Thermal decomposition of ClOOCl

ClOOCl was prepared in situ in a temperature controlled photoreactor (v = 420 L) by photolyzing OClO/N2 mixtures in the wavelength range 300-500 nm at temperatures between 242 and 261 K and total pressures between 2 and 480 mbar. After switching off the lights, excess NO2 was added, and IR and UV spectra were monitored simultaneously as a function of time. By spectral stripping of all other known UV absorbers (in particular, other chlorine oxides and chlorine nitrate), we determined rate constants k-1 of the reaction ClOOCl (+M) --> ClO + ClO (+M) from the first-order decay of the residual UV absorption of ClOOCl at 246 and 255 nm. k-1,0 = [N2] x 7.6 x 10(-9) exp[(-53.6 +/- 6.0) kJ mol(-1)/RT] cm3 molecule(-1) s(-1) (2sigma) was derived for the low-pressure limiting rate constant. Application of Troe's expression for the limiting low-pressure rate constants of unimolecular decomposition reactions leads to E0 = Delta(r)H0(0)(ClOOCl-->ClO+ClO) = 66.4 +/- 3.0 kJ mol(-1). k-1,0 started to fall off from the pressure proportional low pressure behavior at p approximately 30 mbar; however, reliable extrapolation to the high pressure limit was not possible. The decomposition rate constants of ClOOCl were directly measured for the first time, and they are higher, depending on temperature and pressure, by factors between 1.5 and 4.2 as compared to experimental data on k-1 by Nickolaisen et al. [J. Phys. Chem. 1994, 98, 155] which were derived from the approach of ClO to thermal equilibrium with its dimer ClOOCl. Combination of the present dissociation rate constants with recommended temperature and pressure dependent data on the reverse reaction (k1) demonstrate inconsistencies between the dissociation and recombination rate constants. Summarizing laboratory data on k1 and k-1 above 250 K and field measurements on the ClO + ClO <= => ClOOCl equilibrium in the nighttime polar stratosphere close to 200 K, the expression Kc = k1/k-1 = 3.0 x 10(-27) exp(8433 K/T) cm3 molecule(-1) is derived for the temperature range 200-300 K.     

Ab initio studies of CIO, reactions: prediction of the rate constants of CIO + NO for the forward and reverse processes.

The mechanisms for ClO+NO and its reverse reactions were investigated by means of ab initio molecular orbital and statistical theory calculations. The species involved were optimized at the B3LYP/6-311 +G(3df) level, and their energies were refined at the CCSD(T)/6-311+ G(3df)//B3LYP/6-311 + G(3df) level. Five isomers and the transition states among them were located. The relative stability of these isomers is ClNO2 > cis-ClONO > trans-ClONO > cis-OClNO>trans-OClNO. The heats of formation of the three most-stable isomers were predicted using isodesmic reactions by different methods. The predicted bimolecular reaction rate constant shows that, below 100 atm, the formation of Cl+NO2 is dominant and pressure-independent. The total rate constant can be expressed as: k(ClO+NO)= 1.43 x 10(-9)T(-083)exp(92/ T) cm3 molecule(-1)s(-1) in the temperature range of 200-1000 K, in close agreement with experimental data. For the reverse reaction, Cl+NO2-->ClNO2 and ClONO (cis and trans isomers), the sum of the predicted rate constants for the formation of the three isomers and their relative yields also reproduce the experimental data well. The predicted total third-order rate constants in the temperature range of 200-1000 K can be represented by: k0(He) = 4.89 x 10(-6)T(-5.85) exp(-796/T) cm6 molecule(-1)s(-1) and k0(N2) =5.72 x 10(-15)T(-5.80) exp(-814/T) cm6 molecule(-1)s(-1). The predicted high- and low-pressure limit decomposition rates of CINO2 in Ar in the temperature range 400-1500 K can be expressed, respectively, by: k-(ClNO2) = 7.25 x 10(19)T(-1.89) exp(-16875/T) s(-1) and kd(ClNO2) = 2.51 x 10(38)T(-6.8) exp(-18409/T) cm3 molecule(-1) s(-1). The value of k0(ClNO2) is also in reasonable agreement with available experimental data.     

