Collisional Deactivation Research Articles

A trapped ion mobility enabled Fourier transform ion cyclotron resonance mass spectrometer for infrared ion spectroscopy at FELIX

We report the installation of a new infrared ion spectroscopy platform at the free-electron laser facility FELIX, based on a Bruker SolariX Fourier Transform ion cyclotron resonance (FT-ICR) mass spectrometer equipped with a trapped ion mobility spectrometry (TIMS) stage. The instrument allows one to record infrared multiple-photon dissociation (IRMPD) spectra for mass and mobility selected ions, produced by a range of ion sources. We describe two strategies to achieve consistent overlap between the laser beam and the ion packet. Removing the original ECD cathode significantly enhanced the IR transmission and the overlap with the ion cloud, resulting in improved IRMPD yield per pulse. Enhancement of the photodissociation yield is observed when multiple pulses are used, as there is negligible collisional deactivation in the ultra-high vacuum trapping region of the ICR cell. We compare IRMPD spectra recorded on the new platform with IRMPD spectra of the same species recorded on one of our 3D-quadrupole ion trap platforms. We demonstrate the instrument’s performance using a sample containing a mixture of two trisaccharides. Mobility selection allows us to record individual IR spectra for the two isomeric species. This multi-modal platform, encompassing liquid chromatography, ion mobility spectrometry, ultra-high resolution (tandem) mass spectrometry, infrared ion spectroscopy (and soon also mass spectrometry imaging), is available to users at the FELIX facility.

Relaxation behavior of vibrationally excited N2(X1Σg + v″ = 6) collisions with H2

Relaxation behavior of vibrationally excited N2 (X1Σg + v″ = 6) induced by collisions with H2 has been investigated using coherent anti-Stokes Raman spectroscopy (CARS). The total pressure of the N2–H2 mixture was 500 Torr, and the molar ratios of H2 were 0.3, 0.4, 0.5, 0.6 and 0.8, respectively. The v″ = 6 vibrational state of the electronic ground-state manifold X1Σg + of N2 was selectively excited by overtone pumping, and the population evolution was monitored using CARS spectroscopy. The collisional deactivation rate coefficients of the excited state N2 (v″ = 6) with H2 and N2 are approximately 2.59 × 10−14 cm3s−1 and 1.04 × 10−14 cm3s−1 at 300 K, and 2.57 × 10−14 cm3s−1 and 0.54 × 10−14 cm3s−1 at 320 K, respectively. The relaxation rate coefficient of the N2–H2 collision was approximately 2.5 and 5 times that of the self-relaxation rate coefficient. The experimental results show that the population densities of the (1,2), (2,2), (3,5), and (3,6) levels of H2 have a maximum at 320 K, while the population densities of (2,3) and (2,4) show little change with increasing temperature. Simultaneously, the time-resolved CARS profiles of the vibrational levels v = 6,5,4 by preparing v = 6 of N2 also indicated that a near-resonant multi-quantum relaxation process occurred between N2–H2. The collision-induced population distribution of H2 was observed at molar ratios of 0.3, 0.4, 0.5, 0.6 and 0.8, respectively. The ro-vibrational population distribution of H2 after collision with N2 is given by the CARS signal intensity ratio, and the population of hydrogen molecules at v = 2, 3 vibrational states also provides strong experimental evidence for energy near-resonance collisions between N2–H2.

Theoretical Investigations on the Hydroxyl-Initialized Oxidation of Hexafluoro-2-butyne in the Presence of Oxygen.

Hexafluoro-2-butyne (C4F6) is a potential eco-friendly alternative gas in plasma, refrigerants, and electrical insulation applications. Mechanisms for the reactions of C4F6 with OH/O2 have been revealed in detail using various theoretical methods including ROCBS-Q, RCCSD(T), multireference RS2C, and extrapolations to the complete basis-set limit with Aug-cc-pVnZ (n = T, Q, 5) basis sets. Rate coefficients and product branching ratios were predicted for a wide range of temperatures and pressures using the solution of master equations. The vibrationally adiabatic ground-state barrier for the initial C4F6 + OH association was best estimated to be 1.53-2.26 kcal/mol. Energetically preferable decomposition paths for the conformation-dependent C4F6OH adducts include six-center HF elimination, four-center proton migration, and C-C bond cleavage, but the collisional deactivation is dominant under ambient conditions. The subsequent oxidation of C4F6OH by O2 bifurcates in two orientations and proceeds without any well-defined barrier followed by the successive isomerization/elimination steps, forming perfluorobiacetyl to regenerate OH radicals or trifluoroacetic acid with trifluoroacetyl radicals. The OH-recycling path accounts for a branching ratio of 70% under ambient conditions. Theoretical rate coefficients are in good agreement with the available experimental results. The effect of fluorination on the reactivity of alkynes toward OH/O2 is discussed.

