New insights in atmospheric acid-catalyzed gas phase hydrolysis of formaldehyde: a theoretical study

Abstract

The gas phase hydrolysis of HCHO catalyzed via nitric acid and acetic acid, the typical atmospheric acids has been theoretically investigated using M06-2X, CCSD(T), and CCSD(T)-F12A theoretical methods using the 6-311++G(d,p), aug-cc-pVTZ, and VTZ-F12 basis sets and utilizing transition state theory. Our studies predict that when the HNO₃ or CH₃COOH and HCHO⋯H₂O act as reactants, the reactions occur in one step, whereas the reactions of HNO₃⋯H₂O or CH₃COOH⋯H₂O with HCHO proceed via a two-step mechanism. Our results also show that the free energy barrier of the gas phase hydrolysis of HCHO assisted by HNO₃ or CH₃COOH is reduced to 13.95 or 14.27 kcal mol⁻¹ relative to the respective pre-reactive complex from 40.23 kcal mol⁻¹ in the naked HCHO + H₂O reaction. The calculated kinetic data suggests that the HCHO + HNO₃⋯H₂O entrance channel with a two-step mechanism is 1.84–2.76 times faster than HNO₃ + HCHO⋯H₂O with a one-step mechanism, whereas the HCHO⋯H₂O + CH₃COOH entrance path is significantly more favorable than that of HCHO + CH₃COOH⋯H₂O, in the temperature range of 200–300 K. The reaction rates of the gas phase hydrolysis of HCHO catalyzed by HNO₃ or CH₃COOH are much slower than that of the gas phase reaction of HCHO with an OH radical, which demonstrates that the contributions of both catalytic reactions are of minor importance for the sink of HCHO in gas-phase atmospheric chemistry. However, the new findings in this investigation are not only of great necessity and importance for elucidating the gas phase hydrolysis of formaldehyde, but are also of great interest for understanding the importance of other carbonyl compounds in the atmosphere.