Kinetics of the OH + ClOOCl and OH + Cl2O reactions: experiment and theory

The rate coefficients for the reactions OH + ClOOCl --> HOCl + ClOO (eq 5) and OH + Cl2O --> HOCl + ClO (eq 6) were measured using a fast flow reactor coupled with molecular beam quadrupole mass spectrometry. OH was detected using resonance fluorescence at 309 nm. The measured Arrhenius expressions for these reactions are k5 = (6.0 +/- 3.5) x 10(-13) exp((670 +/- 230)/T) cm(3) molecule(-1) s(-1) and k6 = (5.1 +/- 1.5) x 10(-12) exp((100 +/- 92)/T) cm(3) molecule(-1) s(-1), respectively, where the uncertainties are reported at the 2sigma level. Investigation of the OH + ClOOCl potential energy surface using high level ab initio calculations indicates that the reaction occurs via a chlorine abstraction from ClOOCl by the OH radical. The lowest energy pathway is calculated to proceed through a weak ClOOCl-OH prereactive complex that is bound by 2.6 kcal mol(-1) and leads to ClOO and HOCl products. The transition state to product formation is calculated to be 0.59 kcal mol(-1) above the reactant energy level. Inclusion of the OH + ClOOCl rate data into an atmospheric model indicates that this reaction contributes more than 15% to ClOOCl loss during twilight conditions in the Arctic stratosphere. Reducing the rate of ClOOCl photolysis, as indicated by a recent re-examination of the ClOOCl UV absorption spectrum, increases the contribution of the OH + ClOOCl reaction to polar stratospheric loss of ClOOCl.     

High accuracy measurements of OH reaction rate constants and IR absorption spectra: substituted 2-propanols

Rate constants for the gas phase reactions of OH radicals with 2-propanol and three fluorine substituted 2-propanols, (CH(3))(2)CHOH (k(0)), (CF(3))(2)CHOH (k(1)), (CF(3))(2)C(OH)CH(3) (k(2)), and (CF(3))(3)COH (k(3)), were measured using a flash photolysis resonance-fluorescence technique over the temperature range 220-370 K. The Arrhenius plots were found to exhibit noticeable curvature for all four reactions. The temperature dependences of the rate constants can be represented by the following expressions: k(0)(T) = 1.46 × 10(-11) exp{-883/T} + 1.30 × 10(-12) exp{+371/T} cm(3) molecule(-1) s(-1); k(1)(T) = 1.19 × 10(-12) exp{-1207/T} + 7.85 × 10(-16) exp{+502/T } cm(3) molecule(-1) s(-1); k(2)(T) = 1.68 × 10(-12) exp{-1718/T} + 7.32 × 10(-16) exp{+371/T} cm(3) molecule(-1) s(-1); k(3)(T) = 3.0 × 10(-20) × (T/298)(11.3) exp{+3060/T} cm(3) molecule(-1) s(-1). The atmospheric lifetimes due to reactions with tropospheric OH were estimated to be 2.4 days and 1.9, 6.3, and 46 years, respectively. UV absorption cross sections were measured between 160 and 200 nm. The IR absorption cross sections of the three fluorinated compounds were measured between 450 and 1900 cm(-1), and their global warming potentials were estimated.     

