Carlos R. Baiz

Two-Dimensional Infrared Spectroscopy of Metal Carbonyls

Metal carbonyl complexes offer both rich chemistry and complex vibrational spectroscopy due to strong coupling among the carbonyl stretches. Using two-dimensional infrared (2DIR) spectroscopy, it is possible to resolve the underlying transitions between vibrational energy levels, determine the orientations and relative magnitude of the corresponding transition dipole moments, measure the coupling between modes due to the anharmonicity of the potential, and probe energy redistribution among the modes as well as energy relaxation to other degrees of freedom. Measurements on metal carbonyl complexes have played, and continue to play, a crucial role in facilitating the development of 2DIR spectroscopy. These compounds have provided powerful demonstrations of the unique ability of 2DIR spectroscopy to resolve vibrational structure and dynamics in multimode systems. In addition, invaluable new information has been obtained on metal-to-ligand charge transfer processes, solvent-solute interactions and fluxionality. Since transition metal complexes play important roles in catalysis and as dye sensitizers for semiconductor nanoparticle photocatalysis, detailed probes of equilibrium and phototriggered dynamics should aid our understanding of these key catalytic systems. The richness and level of detail provided by the 2DIR spectra of metal carbonyl complexes turn them into extremely useful model systems for testing the accuracy of ab initio quantum chemical calculations. Accurate modeling of the 2DIR spectra of solvated metal carbonyl complexes requires the development of new theoretical and computational tools beyond those employed in the standard analysis of one-dimensional IR spectra, and represents an ongoing challenge to currently available computational methodologies. These challenges are further compounded by the increasing interest in triggered 2DIR experiments that involve nonequilibrium vibrational dynamics on multiple electronic potential surfaces. In this Account, we review the various metal carbonyl complexes studied via 2DIR spectroscopy and outline the theoretical approaches used in order to model the spectra. The capabilities of 2DIR spectroscopy and its interplay with modern ab initio calculations are demonstrated in the context of the metal carbonyl complex Mn(2)(CO)(10) recently studied in our lab. Continued progress in experimental implementation and theoretical analysis will enable transient 2D spectroscopy to provide structurally sensitive details of complex, highly interacting nonequilibrium processes that are central to diverse chemical transformations.

Multilevel vibrational coherence transfer and wavepacket dynamics probed with multidimensional IR spectroscopy.

Multidimensional infrared (MDIR) spectroscopy of a strongly coupled multilevel vibrational system Mn(2)(CO)(10) (dimanganese decacarbonyl) in cyclohexane solution reveals fully resolved excited vibrational state coherences that exhibit slow 0.25-0.50 ps(-1) decay constants. Detailed analysis of the waiting-time dependence of certain cross-peak amplitudes shows modulation at multiple frequencies, providing a direct signature of excited vibrational coherences resulting from coherence transfer. A new signature of coherence transfer is observed as temporally modulated cross-peak amplitudes with more than one modulation frequency. The relative importance of different coherence transfer paths is considered in the context of the orientational response of a system which includes two vibrational modes with parallel dipole moments. Since MDIR spectroscopy enables spectral isolation of individual excited vibrational coherences (i.e., coherences between fundamental excitations), these experiments report directly on the frequency-frequency correlation functions of the excited states relative to each other as well as relative to the ground state. These results highlight the rich information contained in fully exploring three-dimensional third-order spectroscopy, particularly regarding chemically relevant slower dynamics and the importance of intramolecular interactions leading to dephasing by optically dark or low-frequency modes of the molecule.

Ultrafast nonequilibrium Fourier-transform two-dimensional infrared spectroscopy

We present what we believe to be the first implementation of nonequilibrium two-dimensional IR spectroscopy (2DIR) combining electronic excitation within the Fourier transform (FT) approach. Nonequilibrium 2DIR spectra of Mn2(CO)10 and its photoproducts are obtained in two modalities: photoexcitation at 400 nm, either before a 2DIR probe or during the waiting time of the FT 2DIR measurement. Extending FT 2DIR to nonequilibrium systems offers insight into complex condensed-phase reaction dynamics.

Ultrabroadband detection of a mid-IR continuum by chirped-pulse upconversion

Ultrabroadband mid-IR continuum pulses can be detected on a single-shot basis using chirped-pulse upconversion. Converting the mid-IR pulse to the visible reduces the fractional bandwidth and enables use of a silicon CCD camera. Removing the cross-phase modulation of the chirped pulse results in 1 cm(-1) resolution over a 600 cm(-1) detected bandwidth.

Structurally Selective Geminate Rebinding Dynamics of Solvent-Caged Radicals Studied with Nonequilibrium Infrared Echo Spectroscopy

Cyclopentadienylmolybdenum(II) tricarbonyl dimer exists in two different equilibrium conformations: trans and gauche. Ultrafast photoexcitation in the ultraviolet cleaves the Mo-Mo bond, permitting observation of the subsequent geminate rebinding reaction (t(rebinding) = 31.6 ps) by monitoring infrared bleach recoveries at frequencies corresponding to the CO stretches of the trans and gauche isomers, the time-resolved measurements revealed that the monomers rebind in the trans configuration only. Further insight into the rebinding reaction was obtained by mapping the full potential energy surface along the reaction coordinate using electronic-structure methods.

