Vibrational Spectroscopic Map, Vibrational Spectroscopy, and Intermolecular Interaction

© 2020 American Chemical Society. Vibrational spectroscopy is an essential tool in chemical analyses, biological assays, and studies of functional materials. Over the past decade, various coherent nonlinear vibrational spectroscopic techniques have been developed and enabled researchers to study time-correlations of the fluctuating frequencies that are directly related to solute-solvent dynamics, dynamical changes in molecular conformations and local electrostatic environments, chemical and biochemical reactions, protein structural dynamics and functions, characteristic processes of functional materials, and so on. In order to gain incisive and quantitative information on the local electrostatic environment, molecular conformation, protein structure and interprotein contacts, ligand binding kinetics, and electric and optical properties of functional materials, a variety of vibrational probes have been developed and site-specifically incorporated into molecular, biological, and material systems for time-resolved vibrational spectroscopic investigation. However, still, an all-encompassing theory that describes the vibrational solvatochromism, electrochromism, and dynamic fluctuation of vibrational frequencies has not been completely established mainly due to the intrinsic complexity of intermolecular interactions in condensed phases. In particular, the amount of data obtained from the linear and nonlinear vibrational spectroscopic experiments has been rapidly increasing, but the lack of a quantitative method to interpret these measurements has been one major obstacle in broadening the applications of these methods. Among various theoretical models, one of the most successful approaches is a semiempirical model generally referred to as the vibrational spectroscopic map that is based on a rigorous theory of intermolecular interactions. Recently, genetic algorithm, neural network, and machine learning approaches have been applied to the development of vibrational solvatochromism theory. In this review, we provide comprehensive descriptions of the theoretical foundation and various examples showing its extraordinary successes in the interpretations of experimental observations. In addition, a brief introduction to a newly created repository Web site (http://frequencymap.org) for vibrational spectroscopic maps is presented. We anticipate that a combination of the vibrational frequency map approach and state-of-the-art multidimensional vibrational spectroscopy will be one of the most fruitful ways to study the structure and dynamics of chemical, biological, and functional molecular systems in the future

Solvent-Dependent Dynamics of a Series of Rhenium Photoactivated Catalysts Measured with Ultrafast 2DIR

The spectral dynamics of a series of rhenium photocatalysts, <i>fac</i>-Re­(4,4′-R₂-bpy)­(CO)₃Cl, where R = H, methyl, <i>t</i>-butyl, and carboxylic acid, as well as Re­(1,10-phenanthroline)­(CO)₃Cl were observed in multiple aprotic solvents using two-dimensional infrared spectroscopy (2DIR). The carbonyl vibrational stretching frequencies showed slight variations due to the electron-donating or -withdrawing nature of the substituents on the bipyridine. The different substituents had minimal to no influence on the spectral diffusion time scales of the compounds within a particular solvent, but among the three different solvents investigated (DMSO, THF, and CH₃CN), we find the spectral diffusion times to correlate with the solvent’s donor number (DN). Because the donicity is a measure the Lewis basicity of the solvent, these findings may help establish a more complete dynamical picture of the photocatalysis, where the first chemical step following optical excitation is electron transfer from a sacrificial donor to the rhenium complex

NOESY-Like 2D-IR Spectroscopy Reveals Non-Gaussian Dynamics

We have identified an unexpected signature of non-Gaussian dynamics in a conventional 2D IR measurement on a system with rapid intermolecular vibrational energy transfer. In a ternary mixture of the CO₂ reduction photocatalyst, ReCl­(bpy)­(CO)₃, NaSCN, and THF solvent, preferential association between the metal carbonyl catalyst and the NaSCN ion pairs facilitates intermolecular energy transfer on a few picoseconds time scale. Monitoring the cross peak between the highest frequency metal carbonyl band and the CN bands of NaSCN contact ion pairs, we find a striking time evolution of the cross-peak position on the detection axis. This frequency shift, which is due to spectral diffusion following intermolecular energy transfer, occurs with a time scale that is distinct from either the donor or acceptor spectral diffusion measured simultaneously. We argue that the energy transfer, a second-order Förster process, effectively increases the dimensionality of the 2D-IR spectroscopy and thus enables sensitivity to non-Gaussian dynamics

