Time-resolved electron spectroscopy for chemical analysis of photodissociation: Photoelectron spectra of Fe(CO)5, Fe(CO)4, and Fe(CO)3

Mapping Hydrogen Positions along the Proton Transfer Pathway in an Organic Crystal by Computational X-ray Spectra

Understanding proton transfer (PT) dynamics in condensed phases is crucial in chemistry. We computed a 2D map of N 1s X-ray photoelectron/absorption spectroscopy (XPS/XAS) for an organic donor-acceptor salt crystal against two varying N-H distances to track proton motions. Our results provide a continuous spectroscopic mapping of O-H···N↔O-··· H+-N processes via hydrogen bonds at both nitrogens, demonstrating the sensitivity of N 1s transient XPS/XAS to hydrogen positions and PT. By reducing the O-H length at N1 by only 0.2 Å, we achieved excellent theory-experiment agreement in both XPS and XAS. Our study highlights the challenge in refining proton positions in experimental crystal structures by periodic geometry optimizations and proposes an alternative scaled snapshot protocol as a more effective approach. This work provides valuable insights into X-ray spectra for correlated PT dynamics in complex crystals, benefiting future experimental studies.

Theoretical Investigation of Transient Species Following Photodissociation of Ironpentacarbonyl in Ethanol Solution

Photodissociation of ironpentacarbonyl [1Fe(CO)5] in solution generates transient species in different electronic states, which we studied theoretically. From ab initio molecular dynamics simulations in ethanol solution, the closed-shell parent compound 1Fe(CO)5 is found to interact weakly with the solvent, whereas the irontetracarbonyl [Fe(CO)4] species, formed after photodissociation, has a strongly spin-dependent behavior. It coordinates a solvent molecule tightly in the singlet state [1Fe(CO)4] and weakly in the triplet state [3Fe(CO)4]. From the simulations, we have gained insights into intersystem crossing in solvated irontetracarbonyl based on the distinct structural differences induced by the change in multiplicity. Alternative forms of coordination between 1Fe(CO)4 and functional groups of the ethanol molecule are simulated, and a quantum chemical investigation of the energy landscape for the coordinated irontetracarbonyl gives information about the interconversion of different transient species in solution. Furthermore, insights from the simulations, in which we find evidence of a solvent exchange mechanism, challenge the previously proposed mechanism of chain walking for under-coordinated metal carbonyls in solution.

Simulating transient X-ray photoelectron spectra of Fe(CO)5 and its photodissociation products with multireference algebraic diagrammatic construction theory

Accurate simulations of transient X-ray photoelectron spectra (XPS) provide unique opportunities to bridge the gap between theory and experiment in understanding the photoactivated dynamics in molecules and materials. However, simulating X-ray photoelectron spectra along a photochemical reaction pathway is challenging as it requires accurate description of electronic structure incorporating core-hole screening, orbital relaxation, electron correlation, and spin-orbit coupling in excited states or at nonequilibrium ground-state geometries. In this work, we employ the recently developed multireference algebraic diagrammatic construction theory (MR-ADC) to investigate the core-ionized states and X-ray photoelectron spectra of Fe(CO)5 and its photodissociation products (Fe(CO)4, Fe(CO)3) following excitation with 266 nm light. The simulated transient Fe 3p and CO 3σ XPS spectra incorporating spin-orbit coupling and high-order electron correlation effects are shown to be in a good agreement with the experimental measurements by Leitner et al. [J. Chem. Phys., 2018, 149, 044307]. Our calculations suggest that core-hole screening, spin-orbit coupling, and ligand-field splitting effects are similarly important in reproducing the experimentally observed chemical shifts in transient Fe 3p XPS spectra of iron carbonyl complexes. Our results also demonstrate that the MR-ADC methods can be very useful in interpreting the transient XPS spectra of transition metal compounds.

