Daniel E. Rosenfeld

Taking apart the two-dimensional infrared vibrational echo spectra: More information and elimination of distortions

Ultrafast two-dimensional infrared (2D-IR) vibrational echo spectroscopy can probe the fast structural evolution of molecular systems under thermal equilibrium conditions. Structural dynamics are tracked by observing the time evolution of the 2D-IR spectrum, which is caused by frequency fluctuations of vibrational mode(s) excited during the experiment. However, there are a variety of effects that can produce line shape distortions and prevent the correct determination of the frequency-frequency correlation function (FFCF), which describes the frequency fluctuations and connects the experimental observables to a molecular level depiction of dynamics. In addition, it can be useful to analyze different parts of the 2D spectrum to determine if dynamics are different for subensembles of molecules that have different initial absorption frequencies in the inhomogeneously broadened absorption line. Here, an important extension to a theoretical method for extraction of the FFCF from 2D-IR spectra is described. The experimental observable is the center line slope (CLSomega(m)) of the 2D-IR spectrum. The CLSomega(m) is obtained by taking slices through the 2D spectrum parallel to the detection frequency axis (omega(m)). Each slice is a spectrum. The slope of the line connecting the frequencies of the maxima of the sliced spectra is the CLSomega(m). The change in slope of the CLSomega(m) as a function of time is directly related to the FFCF and can be used to obtain the complete FFCF. CLSomega(m) is immune to line shape distortions caused by destructive interference between bands arising from vibrational echo emission, from the 0-1 vibrational transition (positive), and from the 1-2 vibrational transition (negative) in the 2D-IR spectrum. The immunity to the destructive interference enables the CLSomega(m) method to compare different parts of the bands as well as comparing the 0-1 and 1-2 bands. Also, line shape distortions caused by solvent background absorption and finite pulse durations do not affect the determination of the FFCF with the CLSomega(m) method. The CLSomega(m) can also provide information on the cross correlation between frequency fluctuations of the 0-1 and 1-2 vibrational transitions.

Ion–water hydrogen-bond switching observed with 2D IR vibrational echo chemical exchange spectroscopy

The exchange of water hydroxyl hydrogen bonds between anions and water oxygens is observed directly with ultrafast 2D IR vibrational echo chemical exchange spectroscopy (CES). The OD hydroxyl stretch of dilute HOD in H2O in concentrated (5.5 M) aqueous solutions of sodium tetrafluoroborate (NaBF4) displays a spectrum with a broad water-like band (hydroxyl bound to water oxygen) and a resolved, blue shifted band (hydroxyl bound to BF4−). At short time (200 fs), the 2D IR vibrational echo spectrum has 4 peaks, 2 on the diagonal and 2 off-diagonal. The 2 diagonal peaks are the 0–1 transitions of the water-like band and the hydroxyl-anion band. Vibrational echo emissions at the 1–2 transition frequencies give rise to 2 off-diagonal peaks. On a picosecond time scale, additional off-diagonal peaks grow in. These new peaks arise from chemical exchange between water hydroxyls bound to anions and hydroxyls bound to water oxygens. The growth of the chemical exchange peaks yields the time dependence of anion–water hydroxyl hydrogen bond switching under thermal equilibrium conditions as Taw = 7 ± 1 ps. Pump-probe measurements of the orientational relaxation rates and vibrational lifetimes are used in the CES data analysis. The pump-probe measurements are shown to have the correct functional form for a system undergoing exchange.

Structural Dynamics of a Catalytic Monolayer Probed by Ultrafast 2D IR Vibrational Echoes

A method to track fast vibrational motion in solution has been extended to catalytically important solid/liquid interfaces. Ultrafast two-dimensional infrared (2D IR) vibrational echo spectroscopy has proven broadly useful for studying molecular dynamics in solutions. Here, we extend the technique to probing the interfacial dynamics and structure of a silica surface-tethered transition metal carbonyl complex—tricarbonyl (1,10-phenanthroline)rhenium chloride—of interest as a photoreduction catalyst. We interpret the data using a theoretical framework devised to separate the roles of structural evolution and excitation transfer in inducing spectral diffusion. The structural dynamics, as reported on by a carbonyl stretch vibration of the surface-bound complex, have a characteristic time of ~150 picoseconds in the absence of solvent, decrease in duration by a factor of three upon addition of chloroform, and decrease another order of magnitude for the bulk solution. Conversely, solvent-complex interactions increase the lifetime of the probed vibration by 160% when solvent is applied to the monolayer.

Dynamics of the water hydrogen bond network at ionic, nonionic, and hydrophobic interfaces in nanopores and reverse micelles.

The effects of water confinement on hydrogen bond dynamics and hydrogen bond exchange have been analyzed by molecular dynamics simulations for a series of different sizes of spherical nanopores of ionic, nonionic, and hydrophobic interfaces. We have calculated translational diffusion residence times, orientational decay time constants, the infrared spectra, correlation functions describing the hydrogen bond network, the hydrogen bond exchange time and rate constant, and ensemble averages of the hydrogen bond exchange reaction coordinate. We focus on the interfacial layer and bulklike interior of these small water containing nanostructures. Our results indicate a universal slowdown in rotational and hydrogen bond dynamics at the interface relative to bulk water. The interiors of nanopores with highly charged interfaces undergo qualitatively different dynamics than those in other nanopores. The rotational jump hydrogen bond exchange mechanism is shown to be robust and universal, even for this variety of interfaces. The implications of these results are discussed in terms of the role of confinement vs interface structure on water dynamics in nanopores.

