Hydrogen Bond Compression Assisted H-transfer

With any hydrogen bond A-H B one can normally associate two distances, the A-H distance rj = for the diatomic unit AH, and the H---B distance t2 = r B for the diatomic unit HB. According to Pauling [38], one can associate with these distances so-called valence bond orders or bond valences, which correspond to the exponential distances  

3 has been proposed [36]. A similar empirical correction has also been proposed for OFlN-hydrogen bonds [43]. 

The shortest possible equilibrium heavy atom distance is given by [39b] 

The situation is illustrated schematically in Fig. 6.4(a) for the case of degenerate H-transfers. One-dimensional cuts V qi) at different values of 2 through a two-dimensional potential surface of a degenerate H-transfer are displayed. The barrier height Ed of the double well describing the H-transfer decreases when q2 is 

As has been pointed out in Ref [9], such a two-dimensional model can be reduced to a one-dimensional model by setting = constant, as indicated in Fig. 6.4(b) and by assuming a continuous distribution of rapidly interconverting configurations with different values of q2- Such a situation can be reached practically by excitation of low-frequency H-bond vibrations or phonons. The situation of Fig. 6.4(b) can practically be replaced by an inverted parabola as a barrier, with a continuous distribution of vibrational levels on both sides of the barrier. 

---