http://joenschem.com/yahoo_site_admin/assets/docs/atkinson2000r.93190106.pdf

Atmospheric chemistry of VOCs and NOx

Atmospheric degradation of volatile organic compounds

Gas-phase processes were long thought to be the key formation mechanisms for complex organic molecules in star-forming regions. However, recent experimental and theoretical evidence has cast doubt on the efficiency of such processes. Grain-surface chemistry is frequently invoked as a solution, but until now there have been no quantitative models taking into account both the high degree of chemical complexity and the evolving physical conditions of star-forming regions. Here, we introduce a new gas-grain chemical network, wherein a wide array of complex species may be formed by reactions involving radicals. The radicals we consider (H, OH, CO, HCO, CH3, CH3O, CH2OH, NH, and NH2) are produced primarily by cosmic ray-induced photodissociation of the granular ices formed during the colder, earlier stages of evolution. The gradual warm up of the hot core is crucial to the formation of complex molecules, allowing the more strongly bound radicals to become mobile on grain surfaces. This type of chemistry is capable of reproducing the high degree of complexity seen in Sgr B2(N), and can explain the observed abundances and temperatures of a variety of previously detected complex organic molecules, including structural isomers. Many other complex species are predicted by this model, and several of these species may be detectable in hot cores. Differences in the chemistry of high- and low-mass star formation are also addressed; greater chemical complexity is expected where evolution timescales are longer.

http://chemistry.emory.edu/faculty/widicusweaver/papers/74019.pdf

Complex Chemistry in Star-forming Regions: An Expanded Gas-Grain Warm-up Chemical Model

https://backend.orbit.dtu.dk/ws/files/145750384/Nreview_revised.pdf

Modeling nitrogen chemistry in combustion

New experimental profiles of stable species concentrations are reported for formaldehyde oxidation in a variable pressure flow reactor at initial temperatures of 850–950 K and at constant pressures ranging from 1.5 to 6.0 atm. These data, along with other data published in the literature and a previous comprehensive chemical kinetic model for methanol oxidation, are used to hierarchically develop an updated mechanism for CO/H2O/H2/O2, CH2O, and CH3OH oxidation. Important modifications include recent revisions for the hydrogen–oxygen submechanism (Li et al., Int J Chem Kinet 2004, 36, 565), an updated submechanism for methanol reactions, and kinetic and thermochemical parameter modifications based upon recently published information. New rate constant correlations are recommended for CO + OH = CO2 + H (R23) and HCO + M = H + CO + M (R24), motivated by a new identification of the temperatures over which these rate constants most affect laminar flame speed predictions (Zhao et al., Int J Chem Kinet 2005, 37, 282). The new weighted least-squares fit of literature experimental data for (R23) yields k23 = 2.23 × 105T1.89exp(583/T) cm3/mol/s and reflects significantly lower rate constant values at low and intermediate temperatures in comparison to another recently recommended correlation and theoretical predictions. The weighted least-squares fit of literature results for (R24) yields k24 = 4.75 × 1011T0.66exp(−7485/T) cm3/mol/s, which predicts values within uncertainties of both prior and new (Friedrichs et al., Phys Chem Chem Phys 2002, 4, 5778; DeSain et al., Chem Phys Lett 2001, 347, 79) measurements. Use of either of the data correlations reported in Friedrichs et al. (2002) and DeSain et al. (2001) for this reaction significantly degrades laminar flame speed predictions for oxygenated fuels as well as for other hydrocarbons. The present C1/O2 mechanism compares favorably against a wide range of experimental conditions for laminar premixed flame speed, shock tube ignition delay, and flow reactor species time history data at each level of hierarchical development. Very good agreement of the model predictions with all of the experimental measurements is demonstrated. © 2007 Wiley Periodicals, Inc. 39: 109–136, 2007

A comprehensive kinetic mechanism for CO, CH2O, and CH3OH combustion

Vibrationally highly excited toluene molecules with 52 000 cm−1 of vibrational energy have been prepared by UV laser excitation of the isomer cycloheptatriene and subsequent isomerization. The collisional loss of energy from the excited toluene molecules has been observed directly by monitoring hot UV absorption spectra. Direct measurements of the average energies 〈ΔE〉 transferred per collision have been made for about 60 different bath gases. For complex bath gases, 〈ΔE〉 values appear to be considerably smaller than those derived from earlier indirect measurements. 〈ΔE〉 was found not (or only slightly) to depend on the excitation energy.

