利用步進式掃描時域解析傅氏轉換紅外光譜法研究Criegee中間體CH3CHOO之紅外吸收光譜

Abstract

吾人利用步進式掃描時間解析傅氏轉換紅外光譜儀搭配多重吸收槽，以248 nm與308 nm雷射照射CH3CHI2/N2/O2之流動混合氣體，成功觀測到syn-CH3CHOO與anti-CH3CHOO兩種構型異構物。884.0 cm-1、956.0 cm-1、1281.1 cm-1、1479.0 cm-1可分別指派為syn-CH3CHOO之v10、v9、v7、v4振動模， 862.5 cm-1、880.5 cm-1、1285.5 cm-1、1479 cm-1可分別指派為anti-CH3CHOO之v10、v9、v7、v4振動模，其結果與利用MULTIMODE計算之譜帶波數一致，且anti-CH3CHOO與syn-CH3CHOO之濃度比大約等於30：70。吾人亦觀察到syn-CH3CHOO之v10與anti-CH3CHOO之v9、v10振動模具有熱譜帶的躍遷並互相重疊，在800-920 cm-1形成一寬廣的譜帶(A1)。由實驗譜帶之振動波數，CH3CHOO與CH2OO之結構相似，具有較短與雙鍵特性之C=O鍵及鍵長較長之O-O鍵，此結果顯示CH3CHOO之結構如同CH2OO較接近兩性離子。由於兩性離子之特性，CH3CHOO與CH2OO衰減情形相似，符合二級動力學模型，且其反應速率常數約為3.3*10-10 cm3 molecule-1 s-1，此結果與Su等人所提之3.1-4.5*10-10 cm3 molecule-1 s-1相符合。

The highly reactive Criegee intermediate, a carbonyl oxide, was first proposed by Criegee in 1949 but eluded direct detection until recently. The simplest Criegee intermediate, CH2OO, was detected with a step-scan infrared absorption spectrometer by our group in 2013. In this work, the infrared absorption spectra of CH3CHOO, produced from CH3CHI + O2, are detected with a step-scan time-resolved spectrometer; CH3CHI was produced by photolysis of CH3CHI2 at 248 or 308 nm. The methyl-substituted Criegee intermediate CH3CHOO has two conformers: syn- and anti-CH3CHOO. By comparison with the observed rotational contour with vibration-rotational spectra simulated according to quantum-chemical calculations, the absorption bands at 884.0, 956.0 , 1281.1 and 1479.0 cm-1 are assigned to the v10, v9, v7 and v4 modes of syn-CH3CHOO and the absorption bands at 862.5, 880.5, 1285.5, and 1479 cm-1are assigned to the v10, v9, v7 and v4 modes of anti-CH3CHOO, respectively. Furthermore, we also found that the hot bands from v17 and v18 also contribute significantly to the overlapping band in the region of 800-920 cm-1. Compare with the vibrational frequencies of CH2OO, the zwitterionic character of CH3CHOO is similar to CH2OO, which has a stronger C=O bond and a weaker O-O bond.