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HCN is a key ingredient for synthesizing biomolecules such as nucleobases andamino acids. We calculate 42 reaction rate coefficients directly involved withor in competition with the production of HCN in the early Earth or Titanatmospheres. These reactions are driven by methane and nitrogen radicalsproduced via UV photodissociation or lightning. For every reaction in thisnetwork, we calculate rate coefficients at 298 K using canonical variationaltransition state theory (CVT) paired with computational quantum chemistrysimulations at the BHandHLYP/augcc-pVDZ level of theory. We also calculate thetemperature dependence of the rate coefficients for the reactions that havebarriers from 50 to 400 K. We present 15 new reaction rate coefficients with nopreviously known value; 93% of our calculated coefficients are within an orderof magnitude of the nearest experimental or recommended values. Above 320 K,the rate coefficient for the new reaction H2CN -> HCN + H dominates. Contraryto experiments, we find the HCN reaction pathway, N + CH3 -> HCN + H2, to beinefficient and suggest that the experimental rate coefficient actuallycorresponds to an indirect pathway, through the H2CN intermediate. We presentCVT using energies computed with density functional theory as a feasible andaccurate method for calculating a large network of rate coefficients ofsmall-molecule reactions.

A Consistent Reduced Network for HCN Chemistry in Early Earth and Titan Atmospheres: Quantum Calculations of Reaction Rate Coefficients