ON THE ORIGIN OF THE BARRIERS AND THE STRUCTURES OF ACETALDEHYDE IN ITS GROUND AND 1ST SINGLET EXCITED-STATE

An ab initio study of the ground and the first singlet excited states of acetaldehyde has been performed to analyze the molecular properties as a function of the methyl torsion and the aldehydic hydrogen waggin g angles. The structural characteristics and the conformational behavi our in both electronic states have been determined. The important stru ctural changes between the two states have been analyzed by a decompos ition of the total energy into its components. It was found that the m ethyl torsion barriers arise mainly from attractive interactions. Evid ence is presented which shows that these barriers arise from in-plane and out-of-plane hyperconjugative effects involving the oxygen atom. I t is also shown that the pyramidalization experienced by the carbonyl carbon in the first singlet excited state has two sources, namely, a d ecrease in the electronic repulsion and an increase in the electron-nu cleus attraction.