TWO-DIMENSIONAL DISTRIBUTIONS OF TEMPERATURE AND OH, C_2 AND CH RADICALS IN FLAME DETECTED BY LASERINDUCED FLUORESCENCE

The in situ, high spatial resolution, two-dimensional distributions of OH(X), C_2(X) and CH(X) radicals in premixed paraffin/air flames are first reported. The measurements were done with the laser-induced fluorescence spectroscopies (LIFS) of the transitions of OH(A~Σ~+←→X~2Π), C_2(A~3Π←→X~3Π) and CH(B~2Σ←→X~2Π), (A~2△→X~2Π). The results show that OH(X) populated the edge of the outer flame, while C_2(X) and CH(X) the edge of the inner cone of the flame.The local rotational temperature of OH radical for a small zone was detected by the saturated laser-induced fluorescence spectroscopy of A~2Σ~+, v′=0←→X~2Π, v=″0 transition. The two-dimensional temperature distribution for the flame was measured through moving the position of the burner.     

Theoretical study on the atmospheric reaction of CH3O2 with OH

A quantum chemical investigation on the reaction mechanism of CH₃O₂ with OH has been performed. Based on B3LYP and QCISD(T) calculations, seven possible singlet pathways and seven possible triplet pathways have been found. On the singlet potential energy surface (PES), the most favorable channel starts with a barrierless addition of O atom to CH₃O₂ leading to CH₃OOOH and then the O? O bond dissociates to give out CH₃O + HO₂. On the triplet PES, the calculations indicate that the dominant products should be ³CH₂O₂ + H₂O with an energy barrier of 29.95 kJ/mol. The results obtained in this work enrich the theoretical information of the title reaction and provide guidance for analogous atmospheric chemistry reactions. © 2015 Wiley Periodicals, Inc.     

Kinetics of the Gas-Phase Reaction of Hydroxyl Radicals with Dimethyl Methylphosphonate (DMMP) over an Extended Temperature Range (273–837 K)

The kinetics of the reaction of hydroxyl radical (OH) with dimethyl methylphosphonate (DMMP, (CH₃O)₂CH₃PO) (reaction 1) OH + DMMP → products (1) was studied at the bath gas (He) pressure of 1 bar over the 295–837 K temperature range. Hydroxyl radicals were produced in the fast reaction of electronically excited oxygen atoms O(¹D) with H₂O. The time-resolved kinetic profiles of hydroxyl radicals were recorded via UV absorption at around 308 nm using a DC discharge H₂O/Ar lamp. The reaction rate constant exhibits a pronounced V-shaped temperature dependence, negative in the low temperature range, 295–530 K (the rate constant decreases with temperature), and positive in the elevated temperature range, 530–837 K (the rate constant increases with temperature), with a turning point at 530 ± 10 K. The rate constant could not be adequately fitted with a standard 3-parameter modified Arrhenius expression. The data were fitted with a 5-parameter expression as: k₁ = 2.19 × 10⁻¹⁴(T/298)^2.43exp(15.02 kJ mol⁻¹/RT) + 1.71 × 10⁻¹⁰exp(−26.51 kJ mol⁻¹/RT) cm³molecule⁻¹s⁻¹ (295–837 K). In addition, a theoretically predicted pressure dependence for such reactions was experimentally observed for the first time.     

An experimental and theoretical study on the kinetics of the reaction between 4‐hydroxy‐3‐hexanone CH3CH2C(O)CH(OH)CH2CH3 and OH radicals

Experimental and theoretical rate coefficients are determined for the first time for the reaction of 4‐hydroxy‐3‐hexanone (CH₃CH₂C(O)CH(OH)CH₂CH₃) with OH radicals as a function of temperature. Experimental studies were carried out using two techniques. Absolute rate coefficients were measured using a cryogenically cooled cell coupled to the pulsed laser photolysis‐laser‐induced fluorescence technique with temperature and pressure ranges of 280‐365 K and 5‐80 Torr, respectively. Relative values of the studied reaction were measured under atmospheric pressure in the range of 298‐354 K by using a simulation chamber coupled to a FT‐IR spectrometer. In addition, the reaction of 4H3H with OH radicals was studied theoretically by using the density functional theory method over the range of 278‐350 K. Results show that H‐atom abstraction occurs more favorably from the C–H bound adjacent to the hydroxyl group with small barrier height. Theoretical rate coefficients are in good agreement with the experimental data. A slight negative temperature dependence was observed in both theoretical and experimental works. Overall, the results are deliberated in terms of structure–reactivity relationship and atmospheric implications.     

