Molecular Association and Reactivity of the Pyridine Dimer Cation

A recent experimental report has identified the formation of C–N hemi-bonded pyridine dimer cation following vacuum ultraviolet near-threshold photoionization [J. Phys. Chem. Lett. 2021, 12, 4936–4943]. Herein, the dynamics and consequent reactivity of the pyridine dimer cation was investigated employing Born-Oppenheimer Molecular Dynamics (BOMD) simulations. A anti-parallel π-stacked pyridine dimer in the neutral ground state is transformed to a non-covalently interacting C–H∙∙∙N hydrogen-bonded structure which can lead to proton transfer in the cationic state. Additionally, C–N and N–N bonded adducts were formed in the cationic state. Further, metastable C–H∙∙∙H–C bonded cationic was observed, which rearranges to N–N bonded adduct. In contrast to the experimental observation, migration of the proton to the α position was not observed in the C–N bonded adduct owing to a high barrier of about 2 eV. The observed trends in the molecular association, proton transfer and the formation of C–N and N–N bonded adducts is a consequence of roaming dynamics of one pyridine moiety over the other in the cationic state.