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The intermolecular hydrogen bonds in phenol–trimethylamine complexes were investigated by Bader Atom in Molecules (AIM) theory. The AIM parameters of the bond critical points (BCPs) of the O···H, N···H, and CO bonds as well as those of the phenol ring critical point (RCP) were analyzed as functions of the proton‐transfer degree. Transfer of the proton from donor to acceptor changes not only the electron density of the hydrogen bridge, but also the electron cloud in the proton donor. The differences between the proton donors in relation to their pKa values are seen in the systematic changes of the AIM parameters. Copyright © 2008 John Wiley & Sons, Ltd.

The structural consequences of proton transfer in complexes of phenol (1), 2,6-dichlorophenol (2), 4nitrophenol (3) and 2,6-dichloro-4-nitrophenol (4) with pyridine were analyzed on the basis of results of B3LYP/6-31G** calculations. Three methods of describing the progress of proton transfer are proposed: the O-H [d(OH)] and C-O [d(CO)] bond lengths and the fraction X PT of the proton transfer form, calculated from the values of the dipole moments. The d(OH) parameter reveals behaviour near to X PT and can be used as a universal measure of the degree of proton transfer. The d(CO) parameter gives nearly linear dependences for various structural parameters, but independent estimation of the specific effects of the substituents is necessary, as separate correlations for each complex are found. A role of resonance interaction in systems containing a p-NO 2 substituent is demonstrated.

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