl torsions reveal that all the barriers were less than or equal to 13 kcal/mol. The small barriers indicate that all conformers should be accessible under the conditions of hydrothermal synthesis during molecular sieve crystallization. The HF/6-31G* vibrational frequencies of the four conformers indicate that the modes corresponding to methyl torsions are located in the region 238-374 cm(-1) and the modes corresponding to ethyl torsions are located in the region 76-176 cm(-1) From our assignments of the INS spectra, it is concluded that TEA adopts the minimum energy D-2d conformation in the channels of zeolite beta and the C-2 conformation in the chabazite cage of SAPO-34. The conformation in the iodide salt is verified as being S-4. C-13 NMR spectra are consistent with these conformation assignments.