Vibrational force field of solid imidazole from inelastic neutron scattering

Abstract

Inelastic neutron scattering (INS) measurements are presented for solid imidazole at 20 K. Both powder and oriented platelets have been measured. Assignments of bands are consistent with earlier infrared and Raman studies except for a band at 961 cm−1 which is definitely assigned as A″. A fit with the proton-weighted vibrational density of states is presented and a full harmonic force field has been established. The mean deviation between measured eigenvalues and those predicted from the force field is ±3 cm−1. The potential energy distribution is compared to quantum chemistry calculations and is found to be in best agreement with an unscaled calculation [J. Sadlej, A. Jaworski, and K. Miaskiewicz, J. Mol. Struct. 274, 247 (1992)]. The INS spectra from a scaled ab initio force field [K. Fan, Y. Xie, and J. Boggs, Theochem, 29, 339 (1986)] are calculated both before and after refinement against only infrared frequencies and compared to experiment. It is demonstrated that comparison with the INS spectrum is a more stringent test of the force field than with just the infrared frequencies alone.