An ab initio study of the reaction of HOCO radicals with NO2: Addition/elimination mechanism

Pure rotational spectra of the CO-trans-HOCO complex

Pure rotational spectra of the CO-trans-HOCO complex have been observed by Fourier transform microwave (FTMW) and millimeter-wave FTMW double resonance spectroscopy. The complex was produced in a supersonic jet by discharging a mixture gas of CO and H(2)O diluted in Ar. The molecular constants including the fine and hyperfine coupling constants have been precisely determined. The inter-molecular distance between the CO and trans-HOCO monomers has been determined by fixing the structures of the trans-HOCO and CO monomers, where this complex has the OC···HO configuration with the C···HO angle almost linear. The C···H distance, 2.166 Å, is much shorter than those of the closed shell complexes, CO-CH(3)OH and CO-H(2)O. The Fermi contact constant of the proton for the complex was compared with that of the trans-HOCO monomer, leading to a conclusion that there is almost no induced effect for the spin density on the proton of HOCO by the complex formation.

HOCO radical chemistry

Free radicals are important species in atmospheric chemistry, combustion, plasma environments, interstellar clouds, and biochemistry. Therefore, researchers would like to understand the formation mechanism, structure, stability, reactivity, spectroscopy, and dynamics of these chemical species. However, due to the presence of one or more unpaired electrons, radicals are often very reactive and have short lifetimes, which makes it difficult to conduct experiments. The HOCO radical appears in the atmosphere as well as in combustion environments and plays an important role in the conversion of CO to CO(2). Through the interplay between theoretical and experimental investigations, researchers have only recently understood the chemical role of the HOCO radical. In this Account, we systematically describe the current state of knowledge of the HOCO radical based on recent theoretical and experimental studies. This radical's two stable conformers, trans- and cis-HOCO, have been identified by high-level ab initio calculations and experimental spectroscopy. trans-HOCO is more stable by approximately 1.8 kcal/mol. The heat of formation of HOCO (298 K) was determined to be -43.0 ± 0.5 kcal/mol, giving a potential well depth of 30.1 ± 0.5 kcal/mol relative to the asymptote of the reactants OH + CO. The HOCO radical is very reactive. In most reactions between the HOCO radical and atoms, the HOCO radical acts as a hydrogen donor to reaction partners. Generally, the hydrogen is transferred through the formation of an association intermediate, which then proceeds through a molecular elimination step to produce the reaction products. The reaction rates of HOCO with some small radicals fall in the range of 10(-11)-10(-10) cm(3) molecule(-1) s(-1). These results clearly illustrate important features in the reactivity of the HOCO radical with other molecules.