OD Stretch Research Articles

Two-dimensional infrared spectroscopy of isotope-diluted ice Ih

We present experimental 2D IR spectra of isotope diluted ice Ih (i.e., the OH stretch mode of HOD in D(2)O and the OD stretch mode of HOD in H(2)O) at T = 80 K. The main spectral features are the extremely broad 1-2 excited state transition, much broader than the corresponding 0-1 groundstate transition, as well as the presence of quantum beats. We do not observe any inhomogeneous broadening that might be expected due to proton disorder in ice Ih. Complementary, we perform simulations in the framework of the Lippincott-Schroeder model, which qualitatively reproduce the experimental observations. We conclude that the origin of the observed line shape features is the coupling of the OH-vibrational coordinate with crystal phonons and explain the beatings as a coherent oscillation of the O···O hydrogen bond degree of freedom.

Isotope effects in liquid water by infrared spectroscopy. V. A sea of OH4 of C2v symmetry

The two water gas OH stretch vibrations that absorb in the infrared (IR) near 3700 cm(-1) are redshifted to near 3300 cm(-1) upon liquefaction. The bathochromic shift is due to the formation of four H-bonds: two are from the labile hydrogen atoms to neighbors and two are received from neighbors by the oxygen free electron pairs. Therefore, the water oxygen atom is surrounded by four hydrogen atoms, two of these make covalent bonds that make H-bonds and two are oxygen H-bonded. However, these permute at rate in the ps range. When the water molecules are isolated in acetonitrile (MeCN) or acetone (Me(2)CO), only the labile hydrogen atoms make H-bonds with the solvent. The bathochromic shift of the OH stretch bands is then almost 130 cm(-1) with, however, the asymmetric (ν(3)) and symmetric (ν(1)) stretch bands maintained. When more water is added to the solutions, the oxygen lone doublets make H-bonds with the available labile hydrogen atoms from neighboring water molecules. With one bond accepted, the bathochromic shift is further displaced by almost 170 cm(-1). When the second oxygen doublet is filled, another bathochromic shift by almost 100 cm(-1) is observed. The total bathochromic shift is near 400 cm(-1) with a full width at half height of near 400 cm(1). This is the case of pure liquid water. Notwithstanding the shift and the band broadness, the ν(3) and ν(1) band individualities are maintained with, however, added satellite companions that come from the far IR (FIR) absorption. These added to the fundamental bands are responsible for the band broadness and almost featureless shape of the massive OH stretch absorption of liquid water. Comparison of light and heavy water mixture spectra indicates that the OH and OD stretch regions show five different configurations: OH(4); OH(3)D; OH(2)D(2); OHD(3); and OD(4) [J. Chem. Phys. 116, 4626 (2002)]. The comparison of the OH bands of OH(4) with that of OHD(3) indicates that the main component in OHD(3) is ν(OH), whereas in OH(4) two main components are present: ν(3) and ν(1). Similar results are obtained for the OD bands of OD(4) and ODH(3). These results indicate that the C(2) (v) symmetry of H(2)O and D(2)O is preserved in the liquid and aqueous solutions whereas C(s) is that of HDO.

Collective Hydrogen Bond Reorganization in Water Studied with Temperature-Dependent Ultrafast Infrared Spectroscopy

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.

H+ versus D+ transfer from HOD+ to CO2: Bond-selective chemistry and the anomalous effect of bending excitation

Reactions of HOD(+) with CO(2) have been studied for HOD(+) in its ground state, and with one quantum of excitation in each of its vibrational modes: (001)--predominantly OH stretch, 0.396 eV; (010)--bend, 0.153 eV; and (100)--predominantly OD stretch, 0.293 eV. Integral cross sections and product recoil velocities were recorded for collision energies from threshold to 3 eV. The cross sections for both H(+) and D(+) transfer rise with increasing collision energy from threshold to ∼1 eV, then become weakly dependent of the collision energy. All three vibrational modes enhance the total reactivity, but quite mode specifically. The H(+) transfer reaction is enhanced by OH stretch excitation, whereas OD stretch excitation has little effect. Conversely, the D(+) transfer reaction is enhanced by OD stretch excitation, while the OH stretch has little effect. Excitation of the bend strongly enhances both channels. The effects of the stretch excitations are consistent with previous studies of neutral HOD mode-selective chemistry, and can be at least qualitatively understood in terms of a late barrier to product formation. The fact that bend excitation produces the largest overall enhancement is surprising, because this is the lowest energy excitation, and is not obviously connected with the reaction coordinates for either H(+) or D(+) transfer. A rationalization in terms of the effects of water distortion on the potential surface is proposed.