Parity‐Violating Potentials for the Torsional Motion of Methanol (CH3OH) and Its Isotopomers (CD3OH, 13CH3OH,CH3OD,CH3OT,CHD2OH,and CHDTOH)

We present calculations on the parity‐conserving and the parity‐violating potentials in several MeOH isotopomers for the torsional motion by the newly developed methods of electroweak quantum chemistry from our group. The absolute magnitudes of the parity‐violating potentials for MeOH are small compared to H₂O₂ and C₂H₄, but similar to C₂H₆, which is explained by the high (threefold) symmetry of the torsional top in MeOH and C₂H₆. ‘Chiral’ and ‘achiral’ isotopic substitutions in MeOH lead to small changes only, but vibrational averaging is discussed to be important in all these cases. Simple isotopic sum rules are derived to explain and predict the relationships between parity‐violating potentials in various conformations and configurations of the several isotopomers investigated. The parity‐violating energy difference Δ_pvE=E_pv(R)?E_pv(S) between the enantiomers of chiral CHDTOH, first synthesized by Arigoni and co‐workers, is for two conformers ca. ?3.66?10^?17 and for the third one +7.32?10^?17 hc cm^?1. Thus, for Δ_pvE, the conformation is more important than the configuration (at the equilibrium geometries, without vibrational averaging). Averaging over torsional tunneling may lead to further cancellation and even smaller values.     

Rate coefficients for reactions of OH radicals with CH3D,CH2D2, CHD3, and CD4

Fourier transform infrared (FTIR) smog chamber techniques were used to investigate the atmospheric chemistry of the isotopologues of methane. Relative rate measurements were performed to determine the kinetics of the reaction of the isotopologues of methane with OH radicals in cm³ molecule⁻¹ s⁻¹ units: k(CH₃D + OH) = (5.19 ± 0.90) × 10⁻¹⁵, k(CH₂D₂ + OH) = (4.11 ± 0.74) × 10⁻¹⁵, k(CHD₃ + OH) = (2.14 ± 0.43) × 10⁻¹⁵, and k(CD₄ + OH) = (1.17 ± 0.19) × 10⁻¹⁵ in 700 Torr of air diluent at 296 ± 2 K. Using the determined OH rate coefficients, the atmospheric lifetimes for CH_4–xD_x (x = 1–4) were estimated to be 6.1, 7.7, 14.8, and 27.0 years, respectively. The results are discussed in relation to previous measurements of these rate coefficients.     

Dissociative photoionization mechanism of methanol isotopologues (CH3OH, CD3OH, CH3OD and CD3OD) by iPEPICO: energetics, statistical and non-statistical kinetics and isotope effects

The dissociative photoionization of energy selected methanol isotopologue (CH(3)OH, CD(3)OH, CH(3)OD and CD(3)OD) cations was investigated using imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy. The first dissociation is an H/D-atom loss from the carbon, also confirmed by partial deuteration. Somewhat above 12 eV, a parallel H(2)-loss channel weakly asserts itself. At photon energies above 15 eV, in a consecutive hydrogen molecule loss to the first H-atom loss, the formation of CHO(+)/CDO(+) dominates as opposed to COH(+)/COD(+) formation. We see little evidence for H-atom scrambling in these processes. In the photon energy range corresponding to the B[combining tilde] and C[combining tilde] ion states, a hydroxyl radical loss appears yielding CH(3)(+)/CD(3)(+). Based on the branching ratios, statistical considerations and ab initio calculations, this process is confirmed to take place on the first electronically excited ?(2)A' ion state. Uncharacteristically, internal conversion is outcompeted by unimolecular dissociation due to the apparently weak Renner-Teller-like coupling between the X[combining tilde] and the ? ion states. The experimental 0 K appearance energies of the ions CH(2)OH(+), CD(2)OH(+), CH(2)OD(+) and CD(2)OD(+) are measured to be 11.646 ± 0.003 eV, 11.739 ± 0.003 eV, 11.642 ± 0.003 eV and 11.737 ± 0.003 eV, respectively. The E(0)(CH(2)OH(+)) = 11.6454 ± 0.0017 eV was obtained based on the independently measured isotopologue results and calculated zero point effects. The 0 K heat of formation of CH(2)OH(+), protonated formaldehyde, was determined to be 717.7 ± 0.7 kJ mol(-1). This yields a 0 K heat of formation of CH(2)OH of -11.1 ± 0.9 kJ mol(-1) and an experimental 298 K proton affinity of formaldehyde of 711.6 ± 0.8 kJ mol(-1). The reverse barrier to homonuclear H(2)-loss from CH(3)OH(+) is determined to be 36 kJ mol(-1), whereas for heteronuclear H(2)-loss from CH(2)OH(+) it is found to be 210 kJ mol(-1).     