Chemical bond reorganization in intramolecular proton transfer revealed by ultrafast X-ray photoelectron spectroscopy

Time-resolved X-ray photoelectron spectroscopy (TR-XPS) is used in a simulation study to monitor the excited state intramolecular proton transfer between oxygen and nitrogen atoms in 2-(iminomethyl)phenol. Real-time monitoring of the chemical bond breaking and forming processes is obtained through the time evolution of excited-state chemical shifts. By employing individual atomic probes of the proton donor and acceptor atoms, we predict distinct signals with opposite chemical shifts of the donor and acceptor groups during proton transfer. Details of the ultrafast bond breaking and forming dynamics are revealed by extending the classical electron spectroscopy chemical analysis to real time. Through a comparison with simulated time-resolved photoelectron spectroscopy at the valence level, the distinct advantage of TR-XPS is demonstrated thanks to its atom specificity.

Time-resolved x-ray spectroscopy of nucleobases and their thionated analogs

The photoinduced relaxation dynamics of nucleobases and their thionated analogs have been investigated extensively over the past decades motivated by their crucial role in organisms and their application in medical and biochemical research and treatment. Most of these studies focused on the spectroscopy of valence electrons and fragmentation. The advent of ultrashort x-ray laser sources such as free-electron lasers, however, opens new opportunities for studying the ultrafast molecular relaxation dynamics utilizing the site- and element-selectivity of x-rays. In this review, we want to summarize ultrafast experiments on thymine and 2-thiouracil performed at free-electron lasers. We performed time-resolved x-ray absorption spectroscopy at the oxygen K-edge after UV excitation of thymine. In addition, we investigated the excited state dynamics of 2-tUra via x-ray photoelectron spectroscopy at sulfur. For these methods, we show a strong sensitivity to the electronic state or charge distribution, respectively. We also performed time-resolved Auger-Meitner spectroscopy, which shows spectral shifts associated with internuclear distances close to the probed site. We discuss the complementary aspects of time-resolved x-ray spectroscopy techniques compared to optical and UV spectroscopy for the investigation of ultrafast relaxation processes.

Time-Resolved X-ray Photoelectron Spectroscopy: Ultrafast Dynamics in CS2 Probed at the S 2p Edge

Recent developments in X-ray free-electron lasers have enabled a novel site-selective probe of coupled nuclear and electronic dynamics in photoexcited molecules, time-resolved X-ray photoelectron spectroscopy (TRXPS). We present results from a joint experimental and theoretical TRXPS study of the well-characterized ultraviolet photodissociation of CS2, a prototypical system for understanding non-adiabatic dynamics. These results demonstrate that the sulfur 2p binding energy is sensitive to changes in the nuclear structure following photoexcitation, which ultimately leads to dissociation into CS and S photoproducts. We are able to assign the main X-ray spectroscopic features to the CS and S products via comparison to a first-principles determination of the TRXPS based on ab initio multiple-spawning simulations. Our results demonstrate the use of TRXPS as a local probe of complex ultrafast photodissociation dynamics involving multimodal vibrational coupling, nonradiative transitions between electronic states, and multiple final product channels.

Photochemistry of transition metal carbonyls

The purpose of this Tutorial Review is to outline the fundamental photochemistry of metal carbonyls, and to show how the advances in technology have increased our understanding of the detailed mechanisms, particularly how relatively simple experiments can provide deep understanding of complex problems. We recall some important early experiments that demonstrate the key principles underlying current research, concentrating on the binary carbonyls and selected substituted metal carbonyls. At each stage, we illustrate with examples from recent applications. This review first considers the detection of photochemical intermediates in three environments: glasses and matrices; gas phase; solution. It is followed by an examination of the theory underpinning these observations. In the final two sections, we briefly address applications to the characterization and behaviour of complexes with very labile ligands such as N₂, H₂ and alkanes, concentrating on key mechanistic points, and also describe some principles and examples of photocatalysis.

