Theoretical Study of the Effects of Solvation, Substitution, and Structure on the Properties of Imidazolines, Oxazolines, and Thiazolines

Abstract

Different isomers and tautomers of methylphosphino- and phenylphosphino-substituted cyclic imidazoline, oxazoline, and thiazoline have been investigated theoretically in gas and aqueous phases. Special emphasis is put on the relative total energies and on the changes in the structure due to substitution or solvation. The calculations were carried through using the B3LYP/6-31+G(d,p) method. To include the effects of the solvent, we used the polarizable-continuum approach both without and with the inclusion of explicit water molecules. Lacking experimental information on the systems, the results were compared with those for nitrogen-containing compounds. In gas phase the cyclic moiety of these molecules show clear deviations from planarity. Only moderate changes in the structure due to solvation were found. On the other hand, the solvent affects strongly the relative stability of different tautomers and isomers. The inclusion of explicit water molecules changes the order of stability due to the presence of intermolecular hydrogen bonds. On the basis of the Bader theory of atoms in molecules, we identify the critical points of the hydrogen bonds as well as properties of the electron density at those points. Thereby, we can quantify the strength of the hydrogen bonds. Finally, we report energies of solvation for the systems of our study.