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

摘要

Following photodissociation of vinyl fluoride (CH-,CHF) and vinyl bromide (CH2CHBr) at 193 um, fully resolved vibration—rotational emission spectra of HF and HBr in spectral regions 3050— 4900 and 2000—2900 cm’, respectively, are temporally resolved with a step—scan Fourier transform spectrometer. With a data acquisition window 0—5 ~tts 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 moF’ and vibrational energies -83 and 78 kJ moF 1 respectively; the values are corrected for small quenching effects. For CH-,CHBr, these two components of HBr correspond to average rotational energies -4 and 40 kJ moF’, respectively, and similar vibrational energies -68 kJ moF ~. The separate statistical ensemble (SSE) model is suitable for three-center (a, a) 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-] component. An impulse model taking into account geometries and displacement vectors of transition states during bond breaking predicts substantial rotational excitation for thee-center elimination of HX but little rotational excitation for four-center (a, /3) elimination; observed rotational energies of low-] and high-] components are consistent with those predicted for four-center and three-center elimination channels, respectively. The model also evpJn~nr why obrerved rotationni energy of I-IF 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-] and low-] components and considering possible quenching effects within 5 jss. Hence we conclude that, similar to photolysis of CH9CHCl, observed high-f and low-] 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.