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 r_e(ClO) = 2.082, r_e(OO) = 1.208, and θ_e(ClOO) = 115.4°, in remarkable agreement with Endo's semi-experimentally determined values r_e(ClO) = 2.084(1), r_e(OO) = 1.206(2), and θ_e(ClOO) = 115.4(1)°. Moreover, we compute a Cl-O bond dissociation energy of 4.77 kcal mol^-1, which is likewise in excellent agreement with the most recent experimental value of 4.69 ± 0.10 kcal mol^-1.