Effect of Pressure on Molecular and Transition-State Geometries

High pressure significantly influences the chemical behavior of molecules by modifying their geometries, energy levels, and potential energy surfaces. While computational methods exist to study periodic systems under pressure, reliable methods to explore the geometric evolution of isolated molecules under pressure have only recently been developed. Here, we use two such methods, the perturbative approach and full geometry optimization, based on the extreme-pressure polarizable continuum model (XP-PCM) to examine the response of molecular geometry to pressure up to 5 gigapascals. Seven molecules representing covalent bonding, metal-ligand bonding, and nonbonded interactions, as well as six transition states, were examined. Alongside intuitive results (most bonds shorten under pressure), unexpected counterintuitive behavior (some bonds in transition states elongate under pressure) was obtained and rationalized. The strength of the bond, the magnitude of the volume change associated with bond-length variation, and the nature of the structure (minimum or transition state) determine the geometric response to pressure. A unique feature of our perturbative approach, the mode-by-mode analysis, was conducted to decompose the overall geometric change into contributions from each totally symmetric normal mode of molecular vibration, providing a clear understanding of the pressure effect. The XP-PCM perturbative approach and full geometry optimization show good agreement and provide complementary tools for studying molecular geometries under high pressure. Furthermore, experimental data corroborate our results, highlighting the accuracy of these methods and their potential to unravel subtle chemical behavior. These approaches open a path toward a deeper exploration of molecular reactivity and properties under high pressure, providing a solid framework for connecting pressure-induced geometric changes with their resulting chemical consequences.