Proton affinity correlations between hydrogen and dihydrogen bond acceptors

Several series of hydrogen- and dihydrogen-bonded complexes with HCN, C2H2, HF, H2O, CH3CONH2, and CH3COOH as donors and H2O, MeOH, EtOH, MeOMe, NH3, NH2Me, NHMe2, NMe3, NEtMe2, and BH3-NMe3 as acceptors were investigated using the MP2/6-311++G(d,p) level of theory. The total lowering of the X-H stretching frequencies in the hydrogen-bonded complexes were linearly correlated with the proton affinities of the accepting bases. From comparison of hydrogen- and dihydrogen-bonded complexes, a scaling factor to estimate the exact proton affinity of a dihydrogen bond acceptor was developed. Further, the scaling factor involving linear donors (1.204) is marginally higher than that involving nonlinear donor molecules (1.162). Finally, it was found that, given identical conditions, a hydrogen bond will be about 16-20% stronger than a corresponding dihydrogen bond.