Three-center versus four-center elimination in photolysis of vinyl fluoride and vinyl bromide at 193 nm: Bimodal rotational distribution of HF and HBr (v<=5) detected with time-resolved Fourier transform spectroscopy

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Title:	Three-center versus four-center elimination in photolysis of vinyl fluoride and vinyl bromide at 193 nm: Bimodal rotational distribution of HF and HBr (v<=5) detected with time-resolved Fourier transform spectroscopy
Authors:	Lin, Shiaw-Ruey Lin, Shih-Che Lee, Yu-Chang Chou, Yung-Ching Chen, I.-Chia Lee, Yuan-Pern 周永慶
Contributors:	臺北市立教育大學自然科學系
Keywords:	organic compounds photodissociation molecule-photon collisions Fourier transform spectra rotational-vibrational states rotational-vibrational energy transfer reaction kinetics infrared spectra
Date:	2001
Issue Date:	2009-07-31 16:10:03 (UTC+8)
Abstract:	Following photodissociation of vinyl fluoride (CH2CHF) and vinyl bromide (CH2CHBr) at 193 nm, fully resolved vibration-rotational emission spectra of HF and HBr in spectral regions 3050-4900 and 2000-2900 cm-1, respectively, are temporally resolved with a step-scan Fourier transform spectrometer. With a data acquisition window 0-5 μs suitable for spectra with satisfactory ratio of signal-to-noise, emission from HX (with X = F or Br) up to v=6 is observed. All vibrational levels show bimodal rotational distributions. For CH2CHF, these two components of HF have average rotational energies ~2 and 23 kJ mol-1 and vibrational energies ~83 and 78 kJ mol-1, respectively; the values are corrected for small quenching effects. For CH2CHBr, these two components of HBr correspond to average rotational energies ~4 and 40 kJ mol-1, respectively, and similar vibrational energies ~68 kJ mol-1. The separate statistical ensemble (SSE) model is suitable for three-center (α, α) elimination of HX because of the loose transition state and a small exit barrier for this channel; predicted vibrational energy distributions of HX are consistent with those observed for the high-J component. An impulse model taking into account geometries and displacement vectors of transition states during bond breaking predicts substantial rotational excitation for three-center elimination of HX but little rotational excitation for four-center (α, β) elimination; observed rotational energies of low-J and high-J components are consistent with those predicted for four-center and three-center elimination channels, respectively. The model also explains why observed rotational energy of HF produced via three-center elimination of CH2CHF is smaller than that of HCl from CH2CHCl. Ratios of rate coefficients (0.66:0.34 and 0.88:0.12) predicted for three-center or four-center elimination channels based on Rice-Ramsberger-Kassel-Marcus theory are consistent with estimated branching ratios ~0.75:~0.25 and ~0.81:0.19 determined based on counting vibrational distribution of HF and HBr, respectively, to v<=5 for high-J and low-J components and considering possible quenching effects within 5 μs. Hence we conclude that, similar to photolysis of CH2CHCl, observed high-J and low-J components correspond to HX (v,J) produced from three-center and four-center elimination channels, respectively. The results are compared with those from photolysis of vinyl chloride at 193 nm.
Relation:	Journal of Chemical Physics, V114(17), p.7396-7406
Appears in Collections:	[Department of Applied Physics and Chemistry] Periodical Articles

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