Abstract

To assess the substituent effect on the stability of hydrogen-bonded dimers from N-phenyl-N’-sulfinylhydrazines (Ph–NH–NSO), and to confirm the suggested participation of Co–H∙∙∙O interactions in the dimer from protons in ortho position, 1H NMR (proton nuclear magnetic resonance) dilution studies were carried out on the unsubstituted and the meta- and para-methyl substituted compounds. The 1H NMR spectra of the solutions in CDCl3, recorded from the limit of detectability in dilute solution to the solubility limit, at room and low temperature, show characteristic chemical shift changes to lower field, in the resonances of the N–H as well as of the Co–H protons. Upon dilution, a dissociation equilibrium exists, and both the monomer and dimer chemical shifts, for
N–H and Co–H, can be obtained from the concentration-dependence regression curves, which allows to establish a measure of the strength of the hydrogen bonding network in the dimer through the equilibrium (dimerization or association) constant. For dimer formation using the 1H NMR data, a small substituent effect is observed in meta- and para-methyl substituted Ph–NH–NSO, whose dimers are both destabilized to approximately the same degree compared to the stability of the unsubstituted dimer. From the studies, and as had been suggested earlier from computational work, it is clear that the Co–H∙∙∙O interaction is indeed involved in the hydrogen-bonding network in the dimers of unsubstituted and methyl-substituted Ph–NH–NSO’s, based on the characteristic deshielding of the protons that are specifically involved in the hydrogen bond formation. Finally, the experimental 1H NMR results obtained show a remarkable agreement with those obtained from computational work at the density-functional theory