N⋯H[sbnd]N versus O⋯H[sbnd]N intramolecular hydrogen bonding in E- and Z-isomers of para-substituted phenylhydrazones: Differential nature of the H-bonds and the exception of para-CN substituent

Abstract

The present work demonstrates a computational investigation into the N⋯HN (E-isomer) and O⋯HN (Z-isomer) intramolecular H-bonding (IMHB) in a miscellany of para-substituted phenylhydrazone compounds. The application of resonance-assisted H-bonding theory to N⋯HN H-bonding systems, especially those involving an aromatic nucleus, has remained scarcely addressed. The results of NBO and QTAIM methods indicate that resonance-assistance from aromatic pyridyl nucleus leads to a stronger N⋯HN IMHB in the E-isomer in comparison to the O⋯HN IMHB in Z-isomer with the exception of para-CN substitution (Z-isomer) probably due to an extended conjugation. The results of QTAIM analyses reveal that the N⋯HN IMHB is described by a considerable degree of covalence (total electron density at bond critical point, Hc<0) as contrary to the predominantly electrostatic (Hc>0) interaction describing the O⋯HN H-bond in Z-isomer, with the exception of para-CN substitution showing a shared interaction. The existence of non-hydrogen bonding noncovalent interactions is also established.