Experimental investigations on rotation–vibration energy transfer in H2–N2 collisions

We investigated the rotational–vibrational impact energy transfer processes in a H2–N2 gas mixture system. The stimulated Raman pumping technique was used to excite H2 molecules to the (1, 7) high rotational states. The population of the H2(1, 7) level was verified by the coherent anti-Stokes Raman (CARS) spectra, the total pressure of the mixture was maintained at 500 Torr and nitrogen with different molar ratios was filled in the sample cell. The collisional deactivation rate coefficients of the excited state H2(1, 7) with H2 and N2 were obtained by fitting the experimental data with the Stern–Volmer equation. The multi-quantum near-resonant rotational relaxation process of H2(1, 7) colliding with N2 was confirmed by the time-resolved CARS profile measurements of H2(v = 1, J = 7, 5, 3) after the excitation of H2(1, 7). The results can provide data reference for atomic and molecular physics, atomic and molecular collisions, rotation and vibration excitation calculation, etc.

Ab Initio and RRKM/Master Equation Analysis of the Photolysis and Thermal Unimolecular Decomposition of Bromoacetaldehyde

Bromoacetaldehyde (BrCH2CHO) is a major stable brominated organic intermediate of the bromine-ethylene addition reaction during the arctic bromine explosion events. Similar to acetaldehyde, which has been recently identified as a source of organic acids in the troposphere, it may be subjected to photo-tautomerization initially forming brominated vinyl compounds. In this study, we investigate the unimolecular reactions of BrCH2CHO under both photolytic and thermal conditions using high-level quantum chemical calculations and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation analysis. The unimolecular decomposition of BrCH2CHO takes place through 14 dissociation and isomerization channels along a potential energy surface involving eight wells. Under the assumption of singlet ground-state potential energy surface-dominated photodynamics, the primary photodissociation yields of BrCH2CHO are investigated under both collision-free and collision energy transfer conditions. At atmospheric pressure and under tropospheric actinic flux conditions at ground level, depending on the assumed collisional energy transfer parameter, 150 cm-1 < ⟨ΔEdown⟩ < 450 cm-1, 78-33% of BrCH2CHO undergoes direct photodissociation instead of collisional deactivation at an excitation wavelength of 320 nm. This is significantly higher than the 14% reported for acetaldehyde, hence indicating a strong effect of bromine substitution on the product photolysis yield that is related to additional favorable Br and HBr forming dissociation channels. In contrast to the overall photodissociation quantum yield, the relative branching fractions of the photodissociation products are less dependent on the collisional energy transfer parameter. For a representative value of ⟨ΔEdown⟩ = 300 cm-1 and an excitation wavelength of 320 nm, with 27% for C-C bond fission, 11% for C-Br bond fission, 7% for HBr elimination, and only below 2% each for a consecutive O-Br fission reaction and the photo-tautomerization channel yielding brominated vinyl alcohol, the photodissociation is markedly different from the acetaldehyde case. Finally, as brominated halogenated compounds are of interest for flame inhibition purposes, thermal multichannel unimolecular rate constants were calculated for temperatures in the range from 500 to 2000 K. At a temperature of 2000 K and ambient pressure, the two main reaction channels are the C-Br and C-C bond fissions, contributing 35 and 43% to the total reaction flux, respectively.

