Molecular structure and vibrational spectra of methyl cyanoacetate: an FT-IR, raman and ab initio molecular orbital study

Abstract

The results of a combined vibrational and structural study of methyl cyanoacetate undertaken by Raman and infrared spectroscopy, and ab initio SCF-MO calculations are presented. It is shown that for the isolated molecule situation, as well as in the liquid phase, methyl cyanoacetate exists as a mixture of two main conformers of similar energies, differing by the relative orientation of the NC---C---C=O axis (the syn and skew forms, having a NC---C---C=O dihedral angle equal to 0° and in the ± 140° region, respectively). In the crystalline state, only the thermodynamically most stable syn conformer remains. The ab initio SCF-MO optimized geometries of the various possible conformers, their relative stabilities, dipole moments and harmonic force-fields are presented, and the conformational dependence of some relevant structural parameters is used to characterise the most important intramolecular interactions present in the various forms studied. Finally, results of a normal mode analysis based on the ab initio calculated vibrational spectra are used to help interpret the experimental vibrational data, enabling a detailed assignment of both Raman and infrared spectra.