Ab initio study of the ClO + NH2 reaction: prediction of the total rate constant and product branching ratios

The mechanism for ClO + NH2 has been investigated by ab initio molecular orbital and transition-state theory calculations. The species involved have been optimized at the B3LYP/6-311+G(3df,2p) level and their energies have been refined by single-point calculations with the modified Gaussian-2 method, G2M(CC2). Ten stable isomers have been located and a detailed potential energy diagram is provided. The rate constants and branching ratios for the low-lying energy channel products including HCl + HNO, Cl + NH2O, and HOCl + 3NH (X(3)Sigma(-)) are calculated. The result shows that formation of HCl + HNO is dominant below 1000 K; over 1000 K, Cl + NH2O products become dominant. However, the formation of HOCl + 3NH (X(3)Sigma(-)) is unimportant below 1500 K. The pressure-independent individual and total rate constants can be expressed as k1(HCl + HNO) = 4.7 x 10(-8)(T(-1.08)) exp(-129/T), k(2)(Cl + NH2O) = 1.7 x 10(-9)(T(-0.62)) exp(-24/T), k3(HOCl + NH) = 4.8 x 10(-29)(T5.11) exp(-1035/T), and k(total) = 5.0 x 10(-9)(T(-0.67)) exp(-1.2/T), respectively, with units of cm(3) molecule(-1) s(-1), in the temperature range of 200-2500 K.     

Ab initio chemical kinetics for the ClOO + NO reaction: effects of temperature and pressure on product branching formation

The kinetics and mechanism for the reaction of ClOO with NO have been investigated by ab initio molecular orbital theory calculations based on the CCSD(T)/6-311+G(3df)//PW91PW91∕6-311+G(3df) method, employed to evaluate the energetics for the construction of potential energy surfaces and prediction of reaction rate constants. The results show that the reaction can produce two key low energy products ClNO + (3)O(2) via the direct triplet abstraction path and ClO + NO(2) via the association and decomposition mechanism through long-lived singlet pc-ClOONO and ClONO(2) intermediates. The yield of ClNO + O(2) ((1)△) from any of the singlet intermediates was found to be negligible because of their high barriers and tight transition states. As both key reactions initially occur barrierlessly, their rate constants were evaluated with a canonical variational approach in our transition state theory and Rice-Ramspergen-Kassel-Marcus/master equation calculations. The rate constants for ClNO + (3)O(2) and ClO + NO(2) production from ClOO + NO can be given by 2.66 × 10(-16) T(1.91) exp(341/T) (200-700 K) and 1.48 × 10(-24) T(3.99) exp(1711/T) (200-600 K), respectively, independent of pressure below atmospheric pressure. The predicted total rate constant and the yields of ClNO and NO(2) in the temperature range of 200-700 K at 10-760 Torr pressure are in close agreement with available experimental results.     

Ab initio chemical kinetic study on the reactions of ClO with C2H2 and C2H4

The mechanisms for the reactions of ClO with C(2)H(2) and C(2)H(4) have been investigated at the CCSD(T)/CBS level of theory. The results show that in both systems, the interaction between the Cl atom of the ClO radical and the triple and double bonds of C(2)H(2) and C(2)H(4) forms prereaction van der Waals complexes with the O-Cl bond pointing perpendicularly toward the π-bonds, both with 2.1 kcal/mol binding energies. The mechanism is similar to those of the HO-C(2)H(2)/C(2)H(4) systems. The rate constants for the low energy channels have been predicted by statistical theory. For the reaction of ClO and C(2)H(2), the main channels are the production of CH(2)CO + Cl (k(1a)) and CHCO + HCl (k(1b)), with k(1a) = 1.19 × 10(-15)T(1.18) exp(-5814/T) and k(1b) = 6.94 × 10(-21) × T(2.60) exp(-6587/T) cm(3) molecule(-1) s(-1). For the ClO + C(2)H(4) reaction, the main pathway leads to C(2)H(4)O + Cl (k(2a)) with the predicted rate constant k(2a) = 2.13 × 10(-17)T(1.52) exp(-3849/T) in the temperature range of 300-3000 K. These rate constants are pressure-independent below 100 atm.     

Ab initio studies of ClO(x) reactions: prediction of the rate constants of ClO+NO2 for the forward and reverse processes.