Mechanistic and kinetics study on the reaction of CF3CBrCH2 with OH: A theoretical study

AbstractQuantum chemical method (CCSD(T)/cc‐pVTZ//M06‐2X/6‐311++G(d,p)) is employed to research the CF3CBrCH2 + OH reaction. The results indicate that the reaction takes place through the interaction of the oxygen atom of the OH radical with the middle C and terminal C atom of CF3CBrCH2 generating adduct IM1 (CF3CBrCH2OH) and IM2 (CF3CBrOHCH2), respectively, and then further dissociation or rearrangement to many products. The rate constants have been computed at 10−10 to 1010 Torr and 200–3000 K by RRKM theory for various product pathways. The results show that at 200–800 K, the rate constant for the production of IM1 (CF3CBrCH2OH) by collisional deactivation is dominant; at high temperatures, the production of P1 (CF3CBrCHOH + H) becomes predominate. The predicted data for CF3CBrCH2 + OH agree closely with available experimental value. The total rate constants are independent on pressure and dependent on temperature. The rate equation can be fitted as k(T) = 1.77 × 10 −7T−0.65exp(−4518.77/T) at 200–300 K, 30 Torr of Ar. The atmospheric lifetime of CF3CBrCH2 in OH is around 2.77 days. TD‐DFT computations imply that IM1, IM2, IM3, IM4, IM5, and IM6 will photolyze under the sunlight.

Atmospheric chemistry of CF2ClO2: a theoretical study on mechanisms and kinetics of the CF2ClO2 + HO2 reaction.

The singlet and triplet potential energy surfaces of the HO2 with CF2ClO2 reaction have been probed at the BMC-CCSD/cc-pVTZ level according to the B3LYP/6-311++G(d,p) level obtained geometrical structure. On the singlet PES, the association/dissociation, direct H- abstraction, and SN2 displacement mechanisms have been taken into account. On the triplet PES, SN2 displacement and indirect H- abstraction reaction mechanisms have been investigated and the H- abstraction channel makes more contribution to the CF2ClO2 with HO2 reaction. The rate constants have been computed at 10-10 to 1010 atm and 200-3000 K by RRKM-TST theory. The results show that at T ≤ 600 K, the generation of IM1 (CF2ClO4H) by collisional deactivation is dominant pathway; at high temperatures, the production of P8 (CF2ClOOH + O2(3Σ)) becomes predominate. The predicted data for CF2ClO2 + HO2 agrees closely with available experimental value. Moreover, OH radicals act as inhibitors in the CF2ClOOH→CF2O + HOCl and CF2ClOOH→CFClO + HOF reactions. The dominant products for the reaction of CF2ClOOH + OH are CF2ClO2 + H2O.

Collisional Energy Transfer from Vibrationally Excited Hydrogen Isocyanide.

Collisional deactivation of vibrationally excited hydrogen isocyanide (HNC) by inert gas atoms was characterized using nanosecond time-resolved Fourier transform infrared emission spectroscopy. HNC, with an average nascent internal energy of 25.9 ± 1.4 kcal mol-1, was generated following the 193 nm photolysis of vinyl cyanide (CH2CHCN) and collisionally deactivated with the series of inert atomic gases: He, Ar, Kr, and Xe. Time-dependent IR emission allows simultaneous experimental observation of the ν1 NH and ν3 NC stretch emissions from vibrationally excited HNC. Subsequent spectral fit analysis enables direct determination of the average energy of HNC in each spectrum and therefore a measure of the average energy lost per collision, ⟨ΔE⟩, as a function of internal energy. Collisional deactivation of excited HNC is shown to be relatively efficient, exhibiting ⟨ΔE⟩ values more than an order of magnitude larger than comparably sized molecules at similar internal energies. Furthermore, the lighter inert gases are shown to be more efficient quenchers. Both observations can be qualitatively explained by the momentum gap law modeled through the repulsive force dominated vibration-to-translation energy transfer mechanism. The feasibility of efficient collisional deactivation as a contributing factor to the observed overabundance of astrophysical HNC is discussed.

