Kinetic study of IO radical with RO2 (R = CH(3), C2H5, and CF3) using cavity ring-down spectroscopy

The reactions of iodine monoxide radical, IO, with alkyl peroxide radicals, RO(2) (R = CH(3), C(2)H(5), and CF(3)), have been studied using cavity ring-down spectroscopy. The rate constant of the reaction of IO with CH(3)O(2) was determined to be (7.0 +/- 3.0) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K and 100 Torr of N(2) diluent. The quoted uncertainty is two standard deviations. No significant pressure dependence of the rate constant was observed at 30-130 Torr total pressure of N(2) diluent. The temperature dependence of the rate constants was also studied at 213-298 K. The upper limit of the branching ratio of OIO radical formation from IO + CH(3)O(2) was estimated to be <0.1. The reaction rate constants of IO + C(2)H(5)O(2) and IO + CF(3)O(2) were determined to be (14 +/- 6) x 10(-11) and (6.3 +/- 2.7) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K, 100 Torr of N(2) diluent, respectively. The upper limit of the reaction rate constant of IO with CH(3)I was <4 x 10(-14) cm(3) molecule(-1) s(-1).     

LIF studies of iodine oxide chemistry. Part 3. Reactions IO + NO3 --> OIO + NO2, I + NO3 --> IO + NO2, and CH2I + O2 --> (products): implications for the chemistry of the marine atmosphere at night

The technique of pulsed laser photolysis coupled to LIF detection of IO was used to study IO + NO(3) --> OIO + NO(2); I + NO(3) --> (products); CH(2)I + O(2) --> (products); and O((3)P) + CH(2)I(2) --> IO + CH(2)I, at ambient temperature. was observed for the first time in the laboratory and a rate coefficient of k(1 a) = (9 +/- 4) x 10(-12) cm(3) molecule(-1) s(-1) obtained. For , a value of k(2) (298 K) = (1.0 +/- 0.3) x 10(-10) cm(3) molecule(-1) s(-1) was obtained, and a IO product yield close to unity determined. IO was also formed in a close-to-unity yield in , whereas in an upper limit of alpha(3)(IO) < 0.12 was derived. The implications of these results for the nighttime chemistry of the atmosphere were discussed. Box model calculations showed that efficient OIO formation in was necessary to explain field observations of large OIO/IO ratios.     

Laser induced fluorescence studies of iodine oxide chemistry. Part II. The reactions of IO with CH3O2, CF3O2 and O3

The technique of pulsed laser photolysis was coupled to laser induced fluorescence detection of iodine oxide (IO) to measure rate coefficients, k for the reactions IO + CH(3)O(2)--> products (R1, 30-318 Torr N(2)), IO + CF(3)O(2)--> products (R2, 70-80 Torr N(2)), and IO + O(3)--> OIO + O(2) (R3a). Values of k(1) = (2 +/- 1) x 10(-12) cm(3) molecule(-1) s(-1), k(2) = (3.6 +/- 0.8) x 10(-11) cm(3) molecule(-1) s(-1), and k(3a) <5 x 10(-16) cm(3) molecule(-1) s(-1) were obtained at T = 298 K. In the course of this work, the product yield of IO from the reaction of CH(3)O(2) with I was determined to be close to zero, whereas CH(3)OOI was formed efficiently at 70 Torr N(2). Similarly, no evidence was found for IO formation in the CF(3)O(2) + I reaction. An estimate of the rate coefficients k(CH(3)O(2) + I) = 2 x 10(-11) cm(3) molecule(-1) s(-1) and k(CH(3)OOI + I) = 1.5 x 10(-10) cm(3) molecule(-1) s(-1) was also obtained. The results on k(1)-k(3) are compared to the limited number of previous investigations and the implications for the chemistry of the marine boundary layer are briefly discussed.     

