Surprising Quenching of the Spin-Orbit Interaction Significantly Diminishes H2O•••X [X = F, Cl, Br, I] Dissociation Energies

The H2O center dot center dot center dot X complexes, with X = F, Cl, Br, and I, show considerable viability with nonspin-orbit D-e(D-0) dissociation energy values of 3.73(2.42), 3.60(2.68), 3.54(2.72), and 3.36(2.77) kcal mol(-1) for X = F, Cl, Br, and I, respectively, obtained at the CCSD(T)-F12b/aug-cc-pVTZ(-PP) level of theory using relativistic pseudopotentials (PPs) for Br and I. Spin-orbit (SO) corrections, computed with the Breit-Pauli operator in the interacting states approach at the all-electron MRCI+Q/aug-cc-pwCVTZ(-PP) level, are found to depend sensitively and unpredictably on the O center dot center dot center dot X separations. 96% (F), 87% (Cl), 54% (Br), and 30% (I) quenching of the SO corrections significantly reduces the dissociation energies of the H2O center dot center dot center dot X complexes, resulting in D-e(D-0) values of 3.38(2.06), 2.86(1.94), 1.64(0.83), and 1.23(0.64) kcal mol(-1) for X = F, Cl, Br, and I, respectively.