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

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 mus 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 similar to2 and 23 kJ mol(-1) and vibrational energies similar to 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 similar to4 and 40 kJ mol(-1), respectively, and similar vibrational energies similar to 68 kJ mol(-1). The separate statistical ensemble (SSE) model is suitable for three-center (alpha, alpha) 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 (alpha, beta) 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 similar to0.75:similar to0.25 and similar to0.81:0.19 determined based on counting vibrational distribution of HF and HBr, respectively, to v less than or equal to5 for high-J and low-J components and considering possible quenching effects within 5 mus. 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. (C) 2001 American Institute of Physics.