Vibrational assignment and proton tunneling in pyridine–pyridinium complexes

The pyridine–pyridinium cationic complexes [(Py2H)+], with two different counterions (perchlorate and bromide), were prepared. The vibrational spectra of the complexes have been recorded. We then performed theoretical calculations to aid our understanding of the experimental data. To include the proton tunneling effect in our study, we adopted a simple one-dimensional potential energy surface (PES) for the motion of the hydrogen atom (at the center). This PES was derived from quantum mechanical calculations at the B3LYP/6-311++G** level of theory for a fixed skeleton geometry. Based on such a PES, we computed the vibrational energy levels, from which a tunneling splitting of 487 ± 2 and 242 ± 2 cm−1 was obtained in the gas and solution phases, respectively. The calculated barrier height was about 2.0–2.3 and 4.0–4.2 kcal/mol in the gas and solution phases, respectively. After all things considered, our calculations assign the bands at 138, 1255, and 1665 cm−1 to the N⋯N stretching, N–H out-of-plane bending, and N–H in-plane bending modes, respectively.