The dynamics of Br(2Pj) formation in the photodissociation of vinyl and perfluorovinyl bromides (original) (raw)
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The Journal of Chemical Physics, 2011
The photodissociation dynamics of 3-bromo-1,1,1-trifluoro-2-propanol (BTFP) and 2-(bromomethyl) hexafluoro-2-propanol (BMHFP) have been studied at 234 nm, and the C-Br bond dissociation investigated using resonance-enhanced multiphoton ionization coupled with time-of-flight mass spectrometer (REMPI-TOFMS). Br formation is a primary process and occurs on a repulsive surface involving the C-Br bond of BTFP and BMHFP. Polarization dependent time-of-flight profiles were measured, and the translational energy distributions and recoil anisotropy parameters extracted using forward convolution fits. A strong polarization dependence of time-of-flight profiles suggest anisotropic distributions of the Br(2 P 3/2) and Br(2 P 1/2) fragments with anisotropy parameter, β, of respectively 0.5 ± 0.2 and 1.2 ± 0.2 for BTFP, and 0.4 ± 0.1 and 1.0 ± 0.3 for BMHFP. The measured velocity distributions consist of a single velocity component. The average translational energies for the Br(2 P 3/2) and Br(2 P 1/2) channels are 9.2 ± 1.0 and 7.4 ± 0.9 kcal/mol for BTFP, and 15.4 ± 1.8 and 15.1 ± 2.0 kcal/mol for BMHFP. The relative quantum yields of Br(2 P 3/2) and Br(2 P 1/2), which are 0.70 ± 0.14 and 0.30 ± 0.06 in BTFP and 0.81 ± 0.16 and 0.19 ± 0.04 in BMHFP, indicate that the yield of the former is predominant. The measured anisotropy parameters for the Br(2 P 3/2) and Br(2 P 1/2) channels suggest that the former channel has almost equal contributions from both the parallel and the perpendicular transitions, whereas the latter channel has a significant contribution from a parallel transition. Non-adiabatic curve crossing plays an important role in the C-Br bond dissociation of both BTFP and BMHFP. The estimated curve crossing probabilities suggest a greater value in BTFP, which explains a greater observed value of the relative quantum yield of Br(2 P 1/2) in this case.
Photodissociation of C3H5Br and C4H7Br at 234 nm
Bulletin of the Korean Chemical Society, 2012
The photodissociation dynamics of cyclopropyl bromide (C3H5Br) and cyclobutyl bromide (C4H7Br) at 234 nm was investigated. A two-dimensional photofragment ion-imaging technique coupled with a [2+1] resonanceenhanced multiphoton ionization scheme was utilized to obtain speed and angular distributions of the nascent Br(2 P3/2) and Br * (2 P1/2) atoms. The recoil anisotropies for the Br and Br * channels were measured to be βBr = 0.92 ± 0.03 and βBr* = 1.52 ± 0.04 for C3H5Br and βBr = 1.10 ± 0.03 and βBr* = 1.49 ± 0.05 for C4H7Br. The relative quantum yield for Br was found to be ΦBr = 0.13 ± 0.03 and for C3H5Br and C4H7Br, respectively. The soft radical limit of the impulsive model adequately modeled the related energy partitioning. The nonadiabatic transition probability from the 3A' and 4A' potential energy surfaces was estimated and discussed.
Photodissociation Dynamics of C2H4BrCl: Nonadiabatic Dynamics with Intrinsic CsSymmetry
Bulletin of the Korean Chemical Society
The photodissociation dynamics of 1,2-bromochloroethane (CzH^BrCl) was investigated near 234 nm. A two-dimen sional photofragment ion-imaging technique coupled with a [2+1] resonance-enhanced multiphoton ionization scheme was utilized to obtain speed and angular distributions of the nascent Br(2P3/2)and Br (2P1/2)atoms. The total trans lational energy distributions for the Br and Br channels were well characterized by Gaussian functions with average * translational energies of 100 and 84 kJ/mol, respectively. The recoil anisotropies for the Br and Br channels were measured to be g = 0.49 士 0.05 for Br and 1.55 士 0.05 for Br. The relative quantum yield for Br was found to be ①Br* = 0.33 士 0.03. The probability of nonadiabatic transition between A states was estimated to be 0.46. The relevant nonadiabatic dynamics is discussed in terms of interaction between potential energy surfaces in Cs symmetry.
