Feng-Yang Bai

Theoretical Studies of the Reactions CFxH3−xCOOR+Cl and CF3COOCH3+OH

The mechanism and kinetics of the reactions of CF(3)COOCH(2)CH(3), CF(2)HCOOCH(3), and CF(3)COOCH(3) with Cl and OH radicals are studied using the B3LYP, MP2, BHandHLYP, and M06-2X methods with the 6-311G(d,p) basis set. The study is further refined by using the CCSD(T) and QCISD(T)/6-311++G(d,p) methods. Seven hydrogen-abstraction channels are found. All the rate constants, computed by a dual-level direct method with a small-curvature tunneling correction, are in good agreement with the experimental data. The tunneling effect is found to be important for the calculated rate constants in the low-temperature range. For the reaction of CF(3)COOCH(2)CH(3) +Cl, H-abstraction from the CH(2) group is found to be the dominant reaction channel. The standard enthalpies of formation for the species are also calculated. The Arrhenius expressions are fitted within 200-1000 K as kT(1) =8.4×10(-20) T (2.63) exp(381.28/T), kT(2) =2.95×10(-21) T (3.13) exp(-103.21/T), kT(3) =1.25×10(-23) T (3.37) exp(791.98/T), and kT(4) =4.53×10(-22) T (3.07) exp(465.00/T).

Theoretical Study on the Reactions of (CF3)2CFOCH3 + OH/Cl and Reaction of (CF3)2CFOCHO with Cl Atom

Reactions of (CF3)2CFOCH3 and (CF3)2CFOCHO with hydroxyl radical and chlorine atom are studied at the B3LYP and BHandHLYP/6-311+G(d,p) levels along with the geometries and frequencies of all stationary points. This study is further refined by CCSD(T) and QCISD(T)/6-311+G(d,p) methods in the minimum energy paths. For the reaction (CF3)2CFOCH3 + OH, two hydrogen abstraction channels are found. The total rate constants for the reactions (CF3)2CFOCH3 + OH/Cl and (CF3)2CFOCHO + Cl are followed by means of the canonical variational transition state with the small-curvature tunneling correction. The comparison between the hydrogen abstraction rate constants by hydroxyl and chlorine atom is discussed. Calculated rate constants are in reasonable agreement with the available experiment data. The standard enthalpies of formation for the reactants, (CF3)2CFOCH3 and (CF3)2CFOCHO, and two products, (CF3)2CFOCH2 and (CF3)2CFOCO, are evaluated by a series of isodesmic reactions. The Arrhenius expressions for the title reactions are given as follows: k1= 1.08 × 10(-22) T(3.38) exp(-213.31/T), k2= 3.55 × 10(-22) T(3.61) exp(-240.26/T), and k3= 3.00 × 10 (-19) T(2.58) exp(-1294.34/T) cm(3) molecule(-1) s(-1).

Theoretical study of the gaseous hydrolysis of NO2 in the presence of NH3 as a source of atmospheric HONO

Environmental context Nitrous acid is an important atmospheric trace gas, but the sources and the chemical mechanisms of its production are not well understood. This study explores the effects of ammonia and water on the hydrolysis of nitrogen dioxide and nitrous acid production. The calculated results show that ammonia is more effective than water in promoting the hydrolysis reaction of nitrogen dioxide. Abstract The effects of ammonia and water molecules on the hydrolysis of nitrogen dioxide as well as product accumulation are investigated by theoretical calculations of three series of the molecular clusters 2NO2–mH2O (m=1–3), 2NO2–mH2O–NH3 (m=1, 2) and 2NO2–mH2O–2NH3 (m=1, 2). The gas-phase reaction 2NO2 + H2O → HONO + HNO3 is thermodynamically unfavourable. The additional water or ammonia in the clusters can not only stabilise the products by forming stable complexes, but also reduce the energy barrier for the reaction. There is a considerable energy barrier for the reaction at the reactant cluster 2NO2–H2O: 11.7kcalmol–1 (1kcalmol–1=4.18kJmol–1). With ammonia and an additional water in the cluster, 2NO2–H2O–NH3, the thermodynamically stable products t-HONO + NH4NO3–H2O can be formed without an energy barrier. With two ammonia molecules, as in the cluster 2NO2–mH2O–2NH3 (m=1, 2), the reaction is barrierless and the product complex NH4NO2–NH4NO3 is further stabilised. The present study, including natural bond orbital analysis on a series of species, shows that ammonia is more effective than water in promoting the hydrolysis reaction of NO2. The product cluster NH4NO2–NH4NO3 resembles an alternating layered structure containing the ion units NH4+NO2– and NH4+NO3–. The decomposition processes of NH4NO2–NH4NO3 and its monohydrate are all spontaneous and endothermic.

Atmospheric chemistry of ethers, esters, and alcohols on the lifetimes, temperature dependence, and kinetic isotope effect: an example of CF3CX2CX2CX2OX with OX reactions (X = H, D)

The dual-level direct dynamics method is employed to investigate the hydrogen abstraction reaction of CF3CH2CH2CH2OH (CF3CD2CD2CD2OD) with OH (OD) radicals. Four possible reaction channels caused by different positions of hydrogen atom attack are found. All the stationary points are studied with the ab initio and density functional theories. Single points computation is further refined by CCSD(T) and QCISD(T) methods combined with the 6-311++G(d,p) basis set in the minimum energy paths (MEP). Rate constants for each reaction channel, obtained by canonical variational transition state (CVT) coupled with the small curvatures tunneling (SCT) correction, are found to coincide with the available data in experiments. Calculations show that the variational effect was small in 200–2000 K, while the tunneling effect is large for every reaction channel in low-temperature regions. It is shown that the H-abstraction from the –CH2O– group is the primary channel. Standard enthalpies of formation for the species are computed, and the kinetic isotope effects for reactions CF3CH2CH2CH2OH/CF3CD2CD2CD2OD + OH and CF3CH2CH2CH2OH + OH/OD are discussed to provide valuable information for subsequent research. In addition, atmospheric lifetimes of a series of related ethers, esters, and alcohols are estimated. The Arrhenius expression for the title reaction k(T) = 3.43 × 10−21T3.22 exp(741.70/T) cm3 per molecule per s is also provided.