On the structure of the matrix isolated water trimer

Infrared spectra of partially deuterated water trimers have been investigated. It is found that HDO(H(2)O)(2) has a single, bound OD stretching fundamental, (HDO)(2)H(2)O two bound OD stretches. (HDO)(3) has a single, bound OD stretch and (H(2)O)(3) has a pair of bound OH stretches. Ab initio and discrete Fourier transform (DFT) calculations predict that the water trimer has C(1) symmetry with six different, isoenergetic minima. These calculations consequently give three numerically different OD stretches for HDO(H(2)O)(2), six for (HDO)(2)H(2)O, three for (HDO)(3), and three bound OH stretches for (H(2)O)(3). The connection between the observations and the pseudorotation of the trimer is discussed with the help of Wales' pseudorotation model. It is found that pseudorotation is sufficiently fast to average the effective symmetry of the A(3) trimer to C(3h) and to eliminate the difference between the different ab initio minima for A(2)B. The only exception is (H(2)O)(3) where the splitting between the different bound OH stretches is largest. Here a doublet is observed due to incomplete averaging. DFT calculations indicate that the D-bonded form of HDO(H(2)O)(2) is between 50 and 60 cm(-1) more stable than the H-bonded form. The energy difference is determined by differences in zero point vibration energy of intermolecular librations of the two forms. Attempts to measure the energy difference indicate that the energy difference is larger, of the order of 100 cm(-1).

Water dynamics in small reverse micelles in two solvents: Two-dimensional infrared vibrational echoes with two-dimensional background subtraction

Water dynamics as reflected by the spectral diffusion of the water hydroxyl stretch were measured in w(0) = 2 (1.7 nm diameter) Aerosol-OT (AOT)/water reverse micelles in carbon tetrachloride and in isooctane solvents using ultrafast 2D IR vibrational echo spectroscopy. Orientational relaxation and population relaxation are observed for w(0) = 2, 4, and 7.5 in both solvents using IR pump-probe measurements. It is found that the pump-probe observables are sensitive to w(0), but not to the solvent. However, initial analysis of the vibrational echo data from the water nanopool in the reverse micelles in the isooctane solvent seems to yield different dynamics than the CCl(4) system in spite of the fact that the spectra, vibrational lifetimes, and orientational relaxation are the same in the two systems. It is found that there are beat patterns in the interferograms with isooctane as the solvent. The beats are observed from a signal generated by the AOT/isooctane system even when there is no water in the system. A beat subtraction data processing procedure does a reasonable job of removing the distortions in the isooctane data, showing that the reverse micelle dynamics are the same within experimental error regardless of whether isooctane or carbon tetrachloride is used as the organic phase. Two time scales are observed in the vibrational echo data, ~1 and ~10 ps. The slower component contains a significant amount of the total inhomogeneous broadening. Physical arguments indicate that there is a much slower component of spectral diffusion that is too slow to observe within the experimental window, which is limited by the OD stretch vibrational lifetime.

Reaction of HOD+ with NO2: Effects of OD and OH Stretching, Bending, and Collision Energy on Reactions on the Singlet and Triplet Potential Surfaces

Integral cross sections and product recoil velocity distributions were measured for the reaction of HOD(+) with NO(2), in which the HOD(+) reactant was prepared in its ground state and with mode-selective excitation in the 001 (OH stretch), 100 (OD stretch), and 010 (bend) modes. In addition, we measured the 300 K thermal kinetics in a selected ion flow tube reactor and report product branching ratios different from previous measurements. Reaction is found to occur on both the singlet and triplet surfaces with near-unit efficiency. At 300 K, the product branching indicates that triplet → singlet transitions occur in about 60% of triplet-coupled collisions, which we attribute to long interaction times mediated by complexes on the triplet surface. Because the collision times are much shorter in the beam experiments, the product distributions show no signs of such transitions. The dominant product on the singlet surface is charge transfer. Reactions on the triplet surface lead to NO(+), NO(2)H(+), and NO(2)D(+). There is also charge transfer, producing NO(2)(+) (a(3)B(2)); however, this triplet NO(2)(+) mostly predissociates. The NO(2)H(+)/NO(2)D(+) cross sections peak at low collision energies and are insignificant above ~1 eV due to OH/OD loss from the nascent product ions. The effects of HOD(+) vibration are mode-specific. Vibration inhibits charge transfer, with the largest effect from the bend. The NO(2)H(+)/NO(2)D(+) channels are also vibrationally inhibited, and the mode dependence reveals how energy in different reactant modes couples to the internal energy of the product ions.

