Electronic spectra, torsional potential and complete vibrational analysis of methacrolein using semi-empirical molecular orbital and normal coordinate methods

Methacrolein molecule consists of two completely asymmetric rigid parts, th e CH2 = C(CH3) and CHO moieties which may rotate about a C-C single bond an d give rise to different conformers. In order to understand spectral and st ructural characteristics of these conformers and to obtain potential energy curves in the ground and first electronically excited slates, molecular or bital calculations in the MNDO using modified AM1 Hamiltonian and CNDO/2 ap proximations have been reported. Information about the possible conformers, barriers to internal rotation and relative stabilities (Delta H) of the va rious conformers is obtained from the potential functions. Normal coordinat e calculations in the valence force field approximation have also been cond ucted on two of its predominant conformers - trans and cis, and a complete assignment of the vibrational modes provided. The trans conformer is found to be more stable than the cis in the ground state, while reverse is the ca se in the first excited state (n pi*). In addition, a gauche conformer (phi = 90 degrees) may also exist in the excited state.