The increasing number of severe smog weather has aroused public concern. Organic nitrate (RONO2) is an important precursor for the formation of smog and secondary organic aerosol (SOA), but its source is not fully understood. Exploring the formation mechanism of RONO2 in the atmosphere has far-reaching significance for understanding the causes of smog particles and improving air quality. However, for all known sources of RONO2, discrepancies exist between field measurements of its concentration and reported sources of formation, suggesting that there must be an important and as yet unknown source for RONO2.
Recently, some researchers used the density functional theory M06-2X method combined with the 6-311+G(d,p) basis set to optimize the geometric structures of all reactants, complexes, transition states and products, and calculated the harmonic frequencies of all stagnation points at the same level. At the same time, the single-point energy correction of all stagnation points was carried out by using the high-level CCSD(T) method combined with the complete basis set (CBS) method calculated by the combination of aug-cc-pVDZ and aug-cc-pVTZ basis sets. The results show that HNO3 and HCHO can directly generate hydroxymethyl nitrate (HOCH2ONO2) by overcoming the energy barrier of 16.73 kcal/mol, with a rate constant of 1.87×10-22 cm3 molecule-1 s-1 at 296 K. In addition, as the carbon chain grows (C1-C5), the activation energy of RONO2 decreases successively, which are 7.87, 5.33, 5.01, 4.97 and 4.85 kcal/mol, respectively, and the reaction rate constant at room temperature increases successively from 1.87×10-22 to 6.77×10-21cm3 molecule-1 s-1, indicating that there may be a positive correlation between the reactivity and the electron-donating ability of the group. At the same time, the HOMO-LUMO energy gap values of transition states TS1-TS5 are calculated, which are 217.90, 216.91, 215.71, 215.59 and 215.54 kcal/mol, respectively. It can be seen that the same type of energy gap decreases and the law of energy barrier and rate changes is consistent, and further illustrates the positive correlation between carbon chain length and reactivity.
Taking the formation reaction of HOCH2ONO2 as a research model, the catalysis of important inorganic species (H2O, NH3, HNO3, H2SO4) and organic species (HCOOH, HOOCCOOH, CH3NH2, CH3NHCH3) in the atmospheric environment was explored. The results show that the above species all regulate the reaction process through the "double hydrogen transfer" mechanism, and the energy barrier (kcal/mol) order of the catalytic reaction is: CH3NHCH3 (9.61) < CH3NH2 (9.75) < H2SO4 (10.69) < HOOCCOOH (10.87− 11.68) < NH3 (12.11) < HCOOH (13.57) < HNO3 (14.93) < H2O (16.41) < None (16.73). It can be clearly seen that CH3NH2 and CH3NHCH3 have the best catalytic effects, which can reduce the energy barrier of the naked reaction by 6.98 and 7.12 kcal/mol respectively; In addition, the results showed that sulfuric acid was the strongest acidic catalyst, and dicarboxylic acids were more catalytically active than monocarboxylic acids.
The researchers also calculated the minimum effective rate constants for water, nitric acid, sulfuric acid, formic acid, oxalic acid, ammonia, methylamine, and dimethylamine to catalyze the reaction of HCHO and HNO3 to HOCH2ONO2 in the temperature range of 200-296 K. At 200 K, the reaction keff-Xmin values are 3.35×10-25, 1.56×10-23, 6.90×10-26, 5.92×10-16, 3.33×10-16, 4.92×10-22, 2.31×10-23, 6.60×10-23, 6.98×10-23, 1.19×10-19, 1.38×10-11 and 2.90×10-6 cm3 molecule−1 s−1, which shows that except nitric acid, other species can promote the formation of HOCH2ONO2 by accelerating the reaction rate. However, as the temperature increased to 296 K, only the effective catalytic rate constants of CH3NH2 and CH3NHCH3 were about 3 and 6 orders of magnitude higher than those of the bare reaction, respectively, indicating that organic amines have a significant promoting effect on the formation of organic nitrates.
Further experiments also show that CH3NH2 and CH3NHCH3 have a stronger ability to accept protons, making it easier for NO3 in the original HNO3 to add to the C atom of HCHO and finally form HOCH2ONO2.
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Reference
- Metal-free catalysis on the reactions of nitric acid with aliphatic aldehydes: A new potential source of organic nitrates
Atmospheric Environment, Volume 299, 15 April 2023, 119673