Theory of interfacial orientational relaxation spectroscopic observables

The orientational correlation functions measured in the time-resolved second-harmonic generation (TRSHG) and time-resolved sum-frequency generation (TRSFG) experiments are derived. In the laboratory coordinate system, the Y(l) (m)(Omega(lab)(t))Y(2) (m)(Omega(lab)(0)) (l=1,3 and m=0,2) correlation functions, where the Y(l) (m) are spherical harmonics, describe the orientational relaxation observables of molecules at interfaces. A wobbling-in-a-cone model is used to evaluate the correlation functions. The theory demonstrates that the orientational relaxation diffusion constant is not directly obtained from an experimental decay time in contrast to the situation for a bulk liquid. Model calculations of the correlation functions are presented to demonstrate how the diffusion constant and cone half-angle affect the time-dependence of the signals in TRSHG and TRSFG experiments. Calculations for the TRSHG experiments on Coumarin C314 molecules at air-water and air-water-surfactant interfaces are presented and used to examine the implications of published experimental results for these systems.

Hydrogen Bond Migration between Molecular Sites Observed with Ultrafast 2D IR Chemical Exchange Spectroscopy

Hydrogen-bonded complexes between phenol and phenylacetylene are studied using ultrafast two-dimensional infrared (2D IR) chemical exchange spectroscopy. Phenylacetylene has two possible pi hydrogen bonding acceptor sites (phenyl or acetylene) that compete for hydrogen bond donors in solution at room temperature. The OD stretch frequency of deuterated phenol is sensitive to which acceptor site it is bound. The appearance of off-diagonal peaks between the two vibrational frequencies in the 2D IR spectrum reports on the exchange process between the two competitive hydrogen-bonding sites of phenol-phenylacetylene complexes in the neat phenylacetylene solvent. The chemical exchange process occurs in approximately 5 ps and is assigned to direct hydrogen bond migration along the phenylacetylene molecule. Other nonmigration mechanisms are ruled out by performing 2D IR experiments on phenol dissolved in the phenylacetylene/carbon tetrachloride mixed solvent. The observation of direct hydrogen bond migration can have implications for macromolecular systems.

Solute−Solvent Complex Switching Dynamics of Chloroform between Acetone and Dimethylsulfoxide−Two-Dimensional IR Chemical Exchange Spectroscopy

Hydrogen bonds formed between C-H and various hydrogen bond acceptors play important roles in the structure of proteins and organic crystals, and the mechanisms of C-H bond cleavage reactions. Chloroform, a C-H hydrogen bond donor, can form weak hydrogen-bonded complexes with acetone and with dimethylsulfoxide (DMSO). When chloroform is dissolved in a mixed solvent consisting of acetone and DMSO, both types of hydrogen-bonded complexes exist. The two complexes, chloroform-acetone and chloroform-DMSO, are in equilibrium, and they rapidly interconvert by chloroform exchanging hydrogen bond acceptors. This fast hydrogen bond acceptor substitution reaction is probed using ultrafast two-dimensional infrared (2D-IR) vibrational echo chemical exchange spectroscopy. Deuterated chloroform is used in the experiments, and the 2D-IR spectrum of the C-D stretching mode is measured. The chemical exchange of the chloroform hydrogen bonding partners is tracked by observing the time-dependent growth of off-diagonal peaks in the 2D-IR spectra. The measured substitution rate is 1/30 ps for an acetone molecule to replace a DMSO molecule in a chloroform-DMSO complex and 1/45 ps for a DMSO molecule to replace an acetone molecule in a chloroform-acetone complex. Free chloroform exists in the mixed solvent, and it acts as a reactive intermediate in the substitution reaction, analogous to a SN1 type reaction. From the measured rates and the equilibrium concentrations of acetone and DMSO, the dissociation rates for the chloroform-DMSO and chloroform-acetone complexes are found to be 1/24 ps and 1/5.5 ps, respectively. The difference between the measured rate for the complete substitution reaction and the rate for complex dissociation corresponds to the diffusion limited rate. The estimated diffusion limited rate agrees well with the result from a Smoluchowski treatment of diffusive reactions.

Solvent Control of the Soft Angular Potential in Hydroxyl-π Hydrogen Bonds: Inertial Orientational Dynamics

Ultrafast polarization and wavelength selective IR pump-probe spectroscopy is used to measure the inertial and long time orientational dynamics of pi-hydrogen bonding complexes. Inertial orientational relaxation is sensitive to the angular potential associated with the hydrogen bond. The complexes studied are composed of phenol-OD (hydroxyl hydrogen replaced by deuterium) and various pi-base solvents with different electron donating or withdrawing substituents (chlorobenzene, bromobenzene, benzene, toluene, p-xylene, mesitylene, 1-pentyne). The different substituents provide experimental control of the hydrogen bond strength. The inertial orientational relaxation of the complexes, measured at the center frequency of each line, is independent of the hydrogen bond strength, demonstrating the insensitivity of the OD inertial dynamics, and therefore the H-bond angular potential, to the hydrogen bond strength. OD stretch absorption bands are inhomogeneously broadened through interactions with the solvent. The hydrogen bonding complexes all have similar wavelength dependent inertial orientational relaxation across their inhomogeneously broadened OD stretch absorption lines. The wavelength dependence of the inertial reorientation across each line arises because of a correlation between local solvent structure and the angular potential. These two results imply that local solvent structure acts as the controlling influence in determining the extent of inertial orientational relaxation, and therefore the angular potential, and that variation in the pi-hydrogen bond strength is of secondary importance.