Huang Cheng-Liang, Jiang Jyh-Chiang, Dyakov Yuri A, Lin Ming-Fu, Tseng Chien-Ming, Lin S H, Lee Yuan T, Ni Chi-Kung
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.
J Chem Phys. 2006 Oct 7;125(13):133305. doi: 10.1063/1.2219445.
The photodissociation of jet-cooled alpha-fluorotoluene and 4-fluorotoluene at 193 and 248 nm was studied using vacuum ultraviolet (vuv) photoionization/multimass ion imaging techniques as well as electron impact ionization/photofragment translational spectroscopy. Four dissociation channels were observed for alpha-fluorotoluene at both 193 and 248 nm, including two major channels C6H5CH2F-->C6H5CH2 (or C7H7)+F and C6H5CH2F-->C6H5CH (or C7H6)+HF and two minor channels C6H5CH2F-->C6H5CHF+H and C6H5CH2F-->C6H5+CH2F. The vuv wavelength dependence of the C7H7 fragment photoionization spectra indicates that at least part of the F atom elimination channel results from the isomerization of alpha-fluorotoluene to a seven-membered ring prior to dissociation. Dissociation channels of 4-fluorotoluene at 193 nm include two major channels C6H4FCH3-->C6H4FCH2+H and C6H4FCH3-->C6H4F+CH3 and two minor channels C6H4FCH3-->C6H5CH2 (or C7H7)+F and C6H4FCH3-->C6H5CH (or C7H6)+HF. The dissociation rates for alpha-fluorotoluene at 193 and 248 nm are 3.3 x 10(7) and 5.6 x 10(5) s(-1), respectively. The dissociation rate for 4-fluorotoluene at 193 nm is 1.0 x 10(6) s(-1). An ab initio calculation demonstrates that the barrier height for isomerization from alpha-fluorotoluene to a seven-membered ring isomer is much lower than that from 4-fluorotoluene to a seven-membered ring isomer. The experimental observed differences of dissociation rates and relative branching ratios between alpha-fluorotoluene and 4-fluorotoluene may be explained by the differences in the six-membered ring to seven-membered ring isomerization barrier heights, F atom elimination threshold, and HF elimination threshold between alpha-fluorotoluene and 4-fluorotoluene.
利用真空紫外（vuv）光电离/多质量离子成像技术以及电子碰撞电离/光碎片平动光谱，研究了喷射冷却的α-氟甲苯和4-氟甲苯在193和248 nm处的光解离。在193和248 nm处均观察到α-氟甲苯的四个解离通道，包括两个主要通道C6H5CH2F→C6H5CH2（或C7H7）+F和C6H5CH2F→C6H5CH（或C7H6）+HF以及两个次要通道C6H5CH2F→C6H5CHF+H和C6H5CH2F→C6H5+CH2F。C7H7碎片光电离光谱的vuv波长依赖性表明，至少部分F原子消除通道是由于α-氟甲苯在解离前异构化为七元环所致。4-氟甲苯在193 nm处的解离通道包括两个主要通道C6H4FCH3→C6H4FCH2+H和C6H4FCH3→C6H4F+CH3以及两个次要通道C6H4FCH3→C6H5CH2（或C7H7）+F和C6H4FCH3→C6H5CH（或C7H6）+HF。α-氟甲苯在193和248 nm处的解离速率分别为3.3×10^7和5.6×10^5 s^(-1)。4-氟甲苯在193 nm处的解离速率为1.0×10^6 s^(-1)。从头算计算表明，从α-氟甲苯异构化为七元环异构体的势垒高度远低于从4-氟甲苯异构化为七元环异构体的势垒高度。α-氟甲苯和4-氟甲苯之间解离速率和相对分支比的实验观察差异，可能是由α-氟甲苯和4-氟甲苯在六元环到七元环异构化势垒高度、F原子消除阈值和HF消除阈值方面的差异所解释的。
