Relative Stabilities of Weak Hydrogen Bonds to Nitrogen

Enthalpies of formation are reported for the weak complexes formed between nitrogen bases and chloroform. These thermodynamic data are obtained from the temperature variation of equilibrium constants determined from NMR dilution-shift measurements. The value of —ΔH0 for an amine—chloroform complex is 50% greater than that for a nitrile—chloroform complex. This is the trend of basicity with respect to hybridization that is experimentally observed in organic chemistry. This trend is described in terms of a semiclassical model for weak hydrogen bonds, consisting of two effects. These are the classical electrostatic attractive energy and the quantum mechanical overlap repulsion energy. Examination of the electrostatic energy shows that it alone cannot be responsible for the different energies of the amine and nitrile complexes. The repulsive overlap energy is proportional to the square of the overlap integral times an average ionization potential. The overlap integrals for the nitrile and the amine are not significantly different, but the ionization potential of an amine is significantly lower than that of a nitrile. These facts explain the 0.3-Å longer hydrogen-bond length for the nitrile (compared with the amine) calculated previously. Such a difference in bond lengths, in turn, gives rise to differences in the electrostatic energies closely corresponding to the observed differences in ΔH0's.