Theoretical studies of the reactions of HCN with atomic hydrogen

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Abstract
A comprehensive theoretical study has been made of the energetics of the important pathways involved in the reaction of hydrogen atoms with hydrogen cyanide. For each reactiona b i n i t i o GVB‐CI calculations were carried out to determine the structures and vibrational frequencies of the reactants, transition states, and products; then POL‐CI calculations were carried out to more accurately estimate the electronic contribution to the energetics of the reactions. The hydrogen abstraction reaction is calculated to be endoergic by 24 kcal/mol [expt. ΔH (0 K)=16–19 kcal/mol] with a barrier of 31 kcal/mol in the forward direction and 6 kcal/mol in the reverse direction. For the hydrogen addition reactions, addition to the carbon atom is calculated to be exoergic by 19 kcal/mol with a barrier of 11 kcal/mol, while addition to the nitrogen center is essentially thermoneutral with a barrier of 17 kcal/mol. Calculations were also carried out on the isomerizationreactions of the addition products. The c i s→t r a n sisomerization of HCNH has a barrier of only 10 kcal/mol with the t r a n s isomer being more stable by 5 kcal/mol. The (1,2)‐hydrogen migration reaction, converting H2CN to t r a n s‐HCNH, is endoergic by only 14 kcal/mol, but the calculated barrier for the transfer is 52 kcal/mol. The energy of the migration pathway thus lies above that of the dissociation–recombination pathway.

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