The results of electronic structure studies aimed at determining accurate enthalpies of formation and proton affinities for a comprehensive set of C 4 H 7 N and C 3 H 3 ON compounds are reported. The calculated thermodynamic values are rationalized in terms of the nitrogen functionality and the influence of unsaturated C-C bonding environments. Using these results, it is proposed that a previously unidentified mass 70amu tropospheric ion is the protonated form of 1-pyrroline, an unsaturated nitrogen heterocyclic compound emitted into the atmosphere by both plant and animal biological activity.