Reaction kinetics and thermochemistry of the chemically activated and stabilized primary ethyl radical of methyl ethyl sulfide, CH3SCH2CH2•, with O2 to CH3SCH2CH2OO•

AbstractOxidation of methyl ethyl sulfide (CH3SCH2CH3, methylthioethane, MES) under atmospheric and combustion conditions is initiated by hydroxyl radicals, MES radicals, generated after loss of a H atom via OH abstraction, will further react with O2 to form chemically activated and stabilized peroxyl radical adducts. The kinetics of the chemically activated reaction between the CH3SCH2CH2• radical and molecular oxygen are analyzed using quantum Rice‐Ramsperger‐Kassel theory for k(E) with master equation analysis and a modified strong‐collision approach to account for further reactions and collisional deactivation. Thermodynamic properties of reactants, products, and transition states are determined by the B3LYP/6‐31+G(2d,p), M062X/6‐311+G(2d,p), ωB97XD/6‐311+G(2d,p) density functional theory, and CBS‐QB3, G3MP2B3, and G4 composite methods. The reaction of CH3SCH2CH2• with O2 forms an energized peroxy adduct CH3SCH2CH2OO• with a calculated well depth of 34.1 kcal mol−1 at the CBS‐QB3 level of theory. Thermochemical properties of reactants, transition states, and products obtained under CBS‐QB3 level are used for calculation of kinetic parameters. Reaction enthalpies are compared between the methods. The temperature and pressure‐dependent rate coefficients for both the chemically activated reactions of the energized adduct and the thermally activated reactions of the stabilized adducts are presented. Stabilization and isomerization of the CH3SCH2CH2OO• adduct are important under high pressure and low temperature. At higher temperatures and atmospheric pressure, the chemically activated peroxy adduct reacts to new products before stabilization. Addition of the peroxyl oxygen radical to the sulfur atom followed by sulfur‐oxygen double bond formation and elimination of the methyl radical to form S(= O)CCO• + CH3 (branching) is a potentially important new pathway for other alkyl‐sulfide peroxy radical systems under thermal or combustion conditions.

Reaction pathways, kinetics and thermochemistry of the chemically-activated and stabilized primary methyl radical of methyl ethyl sulfide, CH3CH2SCH2•, with 3O2 to CH2CH3SCH2OO•

Oxidation of Methyl ethyl sulfide (CH3SCH2CH3, methylthioethane, MES) under atmospheric and combustion conditions is initiated by reaction with hydroxyl radicals. Methyl ethyl sulfide (MES) radicals generated after losing an H atom via OH abstraction subsequently reacts with O2 to form chemically activated and stabilized peroxyl radical adducts. The kinetics of the chemically activated reaction between the CH2•SCH2CH3 radical and molecular oxygen are analyzed using quantum Rice–Ramsperger–Kassel (QRRK) theory for k(E) with master equation analysis and a modified strong-collision approach to account for further reactions and collisional deactivation. Thermodynamic properties of reactants, products and transition states are determined by the CBS-QB3 composite and M062X/6-311+G(2d, p) DFT methods. The reaction of CH2•SCH2CH3 with O2 forms an energized peroxy adduct •OOCH2SCH2CH3 with a calculated well depth of 26.4 kcal/mol at the CBS-Q//M062x/6-311+g(2d,p) levels of theory. Thermochemical properties of reactants, transition states and products obtained under CBS-QB3 level are used for calculation of the thermochemical and kinetic parameters. The temperature and pressure dependent rate coefficients for both the chemically activated reactions of the energized adduct and the thermally activated reactions of the stabilized adducts are presented. Stabilization and isomerization of the •OOCH2SCH2CH3 adduct are important under high pressure and low temperature. At temperatures between above 600–800 K reactions of the chemically activated peroxy adduct become important relative to stabilization under the atmospheric pressure. Two new pathways are observed, in addition to conventional hydrogen atom transfer reactions from the three carbons to the peroxy oxygen radical. One of these new paths is formation of a Criegee intermediate plus CH2•OO• plus CCS•. A second new path involves the peroxy oxygen radical addition to the sulfur moiety followed by carbon-sulfur bond cleavage with formation of carbon–oxygen and oxygen–sulfur double bonds: CH2O + S•(O)CC. These are potentially important new pathways other alkyl-sulfide peroxy radical systems under thermal or combustion conditions.

Unimolecular Rate Constants versus Energy and Pressure as a Convolution of Unimolecular Lifetime and Collisional Deactivation Probabilities. Analyses of Intrinsic Non-RRKM Dynamics.

