Covalence and π-electron delocalization influence on hydrogen bonds in proton transfer process of o-hydroxy aryl Schiff bases: A combined NMR and QTAIM analysis.

M. NATALIA C. ZARYCZ; M. AYELÉN SCHIEL; EMILIO ANGELIA ; R. DANIEL ENRIZ

JOURNAL OF CHEMICAL PHYSICS

AMER INST PHYSICS

Lugar: New York; Año: 2021 vol. 155

0021-9606

Within the framework of the DFT approach we studied the relationship between the chemical nature of intramolecular hydrogen bonds (HB) and NMR parameters, J-couplings and 1H-chemical shifts (δ(1H)), of atoms involved in such bonds in o-hydroxyaryl Schiff bases during the proton transfer process. For first time the shape of the dependence of the degree of covalence in HB on 1J(N‒H), 1J(O‒H), 2hJ(O‒N), and δ(1H) during the proton transfer process in o-hydroxyaryl Schiff bases was analyzed. Parameters obtained from Bader´s theory of atoms in molecules were used to assess the dependence of covalent character in hydrogen bonds with both NMR properties. Influence of π-electronic delocalization on 2hJ(N‒O) under the proton transfer process was investigated. 2hJ(O‒N) in a Mannich base was also studied in order to compare results with an unsaturated system. In addition substituent effects on the phenolic ring were investigated. Our results indicate that covalent character of HB on both sides of the transition state undergoes a smooth exponential increase as the δ(1H) moves to down field. The degree of covalence of the N⋯H (O⋯H) bond increases linearly as 1J(N‒H) (1J(O‒H)) becomes more negative, even after reaching the transition state. Non-vanishing values of spin dipolar (SD) and paramagnetic spin orbital terms of 2hJ(O‒N) show that π-electronic delocalization has a non-negligible effect on tautomeric equilibrium and give evidence of presence of the resonance assisted hydrogen bond (RAHB). Variation of SD term of 2hJ(O‒N) follows a similar pattern as the change of the PDI aromaticity index of chelate ring.