The pyrimidine-pyrimidinium cationic complex was synthesized. The vibrational spectra of the complex were recorded. Density functional B3LYP/6-311++G(d,p) calculations have been carried out to study the structure and assign the vibrational frequencies of the complex. To describe the proton tunneling in the complex, a one-dimensional potential energy curve for the motion of the hydrogen atom was computed by using quantum mechanical calculations at the B3LYP/6-311++G** level. The tunneling splitting values obtained were 295±1 cm-1 in the gas phase and 89±1 cm-1 in solution phase. A barrier height for proton tunneling of 2.92±0.08 kcal/mol in the gas phase and 6.53±0.07 kcal/mol in solution was calculated. The 1625 and 1242 cm-1 IR bands were assigned to the N-H in-plane and out-of-plane bending modes, respectively. The self-consistent Onsager reaction field model was used to study the solvent effects. By atoms-in-molecules analysis, the hydrogen bond energies in the gas phase and solution were calculated.

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