We investigated rearrangements of the hydrogen-bond network in water by measuring fluctuations in the OH-stretching frequency of HOD in liquid D2O with femtosecond infrared spectroscopy. Using simulations of an atomistic model of water, we relate these frequency fluctuations to intermolecular dynamics. The model reveals that OH frequency shifts arise from changes in the molecular electric field that acts on the proton. At short times, vibrational dephasing reflects an underdamped oscillation of the hydrogen bond with a period of 170 femtoseconds. At longer times, vibrational correlations decay on a 1.2-picosecond time scale because of collective structural reorganizations.

Two-dimensional infrared (2D IR) vibrational spectroscopy is an experimental tool for investigating molecular dynamics in solution on a picosecond time scale. We present experimental and theoretical methods for obtaining a 2D IR correlation spectrum and modeling the underlying microscopic information. Fourier transform 2D spectra are obtained from heterodyne-detected third-order nonlinear signals using a sequence of broad bandwidth femtosecond IR pulses. A 2D IR correlation spectrum with absorptive line shapes results from the addition of 2D rephasing and nonrephasing spectra, which sample conjugate frequencies during the initial evolution time period. The 2D IR spectrum contains peaks with different positions, signs, amplitudes, and line shapes characterizing the vibrational eigenstates of the system and their interactions with the surrounding bath. The positions of the peaks map the transition frequencies between the ground, singly, and doubly excited states of the system and thus describe the anharmonic vibrational potential. Peak amplitudes reflect the relative magnitudes and orientations of the transition dipole moments in the molecular frame, the electrical anharmonicity of the system, and the vibrational relaxation dynamics. The 2D line shapes are sensitive to the system-bath interactions in solution. We illustrate how 2D IR spectra taken with varying polarization conditions and as a function of a variable waiting time can be used to isolate and quantify these spectroscopic observables. As a model vibrational system, we use the strongly coupled asymmetric and symmetric carbonyl stretches of Rh(CO) 2 C 5 H 7 O 2 (RDC) dissolved in hexane and chloroform. The polarization-selective 2D IR spectra of RDC in hexane are analyzed in terms of two coupled local coordinates to obtain their mutual orientation and the magnitude of the coupling between them. The 2D line-shape study of RDC in chloroform performed as a function of the waiting period characterizes the system-bath interactions, revealing that the system transition energies fluctuate in a correlated manner.

Although it is widely accepted that the local structure of liquid water has tetrahedral arrangements of molecules ordered by hydrogen bonds, the mechanism by which water molecules switch hydrogen-bonded partners remains unclear. In this mechanism, the role of nonhydrogen-bonded configurations (NHBs) between adjacent molecules is of particular importance. A molecule may switch hydrogen-bonding partners either (i) through thermally activated breaking of a hydrogen bond that creates a dangling hydrogen bond before finding a new partner or (ii) by infrequent but rapid switching events in which the NHB is a transition state. Here, we report a combination of femtosecond 2D IR spectroscopy and molecular dynamics simulations to investigate the stability of NHB species in an isotopically dilute mixture of HOD in D 2O. Measured 2D IR spectra reveal that hydrogen-bonded configurations and NHBs undergo qualitatively different relaxation dynamics, with NHBs returning to hydrogen-bonded frequencies on the time scale of water's fastest intermolecular motions. Simulations of an atomistic model for the OH vibrational spectroscopy of water yield qualitatively similar 2D IR spectra to those measured experimentally. Analysis of NHBs in simulations by quenching demonstrates that the vast majority of NHBs are in fact part of a hydrogen-bonded well of attraction and that virtually all molecules return to a hydrogen-bonding partner within 200 fs. The results from experiment and simulation demonstrate that NHBs are intrinsically unstable and that dangling hydrogen bonds are an insignificant species in liquid water.femtosecond 2D IR spectroscopy ͉ molecular dynamics ͉ liquids O n average, molecules in liquid water are tetrahedrally coordinated but appear to engage in 10% fewer hydrogen bonds than in ice. Support for this estimate comes broadly, from latent heats of melting and vaporization, from x-ray and neutron scattering, and in very detailed form from molecular dynamics (MD) simulations (1-3). The role of nonhydrogen-bonded configurations (NHBs) in water's rapidly changing structure remains uncertain, lying at the heart of differences between mixture and continuum models of water (1, 3-8). Implicitly or explicitly, the interpretation of many experiments and MD simulations conceives of NHBs as broken or dangling hydrogen bonds, stable species that interconvert with a hydrogen-bonded configuration (HB) at a rate determined by the free energy barrier separating them. But it is also possible that NHBs are intrinsically unstable species that appear transiently during natural fluctuations about a hydrogen bond or when molecules trade hydrogen-bonding partners. These two scenarios not only provide qualitatively different interpretations of water's structure and how it evolves, but also imply different pictures for how water mediates chemical and biological processes. We have distinguished between these scenarios by using a combination of femtosecond 2D IR spectroscopy and MD simulations, finding that NHBs are inherently unstable, reforming ...