A localized view on molecular dissociation via electron-ion partial covariance

Inner-shell photoelectron spectroscopy provides an element-specific probe of molecular structure, as core-electron binding energies are sensitive to the chemical environment. Short-wavelength femtosecond light sources, such as Free-Electron Lasers (FELs), even enable time-resolved site-specific investigations of molecular photochemistry. Here, we study the ultraviolet photodissociation of the prototypical chiral molecule 1-iodo-2-methylbutane, probed by extreme-ultraviolet (XUV) pulses from the Free-electron LASer in Hamburg (FLASH) through the ultrafast evolution of the iodine 4d binding energy. Methodologically, we employ electron-ion partial covariance imaging as a technique to isolate otherwise elusive features in a two-dimensional photoelectron spectrum arising from different photofragmentation pathways. The experimental and theoretical results for the time-resolved electron spectra of the 4d_3/2 and 4d_5/2 atomic and molecular levels that are disentangled by this method provide a key step towards studying structural and chemical changes from a specific spectator site.

Photoinduced bond oscillations in ironpentacarbonyl give delayed synchronous bursts of carbonmonoxide release

Early excited state dynamics in the photodissociation of transition metal carbonyls determines the chemical nature of short-lived catalytically active reaction intermediates. However, time-resolved experiments have not yet revealed mechanistic details in the sub-picosecond regime. Hence, in this study the photoexcitation of ironpentacarbonyl Fe(CO)₅ is simulated with semi-classical excited state molecular dynamics. We find that the bright metal-to-ligand charge-transfer (MLCT) transition induces synchronous Fe-C oscillations in the trigonal bipyramidal complex leading to periodically reoccurring release of predominantly axial CO. Metaphorically the photoactivated Fe(CO)₅ acts as a CO geyser, as a result of dynamics in the potential energy landscape of the axial Fe-C distances and non-adiabatic transitions between manifolds of bound MLCT and dissociative metal-centered (MC) excited states. The predominant release of axial CO ligands and delayed release of equatorial CO ligands are explained in a unified mechanism based on the σ^*(Fe-C) anti-bonding character of the receiving orbital in the dissociative MC states.

The photodissociation of transition metal carbonyls is involved in catalysis and synthetic processes. Here the authors, using semi-classical excited state molecular dynamics, observe details of the early stage dynamics in the photodissociation of Fe(CO)₅, including synchronous bursts of CO at periodic intervals of 90 femtoseconds.

Generation of Highly Vibrationally Excited CO in Sequential Photodissociation of Iron Carbonyl Complexes

Ultraviolet photochemistry of iron pentacarbonyl, Fe(CO)₅, was investigated with resonantly enhanced multiphoton ionization (REMPI) spectroscopy and ion imaging. The REMPI spectrum of CO photofragments, generated by ultraviolet irradiation of Fe(CO)₅, showed the generation in the highly vibrationally excited states with v = 11-15. Analysis of the band intensities observed in the 213-235 nm region indicated that the high-v CO generation was maximized at around 220 nm. Generation yields of the coordinatively unsaturated intermediates, Fe(CO)_n=1-4, were measured as a function of the photolysis wavelength using a nonresonant detection scheme. The yield spectrum of FeCO was correlated with that of the high-v CO fragments, suggesting high-v CO generation in the photodissociation of FeCO. The density functional theory calculations of the excited states of FeCO showed an intense photoabsorption to the metal-centered state near 220 nm. The theoretical results were consistent with the interpretation of FeCO + hν → Fe + high-v CO, which was experimentally indicated. The momentum distribution obtained from the velocity distributions of Fe, Fe(CO)₄, and CO fragments further supported that Fe is the counter-product of the high-v CO fragment. The present results provided selective observation of the photochemistry of the unsaturated iron carbonyl complexes, which has not been well elucidated in laser-based experiments because of the uncontrollable sequential photodissociation producing mixed Fe(CO)_n intermediates.

