利用步進式掃描傅式轉換紅外光譜法研究氣態CH3OO的紅外吸收光譜

Abstract

本實驗利用波長193 nm的雷射光解流動的CH3C(O)CH3/O2混和氣體及波長248 nm的雷射光解流動的CH3I/O2混和氣體，並利用步進式掃描時域解析傅式轉換紅外光譜法搭配多重反射之White cell偵測共同之瞬態產物CH3OO之紅外吸收光譜。吾人觀測到於3023.3 cm−1、2954.3 cm−1、1456.7 cm−1、1182.4 cm−1、1118.0 cm−1、910.7 cm−1、3021.4 cm−1和1440.9 cm−1之吸收峰依序指派為ν1-ν3，ν5-ν6，ν9和ν10之振動模吸收，與黃登瑞等人所觀測於氣態下之CH3OO紅外光譜所得之最大差異不超過0.25 %。此結果亦和Nandi等人於Ar間質環境下得到之結果平均差異小於1 %和Morrison等人於He奈米液滴環境下之觀測 、 、 高解析光譜之差異最大不超過0.1 %。與B3LYP/aug-cc-pVTZ非簡諧計算值平均差異於3 %以內。吾人以近似陀螺對稱分子之模式分析CH3OO的轉動譜線結構而得振動基態之轉動常數，與Endo實驗組以微波測量之結果差異為5 %。此外，吾人根據Just等人計算CH3OO之C–O單鍵內轉能障，並以PGOPHER軟體模擬 振動模與 振動模之熱譜帶躍遷。 振動模模擬光譜之轉動譜線與實驗值吻合，但對於 振動模Q分枝結果並不一致。透過內轉動振動模之熱譜帶躍遷模擬後，吾人認為內轉動對於CH3OO振動模紅外吸收光譜之貢獻是不可忽略的。

Abstract Methylperoxy (CH3OO), the simplest alkylperoxy radical, is an important intermediate in the oxidation of methane both in the atmosphere , and under combustion conditions . In this work, CH3OO radical were produced by irradiation of a flowing mixture of CH3I and O2 with KrF excimer laser at 248 nm. A step-scan time-resolved Fourier-transform spectrometer coupled with a multipath White cell was employed to record temporally resolved IR absorption spectra of reaction intermediate. Previously , transient absorption bands with origins at 3032.3, 2954.3, 1456.7, 1182.6, 1118.1, 3021.4, and 1440.9 cm－1 are assigned ν1-ν3, ν5-ν6, ν9 and ν10 modes of CH3OO, respectively. Recently, ν7 band is observed with origin at 910.7 cm－1. Besides, higher resolution spectra are obtained by irradiation of a flowing mixture of CH3C(O)CH3 and O2 with ArF excimer laser at 193 nm, so that rotational constants are available by using near prolate approximation model. The rotational contours of IR spectra of CH3OO, simulated based on ratios of predicted rotational parameters for the upper and lower states and on experimental rotational parameters of the ground state, agree satisfactorily with experimental results; the mixing ratios of a-, b-, and c-types of rotational structures were evaluated based on the direction of dipole derivatives predicted quantum chemically. Since the contribution of torsional splitting is non-negligible, we apply hot band transiton to simulate ν2 band. Though the result does not perfectly match, Q branch of ν2 band improves quite a lot.