OD Reaction Research Articles

Infrared chemiluminescence from water-forming reactions: Characterization of dynamics and mechanisms

The formation of water molecules via hydrogen atom abstraction by hydroxyl radicals and their formation via unimolecular elimination from vibrationally excited alcohols and organic acids are important processes in a variety of gas-phase chemical environments. The nascent vibrational distributions of the water molecules produced by such reactions have been obtained by analysis of the infrared chemiluminescence from H 2 O, HOD and D 2 O. The analysis required computer simulation of the spectra obtained by recording emission from a fast-flow reactor at 298 K with a low-resolution Fourier transform spectrometer. By combining the information deduced from simulation of the H 2 O and HOD emission spectra from the reactions of OH and OD, the total vibrational energy released to the water molecule and the distribution between the stretch and bending modes have been assigned. The present report provides a summary of results from hydroxyl radicals reacting with inorganic hydride molecules, with the primary, secondary and tertiary bonds of hydrocarbons, and with oxygen- and sulphur-containing organic molecules. The overall fraction of the available energy released to water as vibrational energy is ­ 0.55 for direct abstraction of hydrogen atoms by OH radicals. This fraction declines as the mechanism becomes less direct and reaches 0.15-0.20 for the unimolecular decomposition of vibrationally excited ethanol and acetic acid. The ratio of the energy in the bending mode to that in the stretch mode is sensitive to the dynamics of water-forming reactions. Macroscopic mechanisms, vibrational energy distributions and reaction dynamics are discussed.

The effect of the symmetric and asymmetric stretching vibrations on the [formula omitted] reaction

We report calculations on the CH 3 D+ O( 3 P)→ CH 3+ OD reaction using a reduced dimensionality model with four degrees of freedom. The model has already been used to evaluate the effect of the symmetric and asymmetric stretching vibrations of methane on the CH 4+ O( 3 P) and CH 4+H reactions. Unfortunately, it is not easy to establish a direct comparison between these previous calculations and experimental results. As the difficulties disappear when CH 4 is replaced with CH 3D we decided to try our model on the title reaction. The results of these new calculations are presented in this Letter.

Ab initio studies of ClOx reactions. I. Kinetics and mechanism for the OH+ClO reaction

The reaction of OH with ClO has been investigated by ab initio molecular orbital and variational transition state theory calculations. Both singlet and triplet potential energy surfaces predicted by the G2M method are presented. The reaction was shown to take place primarily over the singlet surface by two main channels producing HO2+Cl and HCl+O2(1Δ), with the former being dominant. The predicted total rate constant, kt=5.27×10−9 T1.03 exp (−40/T) cm3 molecule−1 s−1, and product branching ratios in the temperature range 200–500 K at P<200 atm agree satisfactorily with experimental values. The computed branching ratios, k2/(k1+k2)=0.073 for HCl+1O2 and 0.045–0.048 for DCl+1O2 in the temperature range 200–500 K based on the recent experimental heat of formation for HO2 (4.0±0.8 kcal/mol) compare closely with the experimental values, 0.07±0.03 and 0.05±0.02, respectively. At higher temperatures (1000–2500 K), the branching ratios increase slightly to 0.084–0.137 and 0.061–0.111 for the OH and OD reactions, respectively. The rate constant for HO2+Cl and HCl+O2 production from OH+ClO in the temperature range, 500−2500 K, can be given by k1=3.4×10−13 T0.3 exp (725/T) and k2=5.85×10−19 T1.67 exp (1926/T) cm3 molecule−1 s−1, respectively.

Kinetics and Mechanism of the OH and OD Reactions with BrO

The kinetics and mechanism of the reactions OH + BrO → products (1) and OD + BrO → products (2) have been studied in the temperature ranges of 230−355 K and 230−320 K, respectively, and at total pr...

Kinetic study of the reactions of OH and OD with HBr and DBr

The kinetics of the reactions of OH and OD radicals with HBr and DBr, OH+HBr (1), OD+HBr (3), OH+DBr (4) and OD+DBr (5), have been studied by the mass spectrometric discharge-flow method at temperatures between 230 and 360K and at total pressure of 1Torr of helium. The following Arrhenius expressions were obtained: k1≈k3=(5.3±1.2)×10−12exp{(225±60)/T} and k4≈k5=(4.3±1.2)×10−12exp{(125±80)/T}cm3s−1 per molecule. The isotope exchange reactions OD+HBr→OH+DBr (3′) and OH+DBr→OD+HBr (4′) were found to be slow and the upper limits for the rate constants of these channels were measured at T=298K: k3′<1×10−13 and k4′<3×10−14cm3s−1 per molecule. These results are compared with the literature experimental and computational data.

