The nature of hydrogen bonding in the NN–HF, OC–HF, and HCN–HF complexes

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Abstract
The nature of hydrogen bonding in the complexes NN–HF and OC–HF, as revealed from detailed ab initio electronic structure calculations, is unlike the bonding in the isoelectronic HCN–HF complex. The strong dipole–dipole attraction in HCN–HF is absent in the other two complexes. Competing effects that are possible in all hydrogen bonds are unmasked in NN–HF and OC–HF. Calculations have been performed with a number of large basis sets, extensive treatment of electron correlation, energetic partitioning, and evaluation of molecular properties. The potential energy surfaces that were generated demonstrate the linearity of the equilibrium structures, and show roughly comparable flexibility for bending, thus refining somewhat the experimentally inferred picture. In NN–HF and OC–HF, less than one‐half of the hydrogen bond strength of each is due to just the interaction of the unrelaxed charge distributions of the monomers; while polarization of monomer electron distributions accounts for most of the remaining attraction. There is consequently a substantial electric dipole moment enhancement. The total correlation effect is responsible for about one‐third of the well depth in NN–HF and OC–HF. Charge transfer is unimportant in all three complexes.

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