Two-Dimensional Infrared Spectroscopy of Supercooled Water

We present two-dimensional infrared (2D IR) spectra of the OD stretch vibration of isotope diluted water (HOD/H(2)O) from ambient conditions (293 K) down to the metastable supercooled regime (260 K). We observe that spectral diffusion slows down from 700 fs to 2.6 ps as we lower the temperature. A comparison between measurements performed at the magic angle with those at parallel polarization shows that the 2D IR line shape is affected by the frequency-dependent anisotropy decay in the case of parallel polarization, altering the extracted correlation decay. A fit within the framework of an Arrhenius law reveals an activation energy of E(a) = 6.2 ± 0.2 kcal/mol and a pre-exponential factor of 1/A = 0.02 ± 0.01 fs. Alternatively, a power law fit results in an exponent γ = 2.2 and a singularity temperature T(s) = 221 K. We tentatively conclude that the power law provides the better physical picture to describe the dynamics of liquid water around the freezing point.

Acceptor switching and axial rotation of the water dimer in matrices, observed by infrared spectroscopy

Several isotopologues of the water dimer have been studied in different matrices (Ne, Ar, Kr, and p-H(2)) at very low temperatures. A fine structure, which is more or less matrix independent and very similar for different intramolecular fundamentals of the same isotopologic dimer, is present on the high wavenumber side of the main component. The bound OD (OH) stretches of the donor have temperature dependent components. The fine structure and temperature dependency is interpreted as evidence for acceptor switching and rotation of the water dimer around its O-O axis in the matrices studied here. The slow nuclear spin equilibration in H(2)O inhibits the thermal equilibration between the acceptor switching states in H(2)O-DOH and H(2)O-DOD. The condensed environment slows down the acceptor switching rate compared to the gas phase. The antisymmetric stretch of the proton acceptor is assigned by combining information from different matrices with the rotation-acceptor switching model.

Infrared and Raman line shapes for ice Ih. I. Dilute HOD in H2O and D2O

Vibrational spectroscopy of ice Ih provides information about structure, dynamics, and vibrational coupling in this important substance. Vibrational spectra are simplified for HOD in either H(2)O or D(2)O, as in these instances the OD or OH stretch, respectively, functions as a local chromophore. As a first step in providing a theoretical treatment of the vibrational spectroscopy for the fully coupled system (H(2)O or D(2)O), herein we calculate the infrared and Raman spectra for the isotopically substituted systems. The calculation involves a classical molecular dynamics simulation using a new water model, an initial proton-disordered ice configuration, and ab initio based transition frequency, dipole, and polarizability maps. Our theoretical results are in reasonable agreement with experiment, and from our results we provide molecular and physical interpretations for the spectral features.

Molecular Restructuring of Water and Lipids upon the Interaction of DNA with Lipid Monolayers

Understanding the molecular mechanism of DNA/lipid interaction is critical in optimizing the use of lipid cofactors in gene therapy. Here, we address this question by employing label-free vibrational sum frequency (VSF) spectroscopy to study the interaction of DNA with lipid monolayers of the cationic lipids DPTAP(1,2-dipalmitoyl-3-trimethylammonium-propane) and diC14-amidine as well as the zwitterionic lipid DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) in the presence and absence of calcium. Our approach has the advantage both of allowing us to explicitly probe intermolecular interactions and of providing insight into the structure of water and lipids around DNA at the lipid interface. We find, by examination of the OD stretch of interfacial D(2)O, that water structure differs markedly between systems containing DNA adsorbed to cationic and those that contain DNA adsorbed to zwitterionic lipid monolayers (in the presence or absence of Ca(2+)). The spectral response of interfacial water in the cationic system is consistent with a highly structured, undercoordinated, structural 'type' of water. Further, by investigation of CH stretch modes of the diC14-amidine lipid tails, we demonstrate that the adsorption of DNA to this lipid leads to increased ordering of lipid tails.

