Three-center versus four-center elimination of haloethene: Internal energies of HCl and HF on photolysis of CF2CHCl at 193 nm determined with time-resolved Fourier-transform spectroscopy

Abstract

Following photodissociation of 2-chloro-1,1-difluoroethene CF2CHCl) at 193 nm, vibration–rotationally resolved emission spectra of HCl(v⩽3) and HF(v⩽4) in spectral regions 2000–2900 and 3050–4410 cm−1, respectively, are detected with a step-scan time-resolved Fourier-transform spectrometer. All vibrational levels of HCl and HF show Boltzmann-type rotational distributions. HCl has an average rotational energy of 23±4 kJ mol−1 and a vibrational energy of 25±5 kJ mol−1, whereas HF has an average rotational energy of 20±4 kJ mol−1 and a vibrational energy of 48±6 kJ mol−1. The observed internal energy distribution indicates that HCl is produced via the three-center (α,α), but HF via the four-center (α,β) elimination. A modified separate statistical ensemble model predicts an internal energy distribution of HCl slightly greater than experimental observation. A modified impulse model taking into account geometries and displacement vectors of transition states during bond breaking predicts satisfactorily the rotational excitation of HF produced from four-center elimination. Ratios of rate coefficients (0.87:0.13) predicted for three-center or four-center elimination channels based on Rice–Ramsperger–Kassel–Marcus theory are consistent with a branching ratio of 0.88:0.12 determined based on observed populations of HCl and HF, respectively. We also compare these experimental and theoretical results with those of photolysis of vinyl halides (CH2CHX, X=F, Cl, or Br) at 193 nm.