Infrared and Raman spectra and normal coordinate analysis of trans-1,2-dicyanocyclopropane and its deuterated isotopomers

Abstract

Infrared and Raman spectra are presented for trans-1,2-dicyanocyclopropane and its 1,2- d 2- and 3,3- d 2-isotopomers. Most of the d 0 assignments agree with previous assignments of Strumpf and Dunker encompassing 26 of the 27 fundamentals. The 1,2- d 2 assignments are similar, but for the 3,3- d 2 species only 11 bands could be assigned. A complete ab initio quantum mechanical force field has been calculated for this molecule at the 6-31 G* basis set level. This force-field was scaled and least-squares optimized using eight parameters for functionally related diagonal force constants and their geometric mean for off-diagonals. Both theoretical minimum energy and “experimentally corrected” geometries were used with no significant difference in results. An earlier calculation with a 4-31 G basis set gave a similar frequency fit but different scaling factors. As an alternative approach, the scaled ab initio force field, was also used as a starting point for a more conventional refinement of 27 force constants which had a significant influence on the potential energy distribution. The remainder were fixed at the scaled ab initio values. This empirical force field resulted in a fit to 65 fundamentals of d 0, 1,2- d 2 and 3,3- d 2 to < 1% average error.