Hydrogen bond models and theories: The dual hydrogen bond model and its consequences

The H-bond can be reinterpreted starting from the dual H-bond model, for which any D-H center dot center dot center dot A bond is not a bond donated by D-H to :A but rather consists of two bonds formed by the central proton with two adjacent acceptors. Analogously, the H-bond energy, E-HB, is not the D-H center dot center dot center dot:A dissociation energy but the smaller of two bond-dissociation energies, D-0(D-H) and D-0(H-A), by which D-: and :A are competitively bound to the proton. If one is stronger, the other is weaker, and weak the overall H-bond will be. Strong bonds occur when Delta D-0 = D-0(D-H) - D-0(H-A)= 0 or, in terms of affinity for the proton (pa), when Delta pa = pa(D-) - pa(A) = 0. H-bond properties are then function of two variables, pa(D-) and pa(A), or better of their linear combinations, Sigma pa = pa(D-) + pa(A) and Delta pa = pa(D-) - pa(A), having respective meanings of mean donor/acceptor electronegativity and of energy difference, Delta E-r, between tautomeric D-H center dot center dot center dot:A and D-:center dot center dot center dot H-A(+) forms. Two cases are studied. The case: 'Sigma pa variable for Delta pa = 0' leads to quantitative relationships between D/A electronegativity and maximum energy, E-HB,E-MAX, achievable for each D/A electronegativity class, EC(D,A). The case: 'Delta pa variable for Sigma pa = constant' leads to formulate three different but inter-consistent H-bond theories which are separately discussed. The last one, which is called 'pK(a) equalization principle' and where Delta pa values are empirically estimated from the acid-base dissociation constants in water as Delta pK(a) = pK(AH)(D-H) - pK(BH+)(A-H+), is shown to be a powerful method of large applicability for predicting the H-bond strengths from thermodynamic parameters. (c) 2010 Elsevier B.V. All rights reserved.