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.

Tunneling dynamics and spectroscopic parameters of monodeuterated hydronium, H2DO+, from a combined analysis of infrared and sub-millimeter spectra

The infrared laser direct absorption spectrum of H(2)DO(+) in the OH stretching region was reported quite recently revealing large amplitude tunneling dynamics. The large rotational constants make the jet-cooled spectrum relatively sparse at low rotational temperatures and assignments thus challenging. Transitions were assigned through ground state combination differences, with additional tentative assignments made via comparison of predicted/observed spectra. More recently, 9 rotation-inversion transitions were recorded in the sub-millimeter (sub-mm) region, which yielded tunneling splittings and rotational constants differing slightly from IR results. This has prompted the present reinvestigation of the H(2)DO(+) spectra, which now takes full advantage of the combined data from both studies. While previous analyses considered each tunneling state as independent and non-interacting, the present analysis is based on a tunneling-Hamiltonian model for the well studied, isoelectronic NH(2)D molecule, modified to account for the larger tunneling splitting. The combined analysis revealed rotational interaction between tunneling states as well as between the two OH stretching modes and permitted a substantial number of new assignments to be made, including one sub-millimeter transition while only few IR assignments had to be corrected or omitted. It leads to improvement in parameters for both the ground as well as for the OH stretching states of this important molecular ion, which reproduce the assigned lines within experimental uncertainties, provides guidance, e.g., for the spectral search in the OD stretch region, and yields deeper insight into the tunneling dynamics.

Solvent control of the soft angular potential in hydroxyl-pi 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.

Infrared Spectroscopy of Perdeuterated Protonated Water Clusters in the Vicinity of the Clathrate Cage

We report infrared predissociation spectra of size-selected D(+)(D(2)O)(n) clusters in the size range n = 18-24 for comparison to previous studies of the corresponding H(+)(H(2)O)(n) species (Shin, J.-W.; Hammer, N. I.; Diken, E. G.; Johnson, M. A.; Walters, R. S.; Jaeger, T. D.; Duncan, M. A.; Christie, R. A.; Jordon, K. D. Science 2004, 304, 1137). For n = 18-20, two "free" OD stretch bands are observed and assigned to D(2)O molecules in acceptor-acceptor-donor (AAD) and acceptor-donor (AD) hydrogen bonding arrangements. Only the AAD band is observed for the n = 21 perdeuterated species. This behavior is identical to that observed previously for the corresponding H(+)(H(2)O)(n) clusters. Similar to the all-H protonated species, the AD "free" OD stretch band is also absent for the perdeuterated n = 22 cluster but returns for clusters larger than n = 22. Like the H(+)(H(2)O)(n) systems, the perdeuterated clusters have no spectral band in the lower frequency range where the signature of the hydronium cation is predicted. These observations shed new light on the intriguing spectroscopy and dynamics of large protonated water clusters.

OD stretch vibrational relaxation of HOD in liquid to supercritical H2O

The population relaxation of the OD stretching vibration of HOD diluted in H(2)O is studied by time-resolved infrared pump-probe spectroscopy for temperatures between 278 and 663 K in the density range 0.28<or=rho<or=1.01 g/cm(3). Transient spectra recorded after exciting the v=0-->1 OD stretching transition at low temperatures show a delay between excited state decay and formation of the thermalized spectrum pointing to an intermediately populated state. Above 400 K, the rates of excited state decay and ground state recovery become equivalent and the intermediate state is not detectable anymore. Over the entire thermodynamic range, the derived OD stretch relaxation rate constant k(r) depends linearly on the static dielectric constant epsilon of water, indicating a correlation of k(r) with the average hydrogen bond connectivity of HOD within the H(2)O network. However, in contrast to the OH stretch relaxation rate constant of the complementary system of HOD in D(2)O, the low density data of k(r)(epsilon) extrapolate to a nonzero intercept for epsilon-->1. Our analysis suggests that at ambient conditions the OD excited state is mainly depopulated by a direct v=1-->0 transition, avoiding the excited v=1 HOD bending state. Therefore, at room temperature the detected intermediate is assigned to a nonthermalized state with respect to nuclear degrees of freedom of the solvent molecules, and subsequent formation of the final product spectrum is related to a rearrangement of the hydrogen bond network. Passing over to the gas phase the excited OD stretch state shifts into close resonance with the HOD bend overtone, thereby opening up an additional relaxation channel.

