Theoretical Study of Molecular Structure, Tautomerism, and Geometrical Isomerism of N-Methyl- and N-Phenyl-Substituted Cyclic Imidazolines, Oxazolines, and Thiazolines
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- Remko, M., Van Duijnen, P.T. & Swart, M. Structural Chemistry (2003) 14: 271. doi:10.1023/A:1023811829286
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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 KT 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.