Theoretical study of the ionic hydrogen bond in the isolated proton-bound dimer of cytosine

We report a theoretical study of the ionic hydrogen bonding in the isolated proton-bound cytosine dimer, particularly in view of proton transfer. The theoretical study was carried out by B3LYP density functional and MP2 calculations. This study predicted 41.2 kcal/mol for the dissociation energy of the proton-bound dimer. The calculation of the transition state revealed a low barrier of 1.6 kcal/mol as well as a high rate constant of 5.9 × 1011 s−1 for the proton transfer within the ionic hydrogen bond [B3LYP/6-311+G(2d,p)]. The stabilization in the transition state may provide the low energy pathway for the proton transfer.