Kinetic and mechanistic studies of the I(2)/O(3) photochemistry

The atmospherically relevant chemistry generated by photolysis of I2/O3 mixtures has been studied at 298 K in the pressure range from 10 to 400 hPa by using a laboratory flash photolysis setup combining atomic resonance and molecular absorption spectroscopy. The temporal behaviors of I, I(2), IO, and OIO have been retrieved. Conventional kinetic methods and numerical modeling have been applied to investigate the IO self-reaction and the secondary chemistry. A pressure independent value of k(IO + IO) = (7.6 +/- 1.1) x 10(-11) cm(3) molecule-1 s(-1) has been determined. The pressure dependence of the branching ratios for the I + OIO and IOIO product channels in the IO + IO reaction have been determined and have values of 0.45 +/- 0.10 and 0.44 +/- 0.13 at 400 hPa, respectively. The branching ratios for the 2I + O(2) and I(2) + O(2) product channels are pressure independent with values of 0.09 +/- 0.06 and 0.05 +/- 0.03, respectively. The sensitivity analysis indicates that the isomer IOIO is more thermally stable than predicted by theoretical calculations. A reaction scheme comprising OIO polymerization steps has been shown to be consistent with the temporal behaviors recorded in this study. For simplicity, the rate coefficient has been assumed to be the same for each reaction (OIO)(n) + IO --> (OIO)(n+1), n = 1, 2, 3, 4. The lower limit obtained for this rate coefficient is (1.2 +/- 0.3) x 10(-10) cm(3) molecule(-1) s(-1) at 400 hPa. Evidence for the participation of IO in the polymerization mechanism also has been found. The rate coefficient for IO attachment to OIO and to small polymers has been determined to be larger than (5 +/- 2) x 10(-11) cm(3) molecule(-1) s(-1) at 400 hPa. These results provide supporting evidence for atmospheric particle formation induced by polymerization of iodine oxides.     

Quantum chemical calculations on a selection of iodine-containing species (IO, OIO, INO3, (IO)2, I2O3, I2O4 and I2O5) of importance in the atmosphere

The electronic and geometric structures of the title complexes are studied quantum chemically using ab initio and density functional approaches. Coupled cluster calculations at the scalar relativistic (basis set) level are performed, and the results are corrected for spin-orbit coupling using data from relativistic density functional theory studies. The heats of formation (kJ mol(-1)) at 298 K are found to be: IO3 147.8, INO3 33.1, OIO 110.1, I2O3 64.0, I2O4 111.3, I2O5 33.0, IOIO 141.3, IOOI 179.9 and OI(I)O 157.9. These data are used to draw a number of conclusions regarding three important aspects of iodine chemistry in the marine boundary layer. (i) Although the IO self reaction produces the asymmetric dimer, IOIO, it is unlikely that this species plays a further role in the atmosphere as it is short-lived. (ii) INO3 is sufficiently stable to explain the kinetics of the recombination reaction between IO and NO2, and the reaction between I2 and NO3 to produce I + INO3 is almost certainly the major source of iodine oxides at night. (iii) The higher iodine oxides I2O3 and I2O5 are very stable molecules, by contrast to the OIO dimer, I2O4, which is much less stable but which should still survive long enough in the marine boundary layer to provide a building block for iodine oxide particle formation.     

Computational studies of (HIO3) isomers and the HO2 + IO reaction pathways

The geometries, harmonic vibrational frequencies, relative energetics, and enthalpies of formation of (HIO3) isomers have been examined using quantum mechanical methods. At all levels of theory employed, MP2, B3LYP, and CCSD(T), the lowest energy structure is found to be the HOIO2 form, which shows particular stability. The two isomers HOOOI and HOOIO are closely located at the CCSD(T) level of theory. The higher energy structure is HIO3. The interisomerization transition states have been determined, along with the transition states involved in the various pathways of the reaction HO2 + IO.     

Kinetic study of the reaction of IO with CH3SCH3

The reaction IO + CH₃SCH₃ → products (3) was studied at room temperature and near 1 Torr pressure of He, using the discharge flow mass spectrometric technique. The rate constant was found to be k₃ = (1.5 ± 0.5) × 10^?11 cm³ molecule^?1 s^?1. CH₃S(O)CH₃ was detected as a product suggesting the following channel: IO + CH₃SCH₃ → CH₃S(O)CH₃ + I. The rate constant of the reaction IO + IO → products (1) was also measured: k₁ = (3 ± 0.5) × 10^?11 at 298 K and 1 Torr pressure. The atmospheric implication of reaction (3) is discussed. The results indicate that this reaction could be a potential important sink of CH₃SCH₃ in marine atmosphere.     

