Proton‐transfer reactions within ionized methanol clusters: Mass spectrometric and molecular orbital studies

Proton‐transfer reactions that proceed within methanol cluster ions were studied using an electron impact time‐of‐flight mass spectrometer. When CH3OH seeded in helium is expanded and ionized by electron impact, the protonated species, (CH3OH)nH+, are the predominant cluster ions in the low‐mass region. In CH3OD clusters, both (CH3OD)nH+ and (CH3OD)nD+ ions are observed. The ion abundance ratios, (CH3OD)nH+/(CH3OD)nD+, show a tendency to decrease as the methanol concentration increases, which is apparently related to the cluster structure and reaction energetics. The results suggest that the effective formation of (CH3OD)nH+ ions at low concentration of CH3OD in the expansion is the result of the relatively facile rotation of methanol molecules within the smaller clusters that tend to form at low CH3OD concentration. Ab initio molecular orbital calculations were carried out to investigate the rearrangement and dissociative pathways of ionized methanol dimer. Ion‐neutral complexes, [CH3OH2+…O(H)CH2] and [CH3OH2+…OCH3], are found to play an important role in the low‐energy pathways for production of CH3OH2+ + CH2OH (and OCH3) from ionized methanol dimer.