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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.

Reaction pathways for the hydrogen atom plus cyclopropane (cyc-C3H6) reaction are studied using an extrapolated coupled-cluster/complete basis set (CBS) method based on the cc-pVDZ, cc-pVTZ, and cc-pVQZ basis sets. For this activated reaction, results reveal two reaction mechanisms, a direct H-abstraction and a H-addition/ring-opening. The hydrogen-abstraction reaction yields the H2 and cyclopropyl (cyc-C3H5) radical products. The vibrationally adiabatic ground-state (VAG) barrier height is predicted to be 13.03 kcal/mol. The isomerization barrier height from the product cyclopropyl to allyl radical is 21.98 kcal/mol via a cyc-C3H5 ring-opening process. In addition, the H-addition and ring-opening mechanism will lead to an n-C3H7 radical, which can result in a variety of products such as CH3 + C2H4, H + CH3CHCH2, and H2 + C3H5, etc. The VAG barrier height of the H-addition reaction is 16.49 kcal/mol, which is slightly higher than that of the direct H-abstraction reaction. Although the H + cyc-C3H6 → CH4 + CH2CH reaction is exoergic by 11.90 kcal/mol, this reaction is unlikely due to a high barrier of 43.05 kcal/mol along the minimum energy path.

The stationary point geometries and frequencies on the lowest singlet potential energy surface for the CH3OH system are calculated using the complete-active-space self-consistent-field method. The energetics are refined using a restricted internally contracted multireference configuration interaction (MRCI) method at the complete basis set (CBS) limit. The CBS energy is extrapolated using the scheme of Halkier et al. with two large basis sets:  aug-cc-pVDZ and aug-cc-pVTZ. The implications of our calculated results concerning the O(1D) + CH4 and OH + CH3 reactions are discussed. In addition, the O(1D) + CH4 reaction at a collision energy of 6.8 kcal/mol is investigated using a variant of the “scaling all correlation” (SAC) method of Truhlar et al. and the coupled-cluster double-excitation (CCD) method in a direct dynamics study with a D95(d,p) basis set. The results show that the O(1D) + CH4 → OH + CH3 reaction occurs both via direct and long-lived intermediate pathways. The differential cross section for the direct reaction to form OH is forward peaked with a nearly isotropic background. Finally, the branching fractions for OH, H, H2, and H2O are predicted to be 0.725:0.186:0.025:0.064.

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