VIBRATIONAL-SPECTRA AND STRUCTURE OF METHYL TRANS-CROTONATE

The Raman and infrared spectra of liquid and solid methyl trans-crotonate (MC) and its deuterated analogue, methyl-d3 trans-crotonate (Md3C), were recorded. From variable temperature studies for the pure liquid, it is concluded that these molecules exist as a mixture of two forms, the s-cis and s-trans conformers (C = C-C = O dihedral angles equal to 0-degrees and 180-degrees, respectively), with the s-cis conformer being more stable than the s-trans form by 2.65 +/- 0.27 kJ mol-1. In the solid phase, only the thermodynamically most stable s-cis form exists. In addition, equilibrium geometries, conformer energy differences, dipole moments and vibrational frequencies were also evaluated using ab initio SCF-MO calculations at the 4-31G level. The calculated ab initio frequencies were used to help band assignment and the theoretical harmonic force field scaled to reproduce the observed frequencies.