Kinetic Study of OH + OH and OD + OD Reactions

The kinetics of the disproportionation reactions of OH and OD radicals, OH + OH → O + H2O (1), OD + OD → O + D2O (3), have been studied by the mass spectrometric discharge-flow method at temperatures between 233 and 360 K and at total pressure of 1 Torr of helium. The following Arrhenius expressions were obtained: k1 = (7.1 ± 1.0) × 10-13exp[(210 ± 40)/T] and k3 = (2.5 ± 0.5) × 10-13exp[(170 ± 60)/T] cm3 molecule-1 s-1. Reaction 4 between OH and OD radicals was also investigated and the same rate constant as for reaction 1 was measured. Both the observed temperature dependence of k1 and the measured kinetic isotopic effect (k1/k3) are compatible with the mechanism proposed in a previous theoretical study (Harding, L. B.; Wagner, A. F. 22nd International Symposium on Combustion, 1988). In addition, a temperature-independent rate coefficient of (1.20 ± 0.25) × 10-10 cm3 molecule-1 s-1 was measured for the reaction D + NO2 → OD + NO in the temperature range 230−365 K.

Product Branching Fractions and Kinetic Isotope Effects for the Reactions of OH and OD Radicals with CH3SH and CH3SD

Product branching fractions and kinetic isotope effects for the reactions of OH and OD radicals with CH3SH and CH3SD were determined from the infrared chemiluminescent spectra of the H2O, HDO and D2O products. The spectra were acquired by a Fourier transform spectrometer that viewed the emission from a discharge flow reactor. Abstraction of H atom from the methyl group accounts for (11 ± 4)% of the total reaction rate from the OD + CH3SD (or OD + CH3SH) reaction and for (24 ± 8)% from the OH + CH3SH (or OH + CH3SD) reaction. The major product channel involves interaction with the sulfur end of the molecule. The difference in branching fractions for OD and OH reactions is due to the large inverse secondary kinetic-isotope effect for the addition−elimination channel, which occurs by transfer of the hydrogen atom from the sulfur atom to the oxygen atom in the adduct. Transition state models for the elimination channel are discussed to show that the experimentally determined secondary kinetic isotope effect f...

Rate Coefficients for the Reactions of OH and OD with HCl and DCl between 200 and 400 K

Rate coefficients for the reaction of OH radical with HCl (k1) between 200 and 400 K, were measured to be 3.28 × 10-17 T1.66 exp [184/T] cm3 molecule-1 s-1 by producing OH via pulsed laser photolysis and detecting it via laser induced fluorescence. The rate coefficients for the reactions of OH with DCl (k2), OD with HCl (k3), and OD with DCl (k4) were also measured using the same method to be k2 = 2.9 × 10-12 exp [−728/T], k3 = 8.1 × 10-18 T1.85 exp [300/T], and k4 = 2.5 × 10-12 exp [−660/T] cm3 molecule-1 s-1. k1 − k4 were computed for 200 < T < 400 K using the variational transition theory, including tunneling corrections, to be k1 = 1.49 × 10-16 T1.35 exp [262/T], k2 = 9.04 × 10-20 T2.34 exp[429/T], k3 = 1.01 × 10-16 T1.4 exp[309/T], and k4 = 1.64 × 10-19 T2.27 exp [385/T]cm3 molecule-1 s-1, in reasonable agreement with experiments. They were also used to analyze the origin of the isotope effects. A two-dimensional chemical transport model calculation shows that our measured higher values of k1 at low ...

Infrared Chemiluminescence Study of the Reactions of Hydroxyl Radicals with Formaldehyde and Formyl Radicals with H, OH, NO, and NO2

