RESUMO
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.
Assuntos
Hidrogênio , Prótons , Isomerismo , Ácidos Mandélicos , Ânions/químicaRESUMO
The decarboxylation (CO2 loss) mechanism of cold monodeprotonated phthalic acid was studied in a photodissociation action spectrometer by quantifying mass-selected product anions and neutral particles as a function of the excitation energy. The analysis proceeded by interpreting the translational energy distribution of the generated uncharged products, and with the help of quantum calculations. In particular, this study reveals different fragmentation pathways in the deprotonated anion and in the radical generated upon electron photodetachment. Unlike the behavior found in other deprotonated aryl carboxylic acids, which do not fragment in the anion excited state, a double loss of CO2 molecules takes place in the phthalic monoanion. Moreover, at higher excitation energies the phthalic monoanion experiences decarboxylative photodetachment with a statistical distribution of product translational energies, which contrasts with the impulsive dissociation reactions characteristic of other aryl carboxylic anions.