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[sup [minus]1]. In the theoretical calculations, four local minima corresponding to D[sub 2d], S[sub 4], C[sub 1], and C[sub 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 reveals that all the barriers were [