Extent of Delocalization of Vibrational Modes in Liquids as a Result of Competition between Diagonal Disorder and Off-Diagonal Coupling

Theoretical and experimental OD-stretch vibrational spectroscopy of heavy water

In view of the current situation in which the OD-stretch vibrational spectra have been scarcely computed with non-polarizable rigid D2O models, we investigate the IR and Raman spectra of D2O by using a newly-reported model TIP4P/2005-HW. From the comparison between the calculations and experimental data, we find the excellent performance of TIP4P/2005-HW for vibrational spectroscopy of D2O in the same manner as TIP4P/2005 for H2O, although one may still conveniently employ an alternative method that regards OH as putative OD to calculate the OD-stretch spectra with similar quality from TIP4P/2005 trajectories. We also demonstrate that the appropriate setting for the spectral simulation of D2O under the time-averaging approximation reflects the slower dynamics (i.e., slower motion of translation and rotation due to the heavier mass and stronger hydrogen bond) of D2O than H2O. Moreover, we show from the theoretical calculations that the established interpretation of the OH-stretch spectra of H2O is finely applicable to the OD-stretch of D2O.

Vibrational exciton delocalization precludes the use of infrared intensities as proxies for surfactant accumulation on aqueous surfaces

Surface-sensitive vibrational spectroscopy is a common tool for measuring molecular organization and intermolecular interactions at interfaces. Peak intensity ratios are typically used to extract molecular information from one-dimensional spectra but vibrational coupling between surfactant molecules can manifest as signal depletion in one-dimensional spectra. Through a combination of experiment and theory, we demonstrate the emergence of vibrational exciton delocalization in infrared reflection–absorption spectra of soluble and insoluble surfactants at the air/water interface. Vibrational coupling causes a significant decrease in peak intensities corresponding to C–F vibrational modes of perfluorooctanoic acid molecules. Vibrational excitons also form between arachidic acid surfactants within a compressed monolayer, manifesting as signal reduction of C–H stretching modes. Ionic composition of the aqueous phase impacts surfactant intermolecular distance, thereby modulating vibrational coupling strength between surfactants. Our results serve as a cautionary tale against employing alkyl and fluoroalkyl vibrational peak intensities as proxies for concentration, although such analysis is ubiquitous in interface science.

Coupling between surfactant molecules at the air/water interface bleeds intensity into a diffuse background, such that single-wavelength vibrational intensity is effectively depleted at high surface coverage.

Hydrogen order at the surface of ice Ih revealed by vibrational spectroscopy

A combination of heterodyne-detected sum frequency generation spectroscopy and theoretical modeling elucidates that the surface of ice I_h at 100 K has hydrogen order with the OH group pointing upward to the air (“H-up” orientation).

Computing infrared spectra of proteins using the exciton model

The ability to compute from first principles the infrared spectrum of a protein in solution phase representing a biological system would provide a useful connection to atomistic models of protein structure and dynamics. Indeed, such calculations are a vital complement to 2DIR experimental measurements, allowing the observed signals to be interpreted in terms of detailed structural and dynamical information. In this article, we have studied nine structurally and spectroscopically well-characterized proteins, representing a range of structural types. We have simulated the equilibrium conformational dynamics in an explicit point charge water model. Using the resulting trajectories based on MD simulations, we have computed the one and two dimensional infrared spectra in the Amide I region, using an exciton approach, in which a local mode basis of carbonyl stretches is considered. The role of solvent in shifting the Amide I band (by 30 to 50 cm-1 ) is clearly evident. Similarly, the conformational dynamics contribute to the broadening of peaks in the spectrum. The inhomogeneous broadening in both the 1D and 2D spectra reflects the significant conformational diversity observed in the simulations. Through the computed 2D cross-peak spectra, we show how different pulse schemes can provide additional information on the coupled vibrations. © 2016 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.

