The relationship between the bond length and the difference in proton affinities for the observation of heteronuclear low barrier hydrogen bonds

It is generally assumed that the formation of low barrier hydrogen bonds (LBHBs) requires identical proton affinities (ΔPA = 0) for the bases linked by the LBHB. This is generally true for LBHBs between identical interface atoms, while values of ΔPA ≠ 0 have been reported for LBHBs connecting chemically different atoms. The inherent asymmetry in heteronuclear LBHBs requires ΔPA ≠ 0 to compensate for the asymmetry in the potential energy surface of the LBHB. A PA mismatch between the bases in joined by a heteronuclear LBHB is therefore the standard case and not an exception from the rule.