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Journal of Structural Chemistry

2011 year, number 1

A Theoretical study of keto-enol isomerism and Internal rotation in the H2Salen molecule, N,N′-ethylene-bis(salicylidenimine) - Schiff Base

V. V. Sliznev1, G. V. Girichev2
Keywords: Schiff base, H2Salen, structure, keto-enol isomerism, internal rotation, conformers, intramolecular hydrogen bond, hyperconjugation, electron density functional theory
Pages: 22-32


Geometric parameters, vibrational spectra, and the energies of isomerization of seven keto-enol isomeric forms of the H2Salen molecule (N,N′-ethylene-bis(salicylidenimine)) are calculated using electron density functional theory (DFT/B3LYP) and correlation consistent valence triple-zeta Gaussian basis sets (сс-pvtz). The isomer with two enol groups (EE1) and C2 symmetry configuration is most energetically favorable. Calculations of the keto-enol equilibrium show that at Т ≥ 250 K the H2Salen gas phase is a mixture of four conformers (rotamers of the main isomer EE1). The contribution of other isomers does not exceed a few percent. The NBO analysis reveals that the system of π-conjugated bonds involves not only the atoms of the benzene moiety, but also the O, C, and N atoms nearest to the benzene ring. The energy stabilization of the isomer EE1 is shown to be due to the presence of two strong intramolecular N…H hydrogen bonds. Intramolecular N…H and O…H hydrogen bonds are observed in all other isomers. The bathochromic shift of O-H and N-H vibrational frequencies, caused by the effect of hydrogen bonds, is 520-790 cm-1.