Towards an unified hydrogen-bond theory

Though the hydrogen bond (H-bond) is known since 1920, all attempts to predict its geometry and energetics from the structure of the interacting molecules have been so far unsuccessful. This problem is addressed here through the Electrostatic-Covalent H-Bond Model (ECHBM) derived from the systematic analysis of structural and spectroscopic data of a large number of O-H ... O H-bonds, according to which: (i) weak II-bonds are electrostatic in nature but become increasingly covalent with increasing strength, very strong bonds being essentially three-centre-four-electron covalent bonds; (ii) strong H-bonds belong to a limited number of classes (three for X-H ... X homonuclear and four for X-H ... Y heteronuclear II-bond); and (iii) within each class, H-bonds are the stronger the smaller is Delta PA, the difference between the proton affinities of the I-I-bond donor and acceptor atoms. It is shown that this model leads to a thorough classification of II-bonds in chemical classes which, in turn, can be used to predict the H-bond strength from the simple knowledge of the chemical formula. A number of applications to homonuclear O-H ... O and heteronuclear N-H ... O/O-H ... N H-bonds is illustrated by both systematic analysis of crystal structure data and DFT theoretical calculations at the B3LYP/6-31 + G(d,p)//B3LYP/6-31 + G(d,p) level of theory. (C) 2000 Elsevier Science B.V. All rights reserved.