Water dynamics at the interface in AOT reverse micelles.

The orientational dynamics of water molecules at the interface in large Aerosol-OT (AOT) reverse micelles are investigated using ultrafast infrared spectroscopy of the OD stretch of dilute HOD in H(2)O. In large reverse micelles ( approximately 9 nm diameter or larger), a significant amount of the nanoscopic water is sufficiently distant from the interface that it displays bulk-like characteristics. However, some water molecules interact with the interface and have vibrational absorption spectra and dynamics distinct from bulk water. The different characteristics of these interfacial waters allow their contribution to the data to be separated from the bulk. The infrared absorption spectrum of the OD stretch is analyzed to show that the interfacial water molecules have a spectrum that peaks near 2565 cm(-1) in contrast to 2509 cm(-1) in bulk water. A two-component model is developed that simultaneously describes the population relaxation and orientational dynamics of the OD stretch in the spectral region of the interfacial water. The model provides a consistent description of both observables and demonstrates that water interacting with the interface has slower vibrational relaxation and orientational dynamics. The orientational relaxation of interfacial water molecules occurs in 18 +/- 3 ps, in contrast to the bulk water value of 2.6 ps.

Nonequilibrium molecular dynamics simulations of vibrational energy relaxation of HOD in D2O

Vibrational energy relaxation of HOD in deuterated water is investigated performing classical nonequilibrium molecular dynamics simulations. A flexible SPC/E model is employed to describe the intermolecular interactions and the intramolecular potential of the D(2)O solvent. A more accurate intramolecular potential is used for HOD. Our results for the OH stretch, OD stretch, and HOD bend vibrational relaxation times are 2.7, 0.9, and 0.57 ps, respectively. Exciting the OH stretching mode the main relaxation pathway involves a transition to the bending vibration. These results are in agreement with recent semiclassical Landau-Teller calculations. Contrary to this previous work, however, we observe a strong coupling of bending and OH stretching mode to the HOD rotation. As a result almost half of the total vibrational energy is transferred through the HOD rotation to the bath. At the same time the most efficient acceptor mode is the D(2)O rotation indicating the importance of resonant libration-to-libration energy transfer. We also find significant vibrational excitation of the D(2)O bending mode of the D(2)O solvent by V-V energy transfer from the HOD bending mode.

Sum-frequency vibrational spectroscopic studies of water/vapor interfaces

Phase-sensitive sum-frequency vibrational spectroscopy was used to obtain the OH and OD stretch spectra of the imaginary part of the nonlinear susceptibility, Im χ S ( 2 ) ( ω IR ) , for H 2O/vapor, D 2O/vapor, and 2:1 H 2O–D 2O/vapor interfaces that explicitly characterize their resonances. They all exhibit a positive and a negative resonance band in the bonded-OH region, clearly different from those deduced from fitting of the | χ S ( 2 ) ( ω IR ) | 2 spectra, and from MD simulations. The broad continuum is characteristic of inhomogeneously broadened resonances associated with interfacial water species of various hydrogen-bonding geometries and strengths. Spectral changes due to excess ions at the interfaces support the picture.

Vibrational sum-frequency spectroscopy of the liquid/vapor interface for dilute HOD in D2O

An electronic structure/molecular dynamics approach, originally developed to describe the vibrational spectroscopy of the OH stretch of dilute HOD in liquid D(2)O, is applied to the vibrational sum-frequency spectroscopy of the liquid/vapor interface of this system. In both cases the OH stretch is effectively decoupled from the OD stretches, allowing it to act as a local probe of structure and dynamics. A mixed quantum/classical expression for the vibrational sum-frequency response that includes the effect of motional narrowing is used to calculate the resonant susceptibility. Despite being developed for the bulk liquid, our method works well for the surface in that the real and imaginary parts of the resonant susceptibility are in good agreement with experiment. We explore the nature of hydrogen bonding at the interface as well as its impact on the sum-frequency spectrum. It is found that the spectrum is dominated by single-donor molecules with a total of two or three hydrogen bonds.