Theoretical study and rate constant calculation for reaction of CF(3)CH(2)OH with OH

The reaction mechanism of CF(3)CH(2)OH with OH is investigated theoretically and the rate constants are calculated by direct dynamics method. The potential energy surface (PES) information, which is necessary for dynamics calculation, is obtained at the B3LYP/6-311G (d, p) level. The single-point energy calculations are performed at the MC-QCISD level using the B3LYP geometries. Complexes, with the energies being less than corresponding reactants and products, are found at the entrance and exit channels for methylene-H-abstraction channel, while for the hydroxyl-H-abstraction channel only entrance complex is located. By means of isodesmic reactions, the enthalpies of the formation for the species CF(3)CH(2)OH, CF(3)CHOH, and CF(3)CH(2)O are estimated at the MC-QCISD//B3LYP/6-311G (d, p) level of theory. The rate constants for two kinds of H-abstraction channels are evaluated by canonical variational transition state theory with the small-curvature tunneling correction (CVT/SCT) over a wide range of temperature 200-2000 K. The calculated results are in good agreement with the experimental values in the temperature region 250-430 K. The present results indicate that the two channels are competitive. Below 289 K, hydroxyl-H-abstraction channel has more contribution to the total rate constants than methylene-H-abstraction channel, while above 289 K, methylene-H-abstraction channel becomes more important and then becomes the major reaction channel.     

Direct ab initio dynamics study of the reaction of C2(A3Πu) with CH4

The reaction of C₂(A³Π_u) with CH₄ has been investigated over a wide temperature range 200–3,000 K by direct ab initio dynamics method at the BMC‐CCSD//BB1K/6‐311+G(2d,2p) level of theory. The optimized geometries and frequencies of the stationary points are calculated at the BB1K/6‐311+G(2d,2p) level, and then the energy profiles of the reactions are refined using the BMC‐CCSD method. The activation barrier height for H‐abstraction reaction was calculated to be 4.44 kcal/mol in temperature range (337–605 K), and the electron transfer behavior was also analyzed by quasi‐restricted molecular orbital method in detail. The canonical variational transition‐state theory (CVT) with the small curvature tunneling (SCT) correction method is used to calculate the rate constants over a wide temperature range 200–3,000 K. The theoretical results shows that variational effect is to some extent large in lower temperature range, and small curvature and tunneling effect play important roles to the H‐atom abstraction only at lower temperatures. The CVT/SCT rate constants are in good agreement with the available experimental results. Our theoretical study is expected to provide a direct insight into the reaction mechanism and may be useful for estimating the kinetics of the title reaction over a wide temperature range where no experimental data are available so far. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

The OH* + CH3SH reaction: support for an addition-elimination mechanism from ab initio calculations

