利用時域解析傅式轉換紅外光譜法研究CH2BrOO、 CH2OO及CH2IOO之吸收光譜

Abstract

The Criegee intermediates, which are carbonyl oxides produced in ozonolysis of unsaturated hydrocarbons, play key roles in the formation of OH and organic acid in the atmosphere. However, the Criegee intermediates have eluded direct detection until an efficient metohd to produce CH2OO via the reaction CH2I + O2 was discovered recently. In addiction, the oxidation of this halogenated methyl radical is also of atmospheric importance in the marine boundary layer. In this work, a step-scan Fourier-transform spectrometer coupled with a multipass absorption cell is employed to record temporally resolved infrared (IR) absorption spectra of reaction intermediates including CH2BrOO, the Criegee intermediate CH2OO, and CH2IOO. CH2BrOO radicals were produced upon irradiation, with an excimer laser at 248 nm, of a flowing mixture of CH2Br2 and O2. Transient absorption with origins at 1276.1, 1088.3, 961.0, and 884.9 cm1 are assigned to 4 (CH2-wagging), 6 (OO stretching), 7 (CH2-rocking mixed with CO stretching), and 8 (CO stretching mixed with CH2-rocking) modes of syn-CH2BrOO, respectively. The assignments were made according to the expected photochemistry and a comparison of observed vibrational wavenumbers, relative IR intensities, and rotational contours with those predicted with the B3LYP/aug-cc-pVTZ method. The rotational contours of 7 and 8 indicate that hot bands involving the torsional (12) mode are also present, with transitions 7_0^1 〖12〗_v^v and 8_0^1 〖12〗_v^v, v = 110. The most intense band (4) of anti-CH2BrOO near 1277 cm1 might have a small contribution to the observed spectra. The IR absorption spectra of CH2OO, produced from the reaction of CH2I + O2, was recorded at resolution 0.25 cm1, which showed partially rotationally-resolved structures. The band origins of the 3 (CH2-scissoring mixed with CO stretching),4 (CO stretching mixed with CH2-scissoring), 5 (CH2-rocking), 6 (OO stretching), and 8 (CH2-wagging) modes are determined at 1434.1, 1285.7, 1213.0, 909.2, and 847.44 cm1, respectively. With the analysis of the vibration-rotational spectra, we provide a definitive assignment of these bands to CH2OO. The 5 mode of CH2OO was previously mis-assigned to be near 1241 cm1 due to its small IR intensity and interference from other absorption bands. The 2ν9 overtone transition of CH2OO was also observed at 1233.5 cm1. The rotational contours of 4 and 6 bands suggest that hot bands involving COO deformation moed (7) are also present, with transitions 4_0^1 7_1^1 and 6_0^1 7_1^1, respectively. CH2IOO was also observed in the reaction of CH2I + O2 at high pressure. Bands with origins at 1233.8, 1221.0, 1087.0, and 923.0 cm1 are assigned to the 4 (CH2-wagging), 5 (CH2-twisting), 6 (OO stretching), and 7 (CO stretching) modes of syn-CH2IOO, respectively. The observed band origins and relative IR intensities conform satisfactorily to those observed in p-H¬2 matrix; observed rotational contours of these bands agree with those simulated according to rotational parameters derived from the B3LYP/aug-cc-pVTZ-pp method. Hot bands involving 12 is also present in the 7 mode, with transitions 7_0^1 〖12〗_v^v, v = 09. The observation that hot bands contribute significantly to the CO stretching (ν7) mode but insignificantly to the CH2-wagging (ν4) and O-O stretching (ν6) modes agrees with that for the similar molecule CH2BrOO. The most intense band (4) of anti-CH2IOO centered at 1241 cm1 might have a small contribution to the observed spectra. The direct observation of both CH2OO and CH2IOO in the same experiment shall provide a method to determine accurately the yield of CH2OO at varied pressure.

The Criegee intermediates, which are carbonyl oxides produced in ozonolysis of unsaturated hydrocarbons, play key roles in the formation of OH and organic acid in the atmosphere. However, the Criegee intermediates have eluded direct detection until an efficient metohd to produce CH2OO via the reaction CH2I + O2 was discovered recently. In addiction, the oxidation of this halogenated methyl radical is also of atmospheric importance in the marine boundary layer. In this work, a step-scan Fourier-transform spectrometer coupled with a multipass absorption cell is employed to record temporally resolved infrared (IR) absorption spectra of reaction intermediates including CH2BrOO, the Criegee intermediate CH2OO, and CH2IOO. CH2BrOO radicals were produced upon irradiation, with an excimer laser at 248 nm, of a flowing mixture of CH2Br2 and O2. Transient absorption with origins at 1276.1, 1088.3, 961.0, and 884.9 cm1 are assigned to 4 (CH2-wagging), 6 (OO stretching), 7 (CH2-rocking mixed with CO stretching), and 8 (CO stretching mixed with CH2-rocking) modes of syn-CH2BrOO, respectively. The assignments were made according to the expected photochemistry and a comparison of observed vibrational wavenumbers, relative IR intensities, and rotational contours with those predicted with the B3LYP/aug-cc-pVTZ method. The rotational contours of 7 and 8 indicate that hot bands involving the torsional (12) mode are also present, with transitions 7_0^1 〖12〗_v^v and 8_0^1 〖12〗_v^v, v = 110. The most intense band (4) of anti-CH2BrOO near 1277 cm1 might have a small contribution to the observed spectra. The IR absorption spectra of CH2OO, produced from the reaction of CH2I + O2, was recorded at resolution 0.25 cm1, which showed partially rotationally-resolved structures. The band origins of the 3 (CH2-scissoring mixed with CO stretching),4 (CO stretching mixed with CH2-scissoring), 5 (CH2-rocking), 6 (OO stretching), and 8 (CH2-wagging) modes are determined at 1434.1, 1285.7, 1213.0, 909.2, and 847.44 cm1, respectively. With the analysis of the vibration-rotational spectra, we provide a definitive assignment of these bands to CH2OO. The 5 mode of CH2OO was previously mis-assigned to be near 1241 cm1 due to its small IR intensity and interference from other absorption bands. The 2ν9 overtone transition of CH2OO was also observed at 1233.5 cm1. The rotational contours of 4 and 6 bands suggest that hot bands involving COO deformation moed (7) are also present, with transitions 4_0^1 7_1^1 and 6_0^1 7_1^1, respectively. CH2IOO was also observed in the reaction of CH2I + O2 at high pressure. Bands with origins at 1233.8, 1221.0, 1087.0, and 923.0 cm1 are assigned to the 4 (CH2-wagging), 5 (CH2-twisting), 6 (OO stretching), and 7 (CO stretching) modes of syn-CH2IOO, respectively. The observed band origins and relative IR intensities conform satisfactorily to those observed in p-H¬2 matrix; observed rotational contours of these bands agree with those simulated according to rotational parameters derived from the B3LYP/aug-cc-pVTZ-pp method. Hot bands involving 12 is also present in the 7 mode, with transitions 7_0^1 〖12〗_v^v, v = 09. The observation that hot bands contribute significantly to the CO stretching (ν7) mode but insignificantly to the CH2-wagging (ν4) and O-O stretching (ν6) modes agrees with that for the similar molecule CH2BrOO. The most intense band (4) of anti-CH2IOO centered at 1241 cm1 might have a small contribution to the observed spectra. The direct observation of both CH2OO and CH2IOO in the same experiment shall provide a method to determine accurately the yield of CH2OO at varied pressure.