We present an investigation into hydrogen bonding dynamics and kinetics in water using femtosecond infrared spectroscopy of the OH stretching vibration of HOD in D(2)O. Infrared vibrational echo peak shift and polarization-selective pump-probe experiments were performed with mid-IR pulses short enough to capture all relevant dynamical processes. The experiments are self-consistently analyzed with a nonlinear response function expressed in terms of three dynamical parameters for the OH stretching vibration: the frequency correlation function, the lifetime, and the second Legendre polynomial dipole reorientation correlation function. It also accounts for vibrational-relaxation-induced excitation of intermolecular motion that appears as heating. The long time, picosecond behavior is consistent with previous work, but new dynamics are revealed on the sub-200 fs time scale. The frequency correlation function is characterized by a 50 fs decay and 180 fs beat associated with underdamped intermolecular vibrations of hydrogen bonding partners prior to 1.4 ps exponential relaxation. The reorientational correlation function observes a 50 fs librational decay prior to 3 ps diffusive reorientation. Both of these correlation functions compare favorably with the predictions from classical molecular dynamics simulations. The time-dependent behavior can be separated into short and long time scales by the 340 fs correlation time for OH frequency shifts. The fast time scales arise from dynamics that are mainly local: fluctuations in hydrogen bond distances and angles within relatively fixed intermolecular configurations. On time scales longer than the correlation time, dephasing and reorientations reflect collective reorganization of the liquid structure. Since the OH transition frequency and dipole are only weakly sensitive to these collective coordinates, this is a kinetic regime which gives an effective rate for exchange of intermolecular structures.

We review two-dimensional infrared (2D IR) spectroscopy of the amide I protein backbone vibration. Amide I modes are known for secondary structural sensitivity derived from their protein-wide delocalization. However, amide I FTIR spectra often display little variation for different proteins due to the broad and featureless line shape that arises from different structural motifs. 2D IR offers increased structural resolution by spreading the spectra over a second frequency dimension to reveal two-dimensional line shapes and cross-peaks. In addition, it carries picosecond time resolution, making it an excellent choice for understanding protein dynamics. In 2D IR spectra, cross peaks arise from anharmonic coupling between vibrations. For example, the spectra of ordered antiparallel beta sheets shows a cross peak between the strong nu perpendicular mode at approximately 1620 cm(-1) and the weaker nu parallel mode at approximately 1680 cm(-1). In proteins with beta-sheet content, disorder spreads the cross peaks into ridges, which gives rise to a "Z"-shaped contour profile. 2D IR spectra of alpha helices show a flattened "figure-8" line shape, and random coils give rise to unstructured, diagonally elongated bands. A distinguishing quality of 2D IR is the availability of accurate structure-based models to calculate spectra from atomistic structures and MD simulations. The amide I region is relatively isolated from other protein vibrations, which allows the spectra to be described by coupled anharmonic local amide I vibrations at each peptide unit. One of the most exciting applications of 2D IR is to study protein unfolding dynamics. While 2D IR has been used to study equilibrium structural changes, it has the time resolution to probe all changes resulting from photoinitiated dynamics. Transient 2D IR has been used to probe downhill protein unfolding and hydrogen bond dynamics in peptides. Because 2D IR spectra can be calculated from folding MD simulations, opportunities arise for making rigorous connections. By introduction of isotope labels, amide I 2D IR spectra can probe site-specific structure with picosecond time resolution. This has been used to reveal local information about picosecond fluctuations and disorder in beta hairpins and peptides. Multimode 2D IR spectroscopy has been used to correlate the structure sensitivity of amide I with amide II to report on solvent accessibility and structural stability in proteins.