A CASSCF theoretical study of the vibrational frequencies and structure of formaldehyde, acetaldehyde and acetone valence excited states

The equilibrium geometries of the singlet and triplet n→π*, π→π* and σ→π* valence states of the formaldehyde, acetaldehyde and acetone molecules have been obtained performing a full geometry optimization at the single state CASSCF level. The harmonic vibrational frequencies have been computed analytically at the same level of theory. A common strategy for the various states and molecules has been used in order to allow the comparison of the results. The geometrical structure and the harmonic frequencies of two states of acetaldehyde (S2 and 3(σ→π*)) and two of acetone (3(π→π*) and 3(σ→π*)) are described for the first time. For the 3(σ→π*) state of formaldehyde the first determination of the harmonic frequencies is reported. The strategy here adopted has allowed the identification of various trends for the substitution, on the carbonyl chromophore, of the hydrogen atom with the methyl group.