Vibrational dynamics of iron pentacarbonyl in cryogenic matrices

Iron pentacarbonyl is a textbook example of fluxionality. We trap the molecule in cryogenic matrices to study the vibrational dynamics of CO stretching modes involved in the fluxional rearrangement. The infrared spectrum in Ar and N₂ is composed of about ten narrow bands in the spectral range of interest, indicating the population of various lattice sites and a lowering of the molecular symmetry in the trapping sites. The vibrational dynamics is explored by means of infrared stimulated photon echoes at the femtosecond scale. Vibrational dephasing and population relaxation times are obtained. The non-linear signals exhibit strong oscillations useful to disentangle the site composition of the absorption spectrum. The population relaxation involves at least two characteristic times. An evolution of the photon echo signals with the waiting time is observed. The behavior of all the signals can be reproduced within a simple model that describes the population relaxation occurring in two steps: relaxation of v = 1 (population time T₁ < 100 ps) and return to v = 0 (recovery time > 1 ns). These two steps explain the evolution of the oscillations with the waiting time in the photon echo signals. These results discard fluxional rearrangement on the time scale of hundreds of ps in our samples. Dephasing times are of the same order of magnitude as T₁: dephasing processes due to the matrix environment are rather inefficient. The photon echo experiments also reveal that intermolecular resonant vibrational energy transfers between guest molecules occur at the hundreds of ps time scale in concentrated samples (guest/host > 10⁴).

Following excited-state chemical shifts in molecular ultrafast x-ray photoelectron spectroscopy

The conversion of photon energy into other energetic forms in molecules is accompanied by charge moving on ultrafast timescales. We directly observe the charge motion at a specific site in an electronically excited molecule using time-resolved x-ray photoelectron spectroscopy (TR-XPS). We extend the concept of static chemical shift from conventional XPS by the excited-state chemical shift (ESCS), which is connected to the charge in the framework of a potential model. This allows us to invert TR-XPS spectra to the dynamic charge at a specific atom. We demonstrate the power of TR-XPS by using sulphur 2p-core-electron-emission probing to study the UV-excited dynamics of 2-thiouracil. The method allows us to discover that a major part of the population relaxes to the molecular ground state within 220–250 fs. In addition, a 250-fs oscillation, visible in the kinetic energy of the TR-XPS, reveals a coherent exchange of population among electronic states.

Imaging the charge flow in photoexcited molecules would provide key information on photophysical and photochemical processes. Here the authors demonstrate tracking in real time after photoexcitation the change in charge density at a specific site of 2-thiouracil using time-resolved X-ray photoelectron spectroscopy.

Unimolecular Double Photoionization-Induced Processes in Iron Pentacarbonyl

The dissociations of nascent Fe(CO)₅⁺⁺ ions created by 40.81 eV photoionization of iron pentacarbonyl have been examined using threefold and fourfold electron–ion coincidence measurements. The energies and forms of the ions have been explored by high-level calculations, revealing several new structures. The most stable form of Fe(CO)₅⁺⁺ has a quite different geometry from that of the neutral molecule. The dissociation pattern can be modeled as a sequence of CO evaporations followed by two-body charge separations. Each Fe(CO)_n⁺⁺ (n = 1–4) dication is stable in a restricted energy range; as its internal energy increases, it first ejects a neutral CO, then loses CO⁺ by charge separation at higher energy. In the initial stages, charge-retaining CO evaporations dominate over charge separation, but the latter become more competitive as the number of residual CO ligands decreases. At energies where ionization is mainly from the CO ligands, new Fe–C and C–C bonds are created by a mechanism which might be relevant to catalysis by Fe.

Dissociations of nascent Fe(CO)₅⁺⁺ ions by sequential CO evaporations, leading (in restricted energy ranges) to stable Fe(CO)_n⁺⁺ (n = 1−4) dicationic species. At energies where ionization is mainly from the CO ligands, new Fe−C and C−C bonds are created by a mechanism which might be relevant to catalysis by Fe.

