利用步進式時域解析霍氏轉換紅外光譜法研究氯原子與異丁烯反應產生氯化氫之反應動力學

Abstract

本實驗利用步進式時域解析霍氏紅外放光光譜法研究氯原子與異丁烯之反應，藉由觀測產物HCl之振-轉動放光譜線，分析其內能分佈，並進一步探討其反應動態學。 實驗以248 nm之準分子雷射光解乙二醯氯(P_OC=9 mTorr)生成氯原子與異丁烯(P_iso=209 mTorr)反應生成HCl，並藉由控制系統內氬氣之分壓而於不同壓力條件(P_T=0.22,1.1,2.0,3.0 Torr)下進行實驗。吾人於2350－3250 cm^(-1)光區中觀測到振動激發態HCl之放光譜線，其振-轉動分佈到v≤2、J≤13，各壓力條件下之初生態平均轉動能分別為3.0±0.3 kJ mol^(-1) (0.22 Torr)、3.0±0.1 kJ mol^(-1) (1.1 Torr)、2.9±0.1 kJ mol^(-1) (2.0 Torr)、2.9±0.1 kJ mol^(-1) (3.0 Torr)，且觀察到振動激發態之HCl生成量以及[HCl(v=2)]/[HCl(v=1)]皆隨系統壓力增加而增加。 氯原子與烯類生成HCl主要有兩種反應途徑，分別為直接擷取反應及加成-分解反應。氯原子直接擷取烯類上甲基之氫原子反應生成HCl以及烯類自由基，稱為直接擷取反應。氯原子先對烯類上之碳-碳雙鍵反應生成加成中間產物，隨後分解生成HCl與烯類自由基，合稱為加成-解離反應，又稱為非直接擷取反應。而在近期Suits研究組提出該反應之非直接擷取途徑可能是經由漫遊反應機制而生成；該反應機制中加成中間物之氯原子可藉由大幅度運動而達到一近乎解離之狀態，在C4H7附近漫遊，於該狀態下直到產生適合HCl解離之構型才將HCl解離。本實驗所觀測到振動激發態之HCl生成量以及[HCl(v=2)]/[HCl(v=1)]皆隨系統壓力而增加以及轉動能量隨壓力變化不明顯之現象，與Suits研究組提出之漫遊機制之特性相符，可做為支持漫遊機制之證據。

Reaction of chlorine atom with isobutene has been investigated with a step-scan time-resolved Fourier-transform emission spectrometer. We monitor the IR emission of HCl product and investigate the reaction dynamic. Photolyze oxalyl chloride(P_OC=9 mTorr) at 248 nm produce Cl atom then react with isobutene(P_iso=209 mTorr) to form HCl at varied total pressure(P_T=0.22,1.1,2.0,3.0 Torr) controlled by added Ar. IR emission of HCl in regions 2350－3250 cm-1 are observed; and assigned HCl(v≤2,J≤13). The average rotational energy of each condition is 3.0±0.3 kJ mol^(-1) (0.22 Torr)、3.0±0.1 kJ mol^(-1) (1.1 Torr)、2.9±0.1 kJ mol^(-1) (2.0 Torr)、2.9±0.1 kJ mol^(-1) (3.0 Torr). We observe that the intensity of HCl(v=1,2) and the ratio of HCl(v=2) to HCl(v=1) increased as function of total pressure. HCl formation is a major pathway for which direct abstraction competes with the addition-elimination mechanism. In the study of the reaction of Cl + isobutene using the crossed molecular beam technique, Suits and coworkers found that the angular distribution of product tert-butyl radical becomes isotropic and the kinetic energy release becomes small at low collision energy. They proposed that these results might indicate that a roaming mechanism occurs at low collision energy. This proposal was supported by calculations of transition states and intermediate which the length between Cl and terminal H is long at the CBS-QB3 level. Our result is consistent with the roaming mechanism which is expected with more vibrational excited HCl.