Theoretical study of molecular structure, tautomerism, and geometrical isomerism of N-methyl- and N-phenyl-substituted cyclic imidazolines, oxazolines, and thiazolines

The geometries of various tautomers and isomers of 2-methylamino-2-imidazoline, 2-methylamino-2-oxazoline, 2-methylamino-2-thiazoline, 2-phenylamino-2-imidazoline, 2-phenylamino-2-oxazoline, and 2-phenylamino-2-thiazoline have been studied using the Becke3LYP/6-31+G(d,p) DFT, ONIOM(Becke3LYP/6-31+G(d,p): HF/3-21G*) and ONIOM(Becke3LYP/6-31+G(d,p):AM1) methods. The optimized geometries indicate that these molecules show a distinctly nonplanar configuration of the cyclic moieties. In the gas phase, the amino tautomers ( with exception of 2-phenylamino-2-imidazoline) are computed to be more stable than the imino tautomers. Of the two possible (E and Z) isomers of methyl and phenyl derivatives of imino-oxazolidine and imino-thiazolidine species, the ( Z) isomers have the lowest energy. The iminozation free energies in the gas phase were found to be 5 - 15 kJ/mol. Absolute values of K-T depend strongly on the accuracy of the method used for calculation of free energy. Solvation ( using the MD simulations) causes, in most cases, a shift in tautomeric preference toward the imino species.