以紅外光–真空紫外光游離光譜法研究甲硫醇團聚體之紅外吸收光譜

Abstract

吾人以紅外光削減–真空紫外光游離光譜法研究了甲硫醇團聚體（(CH3SH)n，n = 2–5）之紅外吸收光譜。利用四波混頻產生之132.5 nm真空紫外光游離分子射束中的甲硫醇團聚體，搭配飛行時間質譜儀可分別偵測到不同大小團聚體之離子訊號。實驗時以可調變波長的OPO/OPA雷射作為紅外光源，掃描2470-2670 cm^−1（S–H伸張模光區）及2800-3100 cm^−1（C–H伸張模光區）之波長。若團聚體在被游離前吸收紅外光會造成預解離，使其濃度減少，其相對應之離子訊號也會減少。記錄離子訊號隨波長之變化可得到各團聚體之作用光譜，而根據分子射束中團聚體之分布以及團聚體中分子間束縛能等資訊，可將作用光譜轉換為紅外吸收光譜。 甲硫醇單體之S–H伸張模（ν3）譜帶位於2605 cm^−1，而雙聚體之ν3譜帶位於2601 cm^−1，相較於單體僅有4 cm^−1之紅位移，顯示甲硫醇雙聚體間應不以氫鍵鍵結。此外，在具氫鍵之甲醇雙聚體光譜中可觀察到因質子施體與質子受體而造成各吸收峰分裂的現象，在本實驗中並無觀察到，亦為甲硫醇雙聚體間不以氫鍵鍵結之證據。另一方面，三聚體、四聚體及五聚體之ν3譜帶皆位於2567 cm^−1，相較於單體有38 cm^−1之紅位移，吸收強度也較單體強，顯示這些團聚體中甲硫醇分子間應以氫鍵互相鍵結。本實驗結果證實了前人從理論計算結果得出甲硫醇雙聚體最穩定之結構並無氫鍵鍵結，而三聚體皆為氫鍵鍵結之環狀結構的結論。

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 by using the infrared (IR)-vacuum ultraviolet (VUV) ionization technique. VUV emission at 132.5 nm served as the source of ionization in a time-of-flight mass spectrometer. The tunable IR laser emission served as a source of predissociation. The variations of intensity of methanethiol cluster ions (CH3SH)n^+ and CH3SH^+ 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, only 4 cm^−1 red-shifted 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 C-H stretching region, absorption bands of (CH3SH)2 are locate near 2865, 2890, 2944, and 3010 cm^−1, red-shifted by 3-5 cm^−1 from those of CH3SH. These red shifts increase slightly for higher clusters and bands near 2856, 2884, 2938, and 3005 cm^−1 were observed for (CH3SH)5. The results indicate that the S-H•••S hydrogen bonding exists in clusters with n = 3-5, but not in (CH3SH)2, in agreement with theoretical predictions. The absence of a band near 2605 cm^−1 might indicate that the dominant stable structures of (CH3SH)n, n = 3-5, have cyclic hydrogen-bonding framework.