Infrared Spectrum of the Adamantane⁺ -Water Cation: Hydration-Induced C-H Bond Activation and Free Internal Water Rotation

Diamondoid cations are reactive intermediates in their functionalization reactions in polar solution. Hydration is predicted to strongly activate their C-H bonds in initial proton abstraction reactions. To study the effects of microhydration on the properties of diamondoid cations, we characterize herein the prototypical monohydrated adamantane cation (C₁₀ H₁₆ ⁺ -H₂ O, Ad⁺ -W) in its ground electronic state by infrared photodissociation spectroscopy in the CH and OH stretch ranges and dispersion-corrected density functional theory (DFT) calculations. The water (W) ligand binds to the acidic CH group of Jahn-Teller distorted Ad⁺ via a strong CH⋅⋅⋅O ionic H-bond supported by charge-dipole forces. Although W further enhances the acidity of this CH group along with a proton shift toward the solvent, the proton remains with Ad⁺ in the monohydrate. We infer essentially free internal W rotation from rotational fine structure of the ν₃ band of W, resulting from weak angular anisotropy of the Ad⁺ -W potential.