Publications (10)17.03 Total impact
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Article: Collision dynamics of O(3P) + DMMP using a specific reaction parameters potential form.
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ABSTRACT: Starting from previous benchmark CBS-QB3 electronic structure calculations (Conforti, P. F.; Braunstein, M.; Dodd, J. A. J. Phys. Chem. A 2009, 113, 13752), we develop two global potential energy surfaces for O((3)P) + DMMP collisions, using the specific reaction parameters approach. Each surface is simultaneously fit along the three major reaction pathways: hydrogen abstraction, hydrogen elimination, and methyl elimination. We then use these surfaces in classical dynamics simulations and compute reactive cross sections from 4 to 10 km s(-1) collision velocity. We examine the energy disposal and angular distributions of the reactive and nonreactive products. We find that for reactive collisions, an unusually large amount of the initial collision energy is transformed into internal energy. We analyze the nonreactive and reactive product internal energy distributions, many of which fit Boltzmann temperatures up to ~2000 K.The Journal of Physical Chemistry A 02/2012; 116(10):2506-18. · 2.95 Impact Factor -
Article: Energetics and dynamics of the reactions of O(3P) with dimethyl methylphosphonate and sarin.
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ABSTRACT: Electronic structure and molecular dynamics calculations were performed on the reaction systems O((3)P) + sarin and O((3)P) + dimethyl methylphosphonate (DMMP), a sarin simulant. Transition state geometries, energies, and heats of reaction for the major reaction pathways were determined at several levels of theory, including AM1, B3LYP/6-311+G(d,p), and CBS-QB3. The major reaction pathways for both systems are similar and include H-atom abstraction, H-atom elimination, and methyl elimination, in rough order from low to high energy. The H-atom abstraction channels have fairly low barriers (approximately 10 kcal mol(-1)) and are close to thermoneutral, while the other channels have relatively high energy barriers (>40 kcal mol(-1)) and a wide range of reaction enthalpies. We have also found a two-step pathway leading to methyl elimination through O-atom attack on the phosphorus atom for DMMP and sarin. For sarin, the two-step methyl elimination pathway is significantly lower in energy than the single-step pathway. We also present results of O((3)P) + sarin and O((3)P) + DMMP reaction cross sections over a broad range of collision energies (2-10 km s(-1) collision velocities) obtained using the direct dynamics method with an AM1 semiempirical potential. These excitation functions are intended as an approximate guide to future hyperthermal measurements, which to our knowledge have not yet examined either of these systems. The reaction barriers, reaction enthalpies, transition state structures, and excitation functions are generally similar for DMMP and sarin, with some moderate differences for methyl elimination energetics, which indicates DMMP will likely be a good substitute for sarin in many O((3)P) chemical investigations.The Journal of Physical Chemistry A 10/2009; 113(49):13752-61. · 2.95 Impact Factor -
Article: Hyperthermal atomic oxygen source for near-space simulation experiments.
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ABSTRACT: A hyperthermal atomic oxygen (AO) beam facility has been developed to investigate the collisions of high-velocity AO atoms with vapor-phase counterflow. Application of 4.5 kW, 2.4 GHz microwave power in the source chamber creates a continuous discharge in flowing O(2) gas. The O(2) feedstock is introduced into the source chamber in a vortex flow to constrain the plasma to the center region, with the chamber geometry promoting resonant excitation of the TM(011) mode to localize the energy deposition in the vicinity of the aluminum nitride (AlN) expansion nozzle. The approximately 3500 K environment serves to dissociate the O(2), resulting in an effluent consisting of 40% AO by number density. Downstream of the nozzle, a silicon carbide (SiC) skimmer selects the center portion of the discharge effluent, prior to the expansion reaching the first shock front and rethermalizing, creating a beam with a derived 2.5 km s(-1) velocity. Differential pumping of the skimmer chamber, an optional intermediate chamber and reaction chamber maintains a reaction chamber pressure in the mid-10(-6) to mid-10(-5) Torr range. The beam has been characterized with regard to total AO beam flux, O(2) dissociation fraction, and AO spatial profile using time-of-flight mass spectrometric and Kapton-H erosion measurements. A series of reactions AO+C(n)H(2n) (n=2-4) has been studied under single-collision conditions using mass spectrometric product detection, and at higher background pressure detecting dispersed IR emissions from primary and secondary products using a step-scan Michelson interferometer. In a more recent AO crossed-beam experiment, number densities and predicted IR emission intensities have been modeled using the direct simulation Monte Carlo technique. The results have been used to guide the experimental conditions. IR emission intensity predictions are compared to detected signal levels to estimate absolute reaction cross sections.The Review of scientific instruments 09/2009; 80(9):093104. · 1.52 Impact Factor -
Article: Ab initio energies and product branching ratios for the O+C3H6 reaction.
