QUANTUM-MECHANICAL AND C-13 DYNAMIC NMR-STUDY OF 1,3-DIMETHYLTHIOUREACONFORMATIONAL ISOMERIZATIONS

We present C-13 dynamic NMR results for relative free energies of equi librium structures and free energies of activation for conformational transformations of 1,3-dimethylthiourea in aqueous solution, and we co mpare the results to theoretical predictions. The latter are based on ab initio gas-phase electronic structure calculations of the geometrie s, dipole moments, and energies combined with semiempirical molecular orbital calculations of the free energies of solvation in three differ ent solvents. The gas-phase electronic structure calculations were per formed using Moller-Plesset second-order (MP2) perturbation theory wit h a correlation-consistent polarized valence-double zeta basis set; we calculated relative energies for the three minima Z, Z, E,Z, and E,E and for three transition states on the potential energy surface. The s olvation energy calculations were carried out using the SM5.4/AM1-aque ous, -chloroform, and -organic solvation models; these solvation model s are based on semiempirical molecular orbital theory with class-IV ch arges and geometry-based first-solvation-shell effects. The relative e nergies of the conformers and transition states are compared to experi ment in water and five organic solvents. (C) 1998 Elsevier Science B.V .