Breaking the Symmetry of Pyrimidine: Solvent Effects and Core-Excited State Dynamics

Electronic structure of aqueous nitrite and nitrate ions from resonant inelastic X-ray scattering

In a comparative synchrotron X-ray absorption, non-resonant X-ray emission and resonant inelastic X-ray scattering investigation of aqueous nitrite and nitrate ions, we access both their unoccupied and occupied valence electronic structures. Complementary information is gained through the sensitivity to specific orbitals at the nitrogen and the oxygen 1s absorption edges. In particular, scattering through the pronounced 1s → π* resonances in combination with the scattering anisotropy and symmetry selection rules allow for an unambiguous assignment of molecular orbitals to their detected spectroscopic fingerprints. The nuclear dynamics in the 1s core-excited states are discussed in the context of the vibrational substructure of the detected spectral lines and signatures of core-excited state symmetry breaking are characterized through an analysis of the excitation energy detuning dependent spectra in combination with the involved potentials. A comparison between TD-DFT based spectrum simulations for isolated molecules and sampled structures from a QM/MM simulation reveals signatures of symmetry breaking induced by the solute-solvent interactions and a different response of spectral signatures of in- and out-of-plane orbitals to the solution environment.

Capturing Coupled Structural and Electronic Motions During Excited-State Intramolecular Proton Transfer via Computational Multiedge Resonant Inelastic X-ray Scattering

Proton transfer processes form the foundation of many chemical processes. In excited-state intramolecular proton transfer (ESIPT) processes, ultrafast proton transfer is impulsively initiated through light. Here, we explore time-dependent coupled atomic and electronic motions during and following ESIPT through computational time-resolved resonant inelastic X-ray scattering (RIXS). Excited-state ab initio molecular dynamics simulations combined with time-dependent density functional theory calculations were performed for a model ESIPT system, 10-hydroxybenzo[h]quinoline, to obtain transient RIXS signatures. The RIXS spectra at both the nitrogen and oxygen K-edges were computed to resolve the electronic and atomic structural dynamics from both the proton donor and acceptor perspective. The results demonstrate that RIXS provides unprecedented details of the local electronic structure, the coupling between different core and valence excited electronic states, and the reorganization of the electronic structure coupled to the proton transfer process. We also develop a spectroscopic ruler correlating spectral shifts of a RIXS peak to the proton transfer distance during ESIPT. This work highlights the exciting potential of time-resolved RIXS experiments at newly commissioned soft X-ray free electron laser facilities for measuring coupled electronic and structural changes during ultrafast chemical processes.

A fully dynamical description of time-resolved resonant inelastic X-ray scattering of pyrazine

Recent advancements in ultrashort and intense X-ray sources have enabled the utilisation of resonant inelastic X-ray scattering (RIXS) as a probing technique for monitoring photoinduced dynamics in molecular systems. To account for dynamic phenomena like non-adiabatic transitions across the relevant electronic state manifold, a time-dependent framework is crucial. Here, we introduce a fully time-dependent approach for calculating transient RIXS spectra using wavepacket dynamics simulations, alongside an explicit treatment of the X-ray probe pulse that surpasses Kramers-Heisenberg-Dirac constraints. Our analysis of pyrazine at the nitrogen K-edge underscores the importance of considering nuclear motion effects in all electronic states involved in the transient RIXS process. As a result, we propose a numerically exact approach to computationally support and predict cutting-edge time-resolved RIXS experiments.

Time-Dependent Resonant Inelastic X-ray Scattering of Pyrazine at the Nitrogen K-Edge: A Quantum Dynamics Approach

We calculate resonant inelastic X-ray scattering spectra of pyrazine at the nitrogen K-edge in the time domain including wavepacket dynamics in both the valence and core-excited state manifolds. Upon resonant excitation, we observe ultrafast non-adiabatic population transfer between core-excited states within the core-hole lifetime, leading to molecular symmetry distortions. Importantly, our time-domain approach inherently contains the ability to manipulate the dynamics of this process by detuning the excitation energy, which effectively shortens the scattering duration. We also explore the impact of pulsed incident X-ray radiation, which provides a foundation for state-of-the-art time-resolved experiments with coherent pulsed light sources.

Electronic structure, bonding and stability of fumarate, maleate, and succinate dianions from X-ray spectroscopy

The electronic structure of the fumarate, maleate, and succinate dianions in the context of their stability is determined in a joint experimental and computational study with X-ray absorption spectroscopy and resonant inelastic X-ray scattering at the O K-edge. The study reveals differences in the electronic states and molecular orbitals of the three molecules. In particular, maleate has a non-degenerate oxygen core-orbital with an energy difference of approximately 0.15 eV, visible in a two peak structure in XAS. Polarization-dependent RIXS provides information on the orientation of the occupied valence molecular orbitals with respect to the carboxylate group plane and shows a gradually increasing energy gap between the HOMO and excited π* LUMO from fumarate to maleate to succinate. We also demonstrate the energy excitation dependence of the RIXS spectra of maleate, with the total inelastic RIXS profile shifting towards higher energy loss as the detuning is increased from negative to positive values. Our findings show that maleate is less stable than fumarate and succinate due to the presence of electronic density on its HOMO orbital on the CC bond between carboxylate groups, which can lead to weaker bonding of maleate with molecules or ions.

R-Group stabilization in methylated formamides observed by resonant inelastic X-ray scattering

The inherent stability of methylated formamides is traced to a stabilization of the deep-lying σ-framework by resonant inelastic X-ray scattering at the nitrogen K-edge. Charge transfer from the amide nitrogen to the methyl groups underlie this stabilization mechanism that leaves the aldehyde group essentially unaltered and explains the stability of secondary and tertiary amides.

From the Free Ligand to the Transition Metal Complex: FeEDTA- Formation Seen at Ligand K-Edges

Chelating agents are an integral part of transition metal complex chemistry with broad biological and industrial relevance. The hexadentate chelating agent ethylenediaminetetraacetic acid (EDTA) has the capability to bind to metal ions at its two nitrogen and four of its carboxylate oxygen sites. We use resonant inelastic X-ray scattering at the 1s absorption edge of the aforementioned elements in EDTA and the iron(III)-EDTA complex to investigate the impact of the metal-ligand bond formation on the electronic structure of EDTA. Frontier orbital distortions, occupation changes, and energy shifts through metal-ligand bond formation are probed through distinct spectroscopic signatures.

Targeting Individual Tautomers in Equilibrium by Resonant Inelastic X-ray Scattering

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.

Ligand-Field Effects in a Ruthenium(II) Polypyridyl Complex Probed by Femtosecond X-ray Absorption Spectroscopy

We employ femtosecond X-ray absorption spectroscopy of [Ru(m-bpy)₃]²⁺ (m-bpy = 6-methyl-2,2'-bipyridine) to elucidate the time evolution of the spin and charge density upon metal-to-ligand charge-transfer (MLCT) excitation. The core-level transitions at the Ru L₃-edge reveal a very short MLCT lifetime of 0.9 ps and relaxation to the lowest triplet metal-centered state (³MC) which exhibits a lifetime of about 300 ps. Time-dependent density functional theory relates ligand methylation to a lower ligand field strength that stabilizes the ³MC state. A quarter of the ³MLCT population appears to be trapped which may be attributed to intramolecular vibrational relaxation or further electron transfer to the solvent. Our results demonstrate that small changes in the ligand field allow control of the photophysical properties. Moreover, this study underscores the high information content of femtosecond L-edge spectroscopy as a probe of valence charge density and spin-state in 4d transition metals.