Molecular orbital investigation of multiply hydrogen bonded systems. Formic acid dimer and DNA base pairs

Abstract

Proton transfers within the formic acid dimer are studied using various methods. Minimum atomic basis sets yield single- and double-well potentials for single and double proton transfers, respectively, in agreement with extended basis set calculations. A potential energy surface for the decoupled proton transfers using the 4-31G basis set indicates that a simultaneous and synchronous motion of two protons is an adequate first approximation to the optimal mode of double transfer. All-electron PRDDO calculations are carried out for proton transfers within the base pairs guanine-cytosine and adenine-thymine. Single transfers are again found to be characterized by single-well potentials and double transfers by double-well potentials. Quantum mechanical tunneling rates and equilibrium constants are calculated for the most facile mode which involves a double proton transfer in the guanine-cytosine pair. The values obtained are compared to experimental data and inferences are drawn concerning the importance of the tunneling phenomenon in mutagenesis.