Solvent-dependent structure of molecular iodine probed by picosecond X-ray solution scattering

The effect of solute-solvent interaction on molecular structure and reaction dynamics has been a target of intense studies in solution-phase chemistry, but it is often challenging to characterize the subtle effect of solute-solvent interaction even for the simplest diatomic molecules. Since the I₂ molecule has only one structural parameter and exhibits solvatochromism, it is a good model system for investigating the solvent dependence of the solute structure. By using X-rays as a probe, time-resolved X-ray liquidography (TRXL) can directly elucidate the structures of reacting molecules in solution and can thus determine the solvent-dependent structural change with atomic resolution. Here, by applying TRXL, we characterized the molecular structure of I₂ in methanol and cyclohexane with sub-angstrom accuracy. Specifically, we found that the I-I bond length of I₂ is longer in the polar solvent (methanol) by ∼0.2 Å than in nonpolar solvents (cyclohexane and CCl₄). Density functional theory (DFT) using 22 explicit methanol molecules well reproduces the longer I-I bond of molecular iodine in methanol and reveals that the larger bond length originates from partial negative charge filled in an antibonding σ∗ orbital through solvent-to-solute charge transfer.