Vibrational Quantum Decoherence in Liquid Water

Traditional descriptions of vibrational energy transfer consider a quantum oscillator interacting with a classical environment. However, a major limitation of this simplified description is the neglect of quantum decoherence induced by the different interactions between two distinct quantum states and their environment, which can strongly affect the predicted energy-transfer rate and vibrational spectra. Here, we use quantum-classical molecular dynamics simulations to determine the vibrational quantum decoherence time for an OH stretch vibration in liquid heavy water. We show that coherence is lost on a sub-100 fs time scale due to the different responses of the first shell neighbors to the ground and excited OH vibrational states. This ultrafast decoherence induces a strong homogeneous contribution to the linear infrared spectrum and suggests that resonant vibrational energy transfer in H2O may be more incoherent than previously thought.

Quantifying Surfactant Alkyl Chain Orientation and Conformational Order from Sum Frequency Generation Spectra of CH Modes at the Surfactant-Water Interface

We combine second-order nonlinear vibrational spectroscopy and quantum-chemical calculations to quantify the molecular tilt angle and the structural variation of a decanoic acid surfactant monolayer on water. We demonstrate that there is a remarkable degree of delocalization of the vibrational modes along the backbone of the amphiphilic molecule. A simulation-based on modeled sum frequency generation (SFG) spectra offers quantitative insights into the disorder of surfactant monolayers at the water-air interface. It is shown that an average of one gauche defect in the alkyl chain suffices to give rise to the methylene stretch intensity similar in magnitude to the methyl stretch.

Sum frequency generation image reconstruction: aliphatic membrane under spherical cap geometry

The article explores an opportunity to approach structural properties of phospholipid membranes using Sum Frequency Generation microscopy. To establish the principles of sum frequency generation image reconstruction in such systems, at first approach, we may adopt an idealistic spherical cap uniform assembly of hydrocarbon molecules. Quantum mechanical studies for decanoic acid (used here as a representative molecular system) provide necessary information on transition dipole moments and Raman tensors of the normal modes specific to methyl terminal - a typical moiety in aliphatic (and phospholipid) membranes. Relative degree of localization and frequencies of the normal modes of methyl terminals make nonlinearities of this moiety to be promising in structural analysis using Sum Frequency Generation imaging. Accordingly, the article describes derivations of relevant macroscopic nonlinearities and suggests a mapping procedure to translate amplitudes of the nonlinearities onto microscopy image plane according to geometry of spherical assembly, local molecular orientation, and optical geometry. Reconstructed images indicate a possibility to extract local curvature of bilayer envelopes of spherical character. This may have practical implications for structural extractions in membrane systems of practical relevance.

Nonlinear response of vibrational excitons: simulating the two-dimensional infrared spectrum of liquid water

A simulation formalism for the nonlinear response of vibrational excitons is presented and applied to the OH stretching vibrations of neat liquid H(2)O. The method employs numerical integration of the Schrodinger equation and allows explicit treatment of fluctuating transition frequencies, vibrational couplings, dipole moments, and the anharmonicities of all these quantities, as well as nonadiabatic effects. The split operator technique greatly increases computational feasibility and performance. The electrostatic map for the OH stretching vibrations in liquid water employed in our previous study [A. Paarmann et al., J. Chem. Phys. 128, 191103 (2008)] is presented. The two-dimensional spectra are in close agreement with experiment. The fast 100 fs dynamics are primarily attributed to intramolecular mixing between states in the two-dimensional OH stretching potential. Small intermolecular couplings are sufficient to reproduce the experimental energy transfer time scales. Interference effects between Liouville pathways in excitonic systems and their impact on the analysis of the nonlinear response are discussed.

Electrostatic interactions in phospholipid membranes revealed by coherent 2D IR spectroscopy

The inter- and intramolecular interactions of the carbonyl moieties at the polar interface of a phospholipid membrane are probed by using nonlinear femtosecond infrared spectroscopy. Two-dimensional IR correlation spectra separate homogeneous and inhomogeneous broadenings and show a distinct cross-peak pattern controlled by electrostatic interactions. The inter- and intramolecular electrostatic interactions determine the inhomogeneous character of the optical response. Using molecular dynamics simulation and the nonlinear exciton equations approach, we extract from the spectra short-range structural correlations between carbonyls at the interface.

