Ab Initio Study of Energetics of X−H···π (X = N, O, and C) Interactions Involving a Heteroaromatic Ring

The possibility of the π-face of a heterocyclic ring acting as a hydrogen-bond acceptor has considerable significance in the structure and binding of cofactors and nucleic acids to proteins. This interaction has been modeled using ab initio calculations on various complexes of pyridine with water, ammonia, methane, and benzene. Both Hartree−Fock (HF) and MP2/6-31G(d,p) calculations, including counterpoise corrections, have been carried out on a number of representative geometries. In addition to the expected hydrogen-bonded structure involving the nitrogen lone pair, a number of other orientations in which X−H is placed above the π-face are also found to be energetically favorable. The maximum stabilization is found directly above the pyridine nitrogen for water and ammonia, whereas for methane it is shifted to a point halfway toward the ring center. The corresponding complexation energies are 2.9 (X = O), 1.8 (N), and 0.8 (C) kcal mol-1, which are 0.45, 0.56, and 0.71, respectively, of the values obtained when the interaction is in the conventional hydrogen-bonded geometry. Bifurcated structures, with the XH2 group above the pyridine ring but displaced from the center toward the nitrogen, are also found to be fairly stabilized. A herringbone structure with two of the benzene C−H bonds facing the pyridine ring is computed to have a stabilization energy of 2.7 kcal mol-1, which is greater by 0.4 kcal mol-1 than that for the linear C−H···N hydrogen-bonded geometry involving the nitrogen lone pair. The interaction energies with the π-face are of comparable magnitude for benzene and pyridine. The computed relative energetics for various geometries should be useful in developing potential functions for modeling the binding of cofactors and nucleic acids with proteins.