Quantum mechanical structural studies on 2-butanone in the ground and excited electronic states and electronic spectra of its conformers

2-Butanone (ethyl methyl ketone) may be considered as a two rotor system in which internal rotation may take place about C-C single bond connecting th e ethyl and O=CCH3 moieties and also about C-CCH3 bond leading to hindered rotation of the methyl group. Results of ab initio quantum chemical calcula tions using STO-3G, 4-31G, 6-31G and 6-31G** basis sets and semi-empirical calculations in MNDO approximation using modified AM1 Hamiltonian and CNDO/ 2 approximations are reported The calculations were undertaken for understa nding the structural and spectroscopic characteristics of the rotational is omers of 2-butanone in the electronic ground (S-0) and n-pi* excited (S-1) states and for plotting the potential energy curves for these states. It wa s found that while in S-0 state, the molecule may exist in two stable isome ric Forms - trans and gauche, it may exist in only one isomeric form - gauc he in the S-1 state. Completely optimized geometries of the conformers in t he two states are reported. In the ground state the trans conformer is more stable than gauche by an enthalpy difference of about 1.5 kcal/mol. Ab ini tio calculations predict a shallow potential well for the gauche conformer with rotational barrier height of 1.56 kcal/mol for trans-gauche and 0.025 kcal/mol (0.35 kcal/mol in AM1) for the gauche-trans conformers. The S-0 st ate of 2-butanone has about 93% trans-conformer, which wholly changes to ga uche, in the S-1 state. The methyl group shows hindered rotation both in th e S-0 and S-1 states with rotational barriers of 0.97 kcal/mol (trans) and 1.36 kcal/mol (gauche) in the S-0 state and 1.30 kcal/mol (gauche) in the S -1 states. The ionisation potentials of the trans and gauche conformers in the S-0 state are 11.01 +/- 0.06 eV and 11.03 +/- 0.08 eV, respectively, wh ile that of the gauche conformer in S-1 state is 8.78 eV. Calculations conf irm the assignment of experimental band at 278.0 nm to n-pi* transition and predict transitions near 155 nm, 142 nm and 131 nm corresponding to pi-pi* transition. both for the trans and gauche conformers. The blue shift of n- pi* transition in polar solvent is explained by less polar character of the molecule in the S-1 state than in the S-0 state.