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

Abstract

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 the ethyl and O=CCH₃ moieties and also about C-CCH₃ bond leading to hindered rotation of the methyl group. Results of <i>ab initio </i>quantum chemical calculations using STO-3G,4-31 G, 6-31G and 6-31 G** basis sets and semi-empirical calculations in MNDO approximation using modified AM1 Hamiltonian and CNDO/2 approximations are reported .The calculations were undertaken for understanding the structural and spectroscopic characteristics of the rotational isomers of 2-butanone in the electronic ground (S₀) and n-π* excited (S₁) states and for plotting the potential energy curves for these states. It was found that while in S₀ state, the molecule may exist in two stable isomeric forms- <i>trans</i>and <i>gauche</i>,<i> </i>it may exist in only one isomeric form- <i>gauche </i>in the S₁ state. Completely optimized geometries of the conformers in the two states are reported. In the ground state the <i>trans </i>conformer is more stable than <i>gauche </i>by an enthalpy difference of about 1.5 kcal/mol. <i>Ab initio </i>calculations predict a shallow potential well for the <i>gauche</i> conformer with rotational barrier height of 1.56 kcal/mol for <i>trans-gauche </i>and 0.025kcal/mol (0.35 kcal/mol in AM1) for the <i>gauche-trans </i>conformers. The S₀ state of 2-butanone has about 93% <i>trans</i>-conformer, which wholly changes to <i>gauche </i>in the S₁ state. The methyl group shows hindered rotation both in the S₀ and S₁ states with rotational barriers of 0.97 kcal/mol (<i>trans</i>)<i> </i>and 1.36 kcal/mol (<i>gauche</i>)<i> </i>in the S₀ state and 1.30 kcal/mol (<i>gauche</i>)<i> </i>in the S₁ states. The ionisation potentials of the <i>trans</i>and <i>gauche </i>conformers in the S₀ state are 11.01±0.06 eV and 11.03±0.08 eV, respectively, while that of the <i>gauche </i>conformer in S₁ state is 8.78 eV. Calculations confirm the assignment of experimental band at 278.0 nm to <i>n</i>-π* transition and predict transitions near 155 nm,142 nm and 131 nm corresponding to π-π* transition, both for the <i>trans</i> and <i>gauche </i>conformers. The blue shift of <i>n</i>-π* transition in polar solvent is explained by less polar character of the molecule in the S₁ state than in the S₀ state.