Dissection of methyl internal rotational barrier in thioacetone

The origin of methyl internal rotational barrier in thioacetone has been studied by relaxation effect, natural bond orbital analysis and Pauli exchange interactions. Fully relaxed rotational model, reflecting the true barrier, shows mainly the lengthening of Ccarb–Cme bond, opening of Cme1CcarbCme2 angle and simultaneous wagging of sulfur atom. Calculation shows that the nuclear electron attraction term (ΔVne) is barrier forming and the principal barrier forming term originates from both the Ccarb–S(σ) orbital and lone pair n-orbital. The opening of Cme1CcarbCme2 angle and lengthening of Ccarb–Cme and Ccarb–S(σ) bonds are found to be associated with steric effect and charge transfer interaction between bonding and antibonding orbitals, respectively.