The Structure and Cl–O Dissociation Energy of the ClOO Radical: Finally, the Right Answers for the Right Reason

The chlorine peroxy radical (ClOO) has historically been a highly problematic system for theoretical studies. In particular, the erratic ab initio predictions of the Cl–O bond length reported in the literature thus far exhibit unacceptable errors with respect to the experimental structure. In light of the widespread disagreement observed, we present a careful and systematic investigation of the ClOO geometry toward the basis set and correlation limits of single reference ab initio theory, employing the cc-pVXZ (X = D, T, Q, 5, 6) basis sets extrapolated to the complete basis set limit and coupled cluster theory through single, double, triple, and perturbative quadruple excitations [CCSDT(Q)]. We demonstrate a considerable sensitivity of the Cl–O bond length to both electron correlation and basis set size. The CCSDT(Q)/CBS structure is found to be re(ClO) = 2.082, re(OO) = 1.208, and θe(ClOO) = 115.4°, in remarkable agreement with Endo’s semi-experimentally determined values re(ClO) = 2.084(1), re(OO) = 1....