Probing Composition Effects on the Intrinsic Energetics of Proton Transfer in Binary (HCl)_m(H₂O)_n Clusters

This work examines the effects on intrinsic energetics as the composition of finite (HCl)_m(H₂O)_n cluster families containing at least one water molecule is changed at the molecular level, where m ≥ 2 and m + n = 4 - 6, and the associated (Cl^-)_x(H₃O⁺)_x(HCl)_m-x(H₂O)_n-x clusters resulting from the dissociation of x HCl fragments via proton transfer (PT). They are collectively labeled m:n, and xPT indicates the degree of dissociation, from 0PT (no dissociation) up to 3PT. More than 1000 unique minima were identified via ωB97X-D/6-31++G(d,p) optimizations and frequency computations. Of those, nearly 500 lie within 5 kcal mol^-1 of the corresponding lowest-energy minimum structure as determined by CCSD(T)-F12/haTZ-F12 single point energies: 25 for the 2:2 and 3:1 tetramers, 127 for the 2:3, 3:2, and 4:1 pentamers, and 333 for the 2:4, 3:3, 4:2, and 5:1 hexamers. In four of the nine m:n systems examined (2:2, 3:1, 4:1, 5:1), no low-energy minima exhibited PT, but the other five (2:3, 3:2, 2:4, 3:3, 4:2) have 1PT and 2PT structures with electronic energies near or below the lowest-energy 0PT configurations. In the 2:4, 3:3, and 4:2 hexamers, for example, at least one 1PT structure lies ≈4, 3, and 0.5  kcal mol^-1 below the lowest-energy 0PT structures, respectively, based on CCSD(T)-F12 electronic energies.