Interaction geometry in hydrogen-bond donor-anion complexes: Acidity-dependent formation, fluctuation, and disruption

The hexafluoroisopropyl alcohol (HFIP) group is a strong hydrogen-bond donor that governs interaction geometry in anion complexes. In this study, three model systems with different hydrogen-bond donor acidities-HFIP-substituted ethylbenzene, ethylphenol, and ethylbenzene-were investigated using density functional theory (DFT). Rather than relying on energy-based ranking, the analysis focuses on geometric formation, anchor-induced constraints, and geometric fluctuation. In the HFIP-substituted ethylbenzene system, a strong O-H···O hydrogen-bond anchor is formed, leading to convergence toward an anchor-dominated interaction geometry. The secondary C-H···O contacts exhibit only limited geometric variability around the anchor-dominated geometry. In the ethylphenol system, the anchor is retained but weaker, allowing reproducible secondary C-H···O interactions and limited structural variability. In contrast, in the ethylbenzene system, no primary anchor is formed, and multiple weak interactions produce dispersed interaction geometries without convergence toward a dominant structural regime. These results demonstrate that donor acidity governs the strength of anchor constraints and geometric tolerance, producing distinct interaction geometry regimes separated by geometric boundary regions.