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···: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···:A dissociation energy but the smaller of two bond-dissociation energies, D0(D–H) and D0(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 ΔD0 = D0(D–H) − D0(H–A) = 0 or, in terms of affinity for the proton (pa), when Δpa = 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, Σpa = pa(D−) + pa(A) and Δpa = pa(D−) − pa(A), having respective meanings of mean donor/acceptor electronegativity and of energy difference, ΔrE, between tautomeric D–H···:A and (−)D:···H–A(+) forms. Two cases are studied. The case: ‘Σpa variable for Δpa = 0’ leads to quantitative relationships between D/A electronegativity and maximum energy, E(HB,MAX), achievable for each D/A electronegativity class, EC(D,A). The case: ‘Δpa variable for Σpa = constant’ leads to formulate three different but inter-consistent H-bond theories which are separately discussed. The last one, which is called ‘pKa equalization principle’ and where Δpa values are empirically estimated from the acid–base dissociation constants in water as ΔpKa = pKAH(D–H) − pKBH+(A–H+), is shown to be a powerful method of large applicability for predicting the H-bond strengths from thermodynamic parameters.