Rapid and Accurate Measurement of the Frequency–Frequency Correlation Function

Using an implementation of heterodyne-detected vibrational echo spectroscopy, we show that equilibrium spectral diffusion caused by solvation dynamics can be measured in a fraction of the time required using traditional two-dimensional infrared spectroscopy. Spectrally resolved, heterodyne-detected rephasing and nonrephasing signals, recorded at a single delay between the first two pulses in a photon echo sequence, can be used to measure the full waiting time dependent spectral dynamics that are typically extracted from a series of 2D-IR spectra. Hence, data acquisition is accelerated by more than 1 order of magnitude, while permitting extremely fine sampling of the spectral dynamics during the waiting time between the second and third pulses. Using cymantrene (cyclopentadienyl manganese tricarbonyl, CpMn­(CO)₃) in alcohol solutions, we compare this novel approachdenoted rapidly acquired spectral diffusion (RASD)with a traditional method using full 2D-IR spectra, finding excellent agreement. Though this approach is largely limited to isolated vibrational bands, we also show how to remove interference from cross-peaks that can produce characteristic modulations of the spectral dynamics through vibrational quantum beats

Dynamic Flexibility of Hydrogenase Active Site Models Studied with 2D-IR Spectroscopy

Hydrogenase enzymes enable organisms to use H₂ as an energy source, having evolved extremely efficient biological catalysts for the reversible oxidation of molecular hydrogen. Small-molecule mimics of these enzymes provide both simplified models of the catalysis reactions and potential artificial catalysts that might be used to facilitate a hydrogen economy. We have studied two diiron hydrogenase mimics, μ-pdt-[Fe­(CO)₃]₂ and μ-edt-[Fe­(CO)₃]₂ (pdt = propanedithiolate, edt = ethanedithiolate), in a series of alkane solvents and have observed significant ultrafast spectral dynamics using two-dimensional infrared (2D-IR) spectroscopy. Since solvent fluctuations in nonpolar alkanes do not lead to substantial electrostatic modulations in a solute’s vibrational mode frequencies, we attribute the spectral diffusion dynamics to intramolecular flexibility. The intramolecular origin is supported by the absence of any measurable solvent viscosity dependence, indicating that the frequency fluctuations are not coupled to the solvent motional dynamics. Quantum chemical calculations reveal a pronounced coupling between the low-frequency torsional rotation of the carbonyl ligands and the terminal CO stretching vibrations. The flexibility of the CO ligands has been proposed to play a central role in the catalytic reaction mechanism, and our results highlight that the CO ligands are highly flexible on a picosecond time scale

Site-Specific Coupling of Hydration Water and Protein Flexibility Studied in Solution with Ultrafast 2D-IR Spectroscopy

There is considerable evidence for the slaving of biomolecular dynamics to the motions of the surrounding solvent environment, but to date there have been few direct experimental measurements capable of site-selectively probing both the dynamics of the water and the protein with ultrafast time resolution. Here, two-dimensional infrared spectroscopy (2D-IR) is used to study the ultrafast hydration and protein dynamics sensed by a metal carbonyl vibrational probe covalently attached to the surface of hen egg white lysozyme dissolved in D₂O/glycerol solutions. Surface labeling provides direct access to the dynamics at the protein–water interface, where both the hydration and the protein dynamics can be observed simultaneously through the vibrational probe’s frequency–frequency correlation function. In pure D₂O, the correlation function shows a fast initial 3 ps decay corresponding to fluctuations of the hydration water, followed by a significant static offset attributed to fluctuations of the protein that are not sampled within the <20 ps experimental window. Adding glycerol increases the bulk solvent viscosity while leaving the protein structurally intact and hydrated. The hydration dynamics exhibit a greater than 3-fold slowdown between 0 and 80% glycerol (v/v), and the contribution from the protein’s dynamics is found to slow in a nearly identical fashion. In addition, the magnitude of the dynamic slowdown associated with hydrophobic hydration is directly measured and shows quantitative agreement with predictions from molecular dynamics simulations

Interfacial Hydration Dynamics in Cationic Micelles Using 2D-IR and NMR

Using the thiocyanate anion as a vibrational probe chromophore in conjunction with infrared and NMR spectroscopy, we find that SCN^– strongly associates with the cationic head group of dodecyltrimethylammonium bromide (DTAB) micelles, both in normal-phase and reverse micelles. In competition with chloride and iodide ions, we find no evidence for displacement of thiocyanate, in accord with the chaotropicity of the Hofmeister ordering, while lending support to a direct interaction picture of its origin. Ultrafast 2D-IR spectroscopy of the SCN^– probe in a range of DTAB micelle sizes (<i>w</i>₀ = 4 to <i>w</i>₀ = 12) shows little if any size dependence on the time scale for spectral diffusion, which is found to be ∼3.5 times slower than in bulk water (both D₂O and H₂O). Normal-phase micelles studied with 2D-IR exhibit essentially the same spectral dynamics as do reverse micelles, indicating a lack of sensitivity to interfacial curvature. Combined with ¹H NMR chemical shift perturbations, we conclude that the SCN^– ions tightly associate with the head groups and are partially buried. The 3–4-fold slowdown in spectral diffusion is consistent with the excluded volume model for interfacial perturbation to hydrogen bond reorientation dynamics. On the basis of these observations and comparisons to previous studies of zwitterionic interfaces probed with phosphate transitions, we conclude that the SCN^– spectral dynamics in both reverse- and normal-phase micelles is largely dominated by hydration contributions, and offers a promising probe of interfacial hydration at cationic interfaces. Addition of competitive anions alters neither the IR spectra nor the ultrafast dynamics, indicating that SCN^– is robustly associated with the head groups

