The gas phase oxidation of HCOOH by Cl and NH₂ radicals. Proton coupled electron transfer versus hydrogen atom transfer^*

* This paper is dedicated to the memory of Prof. Dieter Cremer.

ABSTRACT

The reaction of formic acid (HCOOH) with chlorine atom and amidogen radical (NH₂) have been investigated using high level theoretical methods such BH&HLYP, MP2, QCISD, and CCSD(T) with the 6–311 + G(2df,2p), aug-cc-pVTZ, aug-cc-pVQZ and extrapolation to CBS basis sets. The abstraction of the acidic and formyl hydrogen atoms of the acid by the two radicals has been considered, and the different reactions proceed either by a proton coupled electron transfer (pcet) and hydrogen atom transfer (hat) mechanisms. Our calculated rate constant at 298 K for the reaction with Cl is 1.14 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ in good agreement with the experimental value 1.8 ± 0.12/2.0 × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ and the reaction proceeds exclusively by abstraction of the formyl hydrogen atom, via hat mechanism, producing HOCO+ClH. The calculated rate constant, at 298 K, for the reaction with NH₂ is 1.71 × 10⁻¹⁵ cm³ molecule⁻¹ s⁻¹, and the reaction goes through the abstraction of the acidic hydrogen atom, via a pcet mechanism, leading to the formation of HCOO+NH₃.

GRAPHICAL ABSTRACT

KEYWORDS:

• Atmospheric chemistry
• ab-initio calculations
• proton coupled electron transfer
• hydrogen atom transfer
• reaction kinetics

Acknowledgements

We thank the Generalitat de Catalunya (Grant 2017SGR348) for financial support. We also acknowledge the Consorci de Serveis Universitaris de Catalunya (CSUC) for providing computational resources.

Disclosure statement

No potential conflict of interest was reported by the authors.

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials.

Notes

* This paper is dedicated to the memory of Prof. Dieter Cremer.