Hydrogen-bonded ring closing and opening of protonated methanol clusters H⁺(CH₃OH)_n (n = 4–8) with the inert gas tagging

Abstract

The preferential hydrogen bond (H-bond) structures of protonated methanol clusters, H(+)(MeOH)n, in the size range of n = 4-8, were studied by size-selective infrared (IR) spectroscopy in conjunction with density functional theory calculations. The IR spectra of bare clusters were compared with those with the inert gas tagging by Ar, Ne, and N2, and remarkable changes in the isomer distribution with the tagging were found for clusters with n≥ 5. The temperature dependence of the isomer distribution of the clusters was calculated by the quantum harmonic superposition approach. The observed spectral changes with the tagging were well interpreted by the fall of the cluster temperature with the tagging, which causes the transfer of the isomer distribution from the open and flexible H-bond network types to the closed and rigid ones. Anomalous isomer distribution with the tagging, which has been recently found for protonated water clusters, was also found for H(+)(MeOH)5. The origin of the anomaly was examined by the experiments on its carrier gas dependence.

Stepwise Internal Energy Change of Protonated Methanol Clusters By Using the Inert Gas Tagging

“…Its free energy, however, is much higher than those of the linear and cyclic types in all the temperature range, and its population is negligible. 37 The IR spectra of bare and Ar-tagged clusters of H + M 5 have been measured in our previous study. 37 The observed spectra of the bare and Ar-tagged clusters (Figure 1b,h, respectively) are totally different from each other, and they are uniquely attributed to the liner and cyclic isomers, respectively, as demonstrated by the comparison with the simulated spectra of Figure 1a,i.…”

“…37 The IR spectra of bare and Ar-tagged clusters of H + M 5 have been measured in our previous study. 37 The observed spectra of the bare and Ar-tagged clusters (Figure 1b,h, respectively) are totally different from each other, and they are uniquely attributed to the liner and cyclic isomers, respectively, as demonstrated by the comparison with the simulated spectra of Figure 1a,i. The bare cluster is warmer because of the large excess energy in the protonation and limit of the standard jet expansion cooling efficiency, and its effective temperature is roughly estimated to be ∼200 K. In the Ar-tagged cluster, however, the high energy component cannot survive because of thermal evaporation of the Ar tag.…”

“…To estimate the temperature dependence profile of the isomer population based on classical statistical mechanics, we employed the harmonic oscillator approximation. ,, Details of the calculation method are summarized in the Supporting Information. Briefly, each isomer was regarded as an ensemble of harmonic oscillators.…”

“…Therefore, tagging itself has an effect to restrict internal energy of the cluster. Apart from cryogenic ion trap techniques, inert gas tagging of clusters has been widely utilized as a simple technique to observe low energy isomers and to change isomer populations. ,,,,− …”

“…<100 K), and produced protonated clusters are typically warm (ca. 200 K). ,,, Since inert gas tagging provides a cut off for the internal energy according to the binding energy with the tag, it would be possible to stepwise change the internal energy distribution of the protonated cluster by systematic changes of the tag species from strongly bound one to weakly bound one. The internal energy distribution of such a tagged cluster is not necessarily under thermal equilibrium.…”