Quantum effects in hydrogen bonding: Core level photoemission spectroscopy of acetylacetone
The core level photoemission spectroscopy of gaseous acetylacetone, its fully deuterated form, and two derivatives, benzoylacetone and dibenzoylmethane has allowed us to examine the effect of the double-well potential on photoemission spectra. V. Feyer et al. J. Phys. Chem. Lett. (2018)
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The molecule acetylacetone, and two of its derivatives, benzoylacetone and dibenzoylmethane show intramolecular hydrogen bonds, with a proton located in a double well potential, whose barrier height is different for the three compounds. Two distinct O 1s core hole peaks were observed, previously assigned to two chemical states of oxygen. Our alternative assignment of the double peaks takes account of the extended nature of the proton wave function, and the shape of the neutral and ionic potentials in which the proton is located. The peaks are explaineed in terms of an unusual Franck-Condon factor distribution.
Siegbahn won the Nobel prize for Electron Spectroscopy for Chemical Analysis (ESCA), in which the shifts in the binding energies of core electrons measured by photoemission are associated with differences in the chemical environment of the neutral ground state elements.
Photoemission is a powerful spectroscopy in wide use at synchrotrons. For many years it has been known that there are a few exceptions to this interpretation, such as satellite peaks, where the shifts have other origins. Are there any other exceptions? Experiments performed at the Gas Phase beamline at Elettra have shown that there are indeed some other effects to be accounted for.
The molecule acetylacetone is generally believed to have the structure A in the inset of Fig. 1, in which a hydrogen atom is bonded mostly to oxygen atom 1 or 2.
This model pre-supposes that the nucleus of the hydrogen atom, a proton, is a point like object. A few “heretics” have suggested that structure B is more appropriate, with the hydrogen atom located more or less equidistant from the two oxygen atoms. The oxygen core level spectrum, Fig. 1, appears to support structure A: there are two distinct peaks, interpreted as due to oxygen bonded, or not bonded, to hydrogen. The spectrum is slightly sharper when deuterium is substituted for hydrogen as expected: the vibrations of deuterium are at lower energy than those of hydrogen, and the width of the peak is due to vibrations.
Incidentally, there is a weak satellite peak at higher binding energy.
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Is this the end of the story? Our calculations show that it is not, and an alternative interpretation of the spectrum is possible. Protons are not classical point-like charges, but are quantum mechanical objects with an extended wave function, Fig. 2. The green curve is the potential energy in which the proton moves. Its wave function is delocalised, with two extrema, showing the proton is not located mostly close to one oxygen atom or the other: it has two “most likely” locations, near to one or other oxygen atom, but it is also likely to be found in the middle.
Our new interpretation of the O 1s spectrum is that the proton is delocalised in the ground state, see the lower dotted lines in Fig. 2. The wave function of the proton extends over nearly an Ångström.
Upon ionization, it localises close to or far from the ionized oxygen atom, for example in Fig. 2 the proton is localised far from the ionized atom. The upper blue curve shows the potential, and the energy is lower near the neutral oxygen atom on the left. The corresponding wave functions of the vibrational states, upper dotted curves, are now localised to one or two tenths of an Ångström.
This situation corresponds to the lower energy photoemission peak, whereas if the proton is localised close to the positively charged oxygen ion, the energy is higher and this corresponds to the high energy photoemission peak. This interpretation of core level spectra may apply to all systems with hydrogen bonding in a strong, double-well potential. The present case is for an intramolecular double well, but in nature there are numerous situations with both intra- and intermolecular hydrogen bonding, especially in biology and wet chemistry. Our studies provide a fundamental insight into the nature of this bonding, and the interpretation of spectra.
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Quantum effects in hydrogen bonding: Core level photoemission spectroscopy of acetylacetone
Quantum effects in hydrogen bonding: Core level photoemissione spectroscopy of acetylacetone
V. Feyer, K.C. Prince, M. Coreno, S. Melandri, A. Maris, L. Evangelisti, W. Caminati, B.M. Giuliano, H.G. Kjaergaard , V. Carravetta
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