W. Sean McGivern

Photodissociation dynamics of CH2BrCl studied using resonance enhanced multiphoton ionization (REMPI) with time-of-flight mass spectrometry

The photodissociation dynamics of CH2BrCl have been studied using resonance-enhanced multiphoton ionization with time-of-flight mass spectrometry. Polarization dependent time-of-flight profiles were collected for a range of wavelengths from 248 to 268 nm, corresponding to the red wing of the absorption spectrum. Forward convolution fits to the data have provided translational energy distributions and anisotropy parameters over the entire wavelength range for both Br(2P3/2) and Br*(2P1/2). The average translational energies for the Br and Br* channels are 20 and 23 kcal/mol, respectively. The measured anisotropy parameters indicate that both channels arise preferentially from a parallel transition and that the relative contribution of this transition increases with decreasing wavelength. Nonadiabatic transitions appear to play a smaller role in CH2BrCl dissociation than in its monohalogenated analogues, specifically CH3Br. We suggest that this difference is the result of the intrinsic Cs symmetry and lower...

Theoretical study of isomeric branching in the isoprene–OH reaction: implications to final product yields in isoprene oxidation

Abstract The OH initiated oxidation of isoprene is important in tropospheric chemistry and has major implications for local and regional air quality. The reaction of isoprene with OH has been investigated by ab initio molecular orbital theory and unimolecular rate theory calculations. We report the energetics of the four possible adducts and the transition states associated with isomerization, which are energetically comparable to the dissociation of the adduct. We have employed CVTST and RRKM/ME theories based on the ab initio calculations to calculate the rates and branching ratios for the OH–isoprene reaction. The calculated fall-off behavior is consistent with recent experimental measurements.

Isomerization and Decomposition Reactions in the Pyrolysis of Branched Hydrocarbons: 4-Methyl-1-pentyl Radical

The kinetics of the decomposition of 4-methyl-1-pentyl radicals have been studied from 927-1068 K at pressures of 1.78-2.44 bar using a single pulse shock tube with product analysis. The reactant radicals were formed from the thermal C-I bond fission of 1-iodo-4-methylpentane, and a radical inhibitor was used to prevent interference from bimolecular reactions. 4-Methyl-1-pentyl radicals undergo competing decomposition and isomerization reactions via beta-bond scission and 1, x-hydrogen migrations (x = 4, 5), respectively, to form short-chain radicals and alkenes. Major alkene products, in decreasing order of concentration, were propene, ethene, isobutene, and 1-pentene. The observed products are used to validate a RRKM/master equation (ME) chemical kinetics model of the pyrolysis. The presence of the branched methyl moiety has a significant impact on the observed reaction rates relative to analogous reaction rates in straight-chain radical systems. Systems that result in the formation of substituted radical or alkene products are found to be faster than reactions that form primary radical and alkene species. Pressure-dependent reaction rate constants from the RRKM/ME analysis are provided for all four H-transfer isomers at 500-1900 K and 0.1-1000 bar pressure for all of the decomposition and isomerization reactions in this system.

Decomposition and Isomerization of 5-Methylhex-1-yl Radical

The decomposition and isomerization reactions of the 5-methylhex-1-yl radical (1-5MeH) have been studied at temperatures of 889-1064 K and pressures of 1.6-2.2 bar using the single pulse shock tube technique. The radical of interest was generated by shock heating dilute mixtures of 5-methylhexyl iodide to break the weak C-I bond, and the kinetics and reaction mechanism deduced on the basis of the olefin cracking pattern observed by gas chromatographic analysis of the products. In order of decreasing molar yields, alkene products from 1-5MeH decomposition are ethene, isobutene, propene, 3-methylbut-1-ene, but-1-ene, E/Z-hex-2-ene, 4-methylpent-1-ene, and hex-1-ene. The first three products account for almost 90% of the carbon balance. The mechanism involves reversible intramolecular H-transfer reactions that lead to the formation of the radicals 5-methylhex-5-yl (5-5MeH), 5-methylhex-2-yl (2-5MeH), 5-methylhex-4-yl (4-5MeH), 5-methylhex-6-yl (6-5MeH), and 5-methylhex-3-yl (3-5MeH). Competitive with isomerization reactions are decompositions by beta C-C bond scission. The main product forming radical is 5-5MeH, which is formed by intramolecular abstraction of the lone tertiary H in the radical. This reaction is deduced to be a factor of 4.0 +/- 0.7 faster on a per hydrogen basis than the analogous abstraction of a secondary hydrogen in 1-hexyl radical. The estimated uncertainty corresponds to 1 standard deviation. The following relative rates have been deduced under our reaction conditions: k(4-5MeH --> C(2)H(5) + 3-methylbut-1-ene)/k(4-5MeH --> CH(3) + Z-hex-2-ene) = 10((0.39+/-0.12)) exp[(675 +/- 270)K/T]; k(4-5MeH --> C(2)H(5) + 3-methylbut-1-ene)/k(4-5MeH --> CH(3) + E-hex-2-ene) = 10((-0.10+/-0.09)) exp[(1125 +/- 210)K/T]; k(3-5MeH --> iso-C(3)H(7) + but-1-ene)/(k)(3-5MeH --> CH(3) + 4-methylpent-1-ene) = 10((0.26+/-0.55)) exp[(1720 +/- 1300)K/T]. Observed olefin distributions depend on the relative rate constants and the interplay of chemical activation and falloff behavior as the energy distributions of the various radicals relax to steady-state values. A kinetic model using an RRKM/master equation analysis has been developed, and absolute rate expressions have been deduced. The model was used to extrapolate the data to temperatures between 500 and 1900 K and pressures of 0.1-1000 bar, and results for 12 isomerization reactions and 10 beta C-C bond scission reactions are reported.

