Small-angle neutron scattering is a useful new approach to the study of zeolite crystallization from aluminosilicate gels and the action of template molecules. It has been applied to gels for synthesis of zeolite ZSM-5 using tetrapropylammoniurn ions as templates where the scattering length densities of the gel particles and their texture were determined using contrast variation methods. Gels formulated from soluble silicate incorporate template molecules promptly into an amorphous "embryonic" structure and crystallization ensues via a solid hydrogel transformation mechanism. Gels formulated from colloidal silica show different scattering behavior, and a liquid phase transport mechanism is inferred.

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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 T E A in the iodide salt, zeolite beta, and SAPO-34 molecular sieve in the range 0-400 cm-I. In the theoretical calculations, four local minima corresponding to Dzd, Sq, CI, and C2 structures were located in the potential energy surface of the free T E A cation. Their relative energies a t the HF/6-31G1 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 5 1 3 kcal/mol. The small barriers indicate that all conformers should be accessible under the conditions of hydrothermal synthesis during molecular sieve crystallization. The HF/6-3 l G * 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.-' From our assignments of the INS spectra, it is concluded that T E A adopts the minimum energy D2d conformation in the channels of zeolite beta and the CZ conformation in the chabazite cage of SAPO-34. The conformation in the iodide salt is verified as being S4. I3C NMR spectra are consistent with these conformation assignments.

High resolution inelastic neutron scattering (INS) spectra of the n-alkanes (n=2 to 6) have been measured, using the crystal analyzer spectrometer at the intense pulsed neutron source, Argonne National Laboratory, for energy transfer less than 1000 cm−1. In this paper we present these data and extend the valence force field for n-alkanes of Schachtschneider and Snyder by modifying the torsion potential and the C–C–C bending force constants. The modified force constant set provides a much improved match of the frequencies obtained from normal modes calculations to our experimental data in the energy region below 1000 cm−1 and a minimal perturbation of the frequencies at higher energies. The amplitude of the scattering peaks measured by the crystal analyzer spectrometer is shown to be closely related to the hydrogen density of states. The data also show peaks produced by lattice mode excitations. These data are not analyzed but are compared with data from the literature.