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Alcohol/alkane solutions widely used in chemical synthesis and as transportation fuels are highly non-ideal due to the nanoscale clustering of the amphiphilic alcohol molecules within the nonpolar alkanes. Besides impacting reactivity, such as combustion, non-ideal solutions are likely to exhibit unusual solvation dynamics on ultrafast time scales arising from the structurally heterogeneous nature of molecular-scale association. Using a convenient transition metal carbonyl vibrational probe [(C5H5)Mn(CO)3, CMT], linear absorption and nonlinear two-dimensional infrared (2D-IR) spectroscopy reveal composition-dependent solvation dynamics as reported by the frequency fluctuation correlation function in a series of ethanol/heptane solutions. Slow spectral diffusion with dilute ethanol indicates preferential solvation of the polar solute by the alcohol with a mechanism largely dominated by solvent exchange. Comparison with an ethanol/acetonitrile solution series yields no substantial preferential solvation or solvent exchange signatures in the linear or 2D-IR spectra. In ethanol/heptane solutions, increasing the ethanol concentration speeds up the solvation dynamics, which is largely consistent with a model that includes solvent exchange and single-solvent spectral diffusion. Detailed analysis of the deviation from the experimental time constants from the model's optimal parameters yields a remarkable resemblance of the concentration-weighted Kirkwood-Buff integrals for ethanol/heptane solutions. This trend indicates that solution non-ideality alters the spectral diffusion dynamics of the probe solute. Given that nanoscale clustering drives the non-ideality, these experiments reveal a dynamical consequence of nanoscale heterogeneity on the ultrafast dynamics of the solution. Refined understanding of the structural and dynamical aspects of mixed solvents will be necessary for predictive solution strategies in chemistry. [ABSTRACT FROM AUTHOR]

The authors make the following additions and corrections to the paper, C. Baiz et al., Vibrational Spectroscopic Map, Vibrational Spectroscopy, and Intermolecular Interaction. Chem. Rev. 2020, 120, 7152?7218. 1. The below funding sources should be added to the Acknowledgments section on pages 7204 and 7205: NHG acknowledges support from the National Science Foundation (CHE-1905395). AT thanks the Advanced Materials for Energy-Water Systems (AMEWS) Center (DE-AC02-06CH11357) for support. Thus, the Acknowledgments should be as follows: This work was supported by IBS-R023-D1 (MC). BB wishes to thank the European Union’s Horizon 2020 under the Marie Sk?odowska-Curie Grant Agreement No. 665778, as well as the National Science Centre, Poland (grant no. 2016/23/P/ST4/01720). CRB acknowledges generous support from the National Science Foundation (CHE-847199, BIO-1815354 ) , the National Ins t i t u tes o f Hea l t h (R35GM133359), and the Welch Foundation (F-1891). NHG acknowledges support from the National Science Foundation (CHE-1905395). MWDH-H and JDH thank the University of Nottingham, Green Chemicals Beacon for funding toward this research. AT thanks the Advanced Materials for Energy-Water Systems (AMEWS) Center (DEAC02-06CH11357) for support. MCT acknowledges support from Department of Energy (DE-SC0018983), National Science Foundation (1552996), and National Institutes of Health (GM114500). AGD would like to thank Prof. Andrei Tokmakoff for discussing amide modes when he was a Ph.D. student. LW acknowledges the support from the National Institutes of Health through Award R01GM130697. SAC acknowledges support from National Science Foundation (CHE-1565471). 2. The affiliations of Minhaeng Cho and Kijeong Kwak on page 7201 should be corrected as follows: Minhaeng Cho ? Center for Molecular Spectroscopy and Dynamics, Institute for Basic Science (IBS), Seoul 02841, Republic of Korea; Department of Chemistry, Korea University, Seoul 02841, Republic of Korea Kijeong Kwac ? Center for Molecular Spectroscopy and Dynamics, Institute for Basic Science (IBS), Seoul 02841, Republic of Korea 3. Reference 329 on page 7214 should be replaced with the following paper: (329) Sul, S; Karaiskaj, D.; Jiang Y.; Ge, N.-H. Conformations of N-Acetyl-L-Prolinamide by Two-Dimensional Infrared Spectroscopy. J. Phys. Chem. B 2006, 110, 19891?19905. 4. A correction in the biography of Magnus Hanson-Heine on page 7203 should be made. MH-H received his Ph.D. in 2014, not 2010.

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.

