Ab initio study of the reaction of CHO+ with H2O and NH3

An MP4(Full,SDTQ)/6-311 + + G(d,p)/ /MP2(full)/6-311 + + G(d,p) ab initio s tudy was performed of the reactions of formyl and isoformyl cations with H2 O and NH3, which play an important role in flame and interstellar chemistri es. Two different confluent channels were located leading to CO + H3O+/NH4. The first one corresponds to the approach of the neutral molecule to the carbon atom of the cations. The second one leads to the direct proton trans fer from the cations to the neutrals. At 900 K the separate products CO + H 3O+/NH4+ are the most stable species along the Gibbs energy profiles for th e processes. For the reaction with H2O the reaction channel leading to HC(O H)(2)(+) (protonated formic acid) is disfavored with respect to the two CO + H3O+ channels in agreement with the experimental evidence that H3O+ is th e major ion observed in hydrocarbon flames. According to our calculations, NH4+ + H2O are considerably more stable in Gibbs energy than NH3 + H3O+; NH 4+ will predominate in the reaction zone when ammonia is added to CH4 + Ar diffusion flame, as experimentally observed. At 100 It the most stable stru ctures are the intermediate complexes CO...HOH2+/HNH3+. Particularly the CO ...HOH2+ complex has a lifetime large enough to be detected and, therefore, could play a certain role in interstellar chemistry. (C) 1999 John Wiley & Sons, Inc.