The potential-energy surface for the reaction of ClO with NO2 has been constructed at the CCSD(T)/6-311+G(3df)//B3LYP/6-311+G(3df) level of theory. Six ClNO3 isomers are located; these are ClONO2, pc-ClOONO, pt-ClOONO, OClNO2, pt-OClONO, pc-OClONO, with predicted energies relative to the reactants of -25.6, -0.5, 1.0, 1.9, 12.2 and 13.6 kcal mol-1, and heats of formation at 0 K of 7.8, 32.9, 34.4, 35.5, 45.6 and 47.0 kcal mol-1, respectively. Isomerizations among them are also discussed. The rate constants for the low-energy pathways have been computed by statistical theory calculations. For the association reaction producing exclusively ClONO2, the predicted low- and high-pressure-limit rate constants in N2 for the temperature range of 200-600 K can be represented by: (N2)=3.19 x 10-17 T-5.54 exp(-384 K/T) cm6 molecule-2 s-1 and =3.33 x 10-7 T-1.48 exp(-18 K/T) cm3 molecule-1 s-1. The predicted low- and high-pressure-limit rate constants for the decomposition of ClONO2 in N2 at 200-600 K can be expressed, respectively, by =6.08 x 1013 T-6.54 exp(-13813 K/T) cm3 molecule-1 s-1 and =4.59 x 1023 T-2.43 exp(-13437 K/T) s-1. The predicted values compare satisfactorily with available experimental data. The reverse Cl+NO3 reaction was found to be independent of the pressure, giving exclusively ClO+NO2; the predicted rate constant can be expressed as k(Cl+NO3)=1.19 x 10-9 T-0.60 exp(58 K/T) cm3 molecule-1 s-1..     

Ab initio chemical kinetics for the hydrolysis of N2O4 isomers in the gas phase

The mechanism and kinetics for the gas-phase hydrolysis of N(2)O(4) isomers have been investigated at the CCSD(T)/6-311++G(3df,2p)//B3LYP/6-311++G(3df,2p) level of theory in conjunction with statistical rate constant calculations. Calculated results show that the contribution from the commonly assumed redox reaction of sym-N(2)O(4) to the homogeneous gas-phase hydrolysis of NO(2) can be unequivocally ruled out due to the high barrier (37.6 kcal/mol) involved; instead, t-ONONO(2) directly formed by the association of 2NO(2), was found to play the key role in the hydrolysis process. The kinetics for the hydrolysis reaction, 2NO(2) + H(2)O ? HONO + HNO(3) (A) can be quatitatively interpreted by the two step mechanism: 2NO(2) → t-ONONO(2), t-ONONO(2) + H(2)O → HONO + HNO(3). The predicted total forward and reverse rate constants for reaction (A), k(tf) = 5.36 × 10(-50)T(3.95) exp(1825/T) cm(6) molecule(-2) s(-1) and k(tr) = 3.31 × 10(-19)T(2.478) exp(-3199/T) cm(3) molecule(-1) s(-1), respectively, in the temperature range 200-2500 K, are in good agreement with the available experimental data.     

Ab initio chemical kinetics for the OH+HNCN reaction

The kinetics and mechanism of the reaction of the cyanomidyl radical (HNCN) with the hydroxyl radical (OH) have been investigated by ab initio calculations with rate constants prediction. The single and triplet potential energy surfaces of this reaction have been calculated by single-point calculations at the CCSD(T)/6-311+G(3df,2p) level based on geometries optimized at the B3LYP/6-311+G(3df,2p) and CCSD/6-311++G(d,p) levels. The rate constants for various product channels in the temperature range of 300-3000 K are predicted by variational transition-state and Rice-Ramsperger-Kassel-Marcus (RRKM) theories. The predicted total rate constants can be represented by the expressions ktotal=2.66 x 10(+2)xT-4.50 exp(-239/T) in which T=300-1000 K and 1.38x10(-20)xT2.78 exp(1578/T) cm3 molecule(-1) s(-1) where T=1000-3000 K. The branching ratios of primary channels are predicted: k1 for forming singlet HON(H)CN accounts for 0.32-0.28, and k4 for forming singlet HONCNH accounts for 0.68-0.17 in the temperature range of 300-800 K. k2+k7 for producing H2O+NCN accounts for 0.55-0.99 in the high-temperature range of 800-3000 K. The branching ratios of k3 for producing HCN+HNO, k6 for producing H2N+NCO, k8 for forming 3HN(OH)CN, k9 for producing CNOH+3NH, and k5+k10 for producing NH2+NCO are negligible. The rate constants for key individual product channels are provided in a table for different temperature and pressure conditions.     

