Reported here is a theoretical study of possible stratospheric chlorine reservoir species including isomers of chemical formula ClNO4 and ClNO5, in addition to the well-known ClONO2 reservoir species. Density functional theory (DFT) in conjunction with large one-particle basis sets has been used to determine equilibrium structures, dipole moments, rotational constants, harmonic vibrational frequencies, and infrared intensities. The B3LYP functional was used since it has previously been shown to perform well for similar compounds. The equilibrium geometry and vibrational spectra of ClONO2 are shown to be in good agreement with the experimental data and also with high-level coupled-cluster calculations reported previously. Three stable isomers have been identified for each ClNO4 and ClNO5. The vibrational spectrum of O2ClONO2 has been compared with the available experimental data and found to be in good agreement. The relative energetics of the ClNO4 and ClNO5 isomers have been determined using large atomic natural orbital (ANO) basis sets in conjunction with the singles and doubles coupled-cluster method that includes a perturbational correction for triple excitations, denoted CCSD(T). Accurate heats of formation have been evaluated by computing energies for isodesmic reactions involving the ClNO4 and ClNO5 isomers. The stability of these molecules with respect to thermal dissociation is examined. The present study suggests that isomers of ClNO4 and ClNO5 may have no atmospheric chemical relevance because the atmospheric concentrations of the necessary reactants are insufficent, but it is also found that under laboratory conditions the formation of O2ClONO2 cannot be ignored.