Photodissociation of the propargyl and propynyl (CD) radicals at 248 and 193 nm (original) (raw)
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Photofragment translational spectroscopy of propargyl radicals at 248 nm
Chemical Physics, 2008
The dissociation dynamics of allene, propyne, and propyne-d 3 at 193 nm were investigated with photofragment translational spectroscopy. Products were either photoionized using tunable VUV synchrotron radiation or ionized with electron impact. Product time-of-flight data were obtained to determine centre-of-mass translational energy (P(E T )) distributions, and photoionization efficiency (PIE) curves were measured for the hydrocarbon products. The two major product channels evident from this study are atomic and molecular hydrogen loss, with a H:H 2 branching ratio of 90:10, regardless of precursor. The P(E T ) distribution for each channel is also largely independent of precursor. Both channels appear to occur following internal conversion to the ground electronic state. The propyne-d 3 results show that there is extensive isotopic scrambling prior to H(D) atom loss, and that the H:D product ratio is approximately unity. The PIE curves for H(D) atom loss from allene, propyne, and propyne-d 3 indicate that the dominant corresponding C 3 H 3 product is the propargyl radical in all cases. There is some evidence from the PIE curves that the dominant C 3 H 2 products from allene and propyne are propadienylidene (H 2 CCC:) and propargylene (HCCCH), respectively.
UV photodissociation dynamics of allyl radical by photofragment translational spectroscopy
The Journal of Chemical Physics, 1998
Photodissociation of the allyl radical, CH 2 CHCH 2 , has been studied using the method of molecular beam photo fragment translational spectroscopy following excitation to the C(2 2 Bl) and A(l 2 Bl) states by 248 and 351 nm photons. Two different primary channels have been detected at 248 nm excitation: H-atom loss (84%) and CH 3 elimination (16%). From the product translational energy distribution and polarization dependence studies dissociation processes from the ground state C 3 Hs potential energy surface are inferred for both wavelengths. At 248 nm there may also be a contribution to the H-atom loss channel from predissociation by a higher electronic excited state. RRKM calculations show that the formation of cyclopropene is not important while formation of allene and methylacetylene from 1-and 2-propenyl radicals dissociation are important reaction pathways at both wavelengths. Evidence for CH 3 elimination directly from an allylic structure through a four-member cyclic transition state is present in the translational energy distribution peaked well away from zero.
Photodissociation Dynamics of the Phenyl Radical via Photofragment Translational Spectroscopy
2010
Photofragment translational spectroscopy was used to study the photodissociation dynamics of the phenyl radical at 193 and 248 nm. Time of flight data collected for the C_6H_4, C_4H_3, and C_2H_2 photofragments show the presence of two decomposition channels. The only C_6H_5 decomposition channel observed at 248 nm corresponds to C-H bond fission from the cyclic radical producing ortho-benzyne. The translational energy distribution peaks at 0 kcal/mol and is consistent with no exit barrier for the H loss process. At 193 nm photodissociation, however, H loss was observed to be the minor channel, while the major decomposition pathway corresponds with decyclization of the C_6H_5 radical and subsequent fragmentation to n-C_4H_3 and C_2H_2. These two momentum matched photofragments have a translational energy distribution that peaks around 9 kcal/mol, indicative of a process that proceeds through a tighter transition state. Previous theoretical work on the unimolecular decomposition of the phenyl radical predicts a second H loss process that occurs after C_6H_5 decyclization resulting in the linear C_6H_4 photofragment. This channel cannot be unambiguously discerned from the C_6H_4^+ time of flight data, but is believed to take place since decyclization is observed. L. K. Madden, L. V. Moskaleva, S. Kristyan, and M. C. Lin J. Phys. Chem. A 1997, 101, 6790.