Atmospheric chemistry of CF3(CX2)2CH2OH: rate coefficients and temperature dependence of reactions with chlorine atoms and the subsequent pathways of alkyl and alkoxy radicals (X = H, F)

The atmospheric and kinetic properties of CF3(CX2)2CH2OH (X = H, F) with chlorine atoms were studied by density functional and canonical variational transition state theories in conjunction with the small-curvature tunneling correction. The minimum energy path was obtained by the CCSD(T)/6-311++G(d,p)//B3LYP/6-311G(d,p) method. The H-abstraction channel from the –CH2O– group was found to be the dominant channel, whereas that from the –OH site of the title reactions may be negligible because of the high barrier. All rate constants computed within 200–1000 K are in reasonable agreement with the available experimental values. The degradation mechanism of CF3(CX2)2CH2OH is discussed. The subsequent pathways of the CF3(CX2)2C˙HOH and CF3(CX2)2C(O˙)HOH radicals were studied. The atmospheric lifetime and global warming potentials (GWPs) of CF3(CX2)2CH2OH were computed, and it is shown that fluorine substitution may increase the lifetime and GWPs. It is also indicated that fluorine substitution may decrease the reactivity. The reaction enthalpies and reaction Gibbs free energies for all relevant reactions were discussed. The rate coefficient expressions for the title reactions obtained are kT1 = 5.75 × 10−17T2.26exp(428.02/T) and kT2 = 1.30 × 10−17T1.96exp(67.40/T) per cm3 per molecule per s.

Theoretical study of the hydrolysis of HOSO + NO2 as a source of atmospheric HONO: effects of H2O or NH3

Environmental context Nitrous acid (HONO) has long been recognized as an important atmospheric pollutant, with the reaction of HOSO+NO2 being a source of HONO. We explore the effects of an additional water or ammonia molecule on this reaction. Calculations show that the ammonia molecule has a more effective role than the water molecule in assisting the reaction. Abstract Depending on different ways that NO2 approaches the HOSO radical, the main reactant complexes HOS(O)NO2 and HOS(O)ONO–L (lowest energy structure of the isomer) were revealed by Lesar et al. (J. Phys. Chem. A 2011, 115, 11008), and the reaction of HOSO+NO2 is a source of trans (t)-HONO and SO2. In the present work, the water molecule in the hydrolysis reaction of HOSO+NO2 not only acts as a catalyst giving the products of t-HONO+SO2, but also as a reactant giving the products of t-HONO+H2SO3, c-HONO+H2SO3 and HNO3+t-S(OH)2. For the reaction of HOSO+NO2+H2O, the main reaction paths 2, 7, and 9 are further investigated with an additional water or ammonia molecule. The CBS-QB3 calculation result shows that the process of HOS(O)NO2–H2O → t-HONO–SO2–H2O is favourable with a barrier of 0.1kcal mol–1. Although the following process of t-HONO–SO2–H2O → t-HONO–H2SO3 is unfavourable with a barrier 33.6kcal mol–1, the barrier is reduced by 17.3 or 26.3kcal mol–1 with an additional water or ammonia molecule. Starting with HOS(O)ONO–L–H2O, the energy barriers of path 7 and path 9 are reduced by 8.9 and 8.5kcal mol–1 with an additional water molecule and by 9.9 and 9.2kcal mol–1 with an additional ammonia molecule. Ammonia is more beneficial than water for assisting the HOSO+NO2+H2O reaction. Three t-HONO–H2SO3 isomers which contain double intermolecular hydrogen bonds are studied by frequency and natural bond orbital calculations. Frequency calculations show that all hydrogen bonds exhibit an obvious red shift. The larger second-order stabilisation energies are consistent with the shorter hydrogen bonds. H2SO3 can promote the process of t-HONO → HNO2, and reduce the barrier by 45.2kcal mol–1. The product NH3–H2SO3 can further form a larger cluster (NH3–H2SO3)n (n=2, 4) including NH4+HSO3– ion pairs.

Atmospheric chemistry of alkyl iodides: theoretical studies on the mechanisms and kinetics of CH3I/C2H5I + NO3 reactions

The gas-phase reactions of CH3I and C2H5I with NO3 radicals have been studied using a dual-level direct kinetics method. The minimum energy paths have been refined by CCSD(T) and QCISD(T) methods. One displacement and two hydrogen abstraction processes were found for the reaction of CH3I + NO3. For the reaction of C2H5I + NO3, three hydrogen abstraction and one displacement channel were found. The hydrogen abstraction from the –CH2– group was found to be the dominant channel. The displacement channel of the title reactions may be negligible because of the high barrier. The rate constants for the individual reaction channels were followed by means of the canonical variational transition state with the small-curvature tunneling correction. The calculated rate constants were in reasonable agreement with the available data from experiments. The Arrhenius expressions for the title reactions are given as follows: ka = 8.62 × 10−32T6.66 exp(1324.23/T), kb = 9.48 × 10−27T5.75 exp(−655.34/T) cm3 per molecule per s. The atmospheric lifetimes of CH3I and C2H5I determined by reaction with the NO3 radical were about 3.07 and 5.86 h, which indicate that they can be degraded in the gas phase within a short time to serve as a source of reactive iodine compounds at night-time.