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 CH 3 OH seeded in helium is expanded and ionized by electron impact, the protonated species, (CH 3 OH) n H + , are the predominant cluster ions in the low‐mass region. In CH 3 OD clusters, both (CH 3 OD) n H + and (CH 3 OD) n D + ions are observed. The ion abundance ratios, (CH 3 OD) n H + /(CH 3 OD) n D + , 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 (CH 3 OD) n H + ions at low concentration of CH 3 OD in the expansion is the result of the relatively facile rotation of methanol molecules within the smaller clusters that tend to form at low CH 3 OD concentration. Ab initio molecular orbital calculations were carried out to investigate the rearrangement and dissociative pathways of ionized methanol dimer. Ion‐neutral complexes, [CH 3 OH 2 + …O(H)CH 2 ] and [CH 3 OH 2 + …OCH 3 ], are found to play an important role in the low‐energy pathways for production of CH 3 OH 2 + + CH 2 OH (and OCH 3 ) from ionized methanol dimer.