| Summary: | 碩士 === 國立交通大學 === 應用化學系碩博士班 === 108 === Reactions between chlorine atoms and unsaturated hydrocarbons play important roles in atmosphere chemistry. In the marine boundary layer and polluted coastal areas, chlorine atom reactions with alkenes produce alkyl or alkenyl radicals, which may further react with O_2 and become potential contributors to secondary organic aerosols. In the reaction, HCl is produced from either direct abstraction of an allylic hydrogen atom by the chlorine atom or addition-elimination, in which the chlorine atom first adds to the C=C double bond to form a chloroalkyl radical, followed by abstraction of an hydrogen atom of the methyl moiety to produce HCl and an alkenyl radical. Stabilized chloro-alkenyl radicals are produced from the addition reaction at high pressure. The relative importance of the addition reaction and hydrogen atom abstraction reaction depends on the environmental pressure and temperature. Recently, a roaming mechanism mediating the addition-elimination pathway in the reaction of Cl + isobutene was suggested; it becomes significant at low collision energies. This roaming mechanism was further supported through observation of the enhanced rate for the formation of HCl, which is probed with a step-scan FTIR emission spectroscopy, when sufficient Ar was added in the system to enhance the roaming behavior. The added argon quenches efficiently the kinetic energy of photo-induced chlorine atom to reduce the collision energy.
This study is aimed at investigating the HCl formation pathways in the reaction of Cl + trans-2-butene with a step-scan time-resolved FTIR spectrometer. The reaction of Cl + trans-2-butene was carried out at 298 K under pseudo-first-order condition, in which argon (or helium) buffer gas in varied proportion was added to the system so that the total pressure reached 0.48, 1, 2, and 3 Torr, respectively. IR emission signal of HCl in the region of 2400-3300 cm^(-1) was observed. Emission lines of HCl(v=1, J≤11) and HCl(v=2, J≤7) were observed with mean rotational energy of ~3 kJ mol^(-1) and [HCl(v=2)]⁄[HCl(v=1)] =0.10±0.01. We performed the kinetic simulation on temporal profiles of HCl with the MATLAB program according to the proposed mechanism. Literature rate coefficients, kabs=4.6×10^(-11) cm^(3) molecule^(-1) s^(-1), kfor=2.98×10^(-10) cm^(3) molecule^(-1) s^(-1), and kM'(0)=4×10^(-28)-10×10^(-28) cm^(3) molecule^(-1) s^(-1) were fixed during the simulation; ke, krev, kq, ϕ2/ϕ1, and ε2/ε1 were fitted with the temporal profiles. In the reaction of Cl + trans-2-butene, as the total pressure increases, the derived kae decreases, and kadd enhances. At total pressure of 0.48 Torr in which Ar was added as the quenching gas, ke/kfor [alkene] and kae/kfor are greater than those in the He environment; these results implying that under conditions with lower collision energies, the contribution of the addition-elimination to the formation of HCl increases. This observation is consistent with the characteristics of the roaming mechanism proposed by Joalland et al. and Estillore et al., who indicated that roaming mechanism might play an important role in the reaction of Cl + trans-2-butene at small collision energy.
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