Michael Odelius

The structural dynamics of metal carbonyls are central to processes ranging from catalysis to organometallic synthesis. Here we investigate the photodissociation of a prototypical transition metal carbonyl, Fe(CO)₅, using mega-electron-volt ultrafast electron diffraction. By separately tracking structural evolution along the axial and equatorial directions, we provide an atomistic, angle-resolved view of the nuclear motions preceding CO dissociation and infer key features of the excited-state potential energy surface from the experimental observations. We further show that vibrational coupling before reaching the conical intersection facilitates the loss of a random carbonyl ligand via the Berry pseudorotation mechanism.

Time-resolved X-ray and infrared spectroscopy reveal electronic structure changes from the metal and ligand perspect following ligand-exchange in molybdenum hexacarbonyl.

We investigated the non-adiabatic dynamics of photoexcited thiopyridone systems across their ortho-, meta-, and para-isomeric forms. The relaxation pathways of the three isomers in both gas phase and solvent environments are mapped using surface hopping dynamics based on time-dependent density functional theory. Our analysis highlights the influence of isomeric structures on photophysical behavior, offering insights into design principles to control photochemical phenomena. The simulations suggest a systematic reduction in the rate of intersystem crossing (ISC) from ortho- to meta- to para-isomer. Comparisons with multiconfigurational wave function methods in the gas phase further demonstrate how electronic structure influences the predicted dynamical pathways. The simulated dynamics demonstrates that the spin–orbit coupling strength alone does not determine the rate of ISC, as both state energetics and underlying electronic and structural features play decisive roles. These aspects explain the much slower ISC in the para-isomer, as well as the non-negligible role of the El-Sayed forbidden pathway in the computed ISC dynamics.

The trans-cis isomerization in azobenzene is a reversible reaction that has strong application potential ranging from photo-stimulating molecular motors to molecular electronics. Though azobenzene has been intensively and thoroughly investigated in optical spectroscopy, the S0 -> S1 transition is symmetry forbidden. Here, we report on measurements of resonant inelastic X-ray scattering (RIXS) at the nitrogen K-edge of the trans- and cis-forms of azobenzene dissolved in ethanol. In RIXS, we observe isomer-specific spectral features, which contain a distinct signature of the S1 state and further information on the electronic properties of the isomers of azobenzene. For the cis-isomer an additional peak arises, revealing a transition suppressed in the trans-conformation due to diferent orbital hybridization, thereby giving distinct spectral finn=gerprints to the trans- and cis-isomers of azobenzene in RIXS, containing information about valence excitations and their nitrogen character.

The nonadiabatic dynamics of photoexcited thiopyridone systems was studied across their ortho-, meta-, and para-isomeric forms. Surface hopping dynamics, utilizing time-dependent density functional theory, elucidates the relaxation pathways of the three constitutional isomers in both gas-phase and solvent environments. Particular attention is given to intersystem crossing (ISC) and internal conversion in the triplet manifold, as the lowest triplet state is known to play a critical role in driving the excited-state proton transfer process in the ortho-isomer. The efficiency of ISC was analyzed by evaluating electronic features and energy gaps derived from molecular dynamics simulations. The investigations suggest a systematic reduction in the rate of ISC from ortho- to meta- to para-isomer. Furthermore, a comparison in the gas phase with simulations based on multiconfigurational wavefunction methods highlights how variations in electronic structure influence the dynamical pathways.

Transient X-ray absorption spectroscopy at the sulfur K edge probes the ultrafast internal conversion and intersystem crossing of thiopyridone isomers in solution.

Mechanistic insights into photodissociation dynamics of transition metal carbonyls, like Fe(CO)₅, are fundamental for understanding active catalytic intermediates. Although extensively studied, the structural dynamics of these systems remain elusive. Using ultrafast X-ray scattering, we uncover the photochemistry of Fe(CO)₅ in real space and time, observing synchronous oscillations in atomic pair distances, followed by a prompt rotating CO release preferentially in the axial direction. This behavior aligns with simulations, reflecting the interplay between the axial Fe-C distances’ potential energy landscape and non-adiabatic transitions between metal-to-ligand charge-transfer states. Additionally, we characterize a secondary delayed CO release associated with a reduction of Fe-C steady state distances and structural dynamics of the formed Fe(CO)₄. Our results quantify energy redistribution across vibration, rotation, and translation degrees of freedom, offering a microscopic view of complex structural dynamics, enhancing our grasp on Fe(CO)₅ photodissociation, and advancing our understanding of transition metal catalytic systems.

The structural dynamics of metal carbonyls are central to diverse chemical processes, from catalysis to organometallic synthesis. Here we investigate the photodissociation process of a prototypical transition metal carbonyl, Fe(CO) ₅ , using mega-electron-volt ultrafast electron diffraction. By analyzing the structural evolution separately in the axial and equatorial directions of Fe(CO) ₅ , we provide a 3-D atomistic view of the internal conversion process prior to dissociation, and constructed an excited state potential energy surface based on our observation. Additionally, we highlight the significance of wavepacket decoherence preceding the conical intersection, leading to the dissociation of a random carbonyl ligand via Berry pseudorotation mechanism.

Nitrogen K-edge X-ray absorption (XA) spectroscopy of aqueous ammonia reveals a splitting in the main-edge, which through theoretical modeling is shown to be related to symmetry breaking in hydrogen bonding. The XA main-edge of NH₃ is formed by a pair of degenerate core-excitations into extended molecular orbitals. In aqueous solution, these form an antibonding mixture with orbitals of the surrounding water molecules. Although the spectral response to distortions is complex, we show that the degeneracy of the core-excitations is lifted by asymmetry in hydrogen bond donation (NH···O). A quantitative relation between asymmetry in the hydration shell and splitting in the main-edge of the nitrogen K-edge XA spectrum is established from systematic symmetry breaking in well-defined cluster models and through molecular dynamics sampling of simulated XA spectra of aqueous ammonia. The finding indicates that XA spectroscopy is a sensitive probe of asymmetry in solvation also around functional groups in biomolecules.