The reaction of IO with CH3SCH3: products and temperature dependent rate coefficients by laser induced fluorescence

The technique of pulsed laser photolysis was coupled to laser induced fluorescence detection of iodine oxide (IO) to measure rate coefficients, k(1)(T), for the title reaction IO + CH3SCH3 --> products (R1). A value of k1(298 K) = (1.44 +/- 0.15) x 10(-14) cm3 molecule(-1) s(-1) was obtained, independent of bath gas pressure (50 < P((N2 or air))/Torr < 300). The expression k1(T) = (3.2 +/- 1.4)x 10(-13)exp[(-925 +/- 136)/T)] adequately described the data over the range of temperatures (256 < T/K < 341) covered. Uncertainties (2sigma) in the 298 K rate coefficient and the pre-exponential factor include an estimate of systematic error. The conventional Arrhenius behaviour of k1(T) and the lack of pressure dependence are suggestive of an abstraction mechanism, characterised by an energy barrier of E approximately 8 kJ mol(-1). The product yield for production of I-atoms was determined indirectly to be close to unity, indicating that the reaction proceeds via transfer of the O-atom from IO to CH3SCH3 to form CH3S(O)CH3. In general, the values of k1(T) measured in this work indicate that has little impact on the chemistry of the atmosphere.     

Kinetics of the reactions of CH2Br and CH2I radicals with molecular oxygen at atmospheric temperatures

The kinetics of the reactions of CH2Br and CH2I radicals with O2 have been studied in direct measurements using a tubular flow reactor coupled to a photoionization mass spectrometer. The radicals have been homogeneously generated by pulsed laser photolysis of appropriate precursors at 193 or 248 nm. Decays of radical concentrations have been monitored in time-resolved measurements to obtain the reaction rate coefficients under pseudo-first-order conditions with the amount of O2 being in large excess over radical concentrations. No buffer gas density dependence was observed for the CH2I + O2 reaction in the range 0.2-15 x 10(17) cm(-3) of He at 298 K. In this same density range the CH2Br + O2 reaction was obtained to be in the third-body and fall-off area. Measured bimolecular rate coefficient of the CH2I + O2 reaction is found to depend on temperature as k(CH2I + O2)=(1.39 +/- 0.01)x 10(-12)(T/300 K)(-1.55 +/- 0.06) cm3 s(-1)(220-450 K). Obtained primary products of this reaction are I atom and IO radical and the yield of I-atom is significant. The rate coefficient and temperature dependence of the CH2Br + O2 reaction in the third-body region is k(CH2Br + O2+ He)=(1.2 +/- 0.2)x 10(-30)(T/300 K)(-4.8 +/- 0.3) cm6 s(-1)(241-363 K), which was obtained by fitting the complete data set simultaneously to a Troe expression with the F(cent) value of 0.4. Estimated overall uncertainties in the measured reaction rate coefficients are about +/-25%.     

Kinetics and mechanism of the reactions of CH3CO and CH3C(O)CH2 radicals with O2. Low-pressure discharge flow experiments and quantum chemical computations

The reactions CH(3)CO + O(2)--> products (1), CH(3)CO + O(2)--> OH +other products (1b) and CH(3)C(O)CH(2) + O(2)--> products (2) have been studied in isothermal discharge flow reactors with laser induced fluorescence monitoring of OH and CH(3)C(O)CH(2) radicals. The experiments have been performed at overall pressures between 1.33 and 10.91 mbar of helium and 298 +/- 1 K reaction temperature. OH formation has been found to be the dominant reaction channel for CH(3)CO + O(2): the branching ratio, Gamma(1b) = k(1b)/k(1), is close to unity at around 1 mbar, but decreases rapidly with increasing pressure. The rate constant of the overall reaction, k(2), has been found to be pressure dependent: the fall-off behaviour has been analysed in comparison with reported data. Electronic structure calculations have confirmed that at room temperature the reaction of CH(3)C(O)CH(2) with O(2) is essentially a recombination-type process. At high temperatures, the further reactions of the acetonyl-peroxyl adduct may yield OH radicals, but the most probable channel seems to be the O(2)-catalysed keto-enol transformation of acetonyl. Implications of the results for atmospheric modelling studies have been discussed.     

