Quantitative evaluation of transient valence orbital occupations in a 3d transition metal complex as seen from the metal and ligand perspective

It is demonstrated for the case of photo excited ferrocyanide how time resolved soft X ray absorption spectroscopy in transmission geometry at the ligand K edge and metal L3 edge provides quantitatively equivalent valence electronic structure information, where signatures of photo oxidation are assessed locally at the metal as well as the ligand. This allows for a direct and independent quantification of the number of photo oxidized molecules at two soft X ray absorption edges highlighting the sensitivity of X ray absorption spectroscopy to the valence orbital occupation of 3d transition metal complexes throughout the soft X ray rang

Probing Solute Solvent Interactions of Transition Metal Complexes Using L Edge Absorption Spectroscopy

In order to tailor solution phase chemical reactions involving transition metal complexes, it is critical to understand how their valence electronic charge distributions are affected by the solution environment. Here, solute amp; 8722;solvent interactions of a solvatochromic mixed ligand iron complex were investigated using X ray absorption spectroscopy at the transition metal L2,3 edge. Due to the selectivity of the corresponding core excitations to the iron 3d orbitals, the method grants direct access to the valence electronic structure around the iron center and its response to interactions with the solvent environment. A linear increase of the total L2,3 edge absorption cross section as a function of the solvent Lewis acidity is revealed. The effect is caused by relative changes in different metal amp; 8722;ligand bonding channels, which preserve local charge densities while increasing the density of unoccupied states around the iron center. These conclusions are corroborated by a combination of molecular dynamics and spectrum simulations based on time dependent density functional theory. The simulations reproduce the spectral trends observed in the X ray but also optical absorption experiments. Our results underscore the importance of solute amp; 8722;solvent interactions when aiming for an accurate description of the valence electronic structure of solvated transition metal complexes and demonstrate how L2,3 edge absorption spectroscopy can aid in understanding the impact of the solution environment on intramolecular covalency and the electronic charge distributio

Metal water covalency in the photo aquated ferrocyanide complex as seen by multi edge picosecond X ray absorption

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, we carry out a complete characterization of the bonding channels in the [Fe CN 5 H2O ]3 amp; 8722; photo product. We observe clear spectral signatures of covalent bonding between water and the metal, reflecting the mixing of the Fe dz2 orbital with the 3a1 and 4a1 orbitals of H2O. Additional fingerprints related to the symmetry reduction and the resulting loss in orbital degeneracy are also reported. The implications of the elucidated fingerprints in the context of future ultra fast experiments are also discusse

Fundamental electronic changes upon intersystem crossing in large aromatic photosensitizers free base 5,10,15,20 tetrakis 4 carboxylatophenyl porphyrin

Free base 5,10,15,20 tetrakis 4 carboxylatophenyl porphyrin stands for the class of powerful porphyrin photosensitizers for singlet oxygen generation and light harvesting. The atomic level selectivity of dynamic UV pump N K edge probe X ray absorption spectroscopy in combination with time dependent density functional theory TD DFT gives direct access to the crucial excited molecular states within the unusual relaxation pathway. The efficient intersystem crossing, that is El Sayed forbidden and not facilitated by a heavy atom is confirmed to be the result of the long singlet excited state lifetime Qx 4.9 ns and thermal effects. Overall, the interplay of stabilization by conservation of angular momenta and vibronic relaxation drive the de excitation in these chromophore

The nature of frontier orbitals under systematic ligand exchange in pseudo octahedral Fe II complexes

Understanding and controlling properties of transition metal complexes is a crucial step towards tailoring materials for sustainable energy applications. In a systematic approach, we use resonant inelastic X ray scattering to study the influence of ligand substitution on the valence electronic structure around an aqueous iron II center. Exchanging cyanide with 2 20 bipyridine ligands reshapes frontier orbitals in a way that reduces metal 3d charge delocalization onto the ligands. This net decrease of metal ligand covalency results in lower metal centered excited state energies in agreement with previously reported excited state dynamics. Furthermore, traces of solvent effects were found indicating a varying interaction strength of the solvent with ligands of different character. Our results demonstrate how ligand exchange can be exploited to shape frontier orbitals of transition metal complexes in solution phase chemistry; insights upon which future efforts can built when tailoring the functionality of photoactive systems for light harvesting application