Several intermediates for the CH(3)SH + OH(*) --> CH(3)S(*) + H(2)O reaction were identified using MP2(full) 6-311+g(2df,p) ab initio calculations. An adduct, CH(3)S(H)OH(*), I, with electronic energy 13.63 kJ mol(-1) lower than the reactants, and a transition state, II(double dagger), located 5.14 kJ mol(-1) above I, are identified as the entrance channel for an addition-elimination reaction mechanism. After adding zero-point and thermal energies, DeltaH(r,298) ( degrees )(reactants --> I) = -4.85 kJ mol(-1) and DeltaH(298) (double dagger)(I --> II(double dagger)) = +0.10 kJ mol(-1), which indicates that the potential energy surface is broad and flat near the transition state. The calculated imaginary vibrational frequency of the transition state, 62i cm(-1), is also consistent with an addition-elimination mechanism. These calculations are consistent with experimental observations of the OH(*) + CH(3)SH reaction that favored an addition-elimination mechanism rather than direct hydrogen atom abstraction. An alternative reaction, CH(3)SH + OH(*) --> CH(3)SOH + H(*), with DeltaH(r,298) ( degrees ) = +56.94 kJ mol(-1) was also studied, leading to a determination of DeltaH(f,298) ( degrees )(CH(3)SOH) = -149.8 kJ mol(-1).     

Temperature-dependent rate coefficients for the gas-phase OH + furan-2,5-dione (C4H2O3, maleic anhydride) reaction

Rate coefficients, k₁(T), for the gas-phase reaction of the OH radical with furan-2,5-dione (maleic anhydride (MA), C₄H₂O₃), a biomass burning related compound, were measured under pseudo–first-order conditions in OH using the pulsed laser photolysis–laser-induced fluorescence method over a range of temperature (283-374 K) and bath gas pressure (50-200 Torr; He or N₂). k₁(T) was found to be independent of pressure over this range with k₁(283-374 K) = (1.55 ± 0.20) × 10⁻¹² exp[(−410 ± 44)/T) cm³ molecule⁻¹ s⁻¹ and k₁(296 K) = (3.93 ± 0.28) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, where the uncertainties are 2σ and the preexponential term includes the estimated systematic error. The atmospheric lifetime of MA with respect to OH reactive loss is estimated to be ∼15 days. The present results are compared with a previous room temperature relative rate study of the OH + MA reaction, and the significant discrepancy between the studies is discussed; the present results are approximately a factor of 4 lower. It is also noteworthy that the experimentally measured k₁(296 K) value obtained in this work is nearly a factor of 110 less than estimated by a structure activity relationship based on trends in ionization potential. Based in part on a computational evaluation, an atmospheric degradation mechanism of MA is proposed.     

Secondary kinetics of methanol decomposition: theoretical rate coefficients for 3CH2 + OH, 3CH2 + 3CH2, and 3CH2 + CH3

Direct variable reaction coordinate transition state theory (VRC-TST) rate coefficients are reported for the (3)CH(2) + OH, (3)CH(2) + (3)CH(2), and (3)CH(2) + CH(3) barrierless association reactions. The predicted rate coefficient for the (3)CH(2) + OH reaction (approximately 1.2 x 10(-10) cm(3) molecule(-1) s(-1) for 300-2500 K) is 4-5 times larger than previous estimates, indicating that this reaction may be an important sink for OH in many combustion systems. The predicted rate coefficients for the (3)CH(2) + CH(3) and (3)CH(2) + (3)CH(2) reactions are found to be in good agreement with the range of available experimental measurements. Product branching in the self-reaction of methylene is discussed, and the C(2)H(2) + 2H and C(2)H(2) + H2 products are predicted in a ratio of 4:1. The effect of the present set of rate coefficients on modeling the secondary kinetics of methanol decomposition is briefly considered. Finally, the present set of rate coefficients, along with previous VRC-TST determinations of the rate coefficients for the self-reactions of CH(3) and OH and for the CH(3) + OH reaction, are used to test the geometric mean rule for the CH(3), (3)CH(2), and OH fragments. The geometric mean rule is found to predict the cross-combination rate coefficients for the (3)CH(2) + OH and (3)CH(2) + CH(3) reactions to better than 20%, with a larger (up to 50%) error for the CH(3) + OH reaction.     

