Hydrogen-bond network reorganization and reference-state-dependent barriers in the microhydrated CH2OO + H2S reaction: A molecular modeling study.

Explicit water molecules reorganize proton-transfer networks, but cluster recruitment may offset intrinsic catalysis. We model direct (A0), one-water (A1), and two-water (A2) pathways for CH2OO + H2S using a common omegaB97M-V/def2-TZVPP/SMD(water)//omegaB97X-D/def2-TZVP/SMD(water) cluster-continuum protocol with quasi-RRHO thermochemistry. Intrinsic free-energy barriers decrease from 5.52 kcal mol-1 (A0) to 3.86 (A1) and 3.30 (A2), whereas formal 1 M overall barriers increase from 9.88 to 12.18 and 15.20 kcal mol-1 because larger clusters are costlier to assemble. CCSD(T)/def2-TZVP gas-phase single points on the DFT geometries confirm the lowest intrinsic electronic barrier for A2 (1.74 kcal mol-1), while the 0.18 kcal mol-1 A0-A1 difference remains unresolved within this benchmark. NBO analysis identifies a cooperative three-stage donor-acceptor relay in A2. A Wigner sensitivity gives only 1.024-1.057 tunneling factors at 298.15 K and does not change the intrinsic ordering. Thus, hydrogen-bond organization enhances local reactivity without establishing atmospheric pathway dominance; quantitative branching requires explicit association equilibria, water activities, tunneling/recrossing, and master-equation kinetics.