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Neutral and Ionic Hydrogen Bonding in Schiff Bases
Author(s) -
Dominiak Paulina M.,
Grech Eugeniusz,
Barr Gordon,
Teat Simon,
Mallinson Paul,
Woźniak Krzysztof
Publication year - 2003
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200390118
Subject(s) - hydrogen bond , intramolecular force , ionic bonding , chemistry , crystallography , low barrier hydrogen bond , bond length , bond order , computational chemistry , stereochemistry , molecule , ion , crystal structure , organic chemistry
Low‐temperature, high‐resolution X‐ray studies of charge distributions in the three Schiff bases, the dianil of 2‐hydroxy‐5‐methylisophthaldehyde, 3,5‐dinitro‐ N ‐salicylidenoethylamine and 3‐nitro‐ N ‐salicylidenocyclohexylamine, have been carried out. These structures exhibit interesting weak interactions, including two extreme cases of intramolecular hydrogen bonds that are ionic N + H ⋅⋅⋅ O − and neutral OH ⋅⋅⋅ N in nature. These two types of hydrogen bond reflect differences in geometrical parameters and electron density distribution. At the level of geometry, the neutral OH ⋅⋅⋅ N hydrogen bond is accompanied by an increase in the length of the C(1)O(1) bond, opening of the ipso ‐C(1) angle, elongation of the aromatic CC bonds, shortening of the C(7)N(2) bond and increased length of the C(1)C(7) bond, relative to the ionic hydrogen bond type. According to the geometrical and critical point parameters, the neutral OH ⋅⋅⋅ N hydrogen bond seems to be stronger than the ionic ones. There are also differences between charge density parameters of the aromatic rings consistent with the neutral hydrogen bond being stronger than the ionic ones, with a concomitant reduction in the aromaticity of the ring. Compounds with the ionic hydrogen bonds show a larger double‐bond character in the CO bond than appears in the compound containing a neutral hydrogen bond; this suggests that the electronic structure of the former pair of compounds includes a contribution from a zwitterionic canonical form. Furthermore, in the case of ionic hydrogen bonds, the corresponding interaction lines appear to be curved in the vicinity of the hydrogen atoms. In the 3‐nitro‐ N ‐salicylidenocyclohexylamine crystal there exists, in addition to the intramolecular hydrogen bond, a pair of intermolecular O ⋅⋅⋅ H interactions in a centrosymmetric dimer unit.