Kinetics of the CH2Cl + CH3 and CHCl2 + CH3 radical-radical reactions

The CH2Cl + CH3 (1) and CHCl2 + CH3 (2) cross-radical reactions were studied by laser photolysis/photoionization mass spectroscopy. Overall rate constants were obtained in direct real-time experiments in the temperature region 301-800 K and bath gas (helium) density (6-12) x 10(16) atom cm(-3). The observed rate constant of reaction 1 can be represented by an Arrhenius expression k1 = 3.93 x 10(-11) exp(91 K/T) cm3 molecule(-1) s(-1) (+/-25%) or as an average temperature-independent value of k1= (4.8 +/- 0.7) x 10(-11) cm3 molecule(-1) s(-1). The rate constant of reaction 2 can be expressed as k2= 1.66 x 10(-11) exp(359 K/T) cm3 molecule(-1) s(-1) (+/-25%). C2H4 and C2H3Cl were detected as the primary products of reactions 1 and 2, respectively. The experimental values of the rate constant are in reasonable agreement with the prediction based on the "geometric mean rule." A separate experimental attempt to determine the rate constants of the high-temperature CH2Cl + O2 (10) and CHCl2 + O2 (11) reaction resulted in an upper limit of 1.2 x 10(-16) cm(3) molecule(-1) s(-1) for k10 and k11 at 800 K.     

Theoretical study on the reaction CX3 + SiH(CH3)3 (X = H, F)

Theoretical investigations are carried out on the multiple-channel reactions, CH(3) + SiH(CH(3))(3) → products and CF(3) + SiH(CH(3))(3) → products. The minimum energy paths (MEP) are calculated at the MP2/6-311 + G(d,p) level, and energetic information is further refined by the MC-QCISD (single point) method. The rate constants for major reaction channels are calculated by the canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) correction over the temperature range 200-1500 K. The theoretical rate constants are in good agreement with the available experimental data and are found to be k(1a)(T) = 1.93 × 10(-24) T(3.15) exp(-1214.59/T) and k(2a)(T) = 1.33 × 10(-25) T(4.13) exp(-397.94/T) (in unit of cm(3) molecule(-1) s(-1)). Our calculations indicate that hydrogen abstraction channel from SiH group is the major channel due to the smaller barrier height among five channels considered.     

Kinetics of the NCCO + NO2 reaction

The kinetics of the NCCO + NO(2) reaction was studied by transient infrared laser absorption spectroscopy. The total rate constant of the reaction was measured to be k = (2.1 ± 0.1) × 10(-11) cm(3) molecule(-1) s(-1) at 298 K. Detection of products and consideration of possible secondary chemistry shows that CO(2) + NO + CN is the primary product channel. The rate constants of the NCCO + CH(4) and NCCO + C(2)H(4) reactions were also measured, obtaining upper limits of k (NCCO + CH(4)) ≤ 7.0 × 10(-14) cm(3) molecule(-1) s(-1) and k (NCCO + C(2)H(4)) ≤ 5.0 × 10(-15) cm(3) molecule(-1) s(-1). Ab initio calculations on the singlet and triplet potential energy surfaces at B3LYP/6-311++G**//CCSD(T)/6-311++G** levels of theory show that the most favorable reaction pathway occurs on the singlet surface, leading to CO(2) + NO + CN products, in agreement with experiment.     

