Trends in the equilibrium geometries, torsional frequencies, conformational barriers and relative stabilities are investigated for a series of halogenated methyl peroxy nitrites, CXnY3-nOONO (X, Y = H, F, Cl), using density functional theory and ab initio methods. Comparison with the methylperoxy derivatives shows the significant influence of the halogen electron withdrawing effect on the structural parameters and the energetics, leading to higher dissociation energies and a moderate lowering of the conformational barrier for the halogenated nitrites. No influence is found on the relative stability of the two conformers, being similar to that of the methyl analogues. The importance of the results for the kinetics of the reactions CXnY3-nOO + NO (X, Y = H, F, Cl) is discussed. (C) 2007 Elsevier B.V. All rights reserved.