I. Three-center versus four-center HCl-elimination in photolysis of vinyl chloride at 193 nm: Bimodal rotational distribution of HCl (v⩽7) detected with time-resolved Fourier-transform spectroscopy

Abstract

Three-center versus four-center elimination in photolysis of vinyl fluoride and vinyl bromide at 193 nm: Bimodal rotational distribution of HF and HBr (v5) detected with time-resolved Fourier transform spectroscopy Following photodissociation of vinyl chloride at 193 nm, fully resolved vibration-rotational emission spectra of HCl in the spectral region 2000-3310 cm Ϫ1 are temporally resolved with a step-scan Fourier-transform spectrometer. Under improved resolution and sensitivity, emission from HCl up to vϭ7 is observed, with JϾ32 ͑limited by overlap at the band head͒ for vϭ1 -3. All vibrational levels show bimodal rotational distribution with one component corresponding to ϳ500 K and another corresponding to ϳ9500 K for vр4. Vibrational distributions of HCl for both components are determined; the low-J component exhibits inverted vibrational population of HCl. Statistical models are suitable for three-center ͑␣, ␣͒ elimination of HCl because of the loose transition state and a small exit barrier for this channel; predicted internal energy distributions of HCl are consistent but slightly less than those observed for the high-J component. Impulse models considering geometries and displacement vectors of transition states during bond breaking predict substantial rotational excitation for three-center elimination of HCl but little rotational excitation for four-center ͑␣, ␤͒ elimination; observed internal energy of the low-J component is consistent with that predicted for the four-center elimination channel. Rate coefficients 33.8 and 4.9ϫ10 11 s Ϫ1 for unimolecular decomposition predicted for three-center and four-center elimination channels, respectively, based on Rice-Ramsberger-Kassel-Marcus theory are consistent with the branching ratio of 0.81:0.19 determined by counting vibrational distribution of HCl to vр6 for high-J and low-J components. Hence we conclude that observed high-J and low-J components correspond to HCl (v, J) produced from three-center and four-center elimination channels, respectively.

Internal energy of HCl upon photolysis of 2-chloropropene at 193 nm investigated with time-resolved Fourier-transform spectroscopy and quasiclassical trajectories

“…Martínez-Núñez et al 8,9 employed quasiclassical trajectory ͑QCT͒ calculations to investigate energy partitions of HCl upon photodissociation of CH 2 CHCl at 193 nm and reported that three-center elimination and isomerization of vinylidene to acetylene take place in a concerted, nonsynchronous fashion. In agreement with previous experimental results, 1,7 the four-center elimination produces much vibrationally excited HCl, with a population inversion predicted at =1 ͑Ref. 9͒ or =3 ͑Ref.…”

“…Following photodissociation of vinyl chloride at 193 nm, we observed vibration-rotationally resolved emis-sion spectra of HCl ͑ Յ 7͒ with TR-FTS. 1,7 Further work extended to photolysis of vinyl fluoride and vinyl bromide 2 indicated that the HX ͑X = F, Cl, or Br͒ products observed in all vibrational levels show bimodal rotational distributions for which we proposed that the high-J component is associated with production via three-center ͑␣ , ␣͒ molecular elimination and the low-J component with four-center ͑␣ , ␤͒ elimination. Martínez-Núñez et al 8,9 employed quasiclassical trajectory ͑QCT͒ calculations to investigate energy partitions of HCl upon photodissociation of CH 2 CHCl at 193 nm and reported that three-center elimination and isomerization of vinylidene to acetylene take place in a concerted, nonsynchronous fashion.…”

“…According to our previous experience of the photolysis of vinyl halides, even when the pressure of the system was decreased as much as practicable ͑ϳ0.2 Torr͒ while maintaining a satisfactory S/N ratio, slight rotational quenching still occurred within 1 s period after photolysis. 1,7 Previous experiments with vinyl chloride seeded in a supersonic jet eliminate this quenching problem and yielded a nascent distribution of internal states of fragments upon photoirradiation. 7 For this reason we here describe experimental results obtained only upon photolysis of 2-chloropropene expanded through a supersonic slit jet.…”

“…The photolysis of vinyl halides ͑CH 2 CHX, X = F, Cl, or Br͒ and their derivatives is of interest to many researchers partly because this reaction proceeds on multiple potentialenergy surfaces associated with channels for simple bond scission and molecular elimination. 1,2 Experiments with photofragment translational spectroscopy provides detailed dynamics of most dissociation channels, 3 but these channels become indistinguishable when the fragments produced via two different paths possess similar distributions of translational energy. For example, the translational energy distribution of HCl produced upon photolysis of vinyl chloride at 193 nm provides little information about its origin from either a three-center or a four-center molecular elimination.…”

“…Martínez-Núñez et al 8,9 predicted, however, that HCl produced from four-center elimination is more rotationally excited than that from threecenter elimination, in contrast with the revised-impulse model that predicts little rotational excitation for four-center elimination but extensive rotational excitation for three-center elimination. 1 An understanding of the dynamics of reactions of this type requires further investigations.…”