Following work by Slater and Bunker, the unimolecular rate constant versus collision frequency, kuni(ω, E), is expressed as a convolution of unimolecular lifetime and collisional deactivation probabilities. This allows incorporation of nonexponential, intrinsically non-RRKM, populations of dissociating molecules versus time, N( t)/ N(0), in the expression for kuni(ω, E). Previous work using this approach is reviewed. In the work presented here, the biexponential f1 exp(- k1 t) + f2 exp(- k2 t) is used to represent N( t)/ N(0), where f1 k1 + f2 k2 equals the RRKM rate constant k( E) and f1 + f2 = 1. With these two constraints, there are two adjustable parameters in the biexponential N( t)/ N(0) to represent intrinsic non-RRKM dynamics. The rate constant k1 is larger than k( E) and k2 is smaller. This biexponential gives kuni(ω, E) rate constants that are lower than the RRKM prediction, except at the high and low pressure limits. The deviation from the RRKM prediction increases as f1 is made smaller and k1 made larger. Of considerable interest is the finding that, if the collision frequency ω for the RRKM plot of kuni(ω, E) versus ω is multiplied by an energy transfer efficiency factor βc, the RRKM kuni(ω, E) versus ω plot may be scaled to match those for the intrinsic non-RRKM, biexponential N( t)/ N(0), plots. This analysis identifies the importance of determining accurate collisional intermolecular energy transfer (IET) efficiencies.

A Theoretical Study of the Photoisomerization of Glycolaldehyde and Subsequent OH Radical-Initiated Oxidation of 1,2-Ethenediol.

It has recently been discovered that carbonyl compounds can undergo UV-induced isomerization to their enol counterparts under atmospheric conditions. This study investigates the photoisomerization of glycolaldehyde (HOCH2CHO) to 1,2-ethenediol (HOCH═CHOH) and the subsequent (•)OH-initiated oxidation chemistry of the latter using quantum chemical calculations and stochastic master equation simulations. The keto-enol tautomerization of glycolaldehyde to 1,2-ethenediol is associated with a barrier of 66 kcal mol(-1) and involves a double-hydrogen shift mechanism to give the lower-energy Z isomer. This barrier lies below the energy of the UV/vis absorption band of glycolaldehyde and is also considerably below the energy of the products resulting from photolytic decomposition. The subsequent atmospheric oxidation of 1,2-ethenediol by (•)OH is initiated by addition of the radical to the π system to give the (•)CH(OH)CH(OH)2 radical, which is subsequently trapped by O2 to form the peroxyl radical (•)O2CH(OH)CH(OH)2. According to kinetic simulations, collisional deactivation of the latter is negligible and cannot compete with rapid fragmentation reactions, which lead to (i) formation of glyoxal hydrate [CH(OH)2CHO] and HO2(•) through an α-hydroxyl mechanism (96%) and (ii) two molecules of formic acid with release of (•)OH through a β-hydroxyl pathway (4%). Phenomenological rate coefficients for these two reaction channels were obtained for use in atmospheric chemical modeling. At tropospheric (•)OH concentrations, the lifetime of 1,2-ethenediol toward reaction with (•)OH is calculated to be 68 h.

Bath Model for N2 + C6F6 Gas-Phase Collisions. Details of the Intermolecular Energy Transfer Dynamics

Relaxation of vibrationally excited C6F6* in a thermalized bath of N2 molecules is studied by condensed-phase chemical dynamics simulations. The average energy of C6F6 as a function of time, ⟨E(t)⟩, was determined using two different models for the N2–C6F6 intermolecular potential, and both gave statistically the same result. A simulation with a N2 bath density of 20 kg/m3 was performed to check the convergence and validate the results obtained previously with a higher bath density of 40 kg/m3. The initial ensemble of C6F6 is nearly monoenergetically excited, but the ensemble acquires as energy distribution P(E) as it is collisionally relaxed. An evaluation of P(E) and the root-mean-square deviation ⟨ΔE2⟩1/2 of P(E), versus time, shows that P(E) first broadens and then narrows. Simulations with the C6F6 vibrational excitation energy of 85.8 kcal/mol, studied experimentally, show that the width of P(E) does not affect the average collisional deactivation rate. The role of the intramolecular vibrational fre...