The Journal of …, 2001
This paper presents product translational energy spectroscopy measurements of the primary photofragmentation channels of 2-chloropropene excited at 193 nm and of the unimolecular dissociation of the 2-propenyl radical. Tunable vacuum ultraviolet ͑VUV͒ photoionization of the products allows us to distinguish between the various product isomers formed in these processes. The data show evidence for three significant primary reaction channels in the dissociation of 2-chloropropene: An excited-state C-Cl fission channel producing fast Cl atoms, a C-Cl fission channel producing slow Cl atoms, and HCl elimination. A minor C-CH 3 fission channel contributes as well. The measured branching of the major primary product channels is: ͓fast C-Cl͔:͓slow C-Cl͔:͓HCl elimination͔ϭ62%:23%:15%. The experiments also allow us to resolve selectively the product branching between the unimolecular dissociation channels of the 2-propenyl radical, a high energy C 3 H 5 isomer; we measure how the branching ratio between the two competing C-H fission channels changes as a function of the radical's internal energy. The data resolve the competition between the unimolecular Hϩallene and Hϩpropyne product channels from the radical with internal energies from 0 to 18 kcal/mol above the Hϩpropyne barrier. We find that the barrier to Hϩallene formation from this high-energy C 3 H 5 radical is higher than the barrier to Hϩpropyne formation, in agreement with recent theoretical calculations but in sharp contrast to that predicted for the most stable C 3 H 5 isomer, the allyl radical. The experiments demonstrate a general technique for selectively forming a particular C n H m isomer dispersed by internal energy due to the primary photolysis, thus allowing us to determine the branching between unimolecular dissociation channels as a function of the selected radical isomer's internal energy.
Photofragment translational spectroscopy of allene, propyne, and propyne-d3 at 193 nm
Molecular Physics, 2005
The dissociation dynamics of allene, propyne, and propyne-d 3 at 193 nm were investigated with photofragment translational spectroscopy. Products were either photoionized using tunable VUV synchrotron radiation or ionized with electron impact. Product time-of-flight data were obtained to determine centre-of-mass translational energy (P(E T )) distributions, and photoionization efficiency (PIE) curves were measured for the hydrocarbon products. The two major product channels evident from this study are atomic and molecular hydrogen loss, with a H:H 2 branching ratio of 90:10, regardless of precursor. The P(E T ) distribution for each channel is also largely independent of precursor. Both channels appear to occur following internal conversion to the ground electronic state. The propyne-d 3 results show that there is extensive isotopic scrambling prior to H(D) atom loss, and that the H:D product ratio is approximately unity. The PIE curves for H(D) atom loss from allene, propyne, and propyne-d 3 indicate that the dominant corresponding C 3 H 3 product is the propargyl radical in all cases. There is some evidence from the PIE curves that the dominant C 3 H 2 products from allene and propyne are propadienylidene (H 2 CCC:) and propargylene (HCCCH), respectively.
Photodissociation Dynamics of the Thiophenoxy Radical at 248, 193, and 157 nm
The Journal of Physical Chemistry A, 2013
The photodissociation dynamics of the thiophenoxy radical (C 6 H 5 S) have been investigated using fast beam coincidence translational spectroscopy. Thiophenoxy radicals were produced by photodetachment of the thiophenoxide anion followed by photodissociation at 248 nm (5.0 eV), 193 nm (6.4 eV), and 157 nm (7.9 eV). Experimental results indicate two major competing dissociation channels leading to SH + C 6 H 4 (o-benzyne) and CS + C 5 H 5 (cyclopentadienyl) with a minor contribution of S + C 6 H 5 (phenyl). Photofragment mass distributions and translational energy distributions were measured at each dissociation wavelength. Transition states and minima for each reaction pathway were calculated using density functional theory to facilitate experimental interpretation. The proposed dissociation mechanism involves internal conversion from the initially prepared electronic excited state to the ground electronic state followed by statistical dissociation. Calculations show that SH loss involves a single isomerization step followed by simple bond fission. For both SH and S loss, C−S bond cleavage proceeds without an exit barrier. By contrast, the CS loss pathway entails multiple transition states and minima as it undergoes five membered ring formation and presents a small barrier with respect to products. The calculated reaction pathway is consistent with the experimental translational energy distributions in which the CS loss channel has a broader distribution peaking farther away from zero than the corresponding distributions for SH loss.
Ultraviolet Photodissociation Dynamics of the 1-Propenyl Radical
The journal of physical chemistry. A, 2016
Ultraviolet (UV) photodissociation dynamics of jet-cooled 1-propenyl radical (CHCHCH3) were investigated at the photolysis wavelengths from 224 to 248 nm using high-n Rydberg atom time-of-flight (HRTOF) technique. The 1-propenyl radicals were produced from 193 nm photolysis of 1-chloropropene and 1-bromopropene precursors. The photofragment yield (PFY) spectra of the H atom product have a broad peak centered at 230 nm. The H + C3H4 product translational energy P(ET) distribution's peak near ∼8 kcal/mol, and the fraction of average translational energy in the total available energy, ⟨fT⟩, is nearly a constant of ∼0.12 from 224 to 248 nm. The H atom product has an isotropic angular distribution with the anisotropy parameter β ≈ 0. Quasiclassical trajectory calculations were also carried out using an ab initio ground-state potential energy surface for dissociation of 1-propenyl at the excitation energy of 124 kcal/mol (230 nm). The calculated branching ratios are 60% to the methyl ...
