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.

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.

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.

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 CHCHO. The calculated results show that the hydrolysis of CHCHO needs to overcome an energy barrier of 37.21 kcal mol, while the energy barrier is decreased to -9.79 kcal mol with a sulfuric acid catalyst. In addition, the calculated kinetic results show that the HSOHO + CHCHO reaction is faster than HSO + CHCHOHO. Additionally, the HSOHO + CHCHO reaction can play an important role in the sink of CHCHO below 260 K occurring during the night period when OH, HSO, and HO concentrations are 10, 10, and 10 molecules cm, respectively, because it can compete well with the CHCHO + OH reaction. There are wide implications in atmospheric chemistry from these findings because of the potential importance of the catalytic effect of HSO on the hydrolysis of CHCHO in the atmosphere and in the formation of secondary organic aerosols.

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