Quantum mechanical investigation of the atmospheric reaction CH3O2 + NO

The important stationary points on the potential energy surface of the reaction CH(3)O(2) + NO have been investigated using ab initio and density functional theory techniques. The optimizations were carried out at the B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) levels of theory while the energetics have been refined using the G2MP2, G3//B3LYP, and CCSD(T) methodologies. The calculations allow the proper characterization of the transition state barriers that determine the fate of the nascent association conformeric minima of methyl peroxynitrite. The main products, CH(3)O + NO(2), are formed through either rearrangement of the trans-conformer to methyl nitrate and its subsequent dissociation or via the breaking of the peroxy bond of the cis-conformer to CH(3)O + NO(2) radical pair. The important consequences of the proposed mechanism are (a) the allowance on energetic grounds for nitrate formation parallel to radical propagation under favorable external conditions and (b) the confirmation of the conformational preference of the homolytic cleavage of the peroxy bond, discussed in previous literature.     

CH3CH2+O(3P)反应机理的理论研究

The reaction for CH3CH2+O(3P) was studied by ab initio method. The geometries of the reactants, intermediates, transition states and products were optimized at MP2/6-311+G(d,p) level. The corresponding vibration frequencies were calculated at the same level. The single-point calculations for all the stationary points were carried out at the QCISD(T)/6-311+G(d,p) level using the MP2/6-311+G(d,p) optimized geometries. The results of the theoretical study indicate that the major products are the CH2O＋CH3, CH3CHO+H and CH2CH2+OH in the reaction. For the products CH2O＋CH3 and CH3CHO+H, the major production channels are A1: (R)→IM1→TS3→(A) and B1: (R)→IM1→TS4→(B), respectively. The majority of the products CH2CH2+OH are formed via the direct abstraction channels C1 and C2: (R)→TS1(TS2)→(C). In addition, the results suggest that the barrier heights to form the CO reaction channels are very high, so the CO is not a major product in the reaction.     

A single transition state serves two mechanisms. The branching ratio for CH2O*- + CH3Cl on improved potential energy surfaces

The reaction of formaldehyde radical anion with methyl chloride, CH2O*- + CH3Cl, is an example in which a single transition state leads to two products: substitution at carbon (Sub(C), CH3CH2O* + Cl-) and electron transfer (ET, CH2O + CH3* + Cl-). The branching ratio for this reaction has been studied by ab initio molecular dynamics (AIMD). The energies of transition states and intermediates were computed at a variety of levels of theory and compared to accurate energetics calculated by the G3 and CBS-QB3 methods. A bond additivity correction has been constructed to improve the Hartree-Fock potential energy surface (BAC-UHF). A satisfactory balance between good energetics and affordable AIMD calculations can be achieved with BH&HLYP/6-31G(d) and BAC-UHF/6-31G(d) calculations. Approximately 200 ab initio classical trajectories were calculated for each level of theory with initial conditions sampled from a thermal distribution at 298 K at the transition state. Three types of trajectories were distinguished: trajectories that go directly to ET product, trajectories that go to Sub(C) product, and trajectories that initially go into the Sub(C) valley and then dissociate to ET products. The BH&HLYP/6-31G(d) calculations overestimate the number of nonreactive and direct ET trajectories because the transition state is too early. For the BH&HLYP and BAC-UHF methods, about one-third of the trajectories that initially go into the Sub(C) valley dissociate to ET products, compared to just over half with UHF/6-31G(d) in the earlier study. This difference can be attributed to a better value for the calculated energy release from the initial transition state and to an improved Sub(C) --> ET barrier height with the BH&HLYP and BAC-UHF methods.     

Thermal decomposition of ethanol. 4. Ab initio chemical kinetics for reactions of H atoms with CH3CH2O and CH3CHOH radicals

