Theoretical Studies of the Thermal Gas-Phase Decomposition of Vinyl Bromide on the Ground-State Potential-Energy Surface

Abstract

The reaction dynamics of the thermal, gas-phase decomposition of vinyl bromide has been investigated using classical trajectory methods on a global, analytic potential-energy surface fitted to the results of ab initio electronic structure calculations and experimental thermochemical, spectroscopic, and structural data. The saddle-point geometries and energies for several decomposition channels are determined using 6-3 lG(d,p) basis sets for carbon and hydrogen and Huzinaga's (4333/433/4) basis set augmented with split outer s and p orbitals and an f orbital for bromine. Electron correlation is incorporated using Mtiller-Plesset fourthorder perturbation theory with all single, double, triple, and quadruple excitations included. The calculated transition-state energies without zero-point energy corrections relative to vinyl bromide are four-center HBr elimination (3.530 eV), three-center HBr elimination (3.196 eV), four-center H2 elimination (4.159 eV), and three-center HZ elimination (4.618 eV). The global potential is written as a sum of the different reaction channel potentials connected by parametrized switching functions. The average absolute difference between AE values for the various decomposition channels obtained from the global surface and experimental measurement is 0.076 eV. Predicted equilibrium geometries for reactants and products are in good to excellent accord with experiment. The average absolute difference between the fundamental harmonic vibrational frequencies predicted by the global surface and those obtained from Raman and IR spectra varies from 10.2 cm-' for HzC=CHBr to 81.3 cm-' for H2C=CH. The potential barriers for six decomposition channels agree with the ab initio calculations to within an average difference of 0.124 eV. The dissociation dynamics of vinyl bromide on the ground-state surface is investigated at several excitation energies in the range 4.0-6.44 eV. The results show the following: (1) The decomposition dynamics follows a first-order rate law.(2) At thermal energies, the only brominated decomposition product is HBr. The results indicate that a previously reported activation energy for this process is too small. (3) As the excitation energy increases, other decomposition channels become important. At E = 6.44 eV, the reaction channels are, in order of importance, HZ elimination (48.1%), HBr formation (44.5%), Br atom elimination (4.6%), and C-H bond fission (2.6%). (4) The percentage of the total excitation energy partitioned into product relative translational motion and HBr internal energy upon HBr elimination is nearly independent of the total excitation energy. (5) Comparison of the calculated and measured relative translational energy distributions for product Br atoms upon C-Br bond fission and time-of-flight spectra for C2H2 upon HBr elimination indicates that in previously reported photolysis experiments Br atom dissociation is occurring on an excited electronic surface but HBr elimination is taking place on the ground-state surface subsequent to internal conversion....

Photoelectron spectroscopy of vinylbromide and intramolecular dynamics of the ionic B̃ state

“…As a reference, the geometry and vibrational frequencies of the neutral molecule were also calculated; the results summarized in Tables 2-4 are in good agreement with experimental data and previous calculations reported by Abrash et al [30]. Tables 2 and 3.…”

Photodissociation of 1,2‐Dibromoethylene at 248 nm: Br2 Molecular Elimination Probed by Cavity Ring‐Down Absorption Spectroscopy

“…[22] The Br channel carrying a smaller translational energy was anticipated to originate from the dissociation of the ground state and its yield was reported to be one order of magnitude smaller than that of the HBr elimination. [23,24] In contrast to this case, the HBr product was not found in recent studies of the photodissociation of 1,2-dibromoethylene which were carried out at 248 nm by using product translational spectroscopy (PTS). [11] Lin and co-workers proposed the two exclusive dissociation channels Br 2 + C 2 H 2 and BrA C H T U N G T R E N N U N G (fast) + BrA C H T U N G T R E N N U N G (slow) + C 2 H 2 with a branching ratio of about 0.2:0.8.…”

Theoretical Studies of the Photolytic Decomposition of Vinyl Bromide at 193 nm

“…On the ground-state surface, this is the major dissociation channel at internal energies below 6.00 eV. 6 On the excited-state surfaces, however, we find that such dissociations do not occur. None of the 3700 trajectories computed in this study gave HBr as a product.…”

“…6 In that investigation, we found that at internal energies equal to the photon energy in the photolytic experiments2 (6.44 eV), the simulated TOF spectrum obtained from the calculated distribution of translational energies subsequent to HBr elimination is in fair accord with the time-offlight measurementse2 As the excitation energy decreases, the degree of agreement between the computed and measured TOF spectra decreases. It was therefore suggested that the HBr observed in the photolytic beam experiments2 is being produced subsequent to internal conversion to the ground state.…”