Singlet Molecular Oxygen Formation by O-Atom Recombination on Fused-Quartz Surfaces

Experimental results demonstrate () formation by catalytic O-atom surface recombination in a room-temperature fused-quartz flow-tube reactor. Resonance-enhanced multiphoton ionization is used to detect () downstream of a nitrogen discharge flow titrated with nitric oxide to introduce oxygen atoms. A calibration procedure based on ozone photodissociation is developed to quantify () resonance-enhanced multiphoton ionization signals. Partial pressures of () in the range of 2.9 to are measured in the ionization cell for O-atom partial pressures of 1.4–2.9 mtorr atomic oxygen introduced at the titration port. () could not be detected; an upper limit for the () partial pressure is one-fifth of the () partial pressure. A simple chemical kinetics model demonstrates that measured pressures cannot be explained by gas-phase chemistry alone and must involve O atoms participating in surface reactions. It is found that collisional deactivation of on the tube walls must be included to satisfactorily model the experimentally observed pressures and trends. Modeling results also suggest that the surface production yield is 10% or more.

Unimolecular reactions of 1,1,1-trichloroethane, 1,1,1-trichloropropane, and 3,3,3-trifluoro-1,1,1-trichloropropane: determination of threshold energies by chemical activation.

The recombination of CCl3 radicals with CH3, CH3CH2, and CF3CH2 radicals was used to generate CH3CCl3, CH3CH2CCl3, and CF3CH2CCl3 molecules with approximately 87 kcal mol(-1) of vibrational energy in a bath gas at room temperature. The competition between collisional deactivation and unimolecular reaction by HCl elimination was used to obtain the experimental rate constants for each molecule. These experimental rate constants were matched to calculated statistical unimolecular rate constants to assign threshold energies to the three HCl elimination reactions. The models needed for the calculations of the rate constants were obtained from molecular structure calculations using density functional theory (DFT) with the hybrid density-functional MO6-2X recommended by Truhlar for transition states. The assigned threshold energies are 52 ± 2, 50 ± 2, and 52 ± 2 kcal mol(-1) for CH3CCl3, CH3CH2CCl3, and CF3CH2CCl3, respectively, and the CH3 and CF3 groups have only a minor effect on the threshold energies for HCl elimination. The DFT calculated threshold energies are in agreement with the experimentally assigned values. The addition of Cl atoms to the same carbon atom lowers the threshold energy for HCl elimination in the CH3CH2Cl, CH3CHCl2, and CH3CCl3 series. This trend, which is the opposite of that for CH3CH2F, CH3CHF2, and CH3CF3, is discussed in terms of transition-state structure and correlated with the relative stabilities of CH3CH2(+), CH3CHCl(+), and CH3CCl2(+) ions; the relative stabilities are based on the hydride affinities obtained from calculations. Comparison of the reactions of CH3CCl3 and CH2ClCHCl2 shows that the threshold energy is much higher for the isomer with chlorine atoms on both carbon atoms.

Low-temperature collisional quenching of NO A(2)Σ(+)(v' = 0) by NO(X(2)Π) and O2 between 34 and 109 K.

We present measurements of collisional fluorescence quenching cross sections of NO(A(2)Σ(+), v' = 0) by NO(X(2)Π) and O2 between 34 and 109 K using a pulsed converging-diverging nozzle gas expansion, extending the temperature range of previous measurements. The thermally averaged fluorescence quenching cross sections for both species show a monotonic increase as temperature decreases in this temperature range, consistent with earlier observations. These new measurements, however, allow discrimination between predictions obtained by extrapolating fits of previous data using different functional forms that show discrepancies exceeding 120% for NO and 160% for O2 at 34 K. The measured self-quenching cross section is 52.9 Å(2) near 112 K and increases to 64.1 Å(2) at 35 K, whereas the O2 fluorescence quenching cross section is 42.9 Å(2) at 109 K and increases to 58.3 Å(2) at 34 K. Global fits of the quenching cross section temperature dependence show that, when including our current measurements, the low temperature behavior of the quenching cross sections for NO and O2 is better described by a parameterization that accounts for the long-range interactions leading to the collisional deactivation via an inverse power law model.

ChemInform Abstract: Chemical Activation in Azide and Nitrene Chemistry: Methyl Azide, Phenyl Azide, Naphthyl Azides, Pyridyl Azides, Benzotriazoles, and Triazolopyridines

AbstractReview: more than 100 refs.

