•CH3S + H2 is the major product for title reaction without and with water vapor.•The introduction of water vapor reduces the energy of transition state dramatically.•A single H2O molecule can strongly accelerate the reaction rate constant.The reaction mechanism and kinetics have been studied to investigate the possible effect of water vapor on the CH3SH + H reaction. Based on the energy profile for the studied reactions obtained at CCSD(T)/aug-cc-pVTZ//MP2(full)/6–311++G (3df,3pd) levels of theory with ZPE correction, the formation of P1(CH3S + H2) is predominant both in the absence and presence of water vapor. Due to the formation of hydrogen bond, the energies of transition states for reaction with water vapor are lower than those of naked reaction. Accordingly, in the presence of water vapor, the total rate constants computed by transition state theory are about 1–2 orders of magnitude larger than those of naked reaction at 249–605 K, which indicates that the involvement of water vapor would play an important role for the CH3SH + H reaction.Download high-res image (64KB)Download full-size image

The atmospheric oxidation mechanism of 4,4-dimethyl-1-pentene (DMP441) initiated by OH radical has been theoretically investigated at the BH&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 products in the OH-radical-initiated degradation of DMP441 in the presence of O2. However, the epoxide conformers and enols are expected to be minor products because of the high isomerization barriers involved. The calculated results are in qualitative accordance with experimental evidence. Conventional transition state theory has been used to calculate the rate constants of the initial addition channels of the OH + DMP441 reaction over the temperature range 220–500 K. The computed total rate constant at 298 K is 2.20 × 10–11 cm3 molecule–1 s–1, which is in very good agreement with the experimental value. Furthermore, it has been found that the calculated rate constant exhibits a weak non-Arrhenius behavior over the temperature range 220–500 K. The computed expression for the rate constant is k(OH+DMP441) = 1.22 × 10–12 exp[(880 K)/T] cm3 molecule–1 s–1.

In this paper, for the hydrogen abstraction reaction of HCHO by OH radicals assisted by water, formic acid, or sulfur acid, the possible reaction mechanisms and kinetics have been investigated theoretically using quantum chemistry methods and transition-state theory. The potential energy 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 hydrogen bond(s), the relative energies of the transition states involving catalyst are significantly reduced compared to that reaction without catalyst. However, the kinetics calculations show that the rate constants are smaller by about 3, 9, or 10 orders of magnitude for water, formic acid, or sulfur acid assisted reactions than that uncatalyzed reaction, respectively. Consequently, none of the water, formic acid, or sulfur acid can accelerate the title reaction in the atmosphere.

•The addition–elimination mechanism dominates the title reaction.•In the absence of O2, the dominant products are cyclopentanone and enols.•In the presence of O2, the major products are glutaraldehyde and epoxycyclopentanol.We performed the first theoretical potential energy surface investigation on the mechanism and products of the reaction of OH+ cyclopentene in the absence and presence of O2 by using high-level quantum chemical methods CCSD(T)/6-311++G(d,p)//BH&HLYP/6-311++G(d,p)+ZPE × 0.9335. Energies for several species are also refined at the CCSD(T)/cc-pVTZ levels of theory. The calculations indicate that the major products are cyclopentanone, 1-cyclopenten-1-ol, and 2-cyclopenten-1-ol in the absence of O2, which are in qualitative accordance with the available experimental observations. In the presence of O2, the dominant products are predicted to be glutaraldehyde and 1,2-epoxycyclopentanol.

In this study, the reaction mechanism and kinetics of the hydrogen abstraction and addition reactions of NCO with HCHO in the absence and presence of water have been investigated theoretically for the first time. Our theoretical results indicate that direct hydrogen abstraction is favored in the formation of HNCO instead of HOCN and the addition pathways are negligible with and without water. The rate constants calculated at the CCSD(T)/aug-cc-pVTZ//BH&HLYP/6-311++G(3df,3pd) level with zero-point energy correction range from 1.60 × 10–12 to 4.99 × 10–12 cm3 molecule–1 s–1 between 220 and 769 K without water and are in good agreement with the available experimental values. However, with the inclusion of water, the rate constants are slower by 2–8 orders of magnitude than those of the reaction without water. Accordingly, the effect of water on the reaction of NCO with HCHO is negligible in the atmosphere.