MOLECULAR-STRUCTURE AND VIBRATIONAL-SPECTRA OF METHYL CYANOACETATE - AN FT-IR, RAMAN AND AB-INITIO MOLECULAR-ORBITAL STUDY

The results of a combined vibrational and structural study of methyl c yanoacetate undertaken by Raman and infrared spectroscopy, and ab init io SCF-MO calculations are presented. It is shown that for the isolate d molecule situation, as well as in the liquid phase, methyl cyanoacet ate exists as a mixture of two main conformers of similar energies, di ffering 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 degrees and in the +/- 140 degrees region, respectively). In the crystalline state, only the thermodynamically most stable syn conformer remains. The ab i nitio SCF-MO optimized geometries of the various possible conformers, their relative stabilities, dipole moments and harmonic force-fields a re presented, and the conformational dependence of some relevant struc tural parameters is used to characterise the most important intramolec ular interactions present in the various forms studied. Finally, resul ts of a normal mode analysis based on the ab initio calculated vibrati onal spectra are used to help interpret the experimental vibrational d ata, enabling a detailed assignment of both Raman and infrared spectra . (C) 1998 Elsevier Science B.V.