Vibrational kinetics of electronically-excited molecules in nitrogen discharge plasma

A model on formation of the vibrational distribution function of metastable electronically excited states in nitrogen discharge plasma is presented. By means of comparison with the experimental data the rate constants for the reaction have been clarified. It is assumed that the probability of population of states by collisional deactivation of N2(B 3Πg, v′) molecules is proportional to the Franck–Condon factors of correspondent transitions, i.e. that the quenching is essentially vertical. The results of calculations carried out under this assumption are in agreement with measured vibrational distribution at the end of pulsed high current discharge in nitrogen.Study of a variation of the population of vibrational level in the afterglow of streamer discharge in nitrogen at atmospheric pressure has been performed. It is shown that the dynamics of number density depends essentially not only on their loss in the pooling reaction product, but also on additional population due to the quenching of high vibrational levels. The fraction of dissociation channel in the process of deactivation by oxygen molecules have been calculated. It is shown that temporal evolution of the fraction of the dissociation channel is first of all associated with a variation of the relative population of the vibrational level . In air discharge plasma at high pressure, the population of state is relatively low, so the fraction of the dissociation channel in the deactivation of all electronically excited states of nitrogen by oxygen exceeds 90–95%.

Unimolecular Isomerization of CH2FCD2Cl via the Interchange of Cl and F Atoms: Assignment of the Threshold Energy to the 1,2-Dyotropic Rearrangement

The room-temperature gas-phase recombination of CH2F and CD2Cl radicals was used to prepare CH2FCD2Cl molecules with 91 kcal mol(-1) of vibrational energy. Three unimolecular processes are in competition with collisional deactivation of CH2FCD2Cl; HCl and DF elimination to give CHF═CD2 and CH2═CDCl plus isomerization to give CH2ClCD2F by the interchange of F and Cl atoms. The Cl/F interchange reaction was observed, and the rate constant was assigned from measurement of CHCl═CD2 as a product, which is formed by HF elimination from CH2ClCD2F. These experiments plus previously published results from chemically activated CH2ClCH2F and electronic structure and RRKM calculations for the kinetic-isotope effects permit assignment of the three rate constants for CH2FCD2Cl (and for CH2ClCD2F). The product branching ratio for the interchange reaction versus elimination is 0.24 ± 0.04. Comparison of the experimental rate constant with the RRKM calculated rate constant permitted the assignment of a threshold energy of 62 ± 3 kcal mol(-1) for this type-1 dyotropic rearrangement. On the basis of electronic structure calculations, the nature of the transition state for the rearrangement reaction is discussed. The radical recombination reactions in the chemical system also generate vibrationally excited CD2ClCD2Cl and CH2FCH2F molecules, and the rate constants for DCl and HF elimination were measured in order to confirm that the photolysis of CD2ClI and (CH2F)2CO mixtures was giving reliable data for CH2FCD2Cl.

Study of collisional deactivation of O2(b 1Σ g + ) molecules in a hydrogen-oxygen mixture at high temperatures using laser-induced gratings

Collisional deactivation of O2(b 1Σ g + ) molecules resonantly excited by a 10 ns pulse of laser radiation with a wavelength of 762 nm in H2/O2 mixtures is experimentally studied. The radiation intensity and hence the molecule excitation efficiency have a spatially periodic modulation that leads to the formation of laser-induced gratings (LIGs) of the refractive index. The study of LIG temporal evolution allows collisional relaxation rates of molecular excited states and gas temperature to be determined. In this work, the b 1Σ g + state of O2 molecules deactivation rates are measured in a 4.3 vol % H2 mixture at the number density of 2 amg in the temperature range 291–850 K. The physical deactivation is shown to dominate in the collisions of H2 with O2(b 1Σ g + ) and O2(a 1Δ g ) up to temperatures of 780–790 K at time delays up to 10 μs after the excitation pulse. The parameters of the obtained temperature dependence of the (b 1Σ g + state deactivation rate agree well with the data of independent measurements performed earlier at lower temperatures (200–400 K). Tunable diode laser absorption spectroscopy is used to measure the temperature dependence of the number density of the H2O molecules which appear as the mixture, as the result of the dark gross reaction with O2 molecules in the ground state, O2 + 2H2 → 2H2O. The measurements show that this reaction results in complete transformation of H2 into H2O at temperatures of 790–810 K.