Reactivity of Isocyanate Radical with Formic Acid in the Tropospheric and Outer Space Regions: A DFT and MP2 Investigation in Gaseous Phase

Abstract

This work aims to investigate the nature of the structures, mechanisms and kinetics associated with the prototype reaction of formic acid (FA) with NCO radical in either the troposphere or outer space regions. The kinetic properties have been investigated to include temperature in the cold outer space molecular clouds populated regions (typically between 10 and 100 K), temperature in the warm outer space molecular cloud regions (between 90–200 K) and temperature in the troposphere region of the atmosphere (200–400 K). The study has been performed by means of selected Density Functional Theory (DFT), second order Møller–Plesset perturbation theory (MP2) and transition state theory approaches. The results of the investigation suggest that the H2NCO carbamoyl radical and carbon dioxide are the final fate for the reaction involving NCO radical and FA. HOCNH radical is considered the least formed product of the NCO+FA reaction because all pathways leading to its formation are unfavourable with regards to other competitive processes. Of all the investigated reaction channels, the thermodynamically preferred reaction pathways involve cases in which the starting isocyanate radical species has its unpaired electron on the nitrogen atom (N⋅) rather than on its oxygen atom (O⋅).