Infrared absorption of methanethiol clusters (CH3SH) n, n = 2-5, recorded with a time-of-flight mass spectrometer using IR depletion and VUV ionization

Lung Fu, Hui Ling Han, Yuan-Pern Lee*

*Corresponding author for this work

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We investigated IR spectra in the CH- and SH-stretching regions of size-selected methanethiol clusters, (CH3SH)n with n = 2-5, in a pulsed supersonic jet using infrared (IR)-vacuum ultraviolet (VUV) ionization. VUV emission at 132.50 nm served as the source of ionization in a time-of-flight mass spectrometer. Clusters were dissociated with light from a tunable IR laser before ionization. The variations in intensity of methanethiol cluster ions (CH3SH)n + were monitored as the IR laser light was tuned across the range 2470-3100 cm-1. In the SH-stretching region, the spectrum of (CH3SH)2 shows a weak band near 2601 cm-1, red-shifted only 7 cm-1 from that of the monomer. In contrast, all spectra of (CH3SH)n, n = 3-5, show a broad band near 2567 cm-1 with much greater intensity. In the CH-stretching region, absorption bands of (CH 3SH)2 are located near 2865, 2890, 2944, and 3010 cm -1, red-shifted by 3-5 cm-1 from those of CH 3SH. These red shifts increase slightly for larger clusters and bands near 2856, 2884, 2938, and 3005 cm-1 were observed for (CH 3SH)5. These spectral results indicate that the S-H···S hydrogen bond plays an important role in clusters with n = 3-5, but not in (CH3SH)2, in agreement with theoretical predictions. The absence of a band near 2608 cm-1 that corresponds to absorption of the non-hydrogen-bonded SH moiety and the large width of observed feature near 2567 cm-1 indicate that the dominant stable structures of (CH3SH)n, n = 3-5, have a cyclic hydrogen-bonded framework.

Original languageEnglish
Article number234307
JournalJournal of Chemical Physics
Issue number23
StatePublished - 21 Dec 2012

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