利用步進式時域解析傅氏轉換紅外放光光譜法研究二碘甲烷與氧氣之光化學反應

Infrared Emission from UV-Irradiated Mixtures of CH2I2 and O2 Probed with Step-scan Time-resolved Fourier-transform Spectroscopy

陳庭鈺(Ting-Yu Chen)

指導教授 ： 李遠鵬

國立交通大學/理學院/分子科學研究所/碩士(2018年)

摘要

本實驗分別以248 nm及308 nm雷射光解CH2I2後與O2反應利用步進式時域解析傅式紅外放光光譜法研究CH2I2與O2反應，觀測到產物CH2I、CO、CO2、OH與H2CO分子的放光光譜，藉著分析產物CO、CO2及OH振動能階之佈居數，分析其內能分佈並進一步探討反應機構。
於總壓8 torr 248 nm光解實驗中，可觀測產物CO分布到v ≤ 11、J ≤ 19之放光譜線，並觀察到兩種振動能分佈 (定義為channel A及channel B)，平均振動能量(包含v=0)分別為97 kJ mol-1及18 kJ mol-1；兩者之CO比例約為40±4: 60±18。另一方面，可觀測到產物OH分布至v ≤ 3、J ≤ 5.5之放光譜線，平均振動能量為35 kJ mol-1。CO及OH佈居比例約為65±4：35±2。模擬之CO2放光光譜，有兩種振動分佈(定義為channel C及channel D)，最高可用能量分別為275 kJ mol-1及383 kJ mol-1；兩者之CO2比例約為56±2: 44±1。
於總壓8 torr 308 nm光解實驗中，可觀測產物CO分布到v ≤ 5、J ≤ 19之放光譜線，平均振動溫度為6500 K，平均振動能量為32.0 kJ mol-1。另一方面，可觀測到產物OH分布至v ≤ 3、J ≤ 5.5之放光譜線，平均振動溫度為8000 K，平均振動能量為29.0 kJ mol-1。CO及OH佈居比例約為50±4：50±4。模擬之CO2放光光譜，有兩種振動分佈，最高可用能量分別為263 kJ mol-1及359 kJ mol-1；兩者之CO2比例約為51±2: 49±2。
而在總壓16 torr 248 nm與308 nm光解實驗中，可觀測產物CO的振動分佈與總壓8 torr實驗相似，緩弛影響不明顯。另一方面，可觀測到產物OH的平均振動溫度與平均振動能量約為總壓8 torr實驗的85%，略受到O2所造成的緩弛影響。模擬之CO2放光光譜，其振動分佈與平均振動能量與總壓8 torr實驗相同。
參考理論計算之位能面及實驗結果，吾人推測，channel A之高振動分佈的CO由HCO與O2二次反應產生而來，而channel B之低振動分佈的CO為HCOOH經TS4產生H2O + CO而來。OH有可能由c-HCOOH或t-HCOOH分解生成HCO + OH而來。另外，channel C之低振動分佈的CO2為甲酸經由TS5分解生成CO2 + H2而來，而channel D之高振動分佈的CO2來自methylenebisoxy經由TS6分解生成CO2 + H2而來。
Shaub等人[18th Symp. (Int.) Combust., 811 (1981)]在閃光光解CH2I與O2之實驗中，皆觀測到CO的兩種振動分佈，而他們認為HCO二次分解為CO產生低振動之CO與HCOOH分解產生高振動態的CO與吾人所得不同。Alvarez等人[J. Phys. Chem. 98, 174 (1994)]由閃光光解同位素標記的烯酮生成3CH2後與O2反應實驗，其產物CO與OH分支比與吾人以308 nm雷射光解的實驗結果較為相似。近年來實驗室多利用雷射光解CH2I2與O2反應產生Criegee中間體，本實驗的研究成果，對於CH2I與O2反應細節提供進一步的瞭解。

關鍵字

二碘甲烷與氧氣 ； 光化學 ； 時域解析光譜 ； 內能 ； 克里奇中間體

並列摘要

We employed a step-scan Fourier-transform infrared (FTIR) spectrometer to record temporally resolved emission upon irradiation of mixtures of CH2I2, O2, and Ar at 248 and 308 nm. IR emission of CO, CO2, OH, CH2I, and H2CO in the region 1860−4900 cm-1 was recorded. At total pressure 8 Torr and irradiation wavelength 248 nm, rotationally resolved lines of CO (v  11, J  19) in region 1860−2300 cm-1 were observed; the vibrational distribution is bimodal, with two components having averaged vibrational energies of 97 kJ mol-1 for channel A and 18 kJ mol-1 for channel B. The branching ratio of channel A to channel B is 40±4 : 60±18. Emission of OH (v  3, J  5.5) in region 2980−3600 cm-1 was observed with ambient rotational distribution and average vibrational energy of 35 kJ mol-1. The branching ratio of CO : OH is 60±4 : 40±2. Emission of highly internally excited CO2 was also observed; the vibrational distribution is bimodal, with two components having maximal available vibrational energies of 275 kJ mol-1 for channel C and 383 kJ mol-1 for channel D. The population ratio of channel C to channel D is 56±2 : 44±1.
At total pressure 8 Torr and irradiation wavelength 308 nm, rotationally resolved lines of CO (v  5, J  19) in region 1860−2300 cm-1 were observed; the averaged vibrational energy is 32 kJ mol-1. Emission of OH (v  3, J  5.5) in region 2980−3600 cm-1 was observed with ambient rotational distribution and average vibrational energy of 29 kJ mol-1. The branching ratio of CO to OH is 50±4 : 50±4. Emission of highly internally excited CO2 was also observed; the vibrational distribution is bimodal, with two components having averaged vibrational energies of 263 kJ mol-1 for channel C and 359 kJ mol-1 for channel D. The population ratio of channel C to channel D is 51±2 : 49±2.
According to these experimental results and quantum chemical calculations, we attribute channel A (high v) of CO to the secondary reaction of HCO with O2 and channel B (low v) of CO to the decomposition of HCOOH to form H2O+CO. OH is produced from decomposition of cis or trans-HCOOH. CO2 is produced via channel C (low E) from decomposition of HCOOH via TS5 and via channel D (high E) is from decomposition of methylenebis(oxy) via TS6.
These two components of CO were also observed by Shaub et al. [18th Symp. (Int.) Combust., 811 (1981)] from flash photolysis of CH2I +O2；they associated the high-v channel to the molecular channel to form CO+H2O via decomposition of HCOOH and the low-v channel to the secondary decomposition of HCO that was produced from the radical channel. Our conclusion does not agree with theirs. Alvarez et al. [J. Phys. Chem. 98, 174 (1994)] performd experiment to derive the branching ratio of CO to OH similar to our results using irradiated 308 nm.
The reaction CH2I2 + O2 has been commonly employed as a laboratory source of CH2OO. Our results provide further understanding on the details in the reaction of CH2I2 with O2.

並列關鍵字

CH2I2 and O2 ； Photochemistry ； Time-resolved spectroscopy ； Internal energy ； Criegee

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