Steric hindrance
of hydration and hydrogen bond enhancement by
localized charges have been identified as key factors for the massive
chemical differences between the hydroxypyridine/pyridone isomers
in aqueous solution. While all isomers occur mainly in the hydroxypyridine
form in the gas phase, they differ by more than 3 orders of magnitude
both in their acidity and tautomeric equilibrium constants upon hydration.
By monitoring the electronic and solvation structures as a function
of the protonation state and the O
–
substitution
position on the pyridine ring, the amplification of the isomeric differences
in aqueous solution has been investigated. Near-edge X-ray absorption
fine structure (NEXAFS) measurements at the N K-edge served as the
probe of the chemical state. The combination of molecular dynamics
simulations, complete active space self-consistent field (CASSCF),
and time-dependent density functional theory (TD-DFT) spectral calculations
contributes to unraveling the principles of tautomerism and acidity
in multiple biochemical systems based on tautomerism.
In this work we investigate the photo-aquation reaction of the ferrocyanide anion with multi-edge picosecond soft X-ray spectroscopy. Combining the information of the iron L-edge with nitrogen and oxygen K-edges...
While extensive work has been dedicated to the measurement of the demagnetization time following an ultra-short laser pulse, experimental studies of its underlying microscopic mechanisms are still scarce. In transition metal ferromagnets, one of the main mechanism is the spin-flip of conduction electrons driven by electron-phonon scattering. Here, we present an original experimental method to monitor the electron-phonon mediated spin-flip scattering rate in nickel through the stringent atomic symmetry selection rules of x-ray emission spectroscopy. Increasing the phonon population leads to a waning of the 3
d
→ 2
p
3/2
decay peak intensity, which reflects an increase of the angular momentum transfer scattering rate attributed to spin-flip. We find a spin relaxation time scale in the order of 50 fs in the 3
d
-band of nickel at room temperature, while consistantly, no such peak evolution is observed for the diamagnetic counterexample copper, using the same method.
Cr(CO)6 was investigated by X-ray absorption spectroscopy. The spectral signature at the metal edge shows information about the back-bonding of the metal in this class of complexes. Among the processes...
The crucial transient states of free-base porphyrins are characterized by time-resolved X-ray absorption spectroscopy unraveling their unusual relaxation pathway.
Tautomerism
is one
of the most important forms of isomerism, owing
to the facile interconversion between species and the large differences
in chemical properties introduced by the proton transfer connecting
the tautomers. Spectroscopic techniques are often used for the characterization
of tautomers. In this context, separating the overlapping spectral
response of coexisting tautomers is a long-standing challenge in chemistry.
Here, we demonstrate that by using resonant inelastic X-ray scattering
tuned to the core excited states at the site of proton exchange between
tautomers one is able to experimentally disentangle the manifold of
valence excited states of each tautomer in a mixture. The technique
is applied to the prototypical keto–enol equilibrium of 3-hydroxypyridine
in aqueous solution. We detect transitions from the occupied orbitals
into the LUMO for each tautomer in solution, which report on intrinsic
and hydrogen-bond-induced orbital polarization within the π
and σ manifolds at the proton-transfer site.
The electron–phonon scattering is one of the main microscopic mechanisms responsible for the spin-flip in the transient state of ultrafast demagnetization. Here, we present an experimental determination of the temperature-dependent electron–phonon scattering rate in Gd. Using a static x-ray emission spectroscopy method, where the reduction of the decay peak intensities when increasing the temperature is quantified, we measure independently the electron-phonon scattering rate for the 5d and the 4f electrons. We deduce the temperature dependence of scattering for the 5d electrons, while no effect on the phonon population is observed for the 4f electrons. Our results suggest that the ultrafast magnetization dynamics in Gd is triggered by the spin-flip in the 5d electrons. We also evidence the existence of a temperature threshold, above which spin-flip scattering of the 5d electrons takes place. We deduce that during the transient state of ultrafast demagnetization, the exchange energy between 5d electrons has to be overcome before the microscopic electron-phonon scattering process can occur.
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