Nonadiabatic effects in C–Br bond scission in the photodissociation of bromoacetyl chloride
The Journal of Chemical Physics, 2006
Bromoacetyl chloride photodissociation has been interpreted as a paradigmatic example of a process in which nonadiabatic effects play a major role. In molecular beam experiments by Butler and coworkers [J. Chem. Phys. 95, 3843 (1991); J. Chem. Phys. 97, 355 (1992)], BrCH 2 C(O)Cl was prepared in its ground electronic state (S 0 ) and excited with a laser at 248 nm to its first excited singlet state (S 1 ). The two main ensuing photoreactions are the ruptures of the C-Cl bond and of the C-Br bond. A nonadiabatic model was proposed in which the C-Br scission is strongly suppressed due to nonadiabatic recrossing at the barrier formed by the avoided crossing between the S 1 and S 2 states. Recent reduced-dimensional dynamical studies lend support to this model. However, another interpretation that has been given for the experimental results is that the reduced probability of C-Br scission is a consequence of incomplete intramolecular energy redistribution. To provide further insight into this problem, we have studied the energetically lowest six singlet electronic states of bromoacetyl chloride by using an ab initio multiconfigurational perturbative electronic structure method. Stationary points (minima and saddle points) and minimum energy paths have been characterized on the S 0 and S 1 potential energy surfaces. The fourfold way diabatization method has been applied to transform five adiabatic excited electronic states to a diabatic representation. The diabatic potential energy matrix of the first five excited singlet states has been constructed along several cuts of the potential energy hypersurfaces. The thermochemistry of the photodissociation reactions and a comparison with experimental translational energy distributions strongly suggest that nonadiabatic effects dominate the C-Br scission, but that the reaction proceeds along the energetically allowed diabatic pathway to excited-state products instead of being nonadiabatically suppressed. This conclusion is also supported by the low values of the diabatic couplings on the C-Br scission reaction path. The methodology established in the present study will be used for the construction of global potential energy surfaces suitable for multidimensional dynamics simulations to test these preliminary interpretations.
Physical chemistry chemical physics : PCCP, 2018
The photodissociation dynamics of bromoiodomethane (CHBrI) have been investigated at the maximum of the first A and second A' absorption bands, at 266 and 210 nm excitation wavelengths, respectively, using velocity map and slice imaging techniques in combination with a probe detection of both iodine and bromine fragments, I(P), I*(P), Br(P) and Br*(P) via (2 + 1) resonance enhanced multiphoton ionization. Experimental results, i.e. translational energy and angular distributions, are reported and discussed in conjunction with high level ab initio calculations of potential energy curves and absorption spectra. The results indicate that in the A-band, direct dissociation through the 5A' excited state leads to the I(P) channel while I*(P) atoms are produced via the 5A' → 4A'/4A'' nonadiabatic crossing. The presence of Br and Br* fragments upon excitation to the A-band is attributed to indirect dissociation via a curve crossing between the 5A' with upper excit...
Two-and three-body photodissociation dynamics of diiodobromide (I2Br−) anion
The Journal of chemical physics, 2011
The photodissociation of gas-phase I 2 Br − was investigated using fast beam photofragment translational spectroscopy. Anions were photodissociated from 300 to 270 nm (4.13-4.59 eV) and the recoiling photofragments were detected in coincidence by a time-and position-sensitive detector. Both two-and three-body channels were observed throughout the energy range probed. Analysis of the two-body dissociation showed evidence for four distinct channels: Br − + I 2 , I − + IBr, Br + I − 2 , and I + IBr − . In three-body dissociation, Br( 2 P 3/2 ) + I( 2 P 3/2 ) + I − and Br − + I( 2 P 3/2 ) + I( 2 P 3/2 ) were produced primarily from a concerted decay mechanism. A sequential decay mechanism was also observed and attributed to Br − ( 1 S) + I 2 (B 3 + 0u ) followed by predissociation of I 2 (B).
The Journal of …, 2006
Ab initio potential energy curves, transition dipole moments, and spin-orbit coupling matrix elements are computed for HBr. These are then used, within the framework of time-dependent quantum-mechanical wavepacket calculations, to study the photodissociation dynamics of the molecule. Total and partial integral cross sections, the branching fraction for the formation of excited-state bromine atoms Br( 2 P 1/2 ), and the lowest order anisotropy parameters, , for both ground and excited-state bromine are calculated as a function of photolysis energy and compared to experimental and theoretical data determined previously. Higher order anisotropy parameters are computed for the first time for HBr and compared to recent experimental measurements. A new expression for the Re[a 1
Chemical Physics Letters, 2006
The three-center (3C) and four-center (4C) HBr eliminations in the photodissociation of vinyl bromide at 193 nm were investigated by direct quasiclassical trajectory (QCT) calculations using a semiempirical AM1 Hamiltonian with specific reaction parameters. The trajectories were initiated at the corresponding transition states under microcanonical conditions. The calculations show that the 4C distributions are displaced towards higher J values in comparison with the 3C distributions. The total QCT rotational distributions, i.e., those obtained by weighted sums of the 3C and 4C contributions, do not follow the bimodal pattern obtained elsewhere by time-resolved Fourier transform spectroscopy in a flow system.
Photofragment imaging: the 205-nm photodissociation of CH3Br and CD3Br
Chemical Physics, 1992
The photofragment imaging technique IS used to measure the velocity dtstrtbutions of the Br(2P3,2) and Br*(ZP,,2) atoms formed following laser dtssociation of CH,Br and CD,& at 205 nm. The speed and angular distributions are markedly different for the two bromine electromc states. The Br* images are anisotroptc and have the appearance of a co&? angular dlst~butlon mdrcatrve of a paraRe transttion. The Br ground-state images appear more isotroptc but contam a distinct sm% angular component. The speed distnbuttons extracted from the Br* images are narrow compared with the Br speed dtstnbuttons. This indicates that the sibling methyl fragments formed m the Br dissociatron channel have greater Internal excitation than the methyl fragments formed m the Br* dissociation channel. The results are compared wrth methyl todlde drssociatron and the importance of surface crossmgs m the dissociation ts discussed.