Photodetachment of Deprotonated R-Mandelic Acid: The Role of Proton Delocalization on the Radical Stability.

The photodetachment and stability of R-Mandelate, the deprotonated form of the R-Mandelic acid, was investigated by observing the neutral species issued from either simple photodetachment or dissociative photodetachment in a cold anions set-up. R-Mandalate has the possibility to form an intramolecular ionic hydrogen-bond between adjacent hydroxyl and carboxylate groups. The potential energy surface along the proton transfer (PT) coordinate between both groups (O ^- …H ⁺ … ^- OCO) features a single local minima, with the proton localized on the O ^- group (OH… ^- OCO). However, the structure with the proton localized on the ^- OCO group (O ^- …HOCO) is also observed because it falls within the extremity of the vibrational wavefunction of the OH… ^- OCO isomer along the PT coordinate. The stability of the corresponding radicals, produced upon photodetachment, is strongly dependent on the position of the proton in the anion: the radicals produced from the OH… ^- OCO isomer decarboxylate without barrier, while the radicals produced from the O ^- …HOCO isomer are stable.
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