CO(v=1) population lifetimes of metal–carbonyl cluster compounds in dilute CHCl3solution

Time-resolved spectroscopic mapping of vibrational energy flow in proteins: Understanding thermal diffusion at the nanoscale

Vibrational energy exchange between various degrees of freedom is critical to barrier-crossing processes in proteins. Hemeproteins are well suited for studying vibrational energy exchange in proteins because the heme group is an efficient photothermal converter. The released energy by heme following photoexcitation shows migration in a protein moiety on a picosecond timescale, which is observed using time-resolved ultraviolet resonance Raman spectroscopy. The anti-Stokes ultraviolet resonance Raman intensity of a tryptophan residue is an excellent probe for the vibrational energy in proteins, allowing the mapping of energy flow with the spatial resolution of a single amino acid residue. This Perspective provides an overview of studies on vibrational energy flow in proteins, including future perspectives for both methodologies and applications.

Tracking Energy Transfer across a Platinum Center

Rigid, conjugated alkyne bridges serve as important components in various transition-metal complexes used for energy conversion, charge separation, sensing, and molecular electronics. Alkyne stretching modes have potential for modulating charge separation in donor–bridge–acceptor compounds. Understanding the rules of energy relaxation and energy transfer across the metal center in such compounds can help optimize their electron transfer switching properties. We used relaxation-assisted two-dimensional infrared spectroscopy to track energy transfer across metal centers in platinum complexes featuring a triazole-terminated alkyne ligand of two or six carbons, a perfluorophenyl ligand, and two tri(p-tolyl)phosphine ligands. Comprehensive analyses of waiting-time dynamics for numerous cross and diagonal peaks were performed, focusing on coherent oscillation, energy transfer, and cooling parameters. These observables augmented with density functional theory computations of vibrational frequencies and anharmonic force constants enabled identification of different functional groups of the compounds. Computations of vibrational relaxation pathways and mode couplings were performed, and two regimes of intramolecular energy redistribution are described. One involves energy transfer between ligands via high-frequency modes; the transfer is efficient only if the modes involved are delocalized over both ligands. The energy transport pathways between the ligands are identified. Another regime involves redistribution via low-frequency delocalized modes, which does not lead to interligand energy transport.

Fully absorptive 3D IR spectroscopy using a dual mid-infrared pulse shaper

This paper presents the implementation of 3D IR spectroscopy by adding a second pump beam to a two-beam 2D IR spectrometer. An independent mid-IR pulse shaper is used for each pump beam, which can be programmed to collect its corresponding dimension in either the frequency or time-domains. Due to the phase matching geometry employed here, absorptive 3D IR spectra are automatically obtained, since all four of the rephasing and non-rephasing signals necessary to generate absorptive spectra are collected simultaneously. Phase cycling is used to isolate the fifth-order from the third-order signals. The method is demonstrated on tungsten hexacarbonyl (W(CO)6) and dicarbonylacetylacetonato rhodium (I), for which the eigenstates are extracted up to the third excited state. Pulse shaping affords a high degree of control over 3D IR experiments by making possible mixed time- and frequency-domain experiments, fast data acquisition and straightforward implementation.

Ultrafast vibrational relaxation dynamics of carbonyl stretching modes in Os3(CO)12

The vibrational relaxation dynamics of the four infrared active carbonyl (CO) stretching normal modes of Os(3)(CO)(12) at 2068 cm(-1), 2034 cm(-1), 2014 cm(-1), and 2002 cm(-1) were measured using broad-band frequency resolved pump-probe spectroscopy. Transient absorption spectra of these modes were collected, and the fundamental, overtone, and combination bands were assigned. The frequency resolved pump-probe traces measured at the fundamental frequencies for the four stretching normal modes exhibited marked differences: the two axial modes at frequencies of 2068 cm(-1) and 2034 cm(-1) yielded similar bi-exponential decay traces, while the two equatorial modes at 2014 cm(-1) and 2002 cm(-1) showed a rising component, in addition to a bi-exponential decay. Due to the independence of the axial and equatorial stretching modes, it is shown that the axial-equatorial combination anharmonicity constants are near zero. This results in the appearance of the pump-probe signals of these combination bands at the same frequencies as the fundamental transitions, thus leading to interference and the resultant anomalous rising features. If unaccounted for, these interferences may lead to erroneous conclusions about the dynamics of these vibrational stretches. To avoid such pitfalls, it is therefore imperative to resolve such ambiguities. A corrected dynamical picture of the equatorial modes can be obtained by varying the center frequency of the pump pulse. The four modes have a slow vibrational excited population decay time of between 400 to 600 ps. We observe no obvious direct vibrational energy transfer between the axial and equatorial CO stretching modes.

