VIBRATIONAL FORCE-FIELD OF SOLID IMIDAZOLE FROM INELASTIC NEUTRON-SCATTERING

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. (C) 1995 American Institute of Physics.