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

The low-frequency vibrations of tetraethylammonium (TEA) cation have b een investigated by complementary theoretical and experimental approac hes for the purpose of exploring the role of variations in template co nformation on the templating of molecular sieve structures. Ab initio molecular orbital theory has been used to calculate equilibrium struct ures, normal-mode vibration spectra, and torsional potential energy su rfaces of TEA using the STO-3G, 3-21G, and 6-31G basis sets. Inelasti c neutron scattering (INS) has been used to measure the vibrational sp ectra of TEA in the iodide salt, zeolite beta, and SAPO-34 molecular s ieve 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 k cal/mol, respectively. A search of the potential energy surface for th e lowest energy paths between these conformers via ethyl torsions reve al that all the barriers were less than or equal to 13 kcal/mol. The s mall barriers indicate that all conformers should be accessible under the conditions of hydrothermal synthesis during molecular sieve crysta llization. The HF/6-31G vibrational frequencies of the four conformer s indicate that the modes corresponding to methyl torsions are located in the region 238-374 cm(-1) and the modes corresponding to ethyl tor sions 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 conformati on in the chabazite cage of SAPO-34. The conformation in the iodide sa lt is verified as being S-4. C-13 NMR spectra are consistent with thes e conformation assignments.