Investigating vibrational anharmonic couplings in cyanide-bridged transition metal mixed valence complexes using two-dimensional infrared spectroscopy

Understanding the [NiFe] Hydrogenase Active Site Environment through Ultrafast Infrared and 2D-IR Spectroscopy of the Subsite Analogue K[CpFe(CO)(CN)2] in Polar and Protic Solvents

The [CpFe(CO)(CN)2]- unit is an excellent structural model for the Fe(CO)(CN)2 moiety of the active site found in [NiFe] hydrogenases. Ultrafast infrared (IR) pump-probe and 2D-IR spectroscopy have been used to study K[CpFe(CO)(CN)2] (M1) in a range of protic and polar solvents and as a dry film. Measurements of anharmonicity, intermode vibrational coupling strength, vibrational relaxation time, and solvation dynamics of the CO and CN stretching modes of M1 in H2O, D2O, methanol, dimethyl sulfoxide, and acetonitrile reveal that H-bonding to the CN ligands plays an important role in defining the spectroscopic characteristics and relaxation dynamics of the Fe(CO)(CN)2 unit. Comparisons of the spectroscopic and dynamic data obtained for M1 in solution and in a dry film with those obtained for the enzyme led to the conclusion that the protein backbone forms an important part of the bimetallic active site environment via secondary coordination sphere interactions.

Understanding 2D-IR Spectra of Hydrogenases: A Descriptive and Predictive Computational Study

[NiFe] hydrogenases are metalloenzymes that catalyze the reversible cleavage of dihydrogen (), a clean future fuel. Understanding the mechanism of these biocatalysts requires spectroscopic techniques that yield insights into the structure and dynamics of the [NiFe] active site. Due to the presence of CO and ligands at this cofactor, infrared (IR) spectroscopy represents an ideal technique for studying these aspects, but molecular information from linear IR absorption experiments is limited. More detailed insights can be obtained from ultrafast nonlinear IR techniques like IRpump−IRprobe and two-dimensional (2D-)IR spectroscopy. However, fully exploiting these advanced techniques requires an in-depth understanding of experimental observables and the encoded molecular information. To address this challenge, we present a descriptive and predictive computational approach for the simulation and analysis of static 2D-IR spectra of [NiFe] hydrogenases and similar organometallic systems. Accurate reproduction of experimental spectra from a first-coordination-sphere model suggests a decisive role of the [NiFe] core in shaping the enzymatic potential energy surface. We also reveal spectrally encoded molecular information that is not accessible by experiments, thereby helping to understand the catalytic role of the diatomic ligands, structural differences between [NiFe] intermediates, and possible energy transfer mechanisms. Our studies demonstrate the feasibility and benefits of computational spectroscopy in the 2D-IR investigation of hydrogenases, thereby further strengthening the potential of this nonlinear IR technique as a powerful research tool for the investigation of complex bioinorganic molecules.

Femtosecond X-ray Spectroscopy Directly Quantifies Transient Excited-State Mixed Valency

Quantifying charge delocalization associated with short-lived photoexcited states of molecular complexes in solution remains experimentally challenging, requiring local element specific femtosecond experimental probes of time-evolving electron transfer. In this study, we quantify the evolving valence hole charge distribution in the photoexcited charge transfer state of a prototypical mixed valence bimetallic iron-ruthenium complex, [(CN)₅Fe^IICNRu^III(NH₃)₅]^-, in water by combining femtosecond X-ray spectroscopy measurements with time-dependent density functional theory calculations of the excited-state dynamics. We estimate the valence hole charge that accumulated at the Fe atom to be 0.6 ± 0.2, resulting from excited-state metal-to-metal charge transfer, on an ∼60 fs time scale. Our combined experimental and computational approach provides a spectroscopic ruler for quantifying excited-state valency in solvated complexes.

Multimode two-dimensional vibronic spectroscopy. II. Simulating and extracting vibronic coupling parameters from polarization-selective spectra

