New mechanistic pathways for the formation of organosulfates catalyzed by ammonia and carbinolamine formation catalyzed by sulfuric acid in the atmosphere.

In the secondary organic aerosol formation, exploring the formation of nucleation precursors is of paramount importance for understanding the secondary organic aerosol formation. Here, we report new mechanistic pathways for the formation of organosulfates and carbinolamine in the atmospheric gas phase by utilizing a high-level W2X//QCISD/cc-pV(T+d)Z method close to CCSD(T)/CBS accuracy, dual level kinetics strategy by combining multistructural variational the transition state theory, containing small-curvature tunneling at the M08-SO/maug-cc-pVTZ level with the conventional transition state theory at the W2X//QCISD/cc-pV(T+d)Z level, and torsional anharmonicity. However, a previously suggested mechanism indicated that these are only formed in the heterogeneous atmospheric chemical processes. Furthermore, we find that ammonia only exerts a catalytic role in the HCHO + H2SO4 reaction responsible for the formation of organosulfates under some atmospheric conditions, whereas sulfuric acid can significantly promote the HCHO + NH3 reactions, resulting in the formation of carbinolamine in the atmosphere. These calculated results also show that the ammonia-catalyzed reaction of formaldehyde with sulfuric acid can play an important role in the loss of formaldehyde and the sulfuric acid-catalyzed reaction of formaldehyde with ammonia can also contribute towards the loss of ammonia under some atmospheric conditions. In theory, detailed computational strategies have been designed to obtain quantitative rate constants for the reactions investigated here. A remarkable decrease in the rate constant of the reaction between HCHO and H2SO4NH3 is also observed due to recrossing effects. In addition, the calculated results show that M08-SO functional can provide reliable results for describing the reactions studied here with unsigned error bars of less than 1 kcal mol-1.

Atmospheric chemistry of CH3CHO: the hydrolysis of CH3CHO catalyzed by H2SO4.

Elucidating atmospheric oxidation mechanisms and the reaction kinetics of atmospheric compounds is of great importance and necessary for atmospheric modeling and the understanding of the formation of atmospheric organic aerosols. While the hydrolysis of aldehydes has been detected in the presence of sulfuric acid, the reaction mechanism and kinetics remain unclear. Herein, we use electronic structure methods with CCSD(T)/CBS accuracy and canonical variational transition state theory combined with small-curvature tunneling to study the reaction mechanism and kinetics of the hydrolysis of CH3CHO. The calculated results show that the hydrolysis of CH3CHO needs to overcome an energy barrier of 37.21 kcal mol-1, while the energy barrier is decreased to -9.79 kcal mol-1 with a sulfuric acid catalyst. In addition, the calculated kinetic results show that the H2SO4H2O + CH3CHO reaction is faster than H2SO4 + CH3CHOH2O. Additionally, the H2SO4H2O + CH3CHO reaction can play an important role in the sink of CH3CHO below 260 K occurring during the night period when OH, H2SO4, and H2O concentrations are 104, 108, and 1017 molecules cm-3, respectively, because it can compete well with the CH3CHO + OH reaction. There are wide implications in atmospheric chemistry from these findings because of the potential importance of the catalytic effect of H2SO4 on the hydrolysis of CH3CHO in the atmosphere and in the formation of secondary organic aerosols.

The reaction mechanisms and kinetics of CF3CHFOCH 3 and CHF 2CHFOCF 3 with atomic chlorine: a computational study.

Due to their lack of effect on the ozone depletion, hydrofluoroethers are considered as potential candidates for third generation refrigerants. In the present work, the mechanisms and kinetics of reaction of the Cl atom with CF(3)CHFOCH(3) and CHF(2)CHFOCF(3) were investigated theoretically using quantum chemical methods and transition state theory. Four reaction pathways for the title reaction were explored. By using conventional transition state theory with Eckart tunneling correction, the rate constants of the title reaction were obtained over the temperature range 200-300 K. Kinetic calculations demonstrate that H-abstraction from the -CH(3) group in CF(3)CHFOCH(3) and H-abstraction from the -CHF2 group in CHF(2)CHFOCF(3) are major reaction pathways, with the barrier heights of the two paths calculated to be -1.04 and 4.33 kcal mol(-1), respectively. However, the contribution of H-abstraction from the -CHFO- group for the two reactions should also be taken into account with increased temperature. At 298 K, the calculated overall rate constants of the reaction of CHF(2)CHFOCF(3) with the Cl atom are 4.27 × 10(-15) cm(3) molecule(-1) s(-1), which is consistent with the experimental value of (1.2 ± 2.0) × 10(-15) cm(3) molecule(-1) s(-1).

Theoretical studies on gas-phase reactions of sulfuric acid catalyzed hydrolysis of formaldehyde and formaldehyde with sulfuric acid and H2SO4···H2O complex.

