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

George, Martin Andreas Robert; Förstel, Marko; Dopfer, Otto

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 (C10H16+–H2O, 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.
Published in: Angewandte Chemie International Edition, 10.1002/anie.202003637, Wiley