VUV Photoionization Induced Proton Transfer and Formation of New Covalent Bonds in Pyridine–Methanol Complex

Ion-molecule reactions are a crucial form of reaction in space that can reveal the formation mechanisms and evolutionary processes of some complex interstellar molecules and even prebiotic molecules. In this study, infrared spectra of pyridine (Pyd) and methanol (CH₃OH) or deuterated methanol (CH₃OD) clusters were measured in the spectral range of 2400-3800 cm^-1 using an infrared-vacuum ultraviolet (IR-VUV) scheme. The geometric conformer, infrared spectra of the possible products, and reaction paths were investigated utilizing quantum chemical calculations. By comparing the experimental and theoretically calculated spectra, we found that only a unique hydrogen-bonded structure exists within the Pyd-CH₃OH complex, characterized by a linear hydrogen bond conformer between the two constituent molecules. On the other hand, interestingly, we observed a relatively weak NH vibrational peak at 3383 cm^-1 in the infrared spectra of both cationic Pyd-CH₃OH and Pyd-CH₃OD, suggesting that proton transfer has occurred via at least two distinct pathways. These pathways likely involve a proton transfer reaction from either the OH group or the CH₃ group of CH₃OH to the nitrogen atom of Pyd, with the transfer from the OH group being the predominant route. Through the theoretical calculations, we also found that a new C-C bond can be formed between the two moieties after the proton transfer reactions.