Hydrogen‐Bond Rearrangement and Intermolecular Proton Transfer in Protonated Methanol Clusters

Abstract

Vibrational predissociation spectra (VPS) of hydrogen‐bonded and non‐hydrogen‐bonded (free) OH stretches within the frequency range of 2700–3800 cm−1 are closely analyzed to determine the isomeric structures and proton location in protonated methanol clusters, H+(CH3OH)n. The present report emphasizes protonated methanol tetramers and pentamers, which are predicted by ab initio calculations to exist in linear and cyclic forms. Only one isomer with linear structure was identified in the VPS of protonated methanol tetramers in a supersonic expansion. Both linear and cyclic configurations were found for protonated methanol pentamers, and an isomeric transition between these two structures due to hydrogen‐bond rearrangement was observed at a cluster temperature of 190 K. Ab initio calculations indicated that the excess proton in these clusters can be either localized on one methanol unit, as in cyclic H+(CH3OH)4 and linear H+(CH3OH)5 isomers, or delocalized between two methanol molecules, as in linear H+(CH3OH)4 and cyclic H+(CH3OH)5 isomers. Such unique proton delocalization behavior is revealed in the VPS of cyclic H+(CH3OH)5, which is recognized as a symmetric five‐membered pentamer by its characteristic free‐OH stretching absorption at 3647 cm−1 and hydrogen‐bonded OH stretches at 3448 and 3461 cm−1. Using protonated methanol pentamers as a model system, a proton transfer mechanism is suggested to involve intracluster proton transfer mediated by a sequence of hydrogen‐bond breaking and reforming processes. This appealing mechanism can be closely associated with the exceptionally high proton mobility in liquid methanol.

Morphology of Protonated Methanol Clusters: An Infrared Spectroscopic Study of Hydrogen Bond Networks of H+(CH3OH)n(n= 4−15)

“…The observed IR spectra of n = 4 and 5 are essentially the same as those reported by Chang et al , They demonstrated that the IR spectral feature of n = 4 and 5 shows a temperature dependence, which indicates the presence of the cyclic and linear chain isomers. The present IR spectra are similar to those measured under the higher internal temperature condition of Chang et al, and both of the isomers may contribute to the spectra.…”

“…As for the protonated methanol, H + (MeOH) n , experimental information on the hydrogen bond structure is very limited, − despite its importance as a model of the proton solvation by molecules other than water. Chang et al carried out IR spectroscopy of the clusters of n = 4 and 5. , While a linear chain structure was predominant for n = 4, the coexistence of the linear chain and the cyclic form was found for n = 5. They suggested that the interconversion between the linear and cyclic forms causes a fast intracluster proton transfer.…”

The Ionic Hydrogen Bond

“…The enthalpy and entropy effects were investigated using temperature studies by Yamdagni and Kebarle and by Meot-Ner and co-workers in proton-transfer equilibria of diamines and triamines, , protonated polyethers and crown ethers, , and diketones. , Studies by the McMahon and Stone groups found similar effects in dialcohols, , amino alcohols, and methoxy alcohols. , The cyclic internally bonded structures were confirmed by ab initio calculations on diamines, polyethers, methoxy alcohols, ,, and other molecules − and also confirmed by H 2 O and MeOH loss through metastable and collisional dissociation . Cyclic iIHB structures and solvent effects thereon were confirmed also by recent IR predissociation spectroscopy studies, especially by the group of Y. T. Lee and co-workers. ,,− …”

Infrared plus vacuum ultraviolet spectroscopy of neutral and ionic methanol monomers and clusters: New experimental results

“…IR spectroscopy of such ions for n =4,5 has been reported and a linear chain conformation is suggested to be the predominant structure for n = 4, and the coexistence of linear chain and cyclic structures is suggested for n = 5 clusters. 15,16 For larger ͑MeOH͒ n H + , n = 4 -15, IR spectroscopy of the OH stretch region has shown that the free OH stretch feature decreases in intensity with increasing n and finally disappears at n =7. 17 This report suggests that a bicyclic hydrogen bonded structure is appropriate for ͑MeOH͒ n H + , n Ͼ 7.…”