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ABSTRACT: Intermediate and transition-state energies have been calculated for the O+C3H6 (propene) reaction using the compound ab initio CBS-QB3 and G3 methods in combination with density functional theory. The lowest-lying triplet and singlet potential energy surfaces of the O-C3H6 system were investigated. RRKM statistical theory was used to predict product branching fractions over the 300-3000 K temperature and 0.001-760 Torr pressure ranges. The oxygen atom adds to the C3H6 terminal olefinic carbon in the primary step to generate a nascent triplet biradical, CH3CHCH2O. On the triplet surface, unimolecular dissociation of CH3CHCH2O to yield H+CH3CHCHO is favored over the entire temperature range, although the competing H2CO+CH3CH product channel becomes significant at high temperature. Rearrangement of triplet CH3CHCH2O to CH3CH2CHO (propanal) via a 1,2 H-atom shift has a barrier of 122.3 kJ mol(-1), largely blocking this reaction channel and any subsequent dissociation products. Intersystem crossing of triplet CH3CHCH2O to the singlet surface, however, leads to facile rearrangement to singlet CH3CH2CHO, which dissociates via numerous product channels. Pressure was found to have little influence over the branching ratios under most conditions, suggesting that the vibrational self-relaxation rates for p<or=1 atm are negligible compared to the dissociation rates.The Journal of Physical Chemistry A 12/2007; 111(50):12977-84. · 2.95 Impact Factor -
Article: O + CnH2n Products Detected via IR Emission. 1. O + C2H4
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ABSTRACT: Collisions between oxygen atoms and ethene have been investigated by using infrared emission detection of the chemiluminescent product species. A prototypical alkene, ethene, nonetheless exhibits numerous reaction pathways in reactions with O atoms. Oxygen atoms were formed via photolysis of SO2 in the presence of C2H4, and the resultant IR emissions in the 900−3000-cm-1 spectral region were detected by using a time-resolved, step-scan Fourier transform spectrometer. A Welsh cell mirror arrangement was used to maximize the collection efficiency of the product IR emissions. Vibrationally excited products such as CO, CO2, HCO, and H2CO have been identified, with CO and CO2 being the dominant IR emitters. The time-evolving CO and CO2 spectra have been characterized with respect to the SO2 and C2H4 partial pressures and laser fluence. The rate constants for vibrational relaxation of CO2 high-v population by CnH2n (n = 2−4) are in the mid-10-12 cm3 s-1 range; SO2 is a very inefficient relaxer. A chemical kinetics code has been used to model the chemistry and identify the operative reaction mechanisms, including the effects of secondary chemistry.11/2004; -
Article: Formation of OH(v=0,1) by the Reaction of Fast H with O3
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ABSTRACT: A two-laser, pump−probe experiment has been used to determine the rotational level population distribution of OH(v=0,1) resulting from the reaction of fast (2.3 eV) hydrogen atoms with ozone. A trace amount of H2S in slowly flowing O3 was photolyzed at 193 nm, and the resultant OH was detected using laser-induced fluorescence (LIF). The pump−probe delay time was adjusted in order to verify negligible relaxation of the nascent OH product. Initially, side reactions such as O(1D) + H2S → OH + HS were found to contribute to the OH signals; they were subsequently eliminated by adjusting the reactant concentrations and flow velocity. The resultant OH LIF spectra were corrected for several factors using either known or measured experimental quantities, including OH(A) collisional quenching, baseline drift, and partial saturation of the OH(A−X) absorption lines. Near-gas kinetic rate constants for OH(A,v‘=0,N‘≤25) collisional quenching by O3 were derived. The corrected spectra were fit using a nonlinear least-squares routine to infer individual N-level populations for OH(v=0,1). The spin−orbit (F1 and F2) and λ-doublet (Π(A‘) and Π(A‘ ‘)) populations were inferred in a separate least-squares fit by comparing the intensities of different OH(A−X) rotational branches. The OH v=1:v=0 population ratio is equal to 0.37 ± 0.04. For both v levels the rotational level populations increase gradually with N, with the population in N = 20 about 5 times that in N = 1. The F1:F2 and Π(A‘):Π(A‘ ‘) population ratios are equal to 1.03 ± 0.28 and 1.34 ± 0.20, respectively.09/1999; -
Article: Vibrational relaxation of NO(υ = 1) by oxygen atoms