The infrared chemiluminescence in the 1800−3900 cm-1 range was observed from vibrationally excited products generated by the reactions of OH and OD with H2CO in a fast flow reactor with 0.5−1.0 Torr of Ar carrier gas. Computer simulation of the emission spectra from H2O and HOD molecules generated by the primary reaction gave an inverted vibrational distribution in the ν3(O−H) stretching mode of HOD, with a maximum population in v3 = 1; the distribution for the ν1 and ν3 stretch modes of H2O was similar. The vibrational energy disposal to H2O and HOD was 〈fv〉 = 0.54−0.56 with 63% in the newly formed OH bond and 34% in the bending mode. This vibrational distribution is characteristic for a direct abstraction mechanism. The excitation in the bending mode exceeds that from OH reactions with hydrocarbons (<20%), but it is similar to that from the reaction of OH with dimethyl sulfide and HBr. By adjustment of the reaction conditions, infrared emission could be observed from secondary reactions of HCO radical with NO2, NO, OH, and H atoms. The vibrational distributions of H2O and HOD from the primary reaction plus the vibrational distributions of CO2 and CO from the NO2 + HCO reaction, HNO from the NO + HCO reaction, H2O and CO from the OH + HCO reaction, and CO from the H + HCO reaction were analyzed using information theory. The results support an addition mechanism followed by unimolecular decomposition for the HCO radical reactions. The vibrational of H2O distribution from OH + HCO is especially noteworthy, since it can be used to distinguish between direct abstraction vs recombination followed by the decomposition of HCOOH.

Vibrational excitation of H2O and HOD molecules produced by reactions of OH and OD with cyclo-C6H12, n-C4H10, neo-C5H12, HCl, DCl and NH3 as studied by infrared chemiluminescence

The room-temperature reactions of OH(OD) radicals with cyclo-C6H12, n-C4H10, and neo-C5H12 have been investigated by observing the infrared chemiluminescence from the H2O(HOD) molecules generated in a fast-flow reactor. These hydrocarbon molecules are representative for abstraction from secondary and primary C–H bonds. The total vibrational energy released to H2O(HOD) was in the range of 〈fv〉=0.55–0.65. The majority (80%–85%) of the vibrational energy is in the stretching modes and the main energy release is to the local mode associated with the new OH bond. The dynamics associated with the energy disposal to H2O(HOD) resemble the H+L−H dynamics for the analogous reactions of F atoms. The data from H2O and HOD are complementary because of the different collisional coupling between the energy levels of the ν1, ν2, and ν3 modes; however, no specific isotope effect was found for the energy disposal to H2O versus HOD for reactions with the hydrocarbon molecules. In contrast, a very unusual isotope effect was found between the OH+HCl and OD+HCl pairs. The latter reaction gave the expected stretching mode excitation of HOD; however, the OH reaction gave H2O molecules with virtually no vibrational energy. This anomalous situation is partly associated with an inverse secondary kinetic-isotope effect, but the main isotope effect is on the dynamics of the energy disposal process itself.

Dynamics of OH and OD radical reactions with HI and GeH4 as studied by infrared chemiluminescence of the H2O and HDO products

The infrared chemiluminescence of vibrationally excited H2O and HDO from the highly exothermic reactions of OH and OD radicals with HI and GeH4 was observed in the 2200–5500 cm−1 range. The experiments utilized a fast-flow reactor with 0.3–1 Torr of Ar carrier gas at 300 K; the OH(OD) radicals were produced via the H(D)+NO2 reaction and the H or D atoms were generated by a discharge in a H2(D2)/Ar mixture. The H2O and HOD vibrational distributions were determined by computer simulation of the emission spectra in the 2200–3900 cm−1 range. The total vibrational energy released to H2O and HOD molecules is, respectively, 〈fv〉=0.36 and 0.41 from HI and 〈fv〉=0.46 and 0.51 from GeH4. These values are significantly smaller than for the reactions of OH and OD with HBr, 〈fv〉=0.61 and 0.65. The populations of the O–H stretching vibration of HOD and the collisionally coupled ν1 and ν3 stretching modes of H2O decrease with increasing vibrational energy. In contrast, the vibrational distribution from the HBr reaction is inverted. The bending mode distributions in all stretching states of H2O and HOD extend to the thermodynamic limit of each reaction. A surprisal analysis was made for H2O(HOD) distributions from the title reactions and compared with that for OH(OD)+HBr. The surprisal analysis tends to confirm that the dynamics for the HI and GeH4 reactions differ from the HBr reaction. The HI reaction may proceed mainly via addition-migration, while the GeH4 reaction may involve both direct abstraction and addition-migration. A rate constant for the OH+GeH4→H2O+GeH3 reaction was evaluated by comparing the H2O emission intensities with that of the OH+HBr→H2O+Br reaction, kGeH4/kHBr=6.5±0.9. Secondary kinetic-isotope effects, kOH/kOD=1.4±0.1, 1.0±0.2, and 1.3±0.2, were determined for reactions of OH and OD with GeH4, HI, and HBr, respectively, by comparing the relative H2O and HOD emission intensities.

