Pressure-driven switching between N···Cl–C and N···H–C interactions in Diels–Alder cycloaddition

High hydrostatic pressure (HHP) accelerates reactions, but its molecular origins remain elusive. Combining enhanced sampling, quasi-classical trajectories (QCT), and energy decomposition, we show that pressure reduces the barrier and enhances synchronicity by driving a redistribution between N···Cl–C and N···H–C directional solute–solvent contacts in the solvation shell of the Diels–Alder reaction between acrylonitrile and cyclopentadiene in dichloromethane. At 15 kbar, the reactant complex N···Cl–C contacts are more prevalent, while N···H–C contacts become more prominent at the transition state. This redistribution cancels out the destabilizing dispersion contribution observed at ambient pressure, while enhancing orbital interactions, driven by the pressure-induced nitrogen charge redistribution and the greater compression tolerance of the N···Cl–C contact motif. QCT analysis reveals that pressure narrows the reactive entrance channel, and despite increasing transition state synchronicity at 15 kbar, a small subset of trajectories retains dynamic stepwise character. These findings provide an atomistic view of how explicit solvation restructuring mediates HHP-induced reactivity. High hydrostatic pressure accelerates reactions, but its molecular origins remain elusive. Here, the authors combine enhanced sampling, quasi-classical trajectories, and energy decomposition and show that pressure reduces the barrier and enhances synchronicity by driving a halogen to hydrogen bonding switch in the solvation shell of the Diels–Alder reaction between acrylonitrile and cyclopentadiene in dichloromethane.