Selective gating to vibrational modes through resonant X ray scattering

The dynamics of fragmentation and vibration of molecular systems with a large number of coupled degrees of freedom are key aspects for understanding chemical reactivity and properties. Here we present a resonant inelastic X ray scattering RIXS study to show how it is possible to break down such a complex multidimensional problem into elementary components. Local multimode nuclear wave packets created by X ray excitation to different core excited potential energy surfaces PESs will act as spatial gates to selectively probe the particular ground state vibrational modes and, hence, the PES along these modes. We demonstrate this principle by combining ultra high resolution RIXS measurements for gas phase water with state of the art simulation

One dimensional cuts through multidimensional potential energy surfaces by tunable x rays

The concept of the potential energy surface PES and directional reaction coordinates is the backbone of our description of chemical reaction mechanisms. Although the eigenenergies of the nuclear Hamiltonian uniquely link a PES to its spectrum, this information is in general experimentally inaccessible in large polyatomic systems. This is due to near degenerate rovibrational levels across the parameter space of all degrees of freedom, which effectively forms a pseudospectrum given by the centers of gravity of groups of close lying vibrational levels. We show here that resonant inelastic x ray scattering RIXS constitutes an ideal probe for revealing one dimensional cuts through the ground state PES of molecular systems, even far away from the equilibrium geometry, where the independent mode picture is broken. We strictly link the center of gravity of close lying vibrational peaks in RIXS to a pseudospectrum which is shown to coincide with the eigenvalues of an effective one dimensional Hamiltonian along the propagation coordinate of the core excited wave packet. This concept, combined with directional and site selectivity of the core excited states, allows us to experimentally extract cuts through the ground state PES along three complementary directions for the showcase H2O molecul

Cuts through the manifold of molecular H2O potential energy surfaces in liquid water at ambient conditions

The fluctuating hydrogen bridge bonded network of liquid water at ambient conditions entails a varied ensemble of the underlying constituting H2O molecular moieties.This is mirrored in a manifold of the H2O molecular potentials. Subnatural line width resonant inelastic X-ray scattering allowed us to quantify the manifold of molecular potential energy surfaces along the H2O symmetric normal mode and the local asymmetric O–H bond coordinate up to 1 and 1.5 A, respectively. The comparison of the single ˚ H2O molecular potentials and spectroscopic signatures with the ambient conditions liquid phase H2O molecular potentials is done on various levels. In the gas phase, first principles, Morse potentials, and stepwise harmonic potential reconstruction have been employed and benchmarked. In the liquid phase the determination of the potential energy manifold along the local asymmetric O–H bond coordinate from resonant inelastic X-ray scattering via the bound state oxygen 1s to 4a1 resonance is treated within these frameworks. The potential energy surface manifold along the symmetric stretch from resonant inelastic X-ray scattering via the oxygen 1s to 2b2 resonance is based on stepwise harmonic reconstruction. We find in liquid water at ambient conditions H2O molecular potentials ranging from the weak interaction limit to strongly distorted potentials which are put into perspective to established parameters, i.e., intermolecular O–H, H–H, and O–O correlation lengths from neutron scattering.</p

A study of the water molecule using frequency control over nuclear dynamics in resonant X ray scattering

In this combined theoretical and experimental study we report a full analysis of the resonant inelastic X ray scattering RIXS spectra of H 2O, D 2O and HDO. We demonstrate that electronically elastic RIXS has an inherent capability to map the potential energy surface and to perform vibrational analysis of the electronic ground state in multimode systems. We show that the control and selection of vibrational excitation can be performed by tuning the X ray frequency across core excited molecular bands and that this is clearly reflected in the RIXS spectra. Using high level ab initio electronic structure and quantum nuclear wave packet calculations together with high resolution RIXS measurements, we discuss in detail the mode coupling, mode localization and anharmonicity in the studied system