Studies of Hydrogen‐Bonded Systems. I. The Electronic Structure and the Double Well Potential of the N–H···N Hydrogen Bond of the Guanine—Cytosine Base Pair

For hydrogen‐bonded complexes we outline an LCAO—MO—SCF method which considers explicitly the σ electrons of the hydrogen‐bond region and the π electrons of the surrounding ligands. The protonic potential function is obtained by solution of these SCF equations for a series of proton positions. The electronic structure and the potential function of the middle N–H···N hydrogen bond of the guanine—cytosine base pair have been calculated by this method. The integrals appearing in the problem have been approximated semiempirically and several alternative approximations have been tested. The calculated π electronic structure and orbital energies conform with the result of a previous SCF calculation treating the H bond parametrically. The calculated charge distribution in the H‐bond region is in agreement with the mainly covalent character of the N–H bond and mainly electrostatic character of the H···N bond for equilibrium configuration. The empirical and electrostatic models agree well with our model with respect to the qualitative features of the double well potential characterizing a H bond. With respect to the magnitude there is a substantial decrease in the potential barrier calculated by us in comparison to that calculated from classical electrostatic interactions. This decrease can be partly related to the delocalization effect which is treated in our model and partly to the self‐consistent charge distribution utilized for the calculation of electrostatic interactions, which in the simpler models are estimated with more arbitrariness.