AbstractElectronic structures and spectra of “non‐alternant” heterocyclic 2‐chloropurine and its analogs have been computed with different MO techniques, namely, HMO, ω‐SCF‐HMO (ω = 1.4 and 0.5), and PPP semiempirical SCF‐ASMO‐CI. Triplet‐state energies have been calculated by the last method. In general, it is found that at least semiquantitatively consistent results on the ground‐state properties of the molecules are obtained from the different MO methods. As expected, excited states of the molecules are not satisfactorily treated by the HMO and σ techniques. However, an excellent agreement between the observed and calculated π‐π* transition energies has been obtained with the PPP method. In view of the semiquantitative consistency of the ground‐state properties, the use of simple methods such as HMO and ω(=0.5)‐SCF techniques is justifiable for limited application to molecular biology.Results on the molecular geometry, dipole moments, ionization potentials, xanthine oxidase reactivity and spectra have been discussed with regard to the halogen substitutions in the purine base. The significance of these results with reference to the biological uses of substituted purines is briefly described.