Photofragment translational spectroscopy with state-selective 'universal detection': the ultraviolet photodissociation of CS2

We have investigated the photodissociation of CS2 at 193 nm using the technique of photofragment translational spectroscopy. The utilization of vacuum ultraviolet synchrotron radiation for product photoionization has permitted a determination of the vibrationally resolved translational energy distribution for the CS+S(1D) channel and the translational energy distribution for the CS+S(3P) channel. A simulation of the coincident S(1D) translational energy distribution is consistent with a CS vibrational distribution of 0.02:0.17:0.19:0.46:0.15 in ν=0:1:2:3:4 and an average rotational energy of ∼1–3 kcal/mol. We find that the S(3P)/S(1D) branching ratio is 3.0±0.2, in good agreement with previous reports. Both asymptotic channels exhibit similar velocity dependent anisotropy parameters that decrease with decreasing translational energy release. The results extend earlier reports and provide further insight into the dissociation dynamics at 193 nm.

Adiabatic and diabatic dynamics in the photodissociation of CH2BrCl

The photodissociation dynamics of chlorobromomethane (CBM) were investigated between 193 and 242 nm by resonance-enhanced multiphoton ionization (REMPI) with time-of-flight mass spectrometry (TOFMS). Translational energy distributions, anisotropy parameters, and Br:Br* branching ratios were determined at 193 and 235 nm to explore the non-adiabatic dynamics near the avoided crossing. Additional measurements were made at intermediate wavelengths to characterize the wavelength dependence of the Br and Br* anisotropy parameters. The non-adiabatic crossing probabilities calculated by applying a one-dimensional Landau–Zener model were relatively insensitive to the excitation wavelength, indicating that the avoided crossing between 3A′ and 4A′ potentials lies in the exit channel. Additionally, we have determined the partial absorption cross sections for the excited states that contribute to the ultraviolet absorption spectrum of CBM.

The ultraviolet photodissociation dynamics of IBr studied using state-selective translational spectroscopy

Abstract We have investigated the ultraviolet photodissociation of IBr using core-sampled state-selective ion time-of-flight mass spectrometry on the iodine and bromine atom products. The branching ratios and anisotropy parameters were determined for the I ( 2 P 3/2 )+ Br ( 2 P 3/2 ) , I ( 2 P 3/2 )+ Br ( 2 P 1/2 ) , and I ( 2 P 1/2 )+ Br ( 2 P 3/2 ) channels at 248, 267, and 304 nm. We find no evidence for the I ( 2 P 1/2 )+ Br ( 2 P 1/2 ) channel at any wavelength. The results provide information on the relative transition probabilities for the 3 Π 1 (2341) , 3 Π 0+ (2341) , and 1 Π 1 (2341) excited states in the absorption band centered at 270 nm. We have further evaluated the nonadiabatic curve crossing probability for the avoided crossing between the 3 Π 0+ (2341) and 3 Σ − 0+ (2422) states over a range of wavelengths from 250 to 270 nm. The wavelength-dependent curve crossing probability for the avoided crossing is modeled using one-dimensional Landau–Zener theory in order to estimate the location of the avoided crossing and the splitting between adiabats. A comparison with recent work at 267 and 304 nm and analogous interhalogen molecules is also provided.

Quantum yields and energy partitioning in the ultraviolet photodissociation of 1,2 dibromo-tetrafluoroethane (Halon-2402)

The photodissociation of 1,2 dibromo-tetrafluoroethane (Halon-2402) has been investigated at 193 nm using photofragment translational spectroscopy with vacuum ultraviolet ionization and at 193, 233, and 266 nm using state-selected translational spectroscopy with resonance-enhanced multiphoton ionization. The product branching ratios, angular distributions, and translational energy distributions were measured at these wavelengths, providing insight into the ultraviolet photodissociation dynamics of CF2BrCF2Br. The total bromine atom quantum yields were found to be 1.9±0.1 at both 193 and 233 nm and 1.4±0.1 at 266 nm. The first C–Br bond dissociation energy was determined to be 69.3 kcal/mol from ab initio calculations. The second C–Br bond dissociation energy was determined to be 16±2 kcal/mol by modeling of the bromine quantum yield. In addition, variational Rice–Ramsperger–Kassel–Marcus theory was used to calculate the secondary dissociation rates for a range of dissociation energies above threshold. The...

Probing the nature of the K-rotor in unimolecular reactions: Scalar and vector correlations in the photodissociation of NCNO

The photodissociation dynamics of thermal NCNO at 520 and 532 nm have been examined using transient frequency modulation Doppler spectroscopy to measure state-selected CN scalar and vector correlations. Previous work has suggested that the global vibrational and rotational distributions may be described using separate statistical ensembles/phase space theory (SSE/PST). We find that the correlated vibrational and rotational distributions are well described by SSE at 520 nm if the K-rotor is considered inactive. At both wavelengths studied, the correlation between the velocity and the rotational angular momentum vector of the CN product is found to be described by phase space theory with no restriction of the projection of the rotational angular momentum vectors along the relative velocity axis. This is indicative of approximate K-scrambling at the transition state, and a discussion of these results in light of the evolution of the K-quantum number is provided.

Temperature-dependent photodissociation dynamics of ICN at 262 nm

Abstract Transient frequency-modulation (FM) Doppler spectroscopy has been used to measure scalar and vector correlations in state-selected CN fragments arising from the photodissociation of ICN at 262 nm at both 296 and 550 K. We find that the state-selected I * /I branching ratio changes significantly with increased temperature, consistent with the increased width of the state-dependent CN rotational distribution. However, we find little change in the state-selected vector properties suggesting that thermal modification of the initial parent vibrational distribution is not a sensitive means to probe the nonadiabatic dynamics.