TOWARDS A COMPREHENSIVE MODEL OF HYDROGEN BONDING INTERACTIONS: PAST RESULTS AND FUTURE RESEARCH PROJECTS. Gastone Gilli, Dipartimento di Chimica and Centro di Strutturistica Diffrattometrica, Università di Ferrara, via L. Borsari 46, I-44100 Ferrara, Italy

In a series of previous papers (Gilli et al., J. Am. Chem. Soc. 1989, 111, 1023; 1994, 116, 909; Acta Cryst. 1993, B49, 564; Bertolasi et al., J. Am. Chem. Soc. 1991, 113, 4917; Acta Cryst. 1994, B50, 617) it has been shown that (a) very strong homonuclear X-H...X H-bonds are to be conceived as totally delocalized X.-.H...:X <--> X:...H.-.X three-centre-four-electron covalent bonds; (b) their occurrence is limited to the three chemical classes where the H-bond can be assisted by positive or negative charges, or by [[pi]]-resonance; (c) they are the stronger the more similar are the proton affinities of the two H-bond donor and acceptor atoms and, accordingly, (d) heteronuclear X-H...Y bonds can only give weak H-bonds of mostly electrostatic nature.

In this lecture the results of such previous investigations are briefly reviewed and the general problem addressed of whether or not a comprehensive model accounting for both homo- and heteronuclear

H-bond can be conceived. It will be shown that a tentative solution can be inferred by comparing a number of crystal structures where strong or very strong O-H...N, N-H...O and N-H⁺...O^- interactions are known to occur.