Atmospheric chemistry of propionaldehyde: kinetics and mechanisms of reactions with OH radicals and Cl atoms, UV spectrum, and self-reaction kinetics of CH3CH2C(O)O2 radicals at 298 K

The kinetics and mechanism of the reactions of Cl atoms and OH radicals with CH3CH2CHO were investigated at room temperature using two complementary techniques: flash photolysis/UV absorption and continuous photolysis/FTIR smog chamber. Reaction with Cl atoms proceeds predominantly by abstraction of the aldehydic hydrogen atom to form acyl radicals. FTIR measurements indicated that the acyl forming channel accounts for (88 +/- 5)%, while UV measurements indicated that the acyl forming channel accounts for (88 +/- 3)%. Relative rate methods were used to measure: k(Cl + CH3CH2CHO) = (1.20 +/- 0.23) x 10(-10); k(OH + CH3CH2CHO) = (1.82 +/- 0.23) x 10(-11); and k(Cl + CH3CH2C(O)Cl) = (1.64 +/- 0.22) x 10(-12) cm3 molecule(-1) s(-1). The UV spectrum of CH3CH2C(O)O2, rate constant for self-reaction, and rate constant for cross-reaction with CH3CH2O2 were determined: sigma(207 nm) = (6.71 +/- 0.19) x 10(-18) cm2 molecule(-1), k(CH3CH2C(O)O2 + CH3CH2C(O)O2) = (1.68 +/- 0.08) x 10(-11), and k(CH3CH2C(O)O2 + CH3CH2O2) = (1.20 +/- 0.06) x 10(-11) cm3 molecule(-1) s(-1), where quoted uncertainties only represent 2sigma statistical errors. The infrared spectrum of C2H5C(O)O2NO2 was recorded, and products of the Cl-initiated oxidation of CH3CH2CHO in the presence of O2 with, and without, NO(x) were identified. Results are discussed with respect to the atmospheric chemistry of propionaldehyde.     

High-temperature rate constants for CH3OH + Kr --> products, OH + CH3OH --> products, OH + (CH3)(2)CO --> CH2COCH3 + H2O, and OH + CH3 --> CH) + H2O

The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm (corresponding to a total path length of approximately 4.9 m) has been used to study the dissociation of methanol between 1591 and 2865 K. Rate constants for two product channels [CH3OH + Kr --> CH3 + OH + Kr (1) and CH3OH + Kr --> 1CH2 + H2O + Kr (2)] were determined. During the course of the study, it was necessary to determine several other rate constants that contributed to the profile fits. These include OH + CH3OH --> products, OH + (CH3)2CO --> CH2COCH3 + H2O, and OH + CH3 --> 1,3CH2 + H2O. The derived expressions, in units of cm(3) molecule(-1) s(-1), are k(1) = 9.33 x 10(-9) exp(-30857 K/T) for 1591-2287 K, k(2) = 3.27 x 10(-10) exp(-25946 K/T) for 1734-2287 K, kOH+CH3OH = 2.96 x 10-16T1.4434 exp(-57 K/T) for 210-1710 K, k(OH+(CH3)(2)CO) = (7.3 +/- 0.7) x 10(-12) for 1178-1299 K and k(OH+CH3) = (1.3 +/- 0.2) x 10(-11) for 1000-1200 K. With these values along with other well-established rate constants, a mechanism was used to obtain profile fits that agreed with experiment to within <+/-10%. The values obtained for reactions 1 and 2 are compared with earlier determinations and also with new theoretical calculations that are presented in the preceding article in this issue. These new calculations are in good agreement with the present data for both (1) and (2) and also for OH + CH3 --> products.     

A density functional theory study on the atmospheric reaction of CH3O2 with HS: Mechanism and kinetics

The reaction mechanism of CH₃O₂ and HS was systematically investigated by density functional theory (DFT). Six singlet pathways and seven triplet ones are located on the potential surface (PES). The result indicates that the main products are CH₃O and HSO both on the singlet and triplet PES, different from the CH₃O₂ + OH reaction. Moreover, deformation density (ρ_def) and atoms in molecules (AIM) analyses were carried out to further uncover the nature of chemical bonding evolution in the primary pathways. Furthermore, reaction rate constants were calculated in the temperature range from 200 to 1000 K using the transition state theory with the Wigner and Eckart tunneling corrections. Our results can shed light on the title reaction and offer instructions for analogous atmospheric reactions, as well as experimental research in the future.     