Experimental demonstrations of polarization-selection two-dimensional Vibrational-Electronic (2D VE) and 2D Electronic-Vibrational (2D EV) spectroscopies aim to map the magnitudes and spatial orientations of coupled electronic and vibrational coordinates in complex systems. The realization of that goal depends on our ability to connect spectroscopic observables with molecular structural parameters. In this paper, we use a model Hamiltonian consisting of two anharmonically coupled vibrational modes in electronic ground and excited states with linear and bilinear vibronic coupling terms to simulate polarization-selective 2D EV and 2D VE spectra. We discuss the relationships between the linear vibronic coupling and two-dimensional Huang-Rhys parameters and between the bilinear vibronic coupling term and Duschinsky mixing. We develop a description of the vibronic transition dipoles and explore how the Hamiltonian parameters and non-Condon effects impact their amplitudes and orientations. Using simulated polarization-selective 2D EV and 2D VE spectra, we show how 2D peak positions, amplitudes, and anisotropy can be used to measure parameters of the vibronic Hamiltonian and non-Condon effects. This paper, along with the first in the series, provides the reader with a detailed description of reading, simulating, and analyzing multimode, polarization-selective 2D EV and 2D VE spectra with an emphasis on extracting vibronic coupling parameters from complex spectra.

Multimode two-dimensional vibronic spectroscopy. I. Orientational response and polarization-selectivity

Two-dimensional Electronic-Vibrational (2D EV) spectroscopy and two-dimensional Vibrational-Electronic (2D VE) spectroscopy are among the newest additions to the coherent multidimensional spectroscopy toolbox, and they are directly sensitive to vibronic couplings. In this first of two papers, the complete orientational response functions are developed for a model system consisting of two coupled anharmonic oscillators and two electronic states in order to simulate polarization-selective 2D EV and 2D VE spectra with arbitrary combinations of linearly polarized electric fields. Here, we propose analytical methods to isolate desired signals within complicated spectra and to extract the relative orientation between vibrational and vibronic dipole moments of the model system using combinations of polarization-selective 2D EV and 2D VE spectral features. Time-dependent peak amplitudes of coherence peaks are also discussed as means for isolating desired signals within the time-domain. This paper serves as a field guide for using polarization-selective 2D EV and 2D VE spectroscopies to map coupled vibronic coordinates on the molecular frame.

Direct observation of coherent femtosecond solvent reorganization coupled to intramolecular electron transfer

It is well known that the solvent plays a critical role in ultrafast electron-transfer reactions. However, solvent reorganization occurs on multiple length scales, and selectively measuring short-range solute-solvent interactions at the atomic level with femtosecond time resolution remains a challenge. Here we report femtosecond X-ray scattering and emission measurements following photoinduced charge-transfer excitation in a mixed-valence bimetallic (FeⁱⁱRuⁱⁱⁱ) complex in water, and their interpretation using non-equilibrium molecular dynamics simulations. Combined experimental and computational analysis reveals that the charge-transfer excited state has a lifetime of 62 fs and that coherent translational motions of the first solvation shell are coupled to the back electron transfer. Our molecular dynamics simulations identify that the observed coherent translational motions arise from hydrogen bonding changes between the solute and nearby water molecules upon photoexcitation, and have an amplitude of tenths of ångströms, 120-200 cm^-1 frequency and ~100 fs relaxation time. This study provides an atomistic view of coherent solvent reorganization mediating ultrafast intramolecular electron transfer.

Determining the Orientation and Vibronic Couplings between Electronic and Vibrational Coordinates with Polarization-Selective Two-Dimensional Vibrational-Electronic Spectroscopy

We experimentally demonstrate polarization-selective two-dimensional (2D) vibrational-electronic (VE) spectroscopy on a transition-metal mixed-valence complex where the cyanide stretching vibrations are coupled to the metal-to-metal charge-transfer transition. A simultaneous fitting of the parallel and crossed polarized 2D VE spectra quantifies the relative vibronic coupling strengths and angles between the charge-transfer transition and three coupled cyanide stretching vibrations in a mode-specific manner. In particular, we find that the bridging vibration, which modulates the distance between the transition-metal centers, is oriented nearly parallel to the charge-transfer axis and is 9 times more strongly coupled to the electronic transition than the radial vibration, which is oriented almost perpendicular to the charge-transfer axis. The results from this experiment allow us to map the spectroscopically observed vibronic coordinates onto the molecular frame providing a general method to spatially resolve vibronic energy transfer on a femtosecond time scale.

Structural analysis of transient reaction intermediate in formic acid dehydrogenation catalysis using two-dimensional IR spectroscopy

The molecular structure of a catalytically active key intermediate is determined in solution by employing 2D IR spectroscopy measuring vibrational cross-angles. The formate intermediate (2) in the formic acid dehydrogenation reaction catalyzed by a phosphorus-nitrogen PN3P-Ru catalyst is elucidated. Our spectroscopic studies show that the complex features a formate ion directly attached to the Ru center as a ligand, and a proton added to the imine arm of the dearomatized PN3P* ligand. During the catalytic process, the imine arms are not only reversibly protonated and deprotonated, but also interacting with the protic substrate molecules, effectively serving as the local proton buffer to offer remarkable stability with a turnover number (TON) over one million.

