Quantitative Kinetics of HO₂ Reactions with Aldehydes in the Atmosphere: High-Order Dynamic Correlation, Anharmonicity, and Falloff Effects Are All Important

Abstract

Kinetics provides the fundamental parameters for elucidating sources and sinks of key atmospheric species and for atmospheric modeling more generally. Obtaining quantitative kinetics in the laboratory for the full range of atmospheric temperatures and pressures is quite difficult. Here, we use computational chemistry to obtain quantitative rate constants for the reactions of HO 2 with HCHO, CH 3 CHO, and CF 3 CHO. First, we calculate the high-pressure-limit rate constants by using a duallevel strategy that combines conventional transition state theory using a high level of electronic structure wave function theory with canonical variational transition state theory including smallcurvature tunneling using density functional theory. The wavefunction level is beyond-CCSD(T) for HCHO and CCSD(T)-F12a (Level-A) for XCHO (X = CH 3 , CF 3 ), and the density functional (Level-B) is specifically validated for these reactions. Then, we calculate the pressure-dependent rate constants by using system-specific quantum RRK theory (SS-QRRK) and also by an energy-grained master equation. The two treatments of the pressure dependence agree well. We find that the Level-A//Level-B method gives good agreement with CCSDTQ(P)/CBS. We also find that anharmonicity is an important factor that increases the rate constants of all three reactions. We find that the HO 2 + HCHO reaction has a significant dependence on pressure, but the HO 2 + CF 3 CHO reaction is almost independent of pressure. Our findings show that the HO 2 + HCHO reaction makes important contribution to the sink for HCHO, and the HO 2 + CF 3 CHO reaction is the dominant sink for CF 3 CHO in the atmosphere.

Kinetics of Sulfur Trioxide Reaction with Water Vapor to Form Atmospheric Sulfuric Acid

“…We see that the SRP scaling factor differs from the standard CCSD­(T)-F12a/cc-pVDZ-F12 scaling factor by 0.007 or more for four of the cases including both transition states as well as SO 3 and water dimer. This shows the need for specific treatment of the anharmonicity at some of the stationary points, in keeping with previous investigations that showed that standard scale factors are less accurate for some transition states, ,,− although the standard scale factors are good for most stable molecules and some transition states. , The table shows that the SRP anharmonic ZPEs at the transition states differ from standard-scaling-factor anharmonic ZPEs by 0.43 and 0.51 kcal mol –1 and from directly calculated harmonic ZPEs by 1.09 and 1.17 kcal mol –1 .…”

“…Two kinds of vibrational-frequency scale factors were used. For the LL calculations and calculations used to check convergence, we used standard scale factors given in Table S1 (tables with a prefix S are in the Supporting Information), and for HL calculations, we used specific reaction-parameter scale factors , given in Tables S2 and . Standard scale factors take the form λ ZPE = λ Anh λ normalH where λ H corrects a specific combination of method (wave function method or density functional) and basis set for its systematic error in the prediction of harmonic frequencies (and hence also in the prediction of harmonic ZPEs), and λ Anh , which is independent of method and basis set, is a general parameter (equal to 0.986) for the ratio of anharmonic ZPE to the harmonic one.…”

An unexpected feasible route for the formation of organosulfates by the gas phase reaction of sulfuric acid with acetaldehyde catalyzed by dimethylamine in the atmosphere

“…The main sink pathway of acetaldehyde is its reaction with OH with an atmospheric lifetime of about one day. 37,85 To further investigate the important atmospheric contribution of the H 2 SO 4 + (CH 3 ) 2 NH + CH 3 CHO reaction, we compared the H 2 SO 4 + (CH 3 ) 2 NH + CH 3 CHO reaction with CH 3 CHO + OH. The rate ratio is expressed in eqn (4):where k CH 3 CHO+OH is the experimental rate constant of the CH 3 CHO + OH reaction at different temperatures.…”

New Reactions for the Formation of Organic Nitrate in the Atmosphere

“… 50 Thus, it is very difficult to determine whether HCHO + HNO 3 can occur more feasibly than HCHO + HCOOH, as the error bars are produced at the two different theoretical methods and their energy barriers are very close to each other. However, the energy barrier of HCHO + HNO 3 (8.80 kcal/mol) is much higher (at least 5 kcal/mol) than those of the HCHO + H 2 SO 4 reaction (3.5–6.1 kcal/mol) at the W2X//QCISD/cc-pV(T + d)Z level, 52 the HCHO + OH reaction (−0.12 kcal/mol) at the CCSD(T)/aug-cc-pVTZ level, 56 the HCHO + HO 2 reaction (−1.03 kcal/mol) at the MW3X-L//CCSD(T)-F12a/cc-pVTZ-F12 level, 57 the HCHO + CH 2 OO reaction (−5.26 kcal/mol) at the W3X-L//CCSD(T)-F12a/cc-pVTZ-F12 level, 40 and the HCHO + HIO 3 reaction (2.96 kcal/mol) at the CCSD(T)-F12a/def2-TZVPP//M06-2X/aug-cc-pVTZ-PP level. 58 It is noted that these reactions mentioned above were carried out using different theoretical methods; this can cause some different error bars for different theoretical methods.…”