The gas-phase reactions of sulfuric acid catalyzed hydrolysis of formaldehyde and formaldehyde with sulfuric acid and H2SO4···H2O complex are investigated employing the high-level quantum chemical calculations with M06-2X and CCSD(T) theoretical methods and the conventional transition state theory (CTST) with Eckart tunneling correction. The calculated results show that the energy barrier of hydrolysis of formaldehyde in gas phase is lowered to 6.09 kcal/mol from 38.04 kcal/mol, when the sulfuric acid is acted as a catalyst at the CCSD(T)/aug-cc-pv(T+d)z//M06-2X/6-311++G(3df,3pd) level of theory. Furthermore, the rate constant of the sulfuric acid catalyzed hydrolysis of formaldehyde combined with the concentrations of the species in the atmosphere demonstrates that the gas-phase hydrolysis of formaldehyde of sulfuric acid catalyst is feasible and could be of great importance for the sink of formaldehyde, which is in previously forbidden hydrolysis reaction. However, it is shown that the gas-phase reactions of formaldehyde with sulfuric acid and H2SO4···H2O complex lead to the formation of H2C(OH)OSO3H, which is of minor importance in the atmosphere.

Formic acid catalyzed gas-phase reaction of H2O with SO3 and the reverse reaction: a theoretical study.

The formic acid catalyzed gas-phase reaction between H(2)O and SO(3) and its reverse reaction are respectively investigated by means of quantum chemical calculations at the CCSD(T)//B3LYP/cc-pv(T+d)z and CCSD(T)//MP2/aug-cc-pv(T+d)z levels of theory. Remarkably, the activation energy relative to the reactants for the reaction of H(2)O with SO(3) is lowered through formic acid catalysis from 15.97 kcal mol(-1) to -15.12 and -14.83 kcal mol(-1) for the formed H(2)O⋅⋅⋅SO(3) complex plus HCOOH and the formed H(2)O⋅⋅⋅HCOOH complex plus SO(3), respectively, at the CCSD(T)//MP2/aug-cc-pv(T+d)z level. For the reverse reaction, the energy barrier for decomposition of sulfuric acid is reduced to -3.07 kcal mol(-1) from 35.82 kcal mol(-1) with the aid of formic acid. The results show that formic acid plays a strong catalytic role in facilitating the formation and decomposition of sulfuric acid. The rate constant of the SO(3)+H(2)O reaction with formic acid is 10(5) times greater than that of the corresponding reaction with water dimer. The calculated rate constant for the HCOOH+H(2)SO(4) reaction is about 10(-13) cm(3) molecule(-1) s(-1) in the temperature range 200-280 K. The results of the present investigation show that formic acid plays a crucial role in the cycle between SO(3) and H(2)SO(4) in atmospheric chemistry.

Theoretical studies on reactions of the stabilized H2COO with HO2 and the HO2···H2O complex.

The reactions of H(2)COO with HO(2) and the HO(2)···H(2)O complex are studied by employing the high-level quantum chemical calculations with B3LYP and CCSD(T) theoretical methods, the conventional transition-state theory (CTST), and the Rice-Ramsperger-Kassel-Marcus (RRKM) with Eckart tunneling correction. The calculated results show that the proton transfer plus the addition reaction channel (TS1A) is preferable for the reaction of H(2)COO with HO(2) because the barriers are -10.8 and 1.6 kcal/mol relative to the free reactants and the prereactive complex, respectively, at the CCSD(T)/6-311++G(3df,2p)//B3LYP/6-311++G(d,p) level of theory. Furthermore, the rate constant via TS1A (2.23 × 10(-10) cm(3) molecule(-1) s(-1)) combined with the concentrations of the species in the atmosphere demonstrates that the HO(2) radical would be the dominant sink of H(2)COO in some areas, where the concentration of water is less than 10(17) molecules cm(-3). In addition, although the single water molecule would lower the activated barrier of TS1A from 1.0 to 0.1 kcal/mol with respect to the respective complexes, the rate constant is lower than that of the reaction of HO(2) with H(2)COO.

Theoretical study on the gas phase reaction of sulfuric acid with hydroxyl radical in the presence of water.

The reactions of H2SO4 with the OH radical without water and with water are investigated employing the quantum chemical calculations at the B3LYP/6-311+G(2df,2p) and MP2/aug-cc-pv(T+d)z levels of theory, respectively. The calculated results show that the reaction of H2SO4 with OH and H2O is a very complex mechanism because of the formation of the prereactive complex prior to the transition state and product. There are two prereactive complexes with stabilization energies being -20.28 and -20.67 kcal/mol, respectively. In addition, the single water can lower the energy barriers of the hydrogen abstraction and the proton transfer to 7.51 and 6.37 kcal/mol, respectively from 13.79 and 8.82 kcal/mol with respect to the corresponding prereactive complex. The computed rate constants indicate that the water-assisted reaction of sulfuric acid with OH radical is of greater importance than the reaction of the naked sulfuric acid with the OH radical because the rate constant of the water-assisted process is about 10(3) faster than that of the reaction sulfuric acid with OH. Therefore, the conclusion is obtained that the water-assisted process plays an important role in the sink for the gaseous sulfuric acid in the clean area.