Implementation of continuous fast scanning detection in femtosecond Fourier-transform two-dimensional vibrational-electronic spectroscopy to decrease data acquisition time

Femtosecond Fourier transform two-dimensional vibrational-electronic (2D VE) spectroscopy is a recently developed third-order nonlinear spectroscopic technique to measure coupled electronic and vibrational motions in the condensed phase. The viability of femtosecond multidimensional spectroscopy as an analytical tool requires improvements in data collection and processing to enhance the signal-to-noise ratio and increase the amount of data collected in these experiments. Here a continuous fast scanning technique for the efficient collection of 2D VE spectroscopy is described. The resulting 2D VE spectroscopic method gains sensitivity by reducing the effect of laser drift, as well as decreasing the data collection time by a factor of 10 for acquiring spectra with a high signal-to-noise ratio within 3 dB of the more time intensive step scanning methods. This work opens the door to more comprehensive studies where 2D VE spectra can be collected as a function of external parameters such as temperature, pH, and polarization of the input electric fields.

Linear and Nonlinear Infrared Spectroscopies Reveal Detailed Solute-Solvent Dynamic Interactions of a Nitrosyl Ruthenium Complex in Solution

In this work, the solvation of a nitrosyl ruthenium complex, [(CH₃)₄N][RuCl₃(qn)(NO)] (with qn = deprotonated 8-hydroxyquinoline), which is a potential NO-releasing molecule in the bio-environment, was studied in two bio-friendly solvents, namely deuterated dimethyl sulfoxide (dDMSO) and water (D₂O). A blue-shifted NO stretching frequency was observed in water with respect to that in dDMSO, which was believed to be due to ligand-solvent hydrogen-bonding interactions, one N═O···D and particularly three Ru-Cl···D, that show competing effects on the NO bond length. The dynamic differences of the NO stretch in these two solvents were further revealed by transient pump-probe IR and two-dimensional IR results: faster vibrational relaxation and faster spectral diffusion (SD) were observed in D₂O, confirming stronger solvent-solute interaction and also faster solvent structural dynamics in D₂O than in DMSO. Further, a significant non-decaying residual in the SD dynamics was observed in D₂O but not in DMSO, suggesting the formation of a stable solvation shell in water due to strong multi-site ligand-solvent hydrogen-bonding interactions, which is in agreement with the observed blue-shifted NO stretching frequency. This work demonstrates that small solvent molecules such as water can form a relatively rigid solvation shell for certain transition metal complexes due to cooperative ligand-solvent interactions and show slower dynamics.

Two-dimensional vibrational-electronic spectroscopy

Two-dimensional vibrational-electronic (2D VE) spectroscopy is a femtosecond Fourier transform (FT) third-order nonlinear technique that creates a link between existing 2D FT spectroscopies in the vibrational and electronic regions of the spectrum. 2D VE spectroscopy enables a direct measurement of infrared (IR) and electronic dipole moment cross terms by utilizing mid-IR pump and optical probe fields that are resonant with vibrational and electronic transitions, respectively, in a sample of interest. We detail this newly developed 2D VE spectroscopy experiment and outline the information contained in a 2D VE spectrum. We then use this technique and its single-pump counterpart (1D VE) to probe the vibrational-electronic couplings between high frequency cyanide stretching vibrations (νCN) and either a ligand-to-metal charge transfer transition ([Fe(III)(CN)6](3-) dissolved in formamide) or a metal-to-metal charge transfer (MMCT) transition ([(CN)5Fe(II)CNRu(III)(NH3)5](-) dissolved in formamide). The 2D VE spectra of both molecules reveal peaks resulting from coupled high- and low-frequency vibrational modes to the charge transfer transition. The time-evolving amplitudes and positions of the peaks in the 2D VE spectra report on coherent and incoherent vibrational energy transfer dynamics among the coupled vibrational modes and the charge transfer transition. The selectivity of 2D VE spectroscopy to vibronic processes is evidenced from the selective coupling of specific νCN modes to the MMCT transition in the mixed valence complex. The lineshapes in 2D VE spectra report on the correlation of the frequency fluctuations between the coupled vibrational and electronic frequencies in the mixed valence complex which has a time scale of 1 ps. The details and results of this study confirm the versatility of 2D VE spectroscopy and its applicability to probe how vibrations modulate charge and energy transfer in a wide range of complex molecular, material, and biological systems.

