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

Abstract

Proton-transfer reactions that proceed within methanol cluster ions were studied using an electron impact time-offlight mass spectrometer. When CH,OH seeded in helium is expanded and ionized by electron impact, the protonated species, (CH,OH),H+, are the predominant cluster ions in the low-mass region. In CH,OD clusters, both (CH,OD),H+ and (CH,OD),D+ ions are observed. The ion abundance ratios, (CH30D),H+/(CH30D),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,OD),H+ ions at low concentration of CH,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,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,OH,+-.O(H)CH,] and [CH,OH,+-OCH,I, are found to play an important role in the lowenergy pathways for production of CH30H2+ + CH,OH (and OCH,) from ionized methanol dimer.

Proton transfer reactions within the pyridazine–methanol cluster ion

“…Type II is the result of an intracluster reaction in which H in the methyl group of CH 3 OH rearranges onto the O atom. From the energies listed in Table 1, it can be seen that type II is a more stable configuration, which is analogous to the result reported in [11]. Fig.…”

Electron impact ionization mass spectrometry of aliphatic alcohol clusters in helium nanodroplets

“…Previous studies of the electron impact ionization of methanol clusters in the gas phase have revealed no detectable parent methanol cluster ions [25,26]. Equally, in their investigation of ion products from a broader range of clusters, Shukla and Stace also observed no parent cluster ions for any of the alcohols studied [24].…”

“…Vertical ionization results in ions being produced with considerable excess vibrational energies, which then allow rapid conversion to the protonated species. In the specific case of (MeOH) 2 , ab initio calculations predict that vertical ionization results in a cation with a total energy above that for (MeOH)H + + OMe [26]. The calculations show that the transition state for the process…”

“…This work constitutes the first detailed study of the electron impact ionization of a series of molecular clusters embedded in helium droplets. Previous gas-phase studies of the ionization of alcohol cluster ions can be divided into three groups: (1) early multiphoton ionization studies [18][19][20], (2) more recent VUV nearthreshold single-photon ionization studies [21][22][23] and (3) electron impact investigations [24][25][26][27]. This prior work has focused mainly on methanol clusters, but some studies of clusters of the heavier alcohols have also been reported.…”

Dissociative recombination of protonated methanol

“…Reaction (5) probably proceeds over small clusters of protonated methanol, which have been found to be preferentially formed in methanol-noble gas mixtures by electron impact ionisation. 25 3 18 OD + . After extraction of the ions from the source at 40 keV, they were mass selected, injected into the ring and accelerated to 2.90 MeV (CH 3 OH 2 + ) and 2.53 MeV (CD 3 OD 2 + ) translational energy.…”