Ultraviolet photodissociation of gas-phase iron pentacarbonyl probed with ultrafast infrared spectroscopy

It is well known that ultraviolet photoexcitation of iron pentacarbonyl results in rapid loss of carbonyl ligands leading to the formation of coordinatively unsaturated iron carbonyl compounds. We employ ultrafast mid-infrared transient absorption spectroscopy to probe the photodissociation dynamics of gas-phase iron pentacarbonyl following ultraviolet excitation at 265 and 199 nm. After photoexcitation at 265 nm, our results show evidence for sequential dissociation of iron pentacarbonyl to form iron tricarbonyl via a short-lived iron tetracarbonyl intermediate. Photodissociation at 199 nm results in the prompt production of Fe(CO)₃ within 0.25 ps via several energetically accessible pathways. An additional 15 ps time constant extracted from the data is tentatively assigned to intersystem crossing to the triplet manifold of iron tricarbonyl or iron dicarbonyl. Mechanisms for formation of iron tetracarbonyl, iron tricarbonyl, and iron dicarbonyl are proposed and theoretically validated with one-dimensional cuts through the potential energy surface as well as bond dissociation energies. Ground state calculations are computed at the CCSD(T) level of theory and excited states are computed with EOM-EE-CCSD(dT).

Femtosecond X-ray Liquidography Visualizes Wavepacket Trajectories in Multidimensional Nuclear Coordinates for a Bimolecular Reaction

ConspectusVibrational wavepacket motions on potential energy surfaces are one of the critical factors that determine the reaction dynamics of photoinduced reactions. The motions of vibrational wavepackets are often discussed in the interpretation of observables measured with various time-resolved vibrational or electronic spectroscopies but mostly in terms of the frequencies of wavepacket motions, which are approximated by normal modes, rather than the actual positions of the wavepacket. Although the time-dependent positions (that is, the trajectory) of wavepackets are hypothesized or drawn in imagined or calculated potential energy surfaces, it is not trivial to experimentally determine the trajectory of wavepackets, especially in multidimensional nuclear coordinates for a polyatomic molecule. Recently, we performed a femtosecond X-ray liquidography (solution scattering) experiment on a gold trimer complex (GTC), [Au(CN)₂^-]₃, in water at X-ray free-electron lasers (XFELs) and elucidated the time-dependent positions of vibrational wavepackets from the Franck-Condon region to equilibrium structures on both excited and ground states in the course of the formation of covalent bonds between gold atoms.Bond making is an essential process in chemical reactions, but it is challenging to keep track of detailed atomic movements associated with bond making because of its bimolecular nature that requires slow diffusion of two reaction parties to meet each other. Bond formation in the solution phase has been elusive because the diffusion of the reactants limits the reaction rate of a bimolecular process, making it difficult to initiate and track the bond-making processes with an ultrafast time resolution. In principle, if the bimolecular encounter can be controlled to overcome the limitation caused by diffusion, the bond-making processes can be tracked in a time-resolved manner, providing valuable insight into the bimolecular reaction mechanism. In this regard, GTC offers a good model system for studying the dynamics of bond formation in solution. Au(I) atoms in GTC exhibit a noncovalent aurophilic interaction, making GTC an aggregate complex without any covalent bond. Upon photoexcitation of GTC, an electron is excited from an antibonding orbital to a bonding orbital, leading to the formation of covalent bonds among Au atoms. Since Au atoms in the ground state of GTC are located in close proximity within the same solvent cage, the formation of Au-Au covalent bonds occurs without its reaction rate being limited by diffusion through the solvent.Femtosecond time-resolved X-ray liquidography (fs-TRXL) data revealed that the ground state has an asymmetric bent structure. From the wavepacket trajectory determined in three-dimensional nuclear coordinates (two internuclear distances and one bond angle), we found that two covalent bonds are formed between three Au atoms of GTC asynchronously. Specifically, one covalent bond is formed first for the shorter Au-Au pair (of the asymmetric and bent ground-state structure) in 35 fs, and subsequently, the other covalent bond is formed for the longer Au-Au pair within 360 fs. The resultant trimer complex has a symmetric and linear geometry, implying the occurrence of bent-to-linear transformation concomitant with the formation of two equivalent covalent bonds, and exhibits vibrations that can be unambiguously assigned to specific normal modes based on the wavepacket trajectory, even without the vibrational frequencies provided by quantum calculation.