Fast beam photodissociation of the CH2NO2 radical
The Journal of Chemical Physics, 1993
The photodissociation of the nitromethyl radical, CH,N02, has been studied using a fast beam photofragment translational spectrometer. In these experiments, a fast beam of mass selected, internally cold nitromethyl radicals is formed via negative ion photodetachment of CH,NOF and subsequently dissociated. The recoiling photofragments are detected in coincidence using a microchannel plate detector equipped with a time-and position-sensing anode. Two dissociation product channels are observed at each of three dissociation wavelengths investigated in the range 240-270 nm and are identified as (I) CH,NO, -+ CH,NO + 0 and (II) CH,NO, -+ H&O + NO. In marked contrast to the ultraviolet photodissociation of CH3N02, no evidence is found for simple C-N bond fission to give (III) CH2N02+ CH2 + NOz.. Translational energy and angular distributions were obtained for the two observed channels. The translational energy distribution of channel (I) peaks at only 5-8 kcal/mol, while the distribution for channel (II) peaks at -60 kcal/mol. The angular distributions for both channels are largely isotropic. The nature of the electronic excitation and dissociation dynamics are considered at length. The upper state in the electronic transition is assigned to the 1 'Bi state. Results of attempts to model various aspects of the dissociation dynamics as statistical processes on the ground state surface indicate this mechanism is very unlikely. Instead, both dissociation channels are believed to occur primarily on excited state surfaces, and mechanisms for these processes are proposed.
The Journal of Chemical Physics, 2010
The photodissociation of acetaldehyde in the radical channel has been studied at wavelengths between 315 and 325 nm using the velocity-map imaging technique. Upon one-photon absorption at 315 nm, the molecule is excited to the first singlet excited state S 1 , which, in turn, undergoes intersystem crossing to the first excited triplet state T 1. On the triplet surface, the molecule dissociates into CH 3 and HCO radicals with large kinetic energy release ͑KER͒, in accordance with the well characterized exit barrier on T 1. However, at longer wavelengths ͑Ͼ320 nm͒, which correspond to excitation energies just below the triplet barrier, a sudden change in KER is observed. At these photolysis wavelengths, there is not enough energy to surpass the exit barrier on the triplet state, which leaves the possibility of unimolecular dissociation on S 0 after internal conversion from S 1. We have characterized the fragments' KER at these wavelengths, as well as determined the energy partitioning for the radical fragments. A new accurate estimate of the barrier height on T 1 is presented.
Photodissociation of Propargyl Chloride at 193 nm
The Journal of Physical Chemistry A, 2006
The photodissociation of propargyl chloride (C 3 H 3 Cl) has been studied at 193 nm. Ion imaging experiments with state-selective detection of the Cl atoms and single-photon ionization of the C 3 H 3 radicals were performed, along with measurements of the Cl + C 3 H 3 and HCl + C 3 H 2 recoil kinetic energy distributions, using a scattering apparatus with electron bombardment ionization detection to resolve the competing Cl and HCl elimination channels. The experiments allow the determination of the Cl (2 P 3/2) and Cl (2 P 1/2) (hereafter Cl*) branching fractions associated with the C-Cl bond fission, which are determined to be 0.5 (0.1 for both channels. Although prior translational spectroscopy studies by others had concluded that the low velocity signal at the Cl + mass was due to daughter fragments of the HCl elimination products, the present work shows that Cl atoms are produced with a bimodal recoil kinetic energy distribution. The major C-Cl bond fission channel, with a narrow recoil kinetic energy distribution peaking near 40 kcal/mol, produces both Cl and Cl*, whereas the minor (5%) channel, partitioning much less energy to relative kinetic energy, produces only ground spin-orbit state Cl atoms. The maximum internal energy of the radicals produced in the lowrecoil-kinetic-energy channel is consistent with this channel producing electronically excited propargyl radicals. Finally, in contrast to previous studies, the present work determines the HCl recoil kinetic energy distribution and identifies the possible contribution to this spectrum from propargyl radicals cracking to C 3 + ions in the mass spectrometer.