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

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.