Quantum chemical investigation of the molecular structure of some 2, 3-dihydro-1, 4-diazepines and related molecules

Abstract EN

Accurate ab-initio and semi-empirical molecular orbital calculations with full geometry optimization were performed on the various tautomeric forms of some 2, 3-dihydro-1, 4-diazepines and related molecules.

The highly accurate ab-initio calculations at the HF / 6–31G**level with Mo¨ller-Plesset Second-Order Perturbation Theory (MP2) refinement clearly established the higher stability of the enamine tautomer of the 1, 4-diazepine ring over the di-imine form by 27.786 kJ / mol, whereas the semi-empirical calculations at the NDDO level (AM1 and PM3) predicted comparable energies within reported errors of the two methods.

However, both ab-initio and semi-empirical NDDO methods predicted similar geometries in agreement with observed geometrical parameters.

The AM1 calculations predicted small energy differences among the three tautomeric forms of 2, 3- dihydro-5-methyl 7-phenyl 1, 4-diazepine with the more polar enamine tautomer being the more stable tautomer in the half-chair conformation which is likely to predominate in polar media through stabilizing intermolecular solute-solvent interactions.