Ab initio chemical kinetic study on Cl + ClO and related reverse processes

The reaction of ClO with Cl and its related reverse processes have been studied theoretically by ab initio quantum chemical and statistical mechanical calculations. The geometric parameters of the reactants, products, and transition states are optimized by both UMPW1PW91 and unrestricted coupled-cluster single and double excitation (UCCSD) methods with the 6-311+G(3df) basis set. The potential energy surface has been further refined (with triple excitations, T) at the UCCSD(T)/6-311+G(3df) level of theory. The results show that Cl(2) and O ((3)P) can be produced by chlorine atom abstraction via a tight transition state, while ClOCl ((1)A(1)) and ClClO ((1)A') can be formed by barrierless association processes with exothermicities of 31.8 and 16.0 kcal/mol, respectively. In principle the O ((1)D) atom can be generated with a large endothermicity of 56.9 kcal/mol; on the other hand, its barrierless reaction with Cl(2) can readily form ClClO ((1)A'), which fragments rapidly to give ClO + Cl. The rate constants of both forward and reverse processes have been predicted at 150-2000 K by the microcanonical variational transition state theory (VTST)/Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The predicted rate constants are in good agreement with available experimental data within reported errors.     

ClClO2 is the most stable isomer of Cl2O2. Accurate coupled cluster energetics and electronic spectra of Cl2O2 isomers

High level ab initio electronic structure calculations at the coupled cluster level with a correction for triples (CCSD(T)) extrapolated to the complete basis set limit have been made for the thermodynamics of the Cl2O2 isomers: ClClO2, ClOOCl, and ClOClO. The ClClO2 isomer is predicted to be the most stable isomer and is more stable than ClOOCl by 3.1 kcal/mol at 298 K. The ClOClO isomer is less stable than ClOOCl by 8.3 kcal/mol at 298 K. The weakest bond in ClClO2 is the Cl-Cl bond with a bond dissociation energy (BDE) of 24.4 kcal/mol, and the smallest BDE in ClOOCl is the O-O bond with a value of 18.0 kcal/mol. The smallest BDE in ClOClO is for the central O-Cl bond with a BDE of 9.7 kcal/mol. Electronic transitions were calculated with the equations of motion EOM-CCSD method. The calculations clearly demonstrate that singlet states of ClClO2 absorb to longer wavelengths in the visible than do the singlet states of ClOOCl as does ClOClO.     

Quantum chemical prediction of pathways and rate constants for reaction of cyanomethylene radical with NO

High-level ab initio calculations have been performed to study the mechanism and kinetics of the reaction of the cyanomethylene radical (HCCN) with the NO. The species involved have been optimized at the B3LYP/6-311++G(3df,2p) level, and their corresponding single-point energies are improved by the CCSD(T)/aug-cc-PVQZ//B3LYP/6-311++G(3df,2p) approach. From the calculated potential energy surface, we have predicted the favorable pathways for the formation of several isomers of a HCCN-NO complex. Barrierless formation of HCN + NCO (P1) is also possible. Formation of HCNO + CN (P3) is endoergic but may become significant at high temperatures. To rationalize the scenario of our calculated results, we also employ the Fukui functions and hard-and-soft acid-and-base (HSAB) theory to seek possible clues. The predicted total rate coefficient, k(total), at He pressure 760 Torr can be represented with the equation k(total) = 1.40 × 10(-7) T(-2.01) exp(3.15 kcal mol(-1)/RT) at T = 298-3000 K in units of cm(3) molecule(-1) s(-1). The predicted total rate coefficients at some available conditions (He pressures of 6, 18, and 30 Torr in the temperature of 298 K) are in reasonable agreement with experimental observation. In addition, the rate constants for key individual product channels are provided in different temperature and pressure conditions.     

