Energetics of addition versus insertion mechanisms in the Si+(2P) + HCOOH reaction

High-level ab initio calculations have been performed to investigate the preference of insertion processes with respect to the formation of adducts in the Si+ + formic acid reaction in the gas phase. We have found that the reactivity patterns shown by Si+ in reactions with methanol and formaldehyde are significantly different from those exhibited with formic acid, which has both functional groups. The most stable product of the reaction between Si+ and HCOOH corresponds to the insertion of the monocation into the COH bond of the neutral. Mostly importantly, the Si+ association to the carbonyl oxygen atom is only 4.9 kcal/mol less favourable. All investigated local minima lie below the reactants in energy. In agreement with the experimental evidence, the formation of SiOH+ as a possible product of the Si+ + HCOOH reaction is predicted to be exothermic by 41.7 kcal/mol. The distonic character of the products is discussed as well as the harmonic vibrational frequencies of the global minimum.