Importance of Time Scale and Local Environment in Electron-Driven Proton Transfer. The Anion of Acetoacetic Acid.
Anion photoelectron spectroscopy (PES) and electron energy-loss spectroscopy (EELS) probe different regions of the anionic potential energy surface. These complementary techniques provided information about anionic states of acetoacetic acid (AA). Electronic structure calculations facilitated the identification of the most stable tautomers and conformers for both neutral and anionic AA and determined their relative stabilities and excess electron binding energies. The most stable conformers of the neutral keto and enol tautomers differ by less than 1 kcal/mol in terms of electronic energies corrected for zero-point vibrations. Thermal effects favor these conformers of the keto tautomer, which do not support an intramolecular hydrogen bond between the keto and the carboxylic groups. The valence anion displays a distinct minimum which results from proton transfer from the carboxylic to the keto group; thus, we name it an ol structure. The minimum is characterized by a short intramolecular hydrogen bond, a significant electron vertical detachment energy of 2.38 eV, but a modest adiabatic electron affinity of 0.33 eV. The valence anion was identified in the anion PES experiments, and the measured electron vertical detachment energy of 2.30 eV is in good agreement with our computational prediction. We conclude that binding an excess electron in a π* valence orbital changes the localization of a proton in the fully relaxed structure of the AA(-) anion. The results of EELS experiments do not provide evidence for an ultrarapid proton transfer in the lowest π* resonance of AA(-), which would be capable of competing with electron autodetachment. This observation is consistent with our computational results, indicating that major gas-phase conformers and tautomers of neutral AA do not support the intramolecular hydrogen bond that would facilitate ultrarapid proton transfer and formation of the ol valence anion. This is confirmed by our vibrational EELS spectrum. Anions formed by vertical electron attachment to dominant neutrals undergo electron autodetachment with or without vibrational excitations but are unable to relax to the ol structure on a time scale fast enough to compete with autodetachment.
Department of Physics, University of Botswana , Private Bag 0022, Gaborone, Botswana.|Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University , Edinburgh EH14 4AS, United Kingdom.
Zibo G
ZG
Keolopile
Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University , Edinburgh EH14 4AS, United Kingdom.
Maciej
M
Gutowski
Department of Chemistry, Johns Hopkins University , Baltimore, Maryland 21218, United States.
Angela
A
Buonaugurio
Department of Chemistry, Johns Hopkins University , Baltimore, Maryland 21218, United States.
Evan
E
Collins
Department of Chemistry, Johns Hopkins University , Baltimore, Maryland 21218, United States.
Xinxing
X
Zhang
Department of Chemistry, Johns Hopkins University , Baltimore, Maryland 21218, United States.
Jeremy
J
Erb
Department of Chemistry, Johns Hopkins University , Baltimore, Maryland 21218, United States.
Thomas
T
Lectka
Department of Chemistry, Johns Hopkins University , Baltimore, Maryland 21218, United States.
Kit H
KH
Bowen
Department of Chemistry, University of Fribourg , chemin du Musée 9, Fribourg 1700, Switzerland.
Michael
M
Allan