Low Polarity of Acceptor

The typical proton acceptor molecule contains an electronegative atom which is involved in a bond with another atom of much lesser electronegativity. The ensuing bond dipole is an important factor in formation of a H-bond. A number of systems were investigated in which the dipole of the pertinent bond was fairly low. 

Similar considerations apply to C=0, which is a highly nonpolar molecule. The proton of HF prefers to approach the C atom rather than the O, consistent with the direction of the dipole moment of the CO molecule. The red shift of the FDF stretch is severalfold larger for OC---HF than for CO--HF the stretch of this HF bond in the former complex, although small, is consistent with a H-bond, while there is no measurable stretch in CO--HF. The energetics indicate a H-bond may indeed exist in OC---HF, with an electronic contribution to AE of some —3.6 kcal/mol, as compared to only —1.1 kcal/mol in CO—HF. 

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