Oxidation-State-Dependent Vibrational Dynamics Probed with 2D-IR

In an effort to examine the role of electronic structure and oxidation states in potentially modifying intramolecular vibrational dynamics and intermolecular solvation, we have used 2D-IR to study two distinct oxidation states of an organometallic complex. The complex, [1,1′-bis­(diphenyl­phosphino)­ferrocene]­tetracarbonyl chromium (DPPFCr), consists of a catalytic diphenyl­phosphino ferrocene redox-active component as well as a Cr that can be switched from a Cr(0) to a Cr­(I) oxidation state using a chemical oxidant in dichloromethane (DCM) solution. The DPPFCr­(I) radical cation is sufficiently stable to investigate with 2D-IR spectroscopy, which provides dynamical information such as vibrational relaxation, intramolecular vibrational redistribution, as well as solvation dynamics manifested as spectral diffusion. Our measurements show that the primarily intramolecular dynamical processesvibrational relaxation and redistributiondiffer significantly between the two oxidation states, with faster relaxation in the oxidized DPPFCr­(I) radical cation. The primarily intermolecular spectral diffusion dynamics, however, exhibit insignificant oxidation state dependence. We speculate that the low nucleophilicity (i.e., donicity) of the DCM solvent, which is chosen to facilitate the chemical oxidation, masks any potential changes in solvation dynamics accompanying the substantial decrease in the 2.5 D molecular dipole moment of DPPFCr­(I) relative to DPPFCr(0) (7.5 D)

An “Iceberg” Coating Preserves Bulk Hydration Dynamics in Aqueous PEG Solutions

Ultrafast picosecond time scale two-dimensional infrared (2D-IR) spectroscopy of a new water-soluble transition metal complex acting as a vibrational probe shows that over a range of concentration and poly­(ethylene glycol) (PEG) molecular mass (2000, 8000, and 20000 Da) the time scale of the sensed hydration dynamics differs negligibly from bulk water (D₂O). PEG is well-known to establish a highly stable hydration shell because the spacing between adjacent ethereal oxygens nearly matches water’s hydrogen-bonding network. Although these first-shell water molecules are likely significantly retarded, they present an interface to subsequent hydration shells and thus diminish the largely entropic perturbation to water’s orientational dynamics. In addition to the longer PEGs, a series of concentration-dependent 2D-IR measurements using aqueous PEG-400 show a pronounced hydration slowdown in the vicinity of the critical overlap concentration (<i>c</i>*). Comparison between these dynamical results and previously reported steady-state infrared spectroscopy of aqueous PEG-1000 solutions reveals a strikingly identical dependence on number of water molecules per ethylene oxide monomer, scaled according to the critical overlap concentration

WAITING-TIME COHERENCE DECAY IN METAL CARBONYLS AS A FUNCTION OF SOLVENT INTERACTION

Author Institution: Department of Chemistry, University of Michigan, Ann Arbor, MI 48109Multidimensional infrared (MDIR) spectroscopy has been used to study the properties of long-lived coherences in the carbonyl stretching region (1980-2050 cm$^{-1}$) of dimanganese decacarbonyl (DMDC, Mn$_2$(CO)$_{10}$) in different solvents. The coherences are created by the first two pulses in our three pulse sequence. The amplitudes of many peaks in the 2DIR spectrum oscillate as a function of the waiting time (between the second and third pulses); oscillation frequencies correspond to the difference frequency between the states involved in the coherence. The oscillations decay up to three times faster in a strongly interacting solvent (chloroform) than in a weakly interacting one (cyclohexane) despite similar overall signal decays in the two solvents for those peaks which do not oscillate. Further, chloroform is seen to interact differently with each of the three vibrational peaks in the region. These observations are discussed in terms of the effect of vibrational coherence on vibrational energy transfer. We explore the implications of the rate of coherence decay as an indicator of the relative frequency-fluctuation correlations of the excited states relative to one another, rather than referenced to the universal ground state