Collisional deactivation of vibrationally highly excited polyatomic molecules. II. Direct observations for excited toluene

Saturated laser induced fluorescence is used for the sensitive detection of radicals in high pressure gases. The method and its application to a series of addition reactions of HO radicals in the high pressure regime are described. Experiments between 1 and 150 bar of the bath gas He allow for falloff extrapolations to the high pressure limit of the recombination reactions. Limiting rate constants (in cm3 molecule−1 s−1) of 2.2×10−11 for HO+HO→H2O2, of 3.3×10−11 for HO+NO→HONO, of 7.5×10−11 for HO+NO2→HONO2, and of 9.7×10−13 for HO+CO→HOCO (and H+CO2) are derived at 298 K.

High pressure range of the addition of HO to HO, NO, NO2, and CO. I. Saturated laser induced fluorescence measurements at 298 K

Thermal rate constants of the complex‐forming bimolecular reaction HO+CO■HOCO→H+CO2 were measured between 90 and 830 K in the bath gas He over the pressure range 1–700 bar. In addition, the vibrational relaxation of HO in collisions with CO was studied between 300 and 800 K. HO was generated by laser photolysis and monitored by saturated laser‐induced fluorescence. The derived second‐order rate coefficients showed a pronounced pressure and complicated non‐Arrhenius temperature dependence. Above 650 K, the disappearance of HO followed a biexponential time law, indicating thermal instability of collisionally stabilized HOCO. By analyzing the corresponding results, an enthalpy of formation of HOCO of ΔHof,0=−(205±10) kJ mol−1 was derived. On the basis of energy‐ and angular‐momentum‐dependent rates of HOCO formation, activated complex properties for the addition reaction HO+CO→HOCO were derived from the limiting high‐pressure rate constants; with the limiting low‐pressure rate constants, activated complex pr...

High pressure range of addition reactions of HO. II. Temperature and pressure dependence of the reaction HO+CO⇔HOCO→H+CO2

Vibrationally highly excited ethyl‐cycloheptatriene (41 600 cm−1) and isopropyl‐cycloheptatriene (41 800 cm−1) molecules in the electronic ground state have been prepared by UV laser excitation inducing one‐photon absorption and subsequent internal conversion. Collisional energy loss of these molecules has been followed directly by hot UV absorption spectroscopy. Average energies transferred per collision 〈ΔE〉 have been determined for a series of bath gases. 〈ΔE〉 values were found, for a given bath gas, to increase very slightly with the size of the excited molecule.

Collisional deactivation of vibrationally highly excited polyatomic molecules. III. Direct observations for substituted cycloheptatrienes

We experimentally determined complete falloff curves of the rate constant for the unimolecular decomposition of ethoxy radicals. Two different techniques, laser flash photolysis and fast flow reactor were used both coupled to a detection of C2H5O radicals by laser induced fluorescence. Experiments were performed at total pressures between 0.001 and 60 bar of helium and in the temperature range of 391–471 K. Under these conditions the β-C–C scission (1a) CH3CH2O+M→CH2O+CH3+M is the dominating decomposition channel. From a complete analysis of the experimental falloff curves the low and the high pressure limiting rate constants of k1a,0=[He] 3.3×10-8 exp(-58.5 kJ mol-1/RT) cm3 s-1 and k1a,∞=1.1×1013 exp(-70.3 kJ mol-1/RT) s-1 were extracted. We estimate an uncertainty for the absolute values of these rate constants of ±30%. Preexponential factor and activation energy are significantly lower than previous estimations. The rate constants are discussed in terms of statistical unimolecular rate theory. Excellent agreement between the experimental and the statistically calculated rate constants has been found. BAC-MP4, QCISD(T), or higher level of theory provide a reliable picture of the energy and the structure of the transition state of this radical bond dissociation reaction. On the same theoretical basis we predict the high pressure limiting rate constant for the β-C–H scission (1b) CH3CH2O+M→CH3CHO+H+M of k1b,∞=1.3×1013 exp(-84 kJ mol-1/RT) s-1. Atmospheric implications are discussed.

Horst Hippler

Papers

Collisional deactivation of vibrationally highly excited polyatomic molecules. II. Direct observations for excited toluene

High pressure range of the addition of HO to HO, NO, NO2, and CO. I. Saturated laser induced fluorescence measurements at 298 K

High pressure range of addition reactions of HO. II. Temperature and pressure dependence of the reaction HO+CO⇔HOCO→H+CO2

Collisional deactivation of vibrationally highly excited polyatomic molecules. III. Direct observations for substituted cycloheptatrienes

The thermal unimolecular decomposition rate constants of ethoxy radicals