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....

Femtosecond spectroscopic methods are used to study the dynamics following optical excitation of cis-stilbene molecules in hydrocarbon solvents. Transient absorption spectra of cis-stilbene over the range 320–1100 nm are reported. The anisotropies of these transients permit the assignment of the various excited electronic states in this region to A type in C2 symmetry. The excited state absorptions disappear at rates that are weakly dependent on solvent friction and comparisons with simple statistical mechanical theories and various potentials indicate that there is likely to be a barrier crossing process responsible for the observed decay times being in the range 0.7–1.4 ps. For observation times longer than ca. 100 fs an exponential decay of the cis population is observed and the transient spectrum does not appear to change in shape. A Kramers model fits the frictional dependence of the decay rates. Direct observation of what appears to be trans isomer ground states at 335 nm is reported following excitation of cis at 312 nm. The appearence time of this species is indistinguishable from the cis-disappearence time and any intermediate in the process cis→product (trans) is found to have a lifetime of less than 150 fs. This result implies that hot trans-stilbene molecules are produced in the isomerization. Previous studies probing at 312 nm may have detected cooling of these hot molecules. By means of polarized light excitation of cis and detection of the photoproduct it is found that the reactant and product of the isomerization are aligned as if the reaction coordinate were a twisting about the double bond by 180° while maintaining the orientation of the twofold axis in the laboratory frame. This high alignment, in view of obvious possible depolarizing influences in the solution, suggests a more intricate reaction coordinate involving the motion of the ethylene carbons.

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Measurement of the photofragment velocity and angular distributions from the photodissociation of N,N-dimethylformamide at 193 nm in its nb * absorption evidences three competing dissociation channels: HCON͑CH 3 ͒ 2 →HCO(X 2 AЈ)ϩN͑CH 3 ͒ 2 (X 2 B 1); HCO(X 2 AЈ)ϩN͑CH 3 ͒ 2 (Ã 2 A 1); and HCONCH 3 ϩCH 3. ͑H atom eliminations are not probed.͒ These products are formed in a ratio of 0.15Ϯ0.04:0.49Ϯ0.09:0.36Ϯ0.07, determined by use of trimethylamine as a calibrant molecule. Nitrogen-carbonyl bond fission occurs on a rapid time scale with an angular distribution of the dissociation products given by ␤ϭ1.2Ϯ0.2. Excited state N͑CH 3 ͒ 2 products are formed quasidiabatically from the initial planar geometry, whereas symmetry-breaking vibrations allow one-electron matrix elements to couple the initial electronic configuration to the ground state N͑CH 3 ͒ 2 ϩHCO channel. Competition of nitrogen-methyl bond fission is evidence of the strong coupling between the nb * excitation and the nitrogen-methyl reaction coordinate; ab initio calculations confirm that the electronic excitation is not localized on the N-CϭO moiety. We also include here an advance report of the excited state energy of the N͑CH 3 ͒ 2 (Ã 2 A 1) radical, which is found to be 1.59 eV.

Here we report ion mobility experiments and theoretical studies aimed at elucidating the identity of the acetylene dimer cation and its hydrated structures. The mobility measurement indicates the presence of more than one isomer for the C(4)H(4)(●+) ion in the cluster beam. The measured average collision cross section of the C(4)H(4)(●+) isomers in helium (38.9 ± 1 Å(2)) is consistent with the calculated cross sections of the four most stable covalent structures calculated for the C(4)H(4)(●+) ion [methylenecyclopropene (39.9 Å(2)), 1,2,3-butatriene (41.1 Å(2)), cyclobutadiene (38.6 Å(2)), and vinyl acetylene (41.1 Å(2))]. However, none of the single isomers is able to reproduce the experimental arrival time distribution of the C(4)H(4)(●+) ion. Combinations of cyclobutadiene and vinyl acetylene isomers show excellent agreement with the experimental mobility profile and the measured collision cross section. The fragment ions obtained by the dissociation of the C(4)H(4)(●+) ion are consistent with the cyclobutadiene structure in agreement with the vibrational predissociation spectrum of the acetylene dimer cation (C(2)H(2))(2)(●+) [R. A. Relph, J. C. Bopp, J. R. Roscioli, and M. A. Johnson, J. Chem. Phys. 131, 114305 (2009)]. The stepwise hydration experiments show that dissociative proton transfer reactions occur within the C(4)H(4)(●+)(H(2)O)(n) clusters with n ≥ 3 resulting in the formation of protonated water clusters. The measured binding energy of the C(4)H(4)(●+)H(2)O cluster, 38.7 ± 4 kJ/mol, is in excellent agreement with the G3(MP2) calculated binding energy of cyclobutadiene(●+)·H(2)O cluster (41 kJ/mol). The binding energies of the C(4)H(4)(●+)(H(2)O)(n) clusters change little from n = 1 to 5 (39-48 kJ/mol) suggesting the presence of multiple binding sites with comparable energies for the water-C(4)H(4)(●+) and water-water interactions. A significant entropy loss is measured for the addition of the fifth water molecule suggesting a structure with restrained water molecules, probably a cyclic water pentamer within the C(4)H(4)(●+)(H(2)O)(5) cluster. Consequently, a drop in the binding energy of the sixth water molecule is observed suggesting a structure in which the sixth water molecule interacts weakly with the C(4)H(4)(●+)(H(2)O)(5) cluster presumably consisting of a cyclobutadiene(●+) cation hydrogen bonded to a cyclic water pentamer. The combination of ion mobility, dissociation, and hydration experiments in conjunction with the theoretical calculations provides strong evidence that the (C(2)H(2))(2)(●+) ions are predominantly present as the cyclobutadiene cation with some contribution from the vinyl acetylene cation.