High-accuracy measurements of OH(•) reaction rate constants and IR and UV absorption spectra: ethanol and partially fluorinated ethyl alcohols

Rate constants for the gas phase reactions of OH(?) radicals with ethanol and three fluorinated ethyl alcohols, CH(3)CH(2)OH (k(0)), CH(2)FCH(2)OH (k(1)), CHF(2)CH(2)OH (k(2)), and CF(3)CH(2)OH (k(3)) were measured using a flash photolysis resonance-fluorescence technique over the temperature range 220 to 370 K. The Arrhenius plots were found to exhibit noticeable curvature for all four reactions. The temperature dependences of the rate constants can be represented by the following expressions over the indicated temperature intervals: k(0)(220-370 K) = 5.98 × 10(-13)(T/298)(1.99) exp(+515/T) cm(3) molecule(-1) s(-1), k(0)(220-298 K) = (3.35 ± 0.06) × 10(-12) cm(3) molecule(-1) s(-1) [for atmospheric modeling purposes, k(0)(T) is essentially temperature-independent below room temperature, k(0)(220-298 K) = (3.35 ± 0.06) × 10(-12) cm(3) molecule(-1) s(-1)], k(1)(230-370 K) = 3.47 × 10(-14)(T/298)(4.49) exp(+977/T) cm(3) molecule(-1) s(-1), k(2)(220-370 K) = 3.87 × 10(-14)(T/298)(4.25) exp(+578/T) cm(3) molecule(-1) s(-1), and k(3)(220-370 K) = 2.48 × 10(-14)(T/298)(4.03) exp(+418/T) cm(3) molecule(-1) s(-1). The atmospheric lifetimes due to reactions with tropospheric OH(?) were estimated to be 4, 16, 62, and 171 days, respectively, under the assumption of a well-mixed atmosphere. UV absorption cross sections of all four ethanols were measured between 160 and 215 nm. The IR absorption cross sections of the three fluorinated ethanols were measured between 400 and 1900 cm(-1), and their global warming potentials were estimated.     

Kinetics of the unimolecular decomposition of the 2-chloroallyl radical

The thermal decomposition of the 2-chloroallyl radical, CH(2)CClCH(2) --> CH(2)CCH(2) + Cl (1), was studied using the laser photolysis/photoionization mass spectrometry technique. Rate constants were determined in time-resolved experiments as a function of temperature (720-840 K) and bath gas density ([He] = (3-12) x 10(16), [N(2)] = 6 x 10(16) molecule cm(-3)). C(3)H(4) was observed as a primary product of reaction 1. The rate constants of reaction 1 are in the falloff, close to the low-pressure limit, under the conditions of the experiments. The potential energy surface (PES) of reaction 1 was studied using a variety of quantum chemical methods. The results of the study indicate that the minimum energy path of the CH(2)CClCH(2) dissociation proceeds through a PES plateau corresponding to a weakly bound Cl-C(3)H(4) complex; a PES saddle point exists between the equilibrium CH(2)CClCH(2) structure and the Cl-C(3)H(4) complex. The results of quantum chemical calculations, the rate constant values obtained in the experimental study, and literature data on the reverse reaction of addition of Cl to allene were used to create a model of reactions 1 and -1. The experimental dependences of the rate constants on temperature and pressure were reproduced in RRKM/master equation calculations. The reaction model provides expressions for the temperature dependences of the high-pressure-limit and the low-pressure-limit rate constants and the falloff broadening factors (at T = 300-1600 K): k(infinity)(1) = 1.45 x 10(20)T(-1.75) exp(-19609 K/T) s(-1), k(infinity)(-)(1) = 8.94 x 10(-10)T(-0.40) exp(481 K/T) cm(3) molecule(-1) s(-1), k(1)(0)(He) = 5.01 x 10(-32)T(-12.02) exp(-22788 K/T) cm(3) molecule(-1) s(-1), k(1)(0)(N(2)) = 2.50 x 10(-32)T(-11.92) exp(-22756 K/T) cm(3) molecule(-1) s(-1), F(cent)(He) = 0.46 exp(-T/1001 K) + 0.54 exp(-T/996 K) + exp(-4008 K/T), and F(cent)(N(2)) = 0.37 exp(-T/2017 K) + 0.63 exp(-T/142 K) + exp(-4812 K/T). The experimental data are not sufficient to specify all the parameters of the model; consequently, some of the model parameters were obtained from quantum chemical calculations and from analogy with other reactions of radical decomposition. Thus, the parametrization is most reliable under conditions close to those used in the experiments.     

