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Quantum effects in hydrogen bonding: core level photoemission spectroscopy of acetylacetone

Siegbahn won the Nobel prize for ESCA, in which the shifts in the binding energies of core electrons are associated with the chemical environment of the neutral ground state elements. This is a powerful spectroscopy in wide use at synchrotrons. Of course 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 contributed to shed light on the interpretation.
The molecule acetylacetone is generally believed to have the structure A in Figure 1, in which a hydrogen atom is bonded to oxygen atom 1 or 2. A few “heretics” have suggested that structure B is more appropriate.
The oxygen core level spectrum, Figure 2, appears to support structure A: there are two distinct peaks, interpreted as due to oxygen bonded, or not bonded, to hydrogen. Incidentally, there is a weak satellite peak at higher binding energy.

Figure 1 Schematic structure of acetylacetone.

Figure 2 O 1s spectrum of acetylacetone.

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, Figure 3. 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.
The new interpretation of the O 1s spectrum is that the proton is delocalised in the ground state. Upon ionization, it localises close to or far from the ionized oxygen atom and these two cases correspond to the two peaks in the photoemission spectrum. This 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.
 

Figure 3 Electronic ground state potential energy curve (green solid line, left axis), and wave functions of the proton (dotted lines, right scale, units not shown) in the vibrational ground state (black) and first excited vibrational state (red).


 

This research was conducted by the following research team:

Vitaliy Feyer1,8, Kevin C. Prince1,2, Marcello Coreno3, Sonia Melandri4, Assimo Maris4, Luca Evangelisti4, Walther Caminati4, Barbara M. Giuliano5, Henrik G. Kjaergaard6, and Vincenzo Carravetta7
 

Elettra - Sincrotrone Trieste SCpA, in Area Science Park, Trieste, Italy.
Molecular Model Discovery Laboratory, Department of Chemistry and Biotechnology, Swinburne University of Technology, Melbourne, Australia.
CNR – Istituto di Struttura della Materia, Trieste, Italy
Dipartimento di Chimica "G. Ciamician" dell'Università,  Bologna, Italy.
Departamento de Química da Universidade de Coimbra, Coimbra, Portugal.
Department of Chemistry, University of Copenhagen, Copenhagen, Denmark.
CNR, Institute of Chemical Physical Processes, Pisa, Italy.
Present address: Forschungszentrum Julich GmbH, Peter Grunberg Institute, Electronic Properties, Julich, Germany


Contact persons:

Kevin Prince, email: kevin.prince@elettra.eu
 

Reference

Vitaliy Feyer, Kevin C. Prince, Marcello Coreno, Sonia Melandri, Assimo Maris, Luca Evangelisti, Walther Caminati, Barbara M. Giuliano, Henrik G. Kjaergaard, and Vincenzo Carravetta, “Quantum Effects for a Proton in a Low-Barrier, Double-Well Potential: Core Level Photoemission Spectroscopy of Acetylacetone”, J. Phys. Chem. Lett. 9, 521 (2018). DOI: 10.1021/acs.jpclett.7b03175

 

 
Last Updated on Monday, 26 February 2018 14:51