The potential energy surfaces of H-atom reactions with CH(3)CH(2)O and CH(3)CHOH, two major radicals in the decomposition and oxidation of ethanol, have been studied at the CCSD(T)/6-311+G(3df,2p) level of theory with geometric optimization carried out at the BH&HLYP/6-311+G(3df,2p) level. The direct hydrogen abstraction channels and the indirect association/decomposition channels from the chemically activated ethanol molecule have been considered for both reactions. The rate constants for both reactions have been calculated at 100-3000 K and 10(-4) Torr to 10(3) atm Ar pressure by microcanonical VTST/RRKM theory with master equation solution for all accessible product channels. The results show that the major product channel of the CH(3)CH(2)O + H reaction is CH(3) + CH(2)OH under atmospheric pressure conditions. Only at high pressure and low temperature, the rate constant for CH(3)CH(2)OH formation by collisonal deactivation becomes dominant. For CH(3)CHOH + H, there are three major product channels; at high temperatures, CH(3)+CH(2)OH production predominates at low pressures (P < 100 Torr), while the formation of CH(3)CH(2)OH by collisional deactivation becomes competitive at high pressures and low temperatures (T < 500 K). At high temperatures, the direct hydrogen abstraction reaction producing CH(2)CHOH + H(2) becomes dominant. Rate constants for all accessible product channels in both systems have been predicted and tabulated for modeling applications. The predicted value for CH(3)CHOH + H at 295 K and 1 Torr pressure agrees closely with available experimental data. For practical modeling applications, the rate constants for the thermal unimolecular decomposition of ethanol giving key accessible products have been predicted; those for the two major product channels taking place by dehydration and C-C breaking agree closely with available literature data.     

Mechanistic and kinetic study of the CH3CO + O2 reaction

Potential-energy surface of the CH3CO + O2 reaction has been calculated by ab initio quantum chemistry methods. The geometries were optimized using the second-order Moller-Plesset theory (MP2) with the 6-311G(d,p) basis set and the coupled-cluster theory with single and double excitations (CCSD) with the correlation consistent polarized valence double zeta (cc-pVDZ) basis set. The relative energies were calculated using the Gaussian-3 second-order Moller-Plesset theory with the CCSD/cc-pVDZ geometries. Multireference self-consistent-field and MP2 methods were also employed using the 6-311G(d,p) and 6-311++G(3df,2p) basis sets. Both addition/elimination and direct abstraction mechanisms have been investigated. It was revealed that acetylperoxy radical [CH3C(O)OO] is the initial adduct and the formation of OH and alpha-lactone [CH2CO2(1A')] is the only energetically accessible decomposition channel. The other channels, e.g., abstraction, HO2 + CH2CO, O + CH3CO2, CO + CH3O2, and CO2 + CH3O, are negligible. Multichannel Rice-Ramsperger-Kassel-Marcus theory and transition state theory (E-resolved) were employed to calculate the overall and individual rate coefficients and the temperature and pressure dependences. Fairly good agreement between theory and experiments has been obtained without any adjustable parameters. It was concluded that at pressures below 3 Torr, OH and CH2CO2(1A') are the major nascent products of the oxidation of acetyl radicals, although CH2CO2(1A') might either undergo unimolecular decomposition to form the final products of CH2O + CO or react with OH and Cl to generate H2O and HCl. The acetylperoxy radicals formed by collisional stabilization are the major products at the elevated pressures. In atmosphere, the yield of acetylperoxy is nearly unity and the contribution of OH is only marginal.     

Low-energy paths for the unimolecular decomposition of CH3OH: a G2M/statistical theory study

The potential energy surface (PES) of the CH3OH system has been characterized by ab initio molecular orbital theory calculations at the G2M level of theory. The mechanisms for the decomposition of CH3OH and the related bimolecular reactions, CH3 + OH and 1CH2 + H2O, have been elucidated. The rate constants for these processes have been calculated using variational RRKM theory and compared with available experimental data. The total decomposition rate constants of CH3OH at the high- and low-pressure limits can be represented by k infinity = 1.56 x 10(16) exp(-44,310/T) s-1 and kAr0 = 1.60 x 10(36) T-12.2 exp(-48,140/T) cm3 molecule-1 s-1, respectively, covering the temperature range 1000-3000 K, in reasonable agreement with the experimental values. Our results indicate that the product branching ratios are strongly pressure dependent, with the production of CH3 + OH and 1CH2 + H2O dominant under high (P > 10(3) Torr) and low (P < 1 atm) pressures, respectively. For the bimolecular reaction of CH3 and OH, the total rate constant and the yields of 1CH2 + H2O and H2 + HCOH at lower pressures (P < 5 Torr) could be reasonably accounted for by the theory. For the reaction of 1CH2 with H2O, both the yield of CH3 + OH and the total rate constant could also be satisfactorily predicted theoretically. The production of 3CH2 + H2O by the singlet to triplet surface crossing, predicted to occur at 4.3 kcal mol-1 above the H2C...OH2 van der Waals complex (which lies 82.7 kcal mol-1 above CH3OH), was neglected in our calculations.     

