A series of isomeric 4-aminopyrimidinium radicals were used to model hydrogen atom adducts of nucleobases containing the 4-aminopyrimidine structure motif. Relative stabilities and activation energies for dissociations by hydrogen atom loss have been calculated by density functional theory and ab initio methods up to effective QCISD(T)/6-31 1+G(2d,p) for 4-amino-N-1-H- (1), 2-H- (2), N-3-H- (3), 4-H- (4), 5-H- (5), and 6-H(6) pyrimidinium radicals and the 4-pyrimidylammonium radical (7). All these radicals were found to be bound species existing in potential energy wells. The order of stabilities has been established as 5 (most stable) > 3 > 2 > 1 > 6 > 4 much greater than 7 (least stable). Dissociations of the N-H and C-H bonds in 1-7 required activation barriers above the dissociation thresholds. RRKM calculations of unimolecular rate constants for N-H bond dissociations in 1 and 3 predicted substantial stabilization of these radicals by kinetic shift in the gas phase. Additions of hydrogen atoms to the N-1, C-2, N-3, C-4, C-5, and C-6 ring positions in 4-aminopyrimidine were found to be exothermic by 68, 70, 76, 23, 91, and 62 kJ mol(-1) at 0 K, respectively. Hydrogen atom addition to the NH2 group was 58 kJ mol(-1) endothermic. The activation barriers for the hydrogen atom additions to 4-aminopyrimidine were found to inversely correlate with the reaction enthalpies. The calculated rate constants predicted predominant (95%) hydrogen atom addition to C-5. The other positions were substantially less reactive, e.g., N-3 (2%), C-2 (1%), C-6 (0.8%), and N-1 (0.4%).