We study the Walden-inversion, front-side attack retention, and double-inversion retention pathways of the OH + CHY [Y = F, Cl, Br, I] S2 reactions using high-level ab initio methods. Benchmark stationary-point structures and frequencies are computed at the CCSD(T)-F12b/aug-cc-pVTZ level of theory and the best technically feasible relative energies are determined on the basis of CCSD(T)-F12b/aug-cc-pVQZ computations complemented with post-CCSD(T) correlation effects at the CCSDT(Q)/aug-cc-pVDZ level, core correlation corrections at the CCSD(T)/aug-cc-pwCVTZ level, scalar relativistic effects using effective core potentials for Br and I, and zero-point energy corrections at the CCSD(T)-F12b/aug-cc-pVTZ level. Walden inversion proceeds via hydrogen-bonded HO···HCHY (Cl, Br, I) complex → hydrogen-bonded HO···HCHY (Cl, Br, I) transition state → ion-dipole HO···HCY (F, Cl, Br) complex → Walden-inversion [HO-CH-Y] (F, Cl, Br) transition state → hydrogen-bonded CHOH···Y (F, Cl, Br, I) complex, where the Y-dependent existence of the submerged stationary points is indicated in parentheses. Front-side HO···YCH (Cl, Br, I) complexes are also found and HO···ICH is a deeper minimum than HO···HCHI. Front-side attacks go over high barriers of 42.8 (F), 28.7 (Cl), 22.4 (Br), and 17.2 (I) kcal/mol, well above the double-inversion barrier heights of 16.7 (F), 3.4 (Cl), 1.1 (Br), and -3.7 (I) kcal/mol.