Another way to access interfacial dynamics is to study vibrational transitions localized at an interface as reported by Lee I et al. i ,[11] who use the ester carbonyl of model lipids in bilayers in conjunction with MD simulations to characterize hydration structural dynamics. Yang I et al. i [22] have investigated an energetic material, cyclotetramethylene tetranitramine (HMX), using ultrafast transient absorption and 2D-IR spectroscopy of the nitro stretch as a probe, revealing structural dynamics as well as vibrational energy transfer. Solowan I et al. i [4] study azulene using action-based 2D spectroscopy in both the liquid and gas phases, finding differences in the energetics and the rates of conical intersection (CI) and internal conversion in the two conditions. The amide I mode of peptides and proteins offers a powerful means of accessing protein structural dynamics, and the contribution from Tan I et al. i [12] shows how the vibrational anharmonicity is a sensitive probe of protein-water coupling. [Extracted from the article]

Two-dimensional infrared (2D-IR) spectroscopy is used to measure the spectral dynamics of the metal carbonyl complex cyclopentadienyl manganese tricarbonyl (CMT) in a series of linear alkyl nitriles. 2D-IR spectroscopy provides direct readout of solvation dynamics through spectral diffusion, probing the decay of frequency correlation induced by fluctuations of the solvent environment. 2D-IR simultaneously monitors intramolecular vibrational energy redistribution (IVR) among excited vibrations, which can also be influenced by the solvent through the spectral density rather than the dynamical friction underlying solvation. Here, we report that the CMT vibrational probe reveals solvent dependences in both the spectral diffusion and the IVR time scales, where each slows with increased alkyl chain length. In order to assess the degree to which solute–solvent interactions can be correlated with bulk solvent properties, we compared our results with low-frequency dynamics obtained from optical Kerr effect (OKE) spectroscopy—performed by others—on the same nitrile solvent series. We find excellent correlation between our spectral diffusion results and the orientational dynamics time scales from OKE. We also find a correlation between our IVR time scales and the amplitudes of the low-frequency spectral densities evaluated at the 90-cm−1 energy difference, corresponding to the gap between the two strong vibrational modes of the carbonyl probe. 2D-IR and OKE provide complementary perspectives on condensed phase dynamics, and these findings provide experimental evidence that at least at the level of dynamical correlations, some aspects of a solute vibrational dynamics can be inferred from properties of the solvent. [ABSTRACT FROM AUTHOR]

Two-dimensional infrared (2D-IR) spectroscopy provides access to equilibrium dynamics with the extraction of the frequency-fluctuation correlation function (FFCF) from the measured spectra. Several different methods of obtaining the FFCF from experimental spectra, such as the center line slope (CLS), ellipticity, phase slope, and nodal line slope, all depend on the geometrical nature of the 2D line shape and necessarily require spectral extent in order to achieve a measure of the FFCF. Amplitude measures, on the other hand, such as the inhomogeneity index, rely only on signal amplitudes and can, in principle, be computed using just a single point in a 2D spectrum. With a pulse shaper-based 2D-IR spectrometer, in conjunction with phase cycling, we separate the rephasing and nonrephasing signals used to determine the inhomogeneity index. The same measured data provide the absorptive spectrum, needed for the CLS. Both methods are applied to two model molecular systems: tungsten hexacarbonyl (WCO6) and methylcyclopentadienyl manganese tricarbonyl [Cp′Mn(CO)3, MCMT]. The three degenerate IR modes of W(CO)6 lack coherent modulation or noticeable intramolecular vibrational redistribution (IVR) and are used to establish a baseline comparison. The two bands of the MCMT tripod complex include intraband coherences and IVR as well as likely internal torsional motion on a few-picosecond time scale. We find essentially identical spectral diffusion, but faster, non-equilibrium dynamics lead to differences in the FFCFs extracted with the two methods. The inhomogeneity index offers an advantage in cases where spectra are complex and energy transfer can mimic line shape changes due to frequency fluctuations. [ABSTRACT FROM AUTHOR]

Within the conceptual framework of Redfield theory, the optical response function arises from the dynamical evolution of the system's density operator, where nonunitary relaxation is encoded in the Redfield relaxation superoperator. In the conventional approach, the so-called secular approximation neglects terms that induce transitions between distinct coherences and among coherences and populations. The rationale is that these nonsecular terms are small in comparison to the far more dominant population relaxation and coherence dephasing contributions. Since two-dimensional infrared (2D-IR) spectroscopy has significant contributions arising from population relaxation and transfer pathways, it can be challenging to isolate signatures of the nonsecular relaxation. We report here that in three diiron dithiolate hexacarbonyl complexes that serve as small-molecule models of the [FeFe] hydrogenase H-cluster subsite, a fortuitous vibrational energy structure enables direct and clear signatures of vibrational coherence transfer in alkane solution. This finding holds promise towards developing a molecularly detailed understanding of the mechanism of vibrational coherence transfer processes, thanks to the ease of synthesizing derivatives based on the chemical modularity of these well studied diiron compounds. In addition to the fundamental need to characterize coherence transfer in molecular spectroscopy, we find in this set of molecules a practical utility for the nonsecular dynamics: the ability to determine the frequency of an IR-inactive mode. A coherence generated during the waiting time of the 2D-IR measurement transfers to a coherence involving the single dark CO stretching mode, which modulates some peak amplitudes in the 2D spectrum, revealing its transient excitation. [ABSTRACT FROM AUTHOR]

