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CH2OO與甲酸反應之紅外吸收光譜與反應動力學

Abstract

Criegee中間體為不飽和烴類與臭氧反應所生成，其反應性很高，可以和大氣中許多分子，如：NO2、SO2、(H2O)2和HCOOH反應。Welz等人提出Criegee中間體CH2OO與HCOOH之反應很快，其反應速率常數為(1.1±0.1)×10^(-10) cm3 molecule s-1。Long等人之理論計算結果預期反應CH2OO + HCOOH會生成中間物hydroperoxy methylformate (HPMF，HC(O)OCH2OOH)，而HPMF會分解成formic acid anhydride (FAN，(CHO)2O)和水，與Neeb等人之實驗結果相符。 本實驗利用步進式掃描時間解析傅氏轉換紅外吸收光譜儀搭配多重吸收槽，以308 nm雷射照射CH2I2/HCOOH/N2/O2流動混合氣體，成功觀測到Criegee中間體CH2OO和HCOOH反應生成之中間物HPMF於887、925、1052、1115、1170、1342、1391及1760 cm-1之吸收譜帶。利用B3LYP/aug-cc-pVTZ理論計算所得之非簡諧振動波數與相對紅外吸收強度比較，這些譜帶可分別指派為HPMF (P5)之ν16 (860 cm-1)，ν15 (901 cm-1)，ν13 (1060 cm-1)，ν12 (1115 cm-1)，ν11 (1157 cm-1)，ν9 (1346 cm-1)，ν7 (1385 cm-1)及ν5 (1751 cm-1)振動模。比較反應前期(40－60 μs)和反應後期(3640－3660 μs)光譜之差異，可以將反應後期變得較重要之1070、1170和1732 cm-1之吸收譜帶進一步指派為HPMF (P6)之ν13 (1044 cm-1)，ν11 (1147 cm-1)及ν5 (1727 cm-1)振動模。此外，本實驗使用光譜模擬程式PGOPHER模擬HPMF (P5和P6)之基頻吸收光譜並與實驗光譜比較，比較結果顯示實驗光譜之譜帶較模擬光譜寬且無法清楚分辨其P、Q、R分支，有可能是因為低能量振動模之激發態的吸收(即熱譜帶)對於本實驗所觀測到的紅外吸收譜帶之貢獻所致。而在較長的反應時間(〜ms)內，吾人亦觀測到CH2OO和HCOOH之反應中間物HPMF分解所產生之終產物anti-FAN於997、1104、1767及1820 cm-1之吸收譜帶，可分別指派為anti-FAN之ν10、ν7、ν4及ν3振動模。此結果與利用B3LYP/aug-cc-pVTZ理論計算所得之非簡諧振動波數971、1065、1781及1832 cm-1與相對紅外吸收強度符合。 本實驗亦使用擬一級反應之條件，並分別藉由擬合CH2OO衰減和HPMF生成之濃度曲線隨得到CH2OO和HCOOH於溫度298 K下之二級反應速率常數為 (8.7±1.6)×10^(-11) cm3 molecule-1 s-1，而產生HPMF之二級反應速率常數為(5.6±0.3)×10^(-11) cm3 molecule-1 s-1，可能顯示有其他不會產生HPMF之反應途徑存在。此外，在較長的反應時間下，本實驗藉由擬合HPMF衰減以及FAN生成之濃度曲線測得於系統總壓力～119 Torr、溫度298 K之條件下，HPMF (P5)和HPMF (P6)分解成anti-FAN和水之反應速率常數分別為1460 ± 30 s-1和37 ± 1 s-1。

The Criegee intermediates, which are carbonyl oxide intermediate produced in ozonolysis of unsaturated hydrocarbons, play important roles in the production of OH radicals, organic acids and aerosols in the atmosphere. Criegee intermediates react readily with atmospheric species such as NO2, SO2, (H2O)2 and HCOOH. The reaction of CH2OO with HCOOH was reported to be extremely rapid, with a rate coefficient of (1.1±0.1)×10^(−10) cm3 molecule−1 s−1. Quantum-chemical calculations indicate that the reaction of CH2OO + HCOOH proceeds through a barrierless association path to form hydroperoxymethyl formate (HPMF, HC(O)OCH2OOH), in agreement with experimental results by Neeb et al, who observed HPMF and formic acid anhydride (FAN, (CHO)2O) in experiments of ozonolysis of alkene; FAN and water were proposed to be produced from dissociation of HPMF. In this work, a step-scan Fourier-transform spectrometer coupled with a multipass absorption cell was employed to record temporally resolved infrared (IR) absorption spectra of the reactants and products during the reaction of CH2OO with HCOOH in a flow system. CH2OO were produced from the reaction of CH2I with O2; CH2I was produced from photolysis of CH2I2. Observed bands near 887, 925, 1052, 1115, 1170, 1342, 1391 and 1760 cm-1 are assigned to ν16, ν15, ν13, ν12, ν11, ν9, ν7, and ν5 modes of HPMF (P5), respectively. In the later reaction period, bands near 1070, 1170 and 1732 cm-1 can be assigned to ν13, ν11, and ν5 modes of HPMF (P6). The observed wavenumbers and relative intensities agree with the anharmonic vibrational wavenumbers and IR intensities predicted with the B3LYP/aug-cc-pVTZ method. However, the band contour simulated with the PGopher program had much smaller width than observed, indicating contributions from hot bands of low-frequency modes. The bands of the final product, anti-FAN, were also observed near 997, 1104, 1767 and 1820 cm-1. By analyzing the temporal profile of the rise of HPMF and the decay of CH2OO, we obtained the rate coefficient of the reaction CH2OO + HCOOH at 298 K to be (8.7±1.6)×10^(−11) cm3 molecule−1 s−1, and the rate coefficient for formation of HPMF to be (5.6±0.3)×10^(−11) cm3 molecule−1 s−1, indicating the presence of reaction channel producing products other than HPMF. By analyzing the decay of HPMF and the rise of FAN at the later period of reaction, we were able to derive the rate coefficients for HPMF (P5) and HPMF (P6) to convert to FAN to be ～1460 ± 30 s-1 and 37 ± 1 s-1 at 119 Torr, respectively.