Atmospheric fate of several alkyl nitrates Part 1Rate coefficients of the reactions of alkyl nitrates with isotopically labelled hydroxyl radicals

R. K. Talukdar, S. C. Herndon, J. B. Burkholder, J. M. Roberts and A. R. Ravishankara, J. Chem. Soc., Faraday Trans., 1997, 93, 2787 DOI: 10.1039/A701780D

Chemical Dynamics of H Abstraction by OH Radicals: Vibrational Excitation of H2O, HOD, and D2O Produced in Reactions of OH and OD with HBr and DBr

Infrared chemiluminescence from vibrationally excited H2O, HOD, and D2O molecules in the ranges 3200−3900 cm-1 (O−H stretch) and 2400−2900 cm-1 (O−D stretch) was observed from the reactions of OH and OD radicals with hydrogen and deuterium bromide in a fast flow reactor with 0.5−2 Torr of Ar carrier gas at 300 K. Hydroxyl radicals were produced via the H + NO2 reaction; the H atoms were generated by microwave discharge in a H2/Ar mixture. Vibrational distributions for H2O, HOD, and D2O were determined by computer simulation of the experimental emission spectra. The H2O emission from OH + HBr reaction shows inverted populations for both the collisionally coupled stretching modes and the bending mode. Inversion in the bending distribution with a maximum for v2 = 1 is more apparent in the v1,3 = 1 level, which is populated up to the thermochemical limit of v2 = 5. The HOD emission from OD + HBr shows an inverted population in the O−H stretching mode with a maximum for v3 = 2 and shows a decreasing population...

Kinetics of Hydroxyl Radical Reactions with Isotopically Labeled Hydrogen

The rate coefficients for the reactions of hydroxyl radical (OH) with H2 (k1), HD (k2), and D2 (k3) were measured between ∼230 and ∼420 K to be k1= 7.21 × 10-20T2.69 exp(−1150/T), k2 = 5.57 × 10-20T2.7 exp(−1258/T), and k3 = 5.7 × 10-20T2.73 exp(−1580/T) cm3 molecule-1 s-1 using pulsed photolysis to generate OH and laser-induced fluorescence to detect it. Using the same method, the rate coefficients for the reactions of OD with H2 and D2 were measured to be equal to k1 and k3, respectively. In reaction 2, the yield of H was measured to be 0.17 ± 0.03 and 0.26 ± 0.05 at 250 and 298 K, respectively, by detecting it using CW Lyman-α resonance fluorescence. k2 was found to be half the sum of k1 and k3 over the entire temperature range of this study. The quoted uncertainties are at the 95% confidence level and include estimated systematic errors. On the basis of these findings it is suggested that most, if not all, of the reaction in the range of temperatures studied here may be occurring via tunneling of H/D atoms through the barrier.

Primary products of the reactions of fluorine atoms with CH3OH, CH3SH and CH3OCH3 studied with ultraviolet photoelectron spectroscopy

Abstract He I photoelectron spectra have been recorded for the reactions F+CH 3 OH, F+CH 3 SH and F+CH 3 OCH 3 and bands associated with the primary reaction products are identified. The known first photoelectron band of CH 2 OH, with an adiabatic ionization energy (IE) of (7.56±0.01) eV, and a sharp band with an adiabatic IE of (10.72±0.01) eV have been observed early in the F+CH 3 OH reaction. This latter feature is assigned to the first photoelectron band of CH 3 O. The corresponding band of CD 3 O has been observed in the F+CD 3 OH and F+CD 3 OD reactions and is found to be unchanged in shape and position from the first band of CH 3 O. Similarly, the first photoelectron bands of CH 3 S and CH 2 SH have been identified in spectra recorded for the F+CH 3 SH reaction. The CH 3 S first photoelectron band is sharp and has an adiabatic IE of (9.25±0.02) eV whereas the CH 2 SH first band is broad and has an adiabatic IE of (7.52±0.03) eV. These values are in excellent agreement with ionization energies obtained from recent photoionization mass spectrometric studies and molecular orbital calculations on these short-lived molecules. In addition, a broad band observed at low reaction times in the F+CH 3 OCH 3 reaction, with an adiabatic IE of (7.04±0.02) eV, is assigned to the first ionization of the CH 2 OCH 3 radical. The first band of the CD 2 OCD 3 isotopomer has also been identified in the corresponding F+CD 3 OCD 3 reaction. A comparison is made with the results obtained for the F+CH 3 SCH 3 reaction in which the first photoelectron band of CH 2 SCH 3 was recorded.