Ab initio studies on reaction H2C=C(OH)Li+CH3 + (CH3 -)

The possible structures and isomerizations of H₂C=C(OH)Li are studied theoretically by the gradient analytical method at RHF/6-31+G level. According to these results, reactions of H₂C=C(OH)Li with CH₃ ⁺ and CH ₃ ^- are investigated thoroughly. When H₂C=C(OH)Li reacts with CH ₃ ⁺ , H_zC=C(OH)Li firstly changes from structure1 to structure4, and then combines with CH₃ ⁺. In this reaction, the configuration of central carbon is retained. When H₂C=C(OH)Li reacts with CH ₃ ^- , structure1 firstly breaks its C-O bond to give contact ion-pair. Then through transition state16 which is similar to structure2, the attack of CH ₃ ^- from the opposite side of^-OH replaces^-OH group and inverts the configuration of carbenoid carbon atom. All the results show that the ambident reactivity of carbenoid has close relationship with the stability of special structures. Project supported by the National Natural Science Foundation of China (Grant No. 29773025).     

Multichannel RRKM study on the mechanism and kinetics of the CH3Cl + OH reaction

The kinetics and mechanism of the reaction of OH with CH₃Cl have been theoretically studied. The potential energy surface for each possible pathway has been investigated by the G2MP2 method. The rate constants for channels leading to several products have been calculated by multichannel‐Rice‐Ramsperger‐Kassel‐Marcus (RRKM) theory over a temperature range 200–2000 K. The results show the major channel is hydrogen abstraction mechanism. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Fast and Efficient Reductive Amination of Aldehydes by NaBH4/B(OH)3 and NaBH4/Al(OH)3

Reductive amination a variety of aldehydes and anilines to their corresponding secondary amines were carried out with NaBH₄/B(OH)₃ and NaBH₄/Al(OH)₃ as new reducing systems in CH₃CN at room temperature in high to excellent yields of products (90‐96%).     

CF3CH(ONO)CF3: Synthesis,IR spectrum,and use as OH radical source for kinetic and mechanistic studies

The synthesis, IR spectrum, and first‐principles characterization of CF₃CH(ONO)CF₃ as well as its use as an OH radical source in kinetic and mechanistic studies are reported. CF₃CH(ONO)CF₃ exists in two conformers corresponding to rotation about the RCO? NO bond. The more prevalent trans conformer accounts for the prominent IR absorption features at frequencies (cm^?1) of 1766 (N?O stretch), 1302, 1210, and 1119 (C? F stretches), and 761 (O? N? O bend); the cis conformer contributes a number of distinct weaker features. CF₃CH(ONO)CF₃ was readily photolyzed using fluorescent blacklamps to generate CF₃C(O)CF₃ and, by implication, OH radicals in 100% yield. CF₃CH(ONO)CF₃ photolysis is a convenient source of OH radicals in the studies of the yields of CO, CO₂, HCHO, and HC(O)OH products which can be difficult to measure using more conventional OH radical sources (e.g., CH₃ONO photolysis). CF₃CH(ONO)CF₃ photolysis was used to measure k(OH + C₂H₄)/k(OH + C₃H₆) = 0.29 ± 0.01 and to establish upper limits of 16 and 6% for the molar yields of CO and HC(O)OH from the reaction of OH radicals with benzene in 700 Torr of air at 296 K. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 159–165, 2003     

Spin‐Resolved Rotational Energy Transfer for the CH B 2Σ−(v=0, N,F) State by Collisions with Ar

An experimental and theoretical investigation of rotational energy transfers (RET) of CH involving the B ²Σ^? (v=0, 0≤N≤5, F) state by collisions with Ar is undertaken, using the photolysis‐probe technique. Time‐resolved laser‐induced fluorescence resulting from an initially prepared fine‐structure label is dispersed using a step‐scan Fourier transform spectrometer. The spin‐resolved RET rate constants are evaluated with the simulation of a kinetic model. The quantum‐scattering method is used for the calculation of the fine‐structure‐resolved cross sections and rate constants in the rotationally inelastic collisions. The theoretical values are generally consistent with our experimental findings, both in the order of magnitude and trend of N and ΔN dependence. The propensity rules obtained from the experiments are essentially obeyed by theoretical calculations, and are also in accordance with those reported by Kind and Stuhl. The RET rate constants obtained for the v=0 level are smaller than those obtained previously for v=1. The discrepancy in the RET behavior may be caused by an anisotropy difference of the interaction potential resulting from vibrational excitation.