Structure of solvated Fe(CO)5: complex formation during solvation in alcohols

The equilibrium structure of iron pentacarbonyl, Fe(CO)5, solvated in various alcohols has been investigated by Fourier transform infrared (FTIR) measurements and density functional theory calculations. This system was studied because it is prototypical of a larger class of monometallic systems, which are electronically saturated but not sterically crowded. Upon solvation, the Fe(CO)5 is not just surrounded by a solvation shell. Instead, solute-solvent complexes are formed with the oxygen of the alcohol oriented toward an axial ligand of the Fe(CO)5 giving a formation energy on the order of -5 kJ/mol. This complexation is not a chemical reaction but rather a "preassembly" of the solute molecules with a single solvent molecule. For instance, at room temperature the interaction between Fe(CO)5 and ethanol results in 87% of all Fe(CO)5 molecules being complexated with a single ethanol molecule. This complexation was found in all the alcohol systems studied in this paper. The stability of these complexes was found to depend on the alcohol chain length and branching. The observed complexation mechanism is accompanied by an electron density shift from the complexed alcohol molecule toward Fe(CO)5 where it induces a dipole moment. The finding that Fe(CO)5 forms a complex with the hydroxyl group of a single solvent molecule might have significant implications for ligand substitution reactions. This implies that ligand substitution reactions do not have to proceed via a dissociative mechanism. Instead, the reaction might proceed through a concerted mechanism with the leaving CO simultaneously being replaced by the incoming alcohol that was complexed to Fe(CO)5 prior to the photoexcitation.

Intra- and intermolecular vibrational energy transfer in tungsten carbonyl complexes W(CO)5(X) (X=CO, CS, CH3CN, and CD3CN)

Vibrational energy relaxation of degenerate CO stretches of four tungsten carbonyl complexes, W(CO)6, W(CO)5(CS), W(CO)5(CH3CN), and W(CO)5(CD3CN), is observed in nine alkane solutions by subpicosecond time-resolved infrared (IR) pump-probe spectroscopy. Between 0 and 10 ps after the vibrational excitation, the bleaching signal of the ground-state IR absorption band shows anisotropy. Decay of the anisotropic component corresponds either to the rotational diffusion of the molecule or to the intramolecular vibrational energy transfer among the degenerate CO stretch modes. The time constant of the anisotropy decay, tauaniso, shows distinct solvent dependence. By comparing the results for the T1u CO stretch of W(CO)6 and the A1 CO stretch of W(CO)5(CS), the time constant of the rotational diffusion, taur, and the time constant of the intramolecular energy transfer among the three degenerate vibrational modes, taue, are determined as 12 and 8 ps, respectively. The tauaniso value increases as the number of carbon atoms in the alkane solvent increases. After 10 ps, the recovery of the bleaching becomes isotropic. The isotropic decay represents the vibrational population relaxation, from v=1 to v=0. In heptane, the time constant for the isotropic decay, tau1, for W(CO)5(CS) and W(CO)6 was 140 ps. The tau1 for the two acetonitrile-substituted complexes, however, shows a smaller value of 80 ps. The vibrational energy relaxation of W(CO)5(CH3CN) and W(CO)5(CD3CN) is accelerated by the intramolecular energy redistribution from the CO ligand to the acetonitrile ligand. In the nine alkane solutions, the tau1 value of W(CO)6 ranges between 124 and 158 ps, showing the apparent V-shaped solvent dependence with its minimum in decane, while the tau1 value shows little solvent dependence for W(CO)5(CH3CN) and W(CO)5(CD3CN).

