Angular dependence of hydrogen bond energy in neutral and charged systems containing CH and NH proton donors

- Charged H-bonds are stronger, and more sensitive to angular distortion, than neutral systems.
- When sensitivity is normalized to total binding energy, there are only small distinctions between charged and neutral systems.
- k/Eb ratios are slightly larger for NH donors than for CH.
- H-bond weakening associated with bending is due to increased exchange repulsion.

The effects of angular distortions on the H-bond energy are computed in both neutral and ionic complexes. F3CH, NCH, and HNCH+ are taken as CH donors and HCNH, HCNH+, and NH4+ are NH donors. Ionic complexes are more strongly bound and suffer a greater loss of H-bond energy upon angular distortion. However, when bending force constants k are normalized against intrinsic H-bond strength Eb, the k/Eb ratios are similar, only slightly larger for NH than for CH donors, and with only small perturbations caused by overall charge. The source of destabilization arising from angular deformation is traced to exchange repulsion.

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