Theoretical study on structures and internal rotations of methylN,N-dimethylcarbamate and its sulphur, selenium, and tellurium homologues (Me2NC(O)YMe, Y = O,S,Se, Te)

Abstract

A theoretical study on the structures and internal rotations of methyl N,N-dimethylcarbamate and its sulphur, selenium and tellurium homologues [Me2NC(O)YMe, 1 (Y = O), 2 (Y = S), 3 (Y = Se), 4 (Y = Te)] was performed by means of ab initio molecular orbital calculations at the MP2/3-21G(*)//HF/3-21G(*) level. These calculations indicate that 1–4 are all planar with Z-conformation with respect to the central bonds of their O(DOUBLE BOND)C(SINGLE BOND)Y(SINGLE BOND)Me units, whereas the corresponding E-forms are transition states for rotation about Y(SINGLE BOND)C(O) bonds which have energies higher than the E-forms by 20·6, 15·4, 13·9, and 9·6 kcal mol−1, respectively. The energy of 1 increases monotonically from the Z-form to the E-form with rotation about the Y(SINGLE BOND)C(O) bond, but in 2–4 a transition state and a local minimum were found between the two forms. This different phenomenon for 1 compared with its homologues 2–4 arise mainly from the large steric repulsion between a methyl group on the nitrogen and that on the oxygen in E-1. Optimization of the transition states (TSa and TSs) for rotation about N(SINGLE BOND)C(O) bonds showed that TSa is favoured by 2–4 but disfavoured by 1 owing to the repulsion between lone pairs on the nitrogen and oxygen atoms in its TSa. The barriers for rotation about N(SINGLE BOND)C(O) bonds were estimated to be 16·1, 14·7, 14·7, and 15·7 kcal mol−1 for 1,2,3,4, respectively.