Spectroscopic and structural studies on conformers of crotonaldehyde based on quantum-chemical and normal coordinate methods

In order to understand the structural and spectroscopic characteristics of the rotational isomers of crotonaldehyde, semi-emperical MO-calculations in the MNDO (using modified AM1 Hamiltonian) and CNDO/2 approximations and normal coordinate calculations in the GVFF approximation have been conducted on two of its prominent conformers -trans and cis. A complete assignment of the in-plane and out-of-plane vibrational modes for the two conformers has been provided. Based on torsional potential energy analysis, it is found that while the trans conformer is more stable than the cis in the ground state, reverse is the case in the first excited state (n pi*). In addition, as in case of acrolein, a gauche conformer may also exist in the excited state. Rotational barrier heights, enthalpy differences, dipole moments and other parameters have been calculated for the various conformers and compared with experimental findings.