36 research outputs found
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′-R<sub>2</sub>-bpy)(CO)<sub>3</sub>Cl,
where R = H, methyl, <i>t</i>-butyl, and carboxylic acid,
as well as Re(1,10-phenanthroline)(CO)<sub>3</sub>Cl 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 CH<sub>3</sub>CN), 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 CO<sub>2</sub> reduction photocatalyst, ReCl(bpy)(CO)<sub>3</sub>, 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
Dynamic Flexibility of Hydrogenase Active Site Models Studied with 2D-IR Spectroscopy
Hydrogenase
enzymes enable organisms to use H<sub>2</sub> 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)<sub>3</sub>]<sub>2</sub> and μ-edt-[Fe(CO)<sub>3</sub>]<sub>2</sub> (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
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)<sub>3</sub>) in alcohol solutions, we compare this novel approachdenoted
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
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 D<sub>2</sub>O/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 D<sub>2</sub>O, 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<sup>–</sup> 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<sup>–</sup> probe in a range of DTAB micelle sizes
(<i>w</i><sub>0</sub> = 4 to <i>w</i><sub>0</sub> = 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 D<sub>2</sub>O and H<sub>2</sub>O). 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 <sup>1</sup>H NMR chemical
shift perturbations, we conclude that the SCN<sup>–</sup> 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<sup>–</sup> 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<sup>–</sup> 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 processesvibrational relaxation and redistributiondiffer
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 (D<sub>2</sub>O). 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) of dimanganese decacarbonyl (DMDC, Mn(CO)) 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