Vibrational assignment and proton tunneling in pyridine-pyridinium complex

Hydrogen bond is one of the most common binding mechanisms in the nature. The strength of hydrogen bond has been estimated to be in the range of 4-160 kJ/mol. The hydrogen bonded systems have been studied by several spectroscopic techniques. Vibrational spectroscopy plays an important role among these techniques for such a study since the vibrational wavenumber can be used as a strong evidence for estimation of the hydrogen bond strength. Theoretical calculations of the hydrogen bond strength, in conjunction with the vibrational spectroscopy, could be also very important in interpretation of the experimental results and understanding the nature of the hydrogen bonded systems [1,2]. In this study, pyridine-pyridinium complex has been considered to be studied as relatively simple intermolecular hydrogen bonded system (Figure 1). In this system the hydrogen bond proton potential obeys either a symmetric double minimum or a symmetric single minimum potential for proton movement. Pyridine (azabenzene) has attracted a lot of attention owing to its applications in many areas of chemistry. The aim of the present work is reconsidering the vibrational assignment and studying the nature of hydrogen bonded system in the pyridine-pyridinium complex by means of Density Functional Theory (DFT) methods. The results of vibrational assignment will be compared with those reported in the literature.