Hydrogen bonding in Schiff bases – NMR, structural and experimental charge density studies

摘要: A series of sixteen Schiff bases (derivatives salicylaldehydes and aryl amines) was studied to reveal the influence substituents length linker on properties H-bonding formed. In theory, two groups compounds, derivatives 2-(2-hydroxybenzylidenoamine)phenol) 2-hydroxy-N-(2-hydroxybenzylideno)benzylamine, can form different types H-bonds using one or hydroxyl present in molecules. Two other 4-(2-hydroxybenzylidenoamine)phenol N-(2-hydroxybenzyideno)benzylamine, only type H-bond. It confirmed by 15N 13C NMR experiments, that all cases traditional, H-bonded six-membered chelate rings were The positions hydrogen atom depend substituent phase. Generally, OH H-bond dominates solution, with exception nitro derivatives, where NH tautomer is present. solid state tautomeric equilibrium strongly shifted form. Only for 5-Br derivative compound reverse relationship found. According results experimental charge density investigations, intramolecular 5-methoxy 2-hydroxy-N-(2′-hydroxybenzylideno)benzylamine) differ significantly terms properties. intra- intermolecular formed deprotonated oxygen from 2-OH group are strong, significant concentration at bond critical point a straight, well-defined path, whereas second 2′-OH quite weak, ca. five times smaller than previous case bent path. energy densities, latter appears be non-bonding interaction, total being slightly positive. source contributions atoms involved, intermolecular, linear very strong charge-assisted function classification, N(1)–H(1N)⋯O(1) medium-strength, while third extremely weak.