Investigating vibrational relaxation in cyanide-bridged transition metal mixed-valence complexes using two-dimensional infrared and infrared pump-probe spectroscopies

Using polarization-selective two-dimensional infrared (2D IR) and infrared pump-probe spectroscopies, we study vibrational relaxation of the four cyanide stretching (νCN) vibrations found in [(NH3)5Ru(III)NCFe(II)(CN)5](-) (FeRu) dissolved in D2O or formamide and [(NC)5Fe(II)CNPt(IV)(NH3)4NCFe(II)(CN)5](4-) (FePtFe) dissolved in D2O. These cyanide-bridged transition metal complexes serve as models for understanding the role high frequency vibrational modes play in metal-to-metal charge transfers over a bridging ligand. However, there is currently little information about vibrational relaxation and dephasing dynamics of the anharmonically coupled νCN modes in the electronic ground state of these complexes. IR pump-probe experiments reveal that the vibrational lifetimes of the νCN modes are ∼2 times faster when FeRu is dissolved in D2O versus formamide. They also reveal that the vibrational lifetimes of the νCN modes of FePtFe in D2O are almost four times as long as for FeRu in D2O. Combined with mode-specific relaxation dynamics measured from the 2D IR experiments, the IR pump-probe experiments also reveal that intramolecular vibrational relaxation is occurring in all three systems on ∼1 ps timescale. Center line slope dynamics, which have been shown to be a measure of the frequency-frequency correlation function, reveal that the radial, axial, and trans νCN modes exhibit a ∼3 ps timescale for frequency fluctuations. This timescale is attributed to the forming and breaking of hydrogen bonds between each mode and the solvent. The results presented here along with our previous work on FeRu and FePtFe reveal a picture of coupled anharmonic νCN modes where the spectral diffusion and vibrational relaxation dynamics depend on the spatial localization of the mode on the molecular complex and its specific interaction with the solvent.

Measuring Coherently Coupled Intramolecular Vibrational and Charge-Transfer Dynamics with Two-Dimensional Vibrational-Electronic Spectroscopy

We demonstrate Fourier transform (FT) 2D vibrational-electronic (2D VE) spectroscopy employing a novel mid-IR and optical pulse sequence. This new femtosecond third-order nonlinear spectroscopy provides the high time and frequency resolutions of existing 2D FT techniques; however, resulting 2D VE spectra contain IR and electronic dipole moment cross terms. We use 2D VE spectroscopy to help understand the vibrational-electronic couplings in the cyanide-bridged transition-metal mixed valence complex [(CN)5Fe(II)CNRu(III)(NH3)5](-) dissolved in formamide. The amplitudes of the cross-peaks in the 2D VE spectra reveal that three of the intramolecular cyanide stretching vibrations lying along the charge-transfer axis are coherently coupled to the metal-to-metal charge-transfer electronic transition with differing strengths. Analysis of the 2D VE line shapes reveals positive and negative correlations of the cyanide stretching modes with the charge-transfer transition depending on the physical orientation of the vibration in the molecule and its interaction with the solvent. The insights found thus far into the vibronic couplings in the mixed valence model system indicate that the 2D VE technique will be a valuable addition to the existing multidimensional spectroscopy toolbox.

Molecular structure, spectroscopy, and photoinduced kinetics in trinuclear cyanide bridged complex in solution: a first-principles perspective

We investigate the molecular structure of the solvated complex, [(NC)6Fe-Pt(NH3)4-Fe(CN)6](4-), and related dinuclear and mononuclear model complexes using first-principles calculations. Mixed nuclear complexes in both solution and crystal phases were widely studied as models for charge transfer (CT) reactions using advanced spectroscopical and electrochemical tools. In contrast to earlier interpretations, we find that the most stable gas phase and solvated geometries are substantially different from the crystal phase geometry, mainly due to variance in the underlying oxidation numbers of the metal centers. Specifically, in the crystal phase a Pt(IV) metal center resulting from Fe ← Pt backward electron transfers is stabilized by an octahedral ligand field, whereas in the solution phase a Pt(II) metal complex that prefers a square planar ligand field forms a CT salt by bridging to the iron complexes through long-range electrostatic interactions. The different geometry is shown to be consistent with spectroscopical data and measured CT rates of the solvated complex. Interestingly, we find that the experimentally indicated photoinduced process in the solvated complex is of backward CT (Fe ← Pt).