Cluster-size-dependent cooperativity in sulfur-centred hydrogen bonds: ethane-1,2-dithiol with HF, H₂O, and NH₃

Ethanedithiol (EDT) complexed with one to three solvent molecules (HF, H₂O, and NH₃) was computationally investigated systematically to elucidate how hydrogen-bond donor identity and cluster size govern the structure, energetics, and spectroscopic signatures of sulfur-centred hydrogen bonds (SCHBs). All clusters were optimised at the MP2/aug-cc-pVTZ level and confirmed as true minima by harmonic frequency calculations, while accurate interaction energies were obtained from high-level coupled-cluster single-point calculations. DLPNO-CCSD(T)/aug-cc-pVQZ interaction energies were employed as a uniform reference for all cluster sizes (n = 1-3). These were benchmarked against CCSD(T)/aug-cc-pVQZ calculations for monomer and dimer systems (n ≤ 2), showing very good agreement, with deviations typically within ∼0.1-0.7 kcal mol^-1. The most stable assemblies predominantly adopt cyclic or quasi-cyclic hydrogen-bonding networks featuring bifunctional binding, in which the solvent donates X-H⋯S and may also accept S-H⋯X interactions. Interaction energies increase monotonically with cluster size and establish a clear stability hierarchy, EDT-(HF)_n > EDT-(H₂O)_n > EDT-(NH₃)_n, with the most stable trimers bound by approximately 19.2, 18.4, and 12.1 kcal mol^-1, respectively, at the DLPNO-CCSD(T)/aug-cc-pVQZ level. Vibrational analysis revealed direct spectroscopic evidence of hydrogen-bond strengthening and cooperativity with an exceptionally large red shift (up to ∼513 cm^-1) exhibited by the H-F stretching mode, and a moderate red shift shown by the O-H and N-H stretching modes. Complementary atoms-in-molecules (AIM) analysis identified F-H⋯S as the strongest hydrogen bond through elevated electron densities at the bond critical points (up to ∼0.033 a.u.), providing quantitative benchmarks for solvent-dependent SCHB cooperativity and clarifying how hydrogen-bonding motifs govern the stability and spectroscopic response of EDT-solvent clusters.