Advances in H-Bond Theory

Though HB is a well known D•–•H---:A 3-centre-4-electron interaction, the formulation of a general HB theory turned out to be a rather formidable problem because of the extreme variability of HB properties even for a same D...A couple (for instance O-H...O energies range from 0.1 to 31 kcal mol-1). This communication briefly surveys previous results (1989-2000)1-2 and discusses recent advances (2001-2005)3-5 obtained by our lab in this field. Topics treated, in chronological order, are: 1) Chemical Leitmotifs (CLs). Systematic analysis of crystal HB geometries and gas-phase association enthalpies show that there are only 4 classes of short (strong) and 1 of moderately short HBs, while a 6th class collects, for exclusion, all the weak HBs which are the large majority of the existing bonds. 2) PA/pKa Equalization Principle. It can be shown that the four CLs associated with strong HB formation are essentially molecular devices apt to produce acid-base equalization between the donor (D-H) and the Brønsted pair of the acceptor (A-H+). 3) The Driving Variable of HB Strength can therefore be identified as the difference Delta-pKa = pKAH(D-H) – pKBH(A-H+) or, alternatively, Delta-PA = PA(D–) – PA(A). 4) The Transition-State HB Theory. All HB phenomena (energy, geometry and proton transfer (PT)) are unified by assuming that HBs are PT reactions D–H---A <-/-> D---H---A <-/-> D---H–A proceeding via the D---H---A TS which differ from ordinary reactions because reactants and products are pre-bound by the HB, so that rather small PT-barriers are to be expected. So, HBs can be described in terms of shapes of the PT-pathways as symmetric (s) or asymmetric (a) and single- (SW) or double-well (DW), and, in terms of heights of their PT-barrier, as no- (NB), low (LB) or high-barrier (HB) bonds. If Delta-PA/Delta-pKa is actually the steering variable of the HB properties, it must be verified that HB strengths, SW or DW shapes and heights of PT-barriers are completely determined, for any R1X–H---YR2 HB, by the ability of the R1 and R2 substituents to achieve or less the condition of PA/pKa matching between the HB donor and acceptor molecules. This theory can be verified for p-aryl substituted 1-(arylazo)-2-naphthols, a class of molecules where the HB is switched from N–H---O to N---H–O by the electron-donating properties of the substituent. The system is studied in terms of: (i) variable-temperature X-ray crystallography; (ii) DFT emulation of stationary points and full PT pathways; (iii) Marcus rate-equilibrium analysis correlated with substituent LFER Hammett parameters.