A b i n i t i o study of the methyl internal rotation of acetaldehyde in the S1 (n ,π*) state

A b initio SCF calculations have been performed on the methyl internal rotation of acetaldehyde in the S1 (n, π*) state as well as in the ground state. The calculated barrier heights to methyl rotation (481 and 732 cm−1 for the ground and S1 states, respectively) are in good agreement with experimental values. The Mulliken population analysis has revealed that the ‘‘group’’ overlap population between the methyl and formyl groups correlates very well with the potential curve for methyl rotation, suggesting that the intergroup bonding governs the potential curve. It is also shown that the intergroup π bonding (hyperconjugation) is important in determining the barrier. The change of the stable conformation from the H eclipsing O type in the ground state to the ‘‘H antieclipsing O’’ conformation in the S1 state is shown to be due to the presence of an antibonding π electron in the latter.