Electronic Structure of the Hieber Anion [Fe(CO)3(NO)]- Revisited by X-ray Emission and Absorption Spectroscopy

While the Hieber anion [Fe(CO)₃(NO)]^- has been reincarnated in the last years as an active catalyst in organic synthesis, there is still a debate about the oxidation state of the central Fe atom and the resulting charge of the NO ligand. To shed new light on this question and to understand the Fe-NO interaction in the Hieber anion, it is investigated in comparison to the formal 3d⁸ reference Fe(CO)₅ and the formal 3d¹⁰ reference [Fe(CO)₄]^2- by the combination of valence-to-core X-ray emission spectroscopy (VtC-XES), X-ray absorption near-edge structure spectroscopy (XANES), and high-energy-resolution fluorescence-detected XANES. In order to extract information about the electronic structure, time-dependent density functional theory and ground-state density functional theory calculations are applied. This combination of experimental and computational methods reveals that the electron density at the Fe center of the Hieber resembles that of the isoelectronic [Fe(CO)₄]^2-. These observations challenge recent descriptions of the Hieber anion and reopen the debate about the experimentally and computationally determined Fe oxidation state and charge on the NO ligand.

Chemical interactions and dynamics with femtosecond X-ray spectroscopy and the role of X-ray free-electron lasers

X-ray free-electron lasers with intense, tuneable and short-pulse X-ray radiation are transformative tools for the investigation of transition-metal complexes and metalloproteins. This becomes apparent in particular when combining the experimental observables from X-ray spectroscopy with modern theoretical tools for calculations of electronic structures and X-ray spectra from first principles. The combination gives new insights into how charge and spin densities change in chemical reactions and how they determine reactivity. This is demonstrated for the investigations of structural dynamics with metal K-edge absorption spectroscopy, spin states in excited-state dynamics with metal 3p-3d exchange interactions, the frontier-orbital interactions in dissociation and substitution reactions with metal-specific X-ray spectroscopy, and studies of metal oxidation states with femtosecond pulses for 'probe-before-destroy' spectroscopy. The role of X-ray free-electron lasers is addressed with thoughts about how they enable 'bringing back together' different aspects of the same problem and this is thought to go beyond a conventional review paper where these aspects are formulated in italic font type in a prequel, an interlude and in a sequel. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

The effect of coordination of alkanes, Xe and CO2 (η1-OCO) on changes in spin state and reactivity in organometallic chemistry: a combined experimental and theoretical study of the photochemistry of CpMn(CO)3

A combined experimental and theoretical study is presented of several ligand addition reactions of the triplet fragment 3CpMn(CO)2 formed upon photolysis of CpMn(CO)3. Experimental data are provided for reactions in n-heptane and perfluoromethylcyclohexane (PFMCH), as well as in PFMCH doped with C2H6, Xe and CO2. In PFMCH we find that the conversion of 3CpMn(CO)2 to 1CpMn(CO)2(PFMCH) is much slower (τ = 18 (±3) ns) than the corresponding reactions in conventional alkanes (τ = 111 (±10) ps). We measure the effect of the coordination ability by doping PFMCH with alkane, Xe and CO2; these doped ligands form the corresponding singlet adducts with significantly variable formation rates. The reactivity as measured by the addition timescale follows the order 1CpMn(CO)2(C5H10) (τ = 270 (±10) ps) > 1CpMn(CO)2Xe (τ = 3.9 (±0.4) ns) ∼ 1CpMn(CO)2(CO2) (τ = 4.7 (±0.5) ns) > 1CpMn(CO)2(C7F14) (τ = 18 (±3) ns). Electronic structure theory calculations of the singlet and triplet potential energy surfaces and of their intersections, together with non-adiabatic statistical rate theory, reproduce the observed rates semi-quantitatively. It is shown that triplet adducts of the ligand and 3CpMn(CO)2 play a role in the kinetics, and account for the variable timescales observed experimentally.