Analysis of the reaction force for a gas phase S(N)2 process: CH3Cl + H2O --> CH3OH + HCl

The "reaction force" F(R(c)) is the negative derivative of a system's potential energy V(R(c)) along the intrinsic reaction coordinate of a process. If V(R(c)) goes through a maximum, as is commonly the case, then F(R(c)) has a characteristic profile: a negative minimum followed by zero at the transition state and then a positive maximum. These features reflect four phases of the reaction: an initial one of reactant preparation, followed by two of transition to products, and then relaxation of the latter. In this study, we have analyzed, in these terms, a gas-phase S(N)2 substitution, selected to be CH3Cl + H2O --> CH3OH + HCl. We examine, at the B3LYP/6-31G level, the geometries, energetics, and molecular surface electrostatic potentials, local ionization energies, and internal charge separation.     

The effects of water vapor on the CH3O2 self-reaction and reaction with HO2

The gas phase reactions of CH3O2 + CH3O2, HO2 + HO2, and CH3O2 + HO2 in the presence of water vapor have been studied at temperatures between 263 and 303 K using laser flash photolysis coupled with UV time-resolved absorption detection at 220 and 260 nm. Water vapor concentrations were quantified using tunable diode laser spectroscopy operating in the mid-IR. The HO2 self-reaction rate constant is significantly enhanced by water vapor, consistent with what others have reported, whereas the CH3O2 self-reaction and the cross-reaction (CH3O2 + HO2) rate constants are nearly unaffected. The enhancement in the HO2 self-reaction rate coefficient occurs because of the formation of a strongly bound (6.9 kcal mol(-1)) HO2 x H2O complex during the reaction mechanism where the H2O acts as an energy chaperone. The nominal impact of water vapor on the CH3O2 self-reaction rate coefficient is consistent with recent high level ab initio calculations that predict a weakly bound CH3O2 x H2O complex (2.3 kcal mol(-1)). The smaller binding energy of the CH3O2 x H2O complex does not favor its formation and consequent participation in the methyl peroxy self-reaction mechanism.     

Theoretical Studies of the Reaction Mechanisms of CH3S＋NO2

The potential energy surface for the CH3S NO2 reaction has been studied using the ab initio G3(MP2) method. A variety of possible complexes and saddle points along the minimum energy reaction paths have been characterized at UMP2 (full)/6-31G(d) level. The calculations reveal dominating reaction mechanisms of the title reaction: CH3S NO2 firstly produce intermediate CH3SONO,then break up into CH3SO NO. The results are valuable to understand the atmospheric sulfur compounds oxidation mechanism.     

Reflected shock tube studies of high-temperature rate constants for OH + CH4 --> CH3 + H2O and CH3 + NO2 --> CH3O + NO

The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm has been used to study the reactions OH + CH(4) --> CH(3) + H(2)O and CH(3) + NO(2) --> CH(3)O + NO. Over the temperature range 840-2025 K, the rate constants for the first reaction can be represented by the Arrhenius expression k = (9.52 +/- 1.62) x 10(-11) exp[(-4134 +/- 222 K)/T] cm(3) molecule(-1) s(-1). Since this reaction is important in both combustion and atmospheric chemistry, there have been many prior investigations with a variety of techniques. The present results extend the temperature range by 500 K and have been combined with the most accurate earlier studies to derive an evaluation over the extended temperature range 195-2025 K. A three-parameter expression describes the rate behavior over this temperature range, k = (1.66 x 10(-18))T(2.182) exp[(-1231 K)/T] cm(3) molecule(-1) s(-1). Previous theoretical studies are discussed, and the present evaluation is compared to earlier theoretical estimates. Since CH(3) radicals are a product of the reaction and could cause secondary perturbations in rate constant determinations, the second reaction was studied by OH radical production from the fast reactions CH(3)O --> CH(2)O + H and H + NO(2) --> OH + NO. The measured rate constant is 2.26 x 10(-11) cm(3) molecule(-1) s(-1) and is not dependent on temperature from 233 to 1700 K within experimental error.