Theoretical studies of C 2H 4O isomers : Part 1. Acetaldehyde ground, triplet n → π* and triplet n → 3 s electronic states

Abstract

The ground, triplet n → π* and triplet Rydberg n → 3 s states of acetaldehyde have been studied at both the SCF and CI levels. The minimal STO-3G, split valence 3-21G and polarized 6-31G* basis sets have been used in conjunction with both spin unrestricted (UHF) and restricted (RHF) wavefunctions. The CI calculations included all single and double excitations from the SCF reference configuration, excluding three core and the six highest-lying virtual orbitals, using the 3-21G basis set. The Rydberg calculations used the 3-21G basis, augmented by a diffuse s function, plus CI. The agreement between theory and experiment is very good where corresponding data exist. The ground-state methyl rotation barrier is calculated to be 4.3 kJ mol −1 at the 6-31G*/SCF level and 5.2 kJ mol −1 at the 3-21G/CI level, with the syn conformer lowest. The triplet n → π* state is pyramidal, and lies 264 kJ mol −1 above the S 0 state while the vertical excitation energy is 362 kJ mol −1. The main effect of either an RHF versus a UHF wavefunction, a larger basis, or CI, is to increase barrier and excitation energies somewhat. The CO bond length is reduced below the experimentally determined value at the 6-31G* level. The preferred Rydberg state conformation is syn, in agreement with the recent experimental result. The methyl rotation barrier is computed to be 9.0 kJ mol −1 and the excitation energy 595 kJ mol −1 (602 kJ mol −1 vertical).