Orientational Dynamics of Transient Molecules Measured by Nonequilibrium Two-Dimensional Infrared Spectroscopy

Transient two-dimensional infrared (2DIR) spectroscopy is applied to the photodissociation of Mn2(CO)10 to 2 Mn(CO)5 in cyclohexane solution. By varying both the time delay between the 400 nm phototrigger and the 2DIR probe as well as the waiting time in the 2DIR pulse sequence, we directly determine the orientational relaxation of the vibrationally hot photoproduct. The orientational relaxation slows as the photoproduct cools, providing a measure of the transient temperature decay time of 70 +/- 16 ps. We compare the experimental results with molecular dynamics simulations and find near quantitative agreement for equilibrium orientational diffusion time constants but only qualitative agreement for nonequilibrium thermal relaxation. The simulation also shows that the experiment probes an unusual regime of thermal excitation, where the solute is heated while the solvent remains essentially at room temperature.

Ultrafast Vibrational Stark-Effect Spectroscopy: Exploring Charge-Transfer Reactions by Directly Monitoring the Solvation Shell Response

We present the first implementation of transient vibrational Stark-effect spectroscopy as an ultrafast probe of solvation dynamics. The method is applied to the phototriggered intramolecular charge-transfer reaction of Betaine-30, where the vibrational Stark shifts of the nearby solvent molecules--arising from the change in the electrostatic environment--are measured using a three-pulse photon echo probe. This new experiment provides a direct subpicosecond measure of the chromophores excited-state dynamics and back electron transfer as viewed from the solvents perspective. We develop a simple ab initio model that offers semiquantitative prediction of the experimental Stark shifts.

Ultrafast equilibrium and non-equilibrium chemical reaction dynamics probed with multidimensional infrared spectroscopy

Two-dimensional infrared (2D-IR) spectroscopy provides powerful tools to investigate chemical reaction dynamics in the condensed phase. Correlating excitation and detection frequencies grants access to structural and dynamical information that is hidden in a linear absorption spectrum. Low-barrier reactions naturally can occur on the picosecond time scale, and although they are too rapid to study using nuclear magnetic resonance spectroscopy, the intrinsic ultrafast time resolution of coherent 2D-IR spectroscopy enables direct tracking of equilibrium reactive barrier crossings. 2D-IR chemical exchange spectroscopy can monitor the picosecond dynamics of non-triggered chemical reactions by correlating excited reactant frequencies with detected product frequencies. Solvent and temperature-dependent variations enable comparisons with microscopic rate theories at an unprecedented level of detail. 2D-IR spectroscopy is also emerging as a powerful probe of non-equilibrium light-driven chemical transformations. Transient 2D-IR spectroscopy is able to follow nascent photoproducts caused by electronic excitation or by a temperature jump. Soon it will be possible to study transient species with the full range of 2D observables, such as line shapes and waiting-time dynamics that have motivated the wide adoption of equilibrium 2D-IR spectroscopy. This review summarises the general progress in using 2D-IR spectroscopy to study chemical reactions in solution, focusing on our investigations into reactions of isomerisation of CO2(CO)8, photodissociation of Mn2(CO)10, geminate rebinding in [CpMo(CO)3]2 and charge transfer in betaine-30 as viewed from the first solvation shell.

Molecular Theory and Simulation of Coherence Transfer in Metal Carbonyls and Its Signature on Multidimensional Infrared Spectra

We present a general and comprehensive theoretical and computational framework for modeling ultrafast multidimensional infrared spectra of a vibrational excitonic system in liquid solution. Within this framework, we describe the dynamics of the system in terms of a quantum master equation that can account for population relaxation, dephasing, coherence-to-coherence transfer, and coherence-to-population transfer. A unique feature of our approach is that, in principle, it does not rely on any adjustable fitting parameters. More specifically, the anharmonic vibrational Hamiltonian is derived from ab initio electronic structure theory, and the system-bath coupling is expressed explicitly in terms of liquid degrees of freedom whose dynamics can be obtained via molecular dynamics simulations. The applicability of the new approach is demonstrated by employing it to model the recently observed signatures of coherence transfer in the two-dimensional spectra of dimanganese decacarbonyl in liquid cyclohexane. The results agree well with experiment and shed new light on the nature of the molecular interactions and dynamics underlying the spectra and the interplay between dark and bright states, their level of degeneracy, and the nature of their interactions with the solvent.

Cooperative Cold Denaturation: The Case of the C‑Terminal Domain of Ribosomal Protein L9

Cold denaturation is a general property of globular proteins, but it is difficult to directly characterize because the transition temperature of protein cold denaturation, T(c), is often below the freezing point of water. As a result, studies of protein cold denaturation are often facilitated by addition of denaturants, using destabilizing pHs or extremes of pressure, or reverse micelle encapsulation, and there are few studies of cold-induced unfolding under near native conditions. The thermal and denaturant-induced unfolding of single-domain proteins is usually cooperative, but the cooperativity of cold denaturation is controversial. The issue is of both fundamental and practical importance because cold unfolding may reveal information about otherwise inaccessible partially unfolded states and because many therapeutic proteins need to be stabilized against cold unfolding. It is thus desirable to obtain more information about the process under nonperturbing conditions. The ability to access cold denaturation in native buffer is also very useful for characterizing protein thermodynamics, especially when other methods are not applicable. In this work, we study a point mutant of the C-terminal domain of ribosomal protein L9 (CTL9), which has a T(c) above 0 °C. The mutant was designed to allow the study of cold denaturation under near native conditions. The cold denaturation process of I98A CTL9 was characterized by nuclear magnetic resonance, circular dichroism, and Fourier transform infrared spectroscopy. The results are consistent with apparently cooperative, two-state cold unfolding. Small-angle X-ray scattering studies show that the unfolded state expands as the temperature is lowered.