Two-dimensional (2D) Fourier transform (FT) infrared spectroscopy is performed by using a collinear pulse-pair pump and probe geometry with conventional optics. Simultaneous collection of the third-order response and pulse-pair timing permit automated phasing and rapid acquisition of 2D absorptive spectra. To demonstrate the ability of this method to capture molecular dynamics, couplings and structure found in the conventional boxcar 2D FT spectroscopy, a series of 2D spectra of a metal carbonyl, and a beta-sheet protein are acquired.

We demonstrate how multimode 2D IR spectroscopy of the protein amide I' and II' vibrations can be used to distinguish protein secondary structure. Polarization-dependent amide I'-II' 2D IR experiments on poly-l-lysine in the beta-sheet, alpha-helix, and random coil conformations show that a combination of amide I' and II' diagonal and cross peaks can effectively distinguish between secondary structural content, where amide I' infrared spectroscopy alone cannot. The enhanced sensitivity arises from frequency and amplitude correlations between amide II' and amide I' spectra that reflect the symmetry of secondary structures. 2D IR surfaces are used to parametrize an excitonic model for the amide I'-II' manifold suitable to predict protein amide I'-II' spectra. This model reveals that the dominant vibrational interaction contributing to this sensitivity is a combination of negative amide II'-II' through-bond coupling and amide I'-II' coupling within the peptide unit. The empirically determined amide II'-II' couplings do not significantly vary with secondary structure: -8.5 cm(-1) for the beta sheet, -8.7 cm(-1) for the alpha helix, and -5 cm(-1) for the coil.

We use temperature-dependent ultrafast infrared spectroscopy of dilute HOD in H(2)O to study the picosecond reorganization of the hydrogen bond network of liquid water. Temperature-dependent two-dimensional infrared (2D IR), pump-probe, and linear absorption measurements are self-consistently analyzed with a response function formalism that includes the effects of spectral diffusion, population lifetime, reorientational motion, and nonequilibrium heating of the local environment upon vibrational relaxation. Over the range 278-345 K, we find the time scales of spectral diffusion and reorientational relaxation decrease from approximately 2.4 to 0.7 ps and 4.6 to 1.2 ps, respectively, which corresponds to barrier heights of 3.4 and 3.7 kcal/mol, respectively. We compare the temperature dependence of the time scales to a number of measures of structural relaxation and find similar effective activation barrier heights and slightly non-Arrhenius behavior, which suggests that the reaction coordinate for the hydrogen bond rearrangement in water is collective. Frequency and orientational correlation functions computed from molecular dynamics (MD) simulations over the same temperature range support our interpretations. Finally, we find the lifetime of the OD stretch is nearly the same from 278 K to room temperature and then increases as the temperature is increased to 345 K.

2We use temperature-dependent two-dimensional infrared spectroscopy (2D IR) of dilute HOD in H 2 O to investigate hydrogen bond rearrangements in water. The OD stretching frequency is sensitive to its environment, and loss of frequency correlation provides a picture of local and collective hydrogen bond dynamics. The timescales for hydrogen bond rearrangements decrease from roughly 2 ps at 278 K to 0.5 ps at 345 K. We find the barrier to dephasing and hydrogen bond switching to be E a = 3.4 ± 0.5 kcal/mol, although the trend is slightly non-Arrhenius. The value is in good agreement with the reported barrier height for OD reorientation observed in pump-probe anisotropy measurements.This provides evidence for the proposal that hydrogen bond switching occurs through concerted large angular jump reorientation. MD simulations of temperature-dependent OD vibrational dephasing and orientational correlation functions are used to support our conclusions. Table of Contents (TOC) FigureRearrangements in the hydrogen bond (HB) network of water necessarily require collective intermolecular rotations and translations. A number of recent studies using ultrafast infrared spectroscopy and molecular dynamics simulations have led to a selfconsistent description of the dynamics that underlie the mechanism of hydrogen bond rearrangements. 2D IR spectroscopy of HOD in D 2 O has shown that broken or strained hydrogen bonding configurations do not persist in the liquid, but reform a hydrogen bond in <150 fs. 1 2 This observation indicates that hydrogen bond switching is a concerted process that proceeds through a bifurcated HB transition state. Independently, Laage and Hynes presented a picture for molecular jump reorientation in water, in which reorientation upon HB switching is guided by fluctuations in HB coordination. 4 5 This is consistent with frequency-resolved pump-probe anisotropy measurements of HOD in H 2 O, which reveal that non-hydrogen bonded configurations have a higher degree of inertial rotation than strong HB species, 6 and that orientational motion accompanies the formation of a HB from an unstable configuration. 7To further test this proposal for hydrogen bond dynamics, we measured temperaturedependent spectral diffusion of the OD stretch of HOD in H 2 O using 2D IR spectroscopy. 11 Hydrogen bonding fluctuations modulate the OD frequency, and water structural rearrangements dictate the picosecond spectral diffusion kinetics. 8Temperature-dependent studies of these processes provide information on the HB switching barrier, which can be compared to recent simulations 5 and IR pump-probe anisotropy measurements 9 of water reorientation to test their joint contribution to the hydrogen bond rearrangement mechanism. 4 FTIR spectra of dilute HOD in H 2 O at 278, 295, 323, and 345 K are shown along with background H 2 O spectra in Figure 1. Between 278 and 345 K, the OD stretch blue shifts

Rearrangements of the hydrogen bond network of liquid water are believed to involve rapid and concerted hydrogen bond switching events, during which a hydrogen bond donor molecule undergoes large angle molecular reorientation as it exchanges hydrogen-bonding partners. To test this picture of hydrogen bond dynamics, we have performed ultrafast 2D IR spectral anisotropy measurements on the OH stretching vibration of HOD in D 2 O to directly track the reorientation of water molecules as they change hydrogen bonding environments. Interpretation of the experimental data is assisted by modeling drawn from molecular dynamics simulations, and we quantify the degree of molecular rotation on changing local hydrogen bonding environment using restricted rotation models. From the inertial 2D anisotropy decay, we find that water molecules initiating from a strained configuration and relaxing to a stable configuration are characterized by a distribution of angles, with an average reorientation half-angle of 10°, implying an average reorientation for a full switch of ≥20°. These results provide evidence that water hydrogen bond network connectivity switches through concerted motions involving large angle molecular reorientation.2