Using rapidly acquired spectral diffusion, a recently developed variation of heterodyne detected infrared photon echo spectroscopy, we observe ~ 3 ps solvent independent spectral diffusion of benzene chromium tricarbonyl (C6H6Cr(CO)3, BCT) in a series of nonpolar linear alkane solvents. The spectral dynamics is attributed to low-barrier internal torsional motion. This tripod complex has two stable minima corresponding to staggered and eclipsed conformations, which differ in energy by roughly half of kBT. The solvent independence is due to the relative size of the rotor compared with the solvent molecules, which create a solvent cage in which torsional motion occurs largely free from solvent damping. Since the one-dimensional transition state is computed to be only 0.03 kBT above the higher energy eclipsed conformation, this model system offers an unusual, nearly barrierless reaction, which nevertheless is characterized by torsional coordinate dependent vibrational frequencies. Hence, by studying the spectral diffusion of the tripod carbonyls, it is possible to gain insight into the fundamental dynamics of internal rotational motion, and we find some evidence for the importance of non-diffusive ballistic motion even in the room-temperature liquid environment. Using several different approaches to describe equilibrium kinetics, as well as the influence of reactive dynamics on spectroscopic observables, we provide evidence that the low-barrier torsional motion of BCT provides an excellent test case for detailed studies of the links between chemical exchange and linear and nonlinear vibrational spectroscopy. [ABSTRACT FROM AUTHOR]

Two-dimensional infrared (2DIR) spectroscopy is used to study the influence of nanoconfinement on the spectral diffusion dynamics of cyclopentadienyl manganese tricarbonyl (CpMn(CO)3, CMT) free in solution and confined in the cavity of β-cyclodextrin. Contrary to the reorientation correlation function of the solvent molecules, determined through molecular dynamics simulations, measurements in three different solvents indicate that CMT confined in β-cyclodextrin undergoes spectral diffusion that is faster than free CMT. To account for this discrepancy, we propose that spectral diffusion time scales contain a dynamical contribution that is dependent on the effective size of the conformational space presented by the solvation environment. This solvation state space size is related to the number of participating solvent molecules, which in turn is proportional to the solvent accessible surface area (SASA). We test the role of the number of participating solvent molecules using a simple Gaussian-Markov simulation and find that an increase in the number of participating solvent molecules indeed slows the time required to search the available conformational space. Finally, we test this dependence by comparing the spectral diffusion of a previously studied manganese carbonyl, dimanganese decacarbonyl (Mn2(CO)10, DMDC), to CMT and find that DMDC, which has a larger SASA, exhibits slower spectral diffusion. The experimental observations and the supporting simplistic solvation model suggest that vibrational probe molecules, such as CMT, might be able to function as sensors of conformational entropy. [ABSTRACT FROM AUTHOR]

A systematic investigation of the solvent's dynamic influence on activated barrier crossings on an electronic ground state is performed using ultrafast two-dimensional infrared chemical exchange spectroscopy. These measurements facilitate a direct comparison with the widely adopted Kramers theory of condensed phase reaction kinetics, and for the first time avoid the significant complication of electronic excitation to probe directly in the time domain a ground electronic state reaction with a well-defined transition state. The picosecond timescale interconversion between two stable isomers of the metal carbonyl complex Co2(CO)8 in a series of linear alkane solvents shows negligible energetic variation with solvent carbon chain length, providing an exclusive probe of the effects of solvent friction. Relative to the linear alkane series, cyclohexane does alter the potential energy surface by preferentially stabilizing one of the isomers. Despite this pronounced modification of the reaction barrier energetics, combination of experiment and computation enables the removal of the nondynamical barrier contribution to the rate constant, isolating the dynamical influence of solvent friction. The experimental data, supported with quantum and classical computations, show agreement with a simple Markovian Kramers theory for the isomerization rate constant's dependence on solvent viscosity. [ABSTRACT FROM AUTHOR]

Multidimensional infrared (MDIR) spectroscopy of a strongly coupled multilevel vibrational system Mn2(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. [ABSTRACT FROM AUTHOR]

The first implementation of transient vibrational Stark-effect spectroscopy as an ultrafast probe of solvation dynamics which could be applied to the phototriggered intramolecular charge-transfer reaction of Betaine-30 is reported. The newly developed experiment provides a direct subpicosecond measure of the chromophore' s excited-state dynamics and back electron transfer as viewed from the solventEs perspective.

Ultrafast photoexcitation in the ultraviolet has cleaved the Mo-Mo bond and has helped in observing the subsequent geminate rebinding reaction by monitoring infrared bleach recoveries at frequencies corresponding to the CO stretches of the trans and gauche isomers. The rebinding reaction is obtained by mapping the full potential energy surface along the reaction coordinate by using electronic-structure methods.