Accurate Calculation of the Dissociation Energy of the Highly Anharmonic System ClHCl^–

Accurate bond dissociation energies (D₀) are reported for different isotopologues of the highly anharmonic system ClHCl^–. The mass-independent equilibrium dissociation energy D_e was obtained by a composite method with frozen-core (fc) CCSD­(T) as the basic contribution. Basis sets as large as aug-cc-pV8­(+d)­Z were employed, and extrapolation to the complete basis set (CBS) limit was carried out. Explicitly correlated calculations with the CCSD­(T)-F12b method were also performed to support the conventionally calculated values. Core–core and core–valence correlation, scalar relativity, and higher-order correlation were considered as well. Two mass-dependent contributions, namely, the diagonal Born–Oppenheimer correction and the difference in zero-point energies between the complex and the HCl fragment, were then added in order to arrive at precise D₀ values. Results for ³⁵ClH³⁵Cl^– and ³⁵ClD³⁵Cl^– are 23.81 and 23.63 kcal/mol, respectively, with estimated uncertainties of 0.05 kcal/mol. In contrast to FHF^– (Stein, C.; Oswald, R.; Sebald, P.; Botschwina, P.; Stoll, H., Peterson, K. A. Mol. Phys. 2013, 111, 2647−2652), the D₀ values of the bichloride species are larger than their D_e counterparts, which is an unusual situation in hydrogen-bonded systems.