Ab initio study on mechanisms and kinetics for reaction of NCS with NO

The mechanisms and kinetics of the reaction of a thiocyanato radical (NCS) with NO were investigated by a high-level ab initio molecular orbital method in conjunction with variational RRKM calculations. The species involved were optimized at the B3LYP/6-311++G(3df,2p) level, and their single-point energies were refined by the CCSD(T)/aug-cc-PVQZ//B3LYP/6-311+G(3df,2p) method. Our calculated results indicate favorable pathways for the formation of several isomers of an NCSNO complex. Formation of OCS + N 2 also is possible, although this pathway involves a substantial energy barrier. The predicted total rate constants, k total, at a 2 torr He pressure can be represented by the following equations: k total = 9.74 x 10 (26) T (-13.88) exp(-6.53 (kcal mol (-1))/ RT) at T = 298-950 K and 1.17 x 10 (-22) T (2.52) exp(-6.86 (kcal mol (-1))/ RT) at T = 960-3000 K, in units of cm (3) molecule (-1) s (-1), and the predicted values are in good agreement with the experimental results in the temperature range of 298-468 K. The calculated results clearly indicate that the branching ratio for R M1 in the temperature range of 298-950 K has the largest value ( R M1 accounts for 0.53-0.39). However, in the higher temperature range (960-3000 K), the formation of OCS + N 2 ( P5) with branching ratio R P5 (0.40-0.79) becomes dominant. The rate constants for key individual product channels are provided for different temperature and pressure conditions.     

Experimental and theoretical studies of rate coefficients for the reaction O(3P)+CH3OH at high temperatures

Rate coefficients of the reaction O((3)P) + CH(3)OH in the temperature range of 835-1777 K were determined using a diaphragmless shock tube. O atoms were generated by photolysis of SO(2) with a KrF excimer laser at 248 nm or an ArF excimer laser at 193 nm; their concentrations were monitored via atomic resonance absorption excited by emission from a microwave-discharged mixture of O(2) and He. The rate coefficients determined for the temperature range can be represented by the Arrhenius equation, k(T) = (2.29 +/- 0.18) x 10(-10) exp[-(4210 +/- 100)T] cm(3) molecule(-1) s(-1); unless otherwise noted, all the listed errors represent one standard deviation in fitting. Combination of these and previous data at lower temperature shows a non-Arrhenius behavior described as the three-parameter equation, k(T) = (2.74 +/- 0.07) x 10(-18)T(2.25 +/- 0.13) exp[-(1500 +/- 90)T] cm(3)molecule(-1) s(-1). Theoretical calculations at the Becke-3-Lee-Yang-Parr (B3LYP)6-311 + G(3df,2p) level locate three transition states. Based on the energies computed with coupled clusters singles, doubles (triples) [CCSD(T)]/6-311 + G(3df,2p)B3LYP6-311 + G(3df,2p), the rate coefficients predicted with canonical variational transition state theory with small curvature tunneling corrections agree satisfactorily with the experimental observations. The branching ratios of two accessible reaction channels forming OH + CH(2)OH (1a) and OH + CH(3)O (1b) are predicted to vary strongly with temperature. At 300 K, reaction (1a) dominates, whereas reaction (1b) becomes more important than reaction (1a) above 1700 K.