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Kinetics and mechanism of the gas-phase reaction of CH3C(O)OCH(CH3)CH2OCH3 (MPA) with OH radicals in the presence of O2 and NO have been investigated theoretically by performing a high and reliable level of theory, viz., CCSD(T)/6-311 + G(d,p)//BH&HLYP/6-311++G(d,p) + 0.9335×ZPE. The calculations predict that the H-abstraction from the −CH2−O− position of MPA is the most facile channel, which leads to the formation of the corresponding alkoxy radicals CH3C(O)OCH(CH3)C(O •)HOCH3 under atmospheric conditions. This activated radicals CH3C(O)OCH(CH3)C(O •)HOCH3 will undergo further rearrangement, fragmentation and oxidative reactions and predominantly leads to the formation of various products (methyl formate HC(O)OCH3 and acetic anhydride CH3C(O)OC(O)CH3). In the presence of water, acetic anhydride can convert into acetic acid CH3C(O)OH via the hydrolysis reaction. The calculated total rate constants over the temperature range 263–372 K are used to derive a negative activation energy (Ea= −5.88 kJ/mol) and an pre-exponential factor (A = 1.78×10−12 cm3 molecule−1 s−1). The obtained Arrhenius parameters presented here are in strong agreement with the experimental values. Moreover, the temperature dependence of the total rate constant over a temperature range of 263−1000 K can be described by k = 5.60 × 10−14×(T/298 K)3.4×exp(1725.7 K/T) cm3 molecule−1 s−1. [ABSTRACT FROM AUTHOR]

The potential energy surface profiles for the gas-phase pyrolysis of methyl azidoformate (MA, CH3OC(O)N3) in the absence and presence of one water molecule have been investigated by ab initio methods at CCSD(T)/6-311++G(2df,2pd)//MP2(full)/6-311++G(d,p)+0.95×ZPE levels of theory. Three types of mechanisms are discussed for the gas-phase decomposition of CH3OC(O)N3. Ab initio calculations show that a four-membered-ring intermediate can be formed by the stepwise routes. The resulting intermediate can undergo two competitive decomposition channels to generate the major products CO2 + CH2 = NH and HNCO + HC(O)H. The calculated results are in qualitative agreement with the observed experimental data. However, CH3ONCO can be produced from the Curtius-type rearrangement route. This is an intriguing finding in this study. Moreover, the effect of one water molecule on the gas-phase pyrolysis of MA has been also explored. We find that the relative energy of the hydrated transition states is effectively lowered when water is added to the reaction. However, the estimated rate constant at 625 K for the naked reaction is about 30 times faster than the reaction with water. Thus, a single water molecule cannot play an important role in the thermal decomposition of MA. [ABSTRACT FROM AUTHOR]

The mechanism and products of the reaction of (Z)-2-penten-1-ol [(Z)-PO21] with OH radical in the presence of O2have been elucidated by using high-level quantum chemical methods CCSD(T)/6-311+G(d,p)//BH&HLYP/6-311++G(d,p). The calculations clearly indicate that addition channels contribute maximum to the total reaction and H-abstraction channels can be neglected at temperatures of 220–500 K. The rate constant for the reaction of OH radical with (Z)-PO21 at 298 K is computed to be 1.22 × 10−10cm3molecule−1s−1, which is in stronger agreement with the previously reported experimental values. The kinetic data obtained over the temperature range 220−500 K are used to derive an non-Arrhenius expression:k= 3.69 × 10−13× exp(1763.7/T) cm3molecule−1s−1. For the reaction of (Z)-PO21with OH radical in the presence of O2, the major primary reaction products found in this study are propanal [CH3CH2C(O)H] and glycolaldehyde [HOCH2C(O)H], whereas formaldehyde [HC(O)H], 2-hydroxybutanal [CH3CH2CH(OH)C(O)H] and the epoxide P18 are anticipated to be minor products. The calculated results are consistent with the recent experimental observations. [ABSTRACT FROM AUTHOR]

Theatmospheric oxidation mechanism of 4,4-dimethyl-1-pentene (DMP441)initiated by OH radical has been theoretically investigated at theBH&HLYP/6-311++G(d,p) and CCSD(T)/6-31+G(d,p) levels of theory.HC(O)H and 3,3-dimethylbutanal [(CH3)3CCH2C(O)H] are identified in our calculations as major productsin the OH-radical-initiated degradation of DMP441 in the presenceof O2. However, the epoxide conformers and enols are expectedto be minor products because of the high isomerization barriers involved.The calculated results are in qualitative accordance with experimentalevidence. Conventional transition state theory has been used to calculatethe rate constants of the initial addition channels of the OH + DMP441reaction over the temperature range 220–500 K. The computedtotal rate constant at 298 K is 2.20 × 10–11cm3molecule–1s–1, which is in very good agreement with the experimental value. Furthermore,it has been found that the calculated rate constant exhibits a weaknon-Arrhenius behavior over the temperature range 220–500 K.The computed expression for the rate constant is k(OH+DMP441) = 1.22 × 10–12exp[(880 K)/T] cm3molecule–1s–1. [ABSTRACT FROM AUTHOR]