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ABSTRACT: The rate constant kO(υ = 1) for NO(υ = 1) vibrational relaxation by O has been measured at room temperature using a laser photolysis-laser probe technique. Vibrationally excited NO and relaxer O atoms were formed using 355 nm laser photolysis of a dilute mixture of NO2 in argon bath gas. The time evolution of both the NO(υ = 1) and the O atoms was monitored using laser-induced fluorescence (LIF). The required absolute O-atom densities were obtained through a comparison of O-atom LIF signals from the photolysis source and from a titrated cw microwave source. At early times the O atoms constitute the most important loss mechanism for the nascently produced NO(υ = 1). Possible effects from NO(υ = 1) vibrational ladder-climbing and from thermal expansion have been shown to be minimal. The rate constant kO(υ = 1) = (2.4±0.5)×10−11 cm3 s−1 determined herein is a factor of 2 to 3 lower than the generally accepted value of kO(υ = 1) used in thermospheric modeling. The present value for kO(υ = 1) is the same, within the error bars, as the kO(υ = 2,3) previously measured in this laboratory using an entirely different technique, resonant infrared laser excitation of NO(υ = 0). This result suggests that the collisional relaxation rates are independent of υ. A recent quasiclassical trajectory calculation, in which both allowed NO–O surfaces have been explicitly considered, predicts a collisional relaxation rate which is in good agreement with the present result. The kO(υ = 1) value, along with previously measured rate constants for NO–O high-pressure recombination (krec∞) and isotope exchange (kiso), can serve as a proxy for the rate coefficient kC describing the formation of a long-lived NO2∗ intermediate from O+NO collisions. The present value for kO(υ = 1) is significantly lower, however, than a recent determination of krec∞ and also the value of kC derived from kiso. In the latter case the comparison is not as straightforward. © 1999 American Institute of Physics.The Journal of Chemical Physics 08/1999; 111(8):3498-3507. · 3.33 Impact Factor -
Article: Formation and vibrational relaxation of OH (X 2Πi,v) by O2 and CO2
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ABSTRACT: Time‐resolved OH(X 2Πi,v=1–9) populations have been measured and analyzed to determine parameters relating to formation mechanisms and vibrational relaxation. OH(v) was formed in electron‐irradiated Ar/H2/O3 mixtures containing added O2 or CO2 as relaxer species. OH(v→v−1,v−2) emission was observed using time‐resolved Fourier spectroscopy. Spectra were then fit to determine time‐dependent populations. Population data were analyzed using a single‐quantum relaxation model, but the possible effects of multiquantum relaxation were also considered. The model includes provision for OH(v) production via H+O3→OH(v)+O2 after e‐beam termination, which has been found to have a significant effect on the results. The following relaxation rate constants are obtained: kv=1–6(O2)=1.3±0.4, 2.7±0.8, 5.2±1.5, 8.8±3.0, 17±7, 30±15 (10−13 cm3s−1) and kv=1–4(CO2)=1.8±0.5, 4.8±1.5, 14±5, 28±10 (10−13 cm3s−1), respectively. Two different exponential decay rates are necessary to characterize the time dependence of the inferred H atom concentration. The role of O(1D)+H2→OH+H is also discussed.The Journal of Chemical Physics 10/1991; 95(8):5752-5762. · 3.33 Impact Factor -
Article: NH(X3Sigma-, v=1-3) Formation and Vibrational Relaxation in Electron-Irradiated Ar/N2/H2 Mixtures
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ABSTRACT: The imidogen radical NH is a species of fundamental scientific interest. NH is closely analogous to the isoelectronic triplet species CH2 and ground state O (3P), which have received a great deal of attention. Also, because NH has a relatively small number of electrons, its electronic structure and properties are amenable to calculation. However, aspects of the dynamics of NH remain poorly understood. NH is known to play an important role in the combustion of nitrogenous materials, such as hydrazine fuels. NH has also been observed in astrophysical object, including comets and certain types of stars. Thus, knowledge of NH reaction dynamics is needed to characterize NH in numerous chemical and physical processes.03/1991; -
Article: Vibrational Relaxation of OH(X 2 Pi (sub i), nu = 1-3) by O2
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ABSTRACT: Hydroxyl radical (OH) plays a preeminent role in both the chemistry and photophysics of the earths atmosphere. Stratospheric OH takes part in a number of key reactions, including those which determine the concentration of the halogen-containing species involved in the catalytic destruction of ozone. OH also mediates the relative densities of the odd nitrogen oxides (NO and NO2) and nitric acid. Because the presence of OH is a signature of numerous chemical and photochemical processes, its altitude distribution is of fundamental interest, and substantial effort has been made to develop better remote sensing techniques for OH. The steady-state vibrational populations of hydroxyl in the upper atmosphere are determined by the rates of various processes, including the chemical reactions that form and destroy OH, IR fluorescence from OH, and collisional relaxation of OH by bath gas molecules. Reprints.06/1990;
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2007
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LeTourneau University
Longview, TX, USA
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