Vibrational and rotational effects in the Cl+HOD↔HCl+OD reaction

Quantum scattering calculations on the Cl+HODR⇌Cl+OD reaction have been performed at collisional energies up to 1.6 eV. The rotating bond approximation is used. In this method, the OD rotation and HCl vibration as well as the bending motion and OH local stretch of HOD are treated explicitly. Here, the theory is extended to account for thermal HOD reactant rotation. The potential energy surface used has accurate reactant and product rovibrational energy levels, correct bond dissociation energies, and a transition state geometry in accord with ab initio data. Mode selectivity is observed—HOD vibrational stretch energy enhances reaction more than vibrational bend energy. Translational energy enhances reaction more than vibrational stretch energy at low total energies, but not generally at higher total energies. Excitation of the local OH stretch in the reactant HOD produces vibrationally excited HCl product. The OD product rotation depends on the reactant HOD rovibrational state. The OD+HCl(v=0) reaction preferentially produces HOD in the vibrational ground state, while the OD+HCl(v=1) reaction preferentially produces HOD with one quantum of vibrational stretch energy. A calculated OH product rotational distribution for the Cl+H2O reaction agrees quite well with experiment.

Effects of rotational, vibrational, and translational energy on the rate constants for the isotope exchange reactions OH−+D2 and OD−+H2

Rate constants for the isotope exchange reactions of OH− with D2 and OD− with H2 have been measured as a function of average center-of-mass kinetic energy at several temperatures. The reaction of OH− with D2 is slightly exothermic, and the rate constant has a negative temperature dependence. The kinetic energy dependences of the rate constants have minima near 0.1 eV. A strong negative dependence on the D2 rotational temperature was found. The reason for this dependence is unclear at present. In contrast, the reaction of OD− with H2 is slightly endothermic and shows positive dependences on both temperature and kinetic energy. The negative rotational dependence for the reaction of OD− with H2 is not as large as that for OH−+D2, presumably because rotational energy can help overcome the endothermicity in the case of OD−+H2. Vibrational energy is observed to promote reactivity in both reactions.

Effect of reagent vibration on the hydrogen atom + water-d reaction: an example of bond-specific chemistry

OH and OD product branching ratios and internal state distributions have been measured in a state-to-state study of the reaction H+HOD(υ 1 ,υ 2 ,υ 3 )→OH(υ,N)(OD(υ,N))+HD(H 2 ). The HOD molecules are prepared in a specific vibrational level (one or two J states) by infrared excitation of thermal HOD using a tunable optical parametric oscillator. Fast («hot») H atoms are generated by laser photolysis of H1 at 266 nm, and the OH and OD reaction products are probed quantum-state-specifically by laser-induced fluorescence.

Bond-selected reaction of HOD with H atoms

Calculations of vibrationally state-selected cross sections are reported for the H+HOD→H 2+OD reaction. A quantum-mechanical method is used which involves a close-coupling expansion in all three vibrational modes of HOD. It is found that initial vibrational excitation of the OH bond in HOD enhances the reaction much more than excitation of the OD bond. The distributions of vibrational states in the H 2+OD products are found to be closely correlated with the degree of vibrational excitation in the HOD reactant.

ChemInform Abstract: Laser Diagnostics of Nascent Gaseous Product Species Formed in Chemical Reactions on Surfaces

Examples from this laboratory illustrating laser probing of gaseous products formed in chemical reactions on surfaces are taken from the areas of catalytic reactions, silicon etching, and surface photochemistry. The emphasis will be on the unique advantages of laser diagnostics in these dynamical and mechanistic studies in which the chemical reaction product species undergo no collisions after leaving the surface before detection by the laser. UHV experiments on the catalytic oxidation of H2, D2 and NH3 by N02 and H2 by NO2 used the laser-induced fluorescence (LIF) technique to measure the apparent desorption energies of OH radicals and the energy accommodation of the desorbing OH, OD and NO reaction products. The observed different degrees of rotational cooling in the different products will be discussed. In multiphoton ionization mass spectrometry experiments (MPI/MS) spectroscopies of three different MPI processes for the SiF2 radical were studied and two of these were applied to the study of silicon etching by XeF2, F2, and NF3. The only observed gaseous products were SiF4 and the SiF2 radical and their apparent production energies correlated with the case of surface fluorine atom production. In the area of surface photochemistry, an excimer laser was used to photolyze iodobenzene, trimethylgallium, and copper acetylacetonate adsorbed on a cold surface. The time-of-flight (TOF) spectra of the fragments or reaction products were measured by using a mass spectrometer equipped with multiphoton (MPI) or electron (EI) ionization.