Theoretical study on structures and internal rotations of methyl N,N-dimethylcarbamate and its sulphur, selenium, and tellurium homologues (Me2NC(O)YMe, Y = O,S,Se, Te)
✍ Scribed by Wladmir Ferraz de Souza; Nobuaki Kambe; Noboru Sonoda
- Book ID
- 101284184
- Publisher
- John Wiley and Sons
- Year
- 1996
- Tongue
- English
- Weight
- 481 KB
- Volume
- 9
- Category
- Article
- ISSN
- 0894-3230
No coin nor oath required. For personal study only.
✦ Synopsis
A theoretical study on the structures and internal rotations of methyl N,N-dimethylcarbamate and its sulphur, selenium and tellurium homologues [Me,NC(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 0-C-Y-Me units, whereas the corresponding E-forms are transition states for rotation about Y-C(0) bonds which have energies higher than the E-forms by 20.6, 15.4, 13-9, and 9.6 kcalmol-I, respectively. The energy of 1 increases monotonically from the Z-form to the E-form with rotation about the Y-C(0) 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 (TS, and TS,) for rotation about N-C(0) bonds showed that TS, is favoured by 2-4 but disfavoured by 1 owing to the repulsion between lone pairs on the nitrogen and oxygen atoms in its TS,. The barriers for rotation about N-C(0) bonds were estimated to be 16.1,14.7, 14.7, and 15.7 kcal mol-' for 1,2,3,4, respectively.