Quantum mechanical and 13C dynamic NMR study of 1,3-dimethylthiourea conformational isomerizations

We present C-13 dynamic NMR results for relative free energies of equilibrium structures and free energies of activation for conformational transformations of 1,3-dimethylthiourea in aqueous solution, and we compare the results to theoretical predictions. The latter are based on ab initio gas-phase electronic structure calculations of the geometries, dipole moments, and energies combined with semiempirical molecular orbital calculations of the free energies of solvation in three different solvents. The gas-phase electronic structure calculations were performed using Moller-Plesset second-order (MP2) perturbation theory with 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 solvation energy calculations were carried out using the SM5.4/AM1-aqueous, -chloroform, and -organic solvation models; these solvation models are based on semiempirical molecular orbital theory with class-IV charges and geometry-based first-solvation-shell effects. The relative energies of the conformers and transition states are compared to experiment in water and five organic solvents. (C) 1998 Elsevier Science B.V.