Scheme of detecting microscopic inhomogeneity in binary liquid mixtures utilizing resonantly coupled vibrational modes

Influence of microscopic inhomogeneity in binary liquid mixtures on their vibrational spectra is studied by doing calculations on a model liquid system. The concentration dependence of the noncoincidence effect (NCE), which is a feature of vibrational bands related to the intermolecular resonant coupling of vibrational modes, is analyzed. It is suggested that observation of convex behavior of the NCEs for the vibrational bands of both species, especially that of the less polar species, in a binary liquid mixture is an indication of the occurrence of microscopic inhomogeneity.

Vibrational relaxation studies in methyl acetate: role of microenvironment and hydrodynamic forces

The Raman spectra were recorded for the C=O stretching vibration of methyl acetate as solutions in various polar and non-polar solvents. The isotropic component was obtained and the vibrational relaxation rates were calculated. On the basis of dependence of isotropic Raman bandwidth on the hydrodynamic properties of the solute-solvent systems, the information was obtained on the microenvironment prevailing in the neighborhood of the C=O stretching mode. Significant correlation is observed between vibrational relaxation rate and solvent parameters namely viscosity, density, refractive index and molecular radius. Microviscosity, involving the size of solute and solvent molecules, is found to be crucial in determining the bandwidth, hence the relaxation rate. The microenvironment appears to play an important role in the vibrational relaxation process.

Vibrational dynamics of N–H, C–D, and CO modes in formamide

By means of heterodyned two-dimensional IR photon echo experiments on liquid formamide and isotopomers the vibrational frequency dynamics of the N-H stretch mode, the C-D mode, and the C=O mode were obtained. In each case the vibrational frequency correlation function is fitted to three exponentials representing ultrafast (few femtoseconds), intermediate (hundreds of femtoseconds), and slow (many picoseconds) correlation times. In the case of N-H there is a significant underdamped contribution to the correlation decay that was not seen in previous experiments and is attributed to hydrogen-bond librational modes. This underdamped motion is not seen in the C-D or C=O correlation functions. The motions probed by the C-D bond are generally faster than those seen by N-H and C=O, indicating that the environment of C-D interchanges more rapidly, consistent with a weaker C-D...O=C bond. The correlation decays of N-H and C=O are similar, consistent with both being involved in strong H bonding.

Time-Domain Theoretical Analysis of the Noncoincidence Effect, Diagonal Frequency Shift, and the Extent of Delocalization of the CO Stretching Mode of Acetone/Dimethyl Sulfoxide Binary Liquid Mixtures

A time-domain method for simulating vibrational band profiles that simultaneously takes into account both the diagonal and off-diagonal effects is developed and applied to the C=O stretching bands of neat liquid acetone and the acetone/dimethyl sulfoxide (DMSO) binary liquid mixtures. By using this method, it is possible to examine the influence of liquid dynamics on the noncoincidence effect (NCE), which arises from the off-diagonal vibrational interactions, as well as the frequency shifts and band broadening, which are related to both the diagonal and off-diagonal effects. It is shown that the simulations for the C=O stretching bands of acetone in acetone/DMSO binary liquid mixtures on the basis of this method can reproduce the experimentally observed concave curvature of the concentration dependence of the NCE and the unusually large frequency shift of the anisotropic Raman band. The widths of the infrared, isotropic Raman, and anisotropic Raman bands calculated for neat liquid acetone are also in good agreement with those observed. Based on these calculations, the extent of delocalization of the C=O stretching vibrational motions is examined by referring to two quantitative measures of this property, one calculated in the frequency domain and the other in the time domain. It is shown that the extent of delocalization gets larger as the mole fraction of acetone increases, the C=O stretching vibrations being delocalized over a few tens of molecules in neat liquid acetone. It is also shown that the extent of delocalization is related to the quantity called NCE detectability, which is the ratio between the magnitude of NCE and the bandwidth. It is therefore suggested that the extent of delocalization of vibrational motions may be estimated from observable features of Raman band profiles.