Reactions of nitryl chloride (NO2Cl) with imidazole, taken as a model for the guanine base of DNA, leading to the formation of. 2-oxoimidazole (2-oxolm), 2-chloroimidazole (2-chlorolm), and 2-nitroimidazole (2nitroIm) corresponding to the 8-oxo, 8-chloro, and 8-nitro derivatives of guanine were studied at the B3LYP and MP2 levels of theory employing the 6-31+G* and AUG-cc-pVDZ basis sets in gas phase. In order to incorporate solvent effect, a] I the B 3LYP/AUG-cc-pVDZ optimized structures were solvated in aqueous media at the B3LYP/AUG-cc-pV.DZ and MP2/AUG-cc-pVDZ levels of theory using the polarizable continuum model (PCM). A single mechanism was found for the formation of 2-oxolm, while two and three mechanisms were found for the formation of 2-nitrolm and 2-chloroIm respectively. Each of these reaction mechanisms involves two steps. The calculated barrier energies show that the formation of 2-nitrolin would occur more efficiently than those of 2-oxolm and 2-chlorolm. It suggests that formation of 8-nitroguanine would be the main DNA lesion caused by NO2Cl, which is consistent with experimental observations.