利用間質隔離法研究CH3與SO2在p-H2間質中的光解反應

Abstract

此研究利用低溫間質隔離技術搭配霍氏轉換紅外光譜儀(FTIR)研究CH3I與SO2在para-H2 (p-H2) 間質中之光解反應。利用p-H2間質的微弱晶格效應，以254 nm光解CH3I產生CH3自由基再和SO2反應，可觀察到位於633.8、917.5、1071.1、1272.5及1416.0 cm−1的CH3SO2紅外吸收譜線，其吸收譜線分別可指認為CH3SO2之C−S伸張(ν11)，CH3搖擺(ν10)，SO2對稱伸張(ν8)，SO2不對稱伸張(ν7) 及CH2剪式(ν4) 振動模。接著觀察到此產物在以365 nm光源照射時減少並再次產生CH3及SO2。CH3SO2的ν7及ν8振動模吸收頻率和氣態的觀測(1280及1076 cm−1)符合，但本實驗新觀測到了另外三根譜線，其中包括C−S伸張(ν11)振動模之吸收。此實驗中亦另外觀測到在1150.1及1353.1 cm−1之吸收譜線，其特徵峰可指認為ISO2的SO2對稱伸張及不對稱伸張振動模。CH3SO2及ISO2振動模吸收譜線之指認係由比較18O及34S的同位素取代實驗結果及B3P86/aug-cc-pVTZ理論計算預測之IR譜線位置、強度及同位素位移所得到。實驗上只觀測到CH3SO2而未觀測到CH3OSO和理論計算所預測之CH3與SO2反應產生CH3SO2的能障幾乎為零，而產生CH3OSO具有約50–80 kJ mol−1的能障是一致的。

Irradiation with a mercury lamp at 254 nm of a p-H2 matrix containing CH3I and SO2 at 3.2 K, followed by annealing of the matrix, produced prominent features at 633.8, 917.5, 1071.1 (1072.2), 1272.5 (1273.0, 1273.6), and 1416.0 cm−1, attributable to ν11 (C−S stretching), ν10 (CH3 wagging), ν8 (SO2 symmetric stretching), ν7 (SO2 antisymmetric stretching), and ν4 (CH2 scissoring) modes of methylsulfonyl radical (CH3SO2), respectively; lines listed in parentheses are weaker lines likely associated with species in a different matrix environment. Further irradiation at 365 nm diminishes these features and produced SO2 and CH3. Additional features at 1150.1 and 1353.1 (1352.7) cm−1 are tentatively assigned to the SO2 symmetric and antisymmetric stretching modes of ISO2. These assignments are based on comparison of observed vibrational wavenumbers and 18O- and 34S-isotopic shifts with those predicted with the B3P86 method. Our results agree with the previous report of transient IR absorption bands of gaseous CH3SO2 at 1280 and 1076 cm−1. These results demonstrate that the cage effect of solid p-H2 is diminished so that CH3 radicals, produced via UV photodissociation of CH3I in situ, might react with SO2 to form CH3SO2 during irradiation and upon annealing. Observation of CH3SO2 but not CH3OSO is consistent with the theoretical predictions that only the former reactions proceed via a barrierless path.