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The reaction mechanisms for OH-initiated oxidation of trans-β-ocimene (TBO) in the atmosphere have been investigated at the CCSD(T)/6-31 + G(d)//BH&HLYP/6-311++G(d,p) levels of theory. Classical transition state theory (CTST) is used to obtain the rate constants of initial reaction pathways over the temperature range of 220–1000K. The calculated results show that in the initial reaction pathways of TBO with OH radical, the reaction is dominated by OH addition to the double bonds, and H-abstraction pathways can be negligible at low to medium temperature. The total rate constant computed is 4.56 × 10−10 cm3 molecule−1 s−1 at 298 K, which is consistent with the experimental data. The Arrhenius equation of the total rate constants can be fitted as k t o t a l = 2.20 × 10 − 21 T 3.1 e x p (2499.2 / T) in the temperature range of 220–1000 K. In the presence of O2 and NO, the major degradation products of TBO with OH radical are acetone and 4-methyl-3,5-hexadienal, which are in reasonable agreement with the experimental observations. Furthermore, the ozone formation potential of TBO together with the effects of the TBO oxidation products have also been discussed in detail in the paper. [ABSTRACT FROM AUTHOR]

The effect of water (W), ammonia (AM), formic acid (FA), or sulfuric acid (SA) on the HCN + H2O reaction in the aqueous solution has been investigated at the CCSD(T)/6-311++G(2df,2pd), PCM (water)//M06-2X/6-311++G(d,p) + 0.967 × ZPE levels of theory. In addition, transition state theory (TST) has been used to calculate the bimolecular rate constants for the HCN + H2O reaction catalyzed by W, AM, FA, or SA. Our calculations reveal that SA is a significantly more effective catalyst for the hydrolysis of HCN to form formamide (NH2CHO) compared with W, AM, and FA. [ABSTRACT FROM AUTHOR]

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]