Vibrational and rotational dynamics of cyanoferrates in solution

Ultrafast infrared spectroscopy has been used to measure vibrational energy relaxation (VER) and reorientation (Tr) times for the high frequency vibrational bands of potassium ferrocyanide and ferricyanide (CN stretches), and sodium nitroprusside (SNP, CN, and NO stretches) in water and several other solvents. Relatively short VER times (4-43 ps) are determined for the hexacyano species and for the NO band of SNP, but the CN band of SNP relaxes much more slowly (55-365 ps). The solvent dependence of the VER times is similar for all the solutes and resembles what has been previously observed for triatomic molecular ions [Li et al., J. Chem. Phys. 98, 5499 (1993)]. Anisotropy decay times are also measured from the polarization dependence of the transient absorptions. The Tr times determined for SNP are different for the different vibrational bands; for the nondegenerate NO mode of nitroprusside (SNP) they are much longer (>15 ps), correlate with solvent viscosity, and are attributed to overall molecular rotation. The short Tr (<10 ps) times for the CN band in SNP and for the hexacyanoferrates are due to dipole orientational relaxation in which the transition moment rapidly redistributes among the degenerate modes. There is no evidence of intramolecular vibrational relaxation (IVR) to other high frequency modes. VER times measured for hexacarbonyls and SNP in methanol are similar, which suggests that the generally faster VER for the latter is in part because they are soluble in more strongly interacting polar solvents. The results are compared to those for small ions and metal carbonyls and are discussed in terms of the importance of solute charge and symmetry on VER.

Vibrational coherence transfer characterized with Fourier-transform 2D IR spectroscopy

Two-dimensional infrared (2D IR) spectroscopy of the symmetric and asymmetric C[Triple Bond]O stretching vibrations of Rh(CO)(2)acac in hexane has been used to investigate vibrational coherence transfer, dephasing, and population relaxation in a multilevel vibrational system. The transfer of coherence between close-lying vibrational frequencies results in extra relaxation-induced peaks in the 2D IR spectrum, whose amplitude depends on the coherence transfer rate. Coherence transfer arises from the mutual interaction of the bright CO stretches with dark states, which in this case reflects the mutual d-pi(*) back bonding of the Rh center to both the terminal carbonyls and the acetylacenonate ligand. For 2D IR relaxation experiments with variable waiting times, coherent dynamics lead to the modulation of peak amplitudes, while incoherent population relaxation and exchange results in the growth of the relaxation-induced peaks. We have modeled the data by propagating the density matrix with the Redfield equation, incorporating all vibrational relaxation processes during all three experimental time periods and including excitation reorientation effects arising from relaxation. Coherence and population transfer time scales from the symmetric to the asymmetric stretch were found to be 350 fs and 3 ps, respectively. We also discuss a diagrammatic approach to incorporating all vibrational relaxation processes into the nonlinear response function, and show how coherence transfer influences the analysis of structural variables from 2D IR spectroscopy.

Picosecond infrared study of the photodynamics of carbonmonoxy-cytochrome c oxidase

Time-resolved infrared (TRIR) techniques have been employed to study the reactions of carbon monoxide with the cytochrome alpha 3-Cu(B) site of cytochrome c oxidase (CcO). The ligation dynamics immediately following photodissociation have been investigated using picosecond TRIR spectroscopy and linear dichroism. The rate of photoinitiated transfer of CO from cytochrome alpha 3 to CuB was measured directly by monitoring the development of the transient CuBCO absorption. In less than 1 ps, a stationary CuBCO spectrum develops, which together with the CO infrared linear dichroism is constant until the CO dissociates from CuB on a microsecond time scale. These observations indicate that the CO is transferred between metals and reaches its equilibrium conformation in less than 1 ps. This unprecedented ligand transfer rate has profound implications with regard to the structure and dynamics of the cytochrome alpha 3-CuB site, the functional architecture of the protein, and coordination dynamics in general.

High-repetition-rate infrared-pump, infrared-probe spectrometer

A method for generating two independently tunable 25-ps infrared pulses at 500 Hz was demonstrated. The apparatus is based on two dye lasers, a regenerative amplifier, and two lithium iodate optical-parametric amplifiers. Using the pump-probe technique, we measured the lifetime of the excited vibrational-state absorption of tungsten hexacarbonyl in solution.