Oxidation of nontraditional nucleobases 1-methylcytosine (hachimoji base S) and isoguanine (hachimoji base B) in gas-phase ternary complexes with Cu-II (terpyridine)' formed cation radicals that were characterized by tandem mass spectrometry, UV-vis photodissociation action spectroscopy in the 210-700 nm region, and ab initio calculations up to the CCSD(T)/complete basis set level of theory. Oxidation of S was accompanied by exothermic isomerization in the 1-methylcytosine ion (1(+center dot)) forming 1-methylene-2-hydroxy-4-aminopyrimidine cation radical (9(+center dot)) as a noncanonical distonic isomer of the nucleobase. Ion 9(+center dot) was characterized by deuterium exchange experiments and provided a matching UV-vis action spectrum with the vibronic absorption spectrum from time-dependent density functional theory calculations. Oxidation of B resulted in the formation of a canonical isoguanine cation radical (12(+center dot)) as judged from the match of the experimental action spectrum with the calculated vibronic absorption spectrum. The calculated adiabatic ionization energies of canonical S and B, 8.51 and 7.76 eV, respectively, indicated exothermic electron transfer from B to S+center dot to proceed in an ionized base pair. Contrasting this, the lowest energy tautomer of ionized S (9(+center dot)) had a low adiabatic recombination energy, REadiab = 5.70 eV, that would prevent it from oxidizing other nucleobases. Recombination energies of several nucleobase tautomers are reported and discussed.