Inthis paper, for the hydrogen abstraction reaction of HCHO byOH radicals assisted by water, formic acid, or sulfur acid, the possiblereaction mechanisms and kinetics have been investigated theoreticallyusing quantum chemistry methods and transition-state theory. The potentialenergy surfaces calculated at the CCSD(T)/6-311++G(df,pd)//MP2(full)/6-311++G(df,pd)levels of theory reveal that, due to the formation of strong hydrogenbond(s), the relative energies of the transition states involvingcatalyst are significantly reduced compared to that reaction withoutcatalyst. However, the kinetics calculations show that the rate constantsare smaller by about 3, 9, or 10 orders of magnitude for water, formicacid, or sulfur acid assisted reactions than that uncatalyzed reaction,respectively. Consequently, none of the water, formic acid, or sulfuracid can accelerate the title reaction in the atmosphere. [ABSTRACT FROM AUTHOR]

In this study, the reaction mechanismand kinetics of the hydrogenabstraction and addition reactions of NCO with HCHO in the absenceand presence of water have been investigated theoretically for thefirst time. Our theoretical results indicate that direct hydrogenabstraction is favored in the formation of HNCO instead of HOCN andthe addition pathways are negligible with and without water. The rateconstants calculated at the CCSD(T)/aug-cc-pVTZ//BH&HLYP/6-311(3df,3pd)level with zero-point energy correction range from 1.60 × 10–12to 4.99 × 10–12cm3molecule–1s–1between 220 and769 K without water and are in good agreement with the available experimentalvalues. However, with the inclusion of water, the rate constants areslower by 2–8 orders of magnitude than those of the reactionwithout water. Accordingly, the effect of water on the reaction ofNCO with HCHO is negligible in the atmosphere. [ABSTRACT FROM AUTHOR]

Abstract: The potential energy surfaces for the degradation mechanism of OH-initiated allyl alcohol oxidation in the atmosphere have been investigated in detail using QCISD(T)/6-311++G(d,p)//MP2(full)/6-311++G(d,p)+ZPE×0.95 level of theory for the first time. The theoretical results indicate that the initial OH additions to the double bond will be more feasible than the H abstraction reactions. Two initial formed OH radical adducts IM1 and IM2 will prefer to react with O2 and NO rather than undergo its self isomerization and dissociation reactions. Our calculational investigations suggest that the major products for the title reaction in the atmosphere are HCHO+CH2OHCHO, and the minor product is CH2 ich all agree well with the available experimental results. [Copyright &y& Elsevier]

The potential energy surface (PES) for the CF3CFHO2+HO2 reaction has been theoretically investigated using the DFT [B3LYP/6-311G(d,p)] and B3LYP/6-311++G(3df,3pd)//B3LYP/6-311G(d,p) levels of theory. Both singlet and triplet PESs are investigated. The reaction mechanism on the triplet surface is simple. It is revealed that the formation of CF3CFHOOH+3O2 is the dominant channel on the triplet surface. On the basis of the ab initio data, the total rate constants for the reaction CF3CFHO2+HO2 in the T = 210–500 K range have been computed using conventional transition state theory with Wigner's tunneling correction and have been fitted by a rate constant expression as k = 1.04 ×10-12(cm3 molecule-1 s-1) exp (700.33/T). Calculated transition state rate constants with Wigner's tunneling correction for the reaction CF3CFHO2+HO2 are in good agreement with the available experimental values. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [ABSTRACT FROM AUTHOR]

The potential energy surface (PES) for the CF3CFHO2+HO2 reaction has been theoretically investigated using the DFT [B3LYP/6-311G(d,p)] and B3LYP/6-311++G(3df,3pd)//B3LYP/6-311G(d,p) levels of theory. Both singlet and triplet PESs are investigated. The reaction mechanism on the triplet surface is simple. It is revealed that the formation of CF3CFHOOH+3O2 is the dominant channel on the triplet surface. On the basis of the ab initio data, the total rate constants for the reaction CF3CFHO2+HO2 in the T = 210–500 K range have been computed using conventional transition state theory with Wigner's tunneling correction and have been fitted by a rate constant expression as k = 1.04 ×10-12(cm3 molecule-1 s-1) exp (700.33/T). Calculated transition state rate constants with Wigner's tunneling correction for the reaction CF3CFHO2+HO2 are in good agreement with the available experimental values. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [ABSTRACT FROM AUTHOR]

Six product channels have been found in the association reaction of CN + CH2CO, and a variety of possible complexes and saddle points along the minimum energy reaction paths have been characterized at the UMP2(full)/6-31G(d) level. The dominant reaction channels are the production of CH2CN + CO and CH2NC + CO. The isomerization and dissociation reactions of the major products of CH2CN and CH2NC have been investigated using the G2MP2 level. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [ABSTRACT FROM AUTHOR]