Testata

The Dual Hydrogen Bond Web Site

    Introduction


 But all the research programmes I admire have one characteristic in common. They all predict novel facts, facts which had been either undreamt of, or have indeed been contradicted by previous or rival programmes.
Imre Lakatos, Science and Pseudoscience (The Open University Arts Course A303, ‘Problems of Philosophy’, broadcast on 30 June 1973)


  If someone ask me what has been my greatest concern during the last 25 years, I must answer ‘the nature of the hydrogen bond (H-bond)’. I always disregarded the widespread idea that the H-bond was a well-known weak interaction of electrostatic nature as a nonsense, just because all interatomic forces are electrostatic in nature and we cannot call weak an interaction whose binding energy ranges from less than one to more than 45 kcal/mol. For me, the H-bond should have a quite different and still unknown and mysterious raison d’Ítre and the only sensible way for finding it out was to make tabula rasa of all previous theories, to embark on a new thorough analysis of the extraordinary wealth of structural data in the solid state and thermodynamic data in the gas phase made available in the last twenty years, and to try to infer from this data the empirical rules all H-bonds were conforming to.

  This choice was lucky and I found myself involved, together with my coworkers at the University of Ferrara, in one of those research programmes that endlessly predict novel facts and, because of that, Imre Lakatos has called scientific or progressive (in opposition to pseudoscientific or degenerating ones). Its genuine progressive character can be easily appreciated by the number of acronyms and neologisms that, over the years, should be invented to account for the new concepts that were emerging: RAHB (resonance-assisted H-bond; 1989, 1991), ECHBM (electrostatic-covalent H-bond model; 2000.2), CAHB (charge-assisted H-bond; 1994.1, 2002.2), PAHB (polarization-assisted H-bond; 2000.2), the six chemical leitmotifs (2002.3, 2004), the PA/pKa equalization principle (2004, 2005), the pKa slide rule (2009.1, 2009.2) and, finally, the concept of functional H-bond (2009.2). The programme was so disclosing that the legendary ‘well-known weak interaction’ was instead a complex shared-proton interaction, fully modulated by the acid-base properties of the donor and the acceptor and with a rich internal structure in different classes, each one having specific functional roles in important aspects of chemistry, biochemistry, and material sciences.

  It is interesting to note that, while this progression of new concepts was discovering (and sometimes rediscovering) all empirical laws governing the H-bond, no significant progress was being made in the identification of the ‘unknown and mysterious raison d’Ítre’ of the H-bond itself. The reason was that the laws discovered were establishing a strict net of relations among the physical variables describing the H-bond but were clearly unable to tell us which independent variables were deputed to drive the process of H-bond formation. It is generally recognized that this type of problem can be overcome only by assessing an aprioristic model which, having arbitrarily chosen the driving variables, becomes the original idea of how things may work.

  Such a model came out, eventually, in 2002 during a DFT study of the effect of substituents on the shape of the N−H∙∙∙O ↔ N∙∙∙H∙∙∙O ↔ N∙∙∙H−O proton-transfer (PT) reaction pathway associated with strong intramolecular RAHB formation in ketohydrazones. Data analysis by the well-known Marcus method showed that the reaction studied had all the features of a traditional SN2 nucleophilic substitution reaction and that, accordingly, what we call H-bond can well be considered as a point (or two points separated by a barrier) of minimum along a same reaction pathway which assumes different, but characteristic, shapes according to the strengths of the H-bonds formed. At the time, we interpreted the facts in terms of a novel but rather specialized transition-state H-bond theory (TSHBT; 2002.2, 2005, 2006.1).

  The resulting message was, however, quite clear: the many difficulties encountered in treating the H-bond as a physical interaction could perhaps be overcome by considering it as a chemical reaction and this message, eventually set free from the subtleties of chemical kinetics, has become the dual H-bond model (2009.2, 2010) which is the object of this website and that, in spite of its simplicity, has already proved to explain many H-bond aspects that had resisted for a long time to all other competing approaches.

  Following Lakatos’ ideas, this dual H-bond model is not a new theory but just the logical nucleus of a new research programme whose value is to be evaluated by how much it is progressive or, in other words, by how many new facts it is able to predict. So far it has contributed, besides to stress the unexpected importance of chemical kinetics in H-bond studies (2002.2, 2004, 2005, 2006.1), to shed new light on the concept of PA/pKa equalization by separating the joint roles of electronegativity and proton affinity in H-bond formation (2009.1, 2009.2).

  To make an educated guess of possible future advances I can only rely on the limited experience of these last two years. For sure, the fact of considering the H-bond as a PT chemical reaction has modified the relative importance of the various subjects, contributing to reintroduce the Mulliken’s interpretation (1951) of the H-bond as a n→σ* charge-transfer (CT) interaction, to suggest the need of a more direct comparison between halogen bond (X-bond) and H-bond, and to prompt a more general classification of CT complexes which was inclusive of H-bond phenomena (2011.1, 2011.2). Only recently, this comprehensive CT/PT approach to molecular interactions has been shown to provide a particularly easy way to look at the packing of molecular crystals (2011.1, 2012, 2013).

The Dual Hydrogen Bond Web Site

 © Gastone Gilli 2010 2012