Vibrational sum-frequency spectroscopy (VSFS) studies of a series of HOD/H2O/D2O mixtures ranging from pure D2O to pure H2O have been performed at the vapor/water interface. The various ...concentrations allow an iterative fitting procedure to be applied, resulting in a set of resonant peaks which consistently describe the vibrational modes of water molecules present in the interfacial region. The resonant sum-frequency response from the contributing vibrational modes allows more definitive characterization than in previous studies of the bonding interactions between surface water molecules. Comparison of the resonant spectrum of the vapor/H2O interface with the sum-frequency spectrum obtained at the CCl4/H2O interface reveals more similarity between the interfacial hydrogen-bonding environments than previously determined. Recent molecular dynamics simulations of VSF spectra of the vapor/H2O interface are in good agreement with the experimentally obtained spectra, and give insight into the molecular interactions in the interfacial region, as well as an estimate of the interfacial depth probed.
The vibrational spectra of interfacial water at a series of alkane/water interfaces have been measured using vibrational sum-frequency spectroscopy. The OH stretching modes of water have been used to ...characterize the water−water and alkane−water interactions present at these hydrophobic/aqueous interfaces; these results are then compared with previous studies of the CCl4/H2O interface. The results for all alkanes examined are similar and have general spectral characteristics that coincide with the CCl4/H2O interface. All spectra show weaker hydrogen bonding interactions than have been observed for the vapor/water interface. Molecular dynamics calculations are used to complement the experimental results in obtaining a more complete picture of the interfacial interactions.
Vibrational sum frequency (VSF) spectroscopy has been increasingly used in recent years to measure the vibrational spectroscopy of molecules at surfaces. Some of the more important systems examined ...in such studies involve the surface of liquid water. Unfortunately, obtaining spectral fits to vibrational spectra acquired in these studies can be challenging. The difficulty arises from the wide range of contributing vibrational modes, the breadth of the spectral peaks for hydrogen-bonded water molecules, and the complex interference effects that can arise between adjacent vibrational modes because of the coherent nature of the sum frequency process. In this paper, we describe a detailed method for obtaining spectral fits to such VSF data that takes into account a range of water species present at a water surface and the possible interference between these contributing modes. The relationship between the spectral information derived and the molecular orientation is also given. This analysis is applied to two cases, VSF spectra of water measured at the CCl4/H2O interface and at the air/H2O interface.
The 1,2-dichloroethane (DCE)/water interface is important because of its suitability as an electrochemical interface between two immiscible electrolyte solutions (ITIES). An issue of particular ...interest is whether the interfacial region is molecularly sharp or whether the interface is comprised of a diffuse mixed interfacial region. These studies using vibrational sum-frequency spectroscopy as a probe of the structure, orientation, and bonding of interfacial water show that the interface is molecularly disordered with properties similar to a mixed phase interfacial region. It does not have the characteristics of a sharp interface that have been shown to occur at other liquid/liquid interfaces such as CCl4/H2O and alkane/H2O.
A vibration−rotation-tunneling (VRT) spectrum of (H2O)4 has been recorded near 2.04 THz (67.9 cm-1). The band origin of this ΔK = 0 symmetric rotor spectrum is shifted by 0.1 cm-1 to the blue of that ...of a (D2O)4 band reported previously Science, 1996, 271, 59. Similar to that spectrum, each transition in the (H2O)4 spectrum exhibits a regular doublet splitting. The (D2O)4 and (H2O)4 doublet spacings, 5.6 and 2260 MHz, respectively, are constant as a function of rotation, indicative of degenerate tunneling splittings. They have been rationalized in terms of a concerted flipping motion of the H atoms that do not participate in hydrogen bonding in the S 4 equlibrium structure, analogous to ammonia inversion. Both bands have been assigned to the lowest Ag→ Bg intermolecular vibration in the molecular symmetry group C 4 h (M). Additional K = 2 splittings, which increase nonlinearly with increasing J, were found in both D2O and H2O tetramer spectra. Evidence for the bifurcation tunneling motion observed in the water trimer VRT data, but not in evidence for (D2O)4, was found in the (H2O)4 spectrum in the form of broadened lines probably containing unresolved bifurcation tunneling splittings. Structural estimates from the data indicate that the interoxygen separations in both (D2O)4 and (H2O)4 are consistent with an exponential contraction in that parameter as a function of increasing cluster size.
Resonance Raman spectroelectrochemistry (RR-SEC) at −20 °C has been performed on the pyrazine-bridged dimer of μ-oxo-centered trinuclear ruthenium−acetate “clusters” ...(dmap)(CO)(μ-OAc)6(μ3-O)Ru3(μ-Lb)Ru3(μ3-O)(μ-OAc)6(CO)(dmap) n (where dmap = 4-(dimethylamino)pyridine and Lb = pyrazine-h 4 and pyrazine-d 4)in three oxidation states: n = 0, −1, and −2. In the one-electron reduced, “mixed-valent” state (overall −1 charge and a single odd electron; formal oxidation states II, II, III−III, III, II on the metal centers), the Raman excitation at 800 nm is resonant with a cluster-to-cluster intervalence charge-transfer (IVCT) band. Under these conditions, scattering enhancement is observed for all four totally symmetric vibrational modes of the bridging pyrazine ligand (ν8a, ν9a, ν1, and ν6a) in the investigated spectral range (100−2000 cm-1), and there is no evidence of activity in non-totally symmetric vibrations. Resonantly enhanced Raman peaks related to peripheral pyridyl (dmap) ligand modes and low-frequency features arising from the trigonal Ru3O cluster core and the clusterRu−Nligand vibrations were also observed in the spectra of the intermediate-valence (n = −1) cluster dimer. The vibrational assignments and interpretations proposed in this work were reinforced by observation of characteristic isotopic frequency shifts accompanying deuteration of the bridging pyrazine. The results reveal that the fully symmetric (A g) vibrational motions of the organic bridge are coupled to the nominally metal cluster-to-metal cluster fast intramolecular electron transfer (ET) and provide validation of the near-delocalized description according to a predicted three-site/three-state (e.g., metal−bridge−metal) vibronic coupling model, in which the important role of the bridging ligand in mediating electronic communication and delocalization between charge centers is explicitly considered. Further compelling evidence supporting an extended five-state model, which incorporates the peripheral cluster-bound pyridyl ligands, is also presented.
The novel charge-transfer ground state found in α,α‘-diimine adducts of ytterbocene (C5Me5)2Yb(L) L = 2,2‘-bipyridine (bpy) and 1,10-phenanthroline (phen) in which an electron is spontaneously ...transferred from the f14 metal center into the lowest unoccupied (π*) molecular orbital (LUMO) of the diimine ligand to give an f13−L•- ground-state electronic configuration has been characterized by cyclic voltammetry, UV−vis−near-IR electronic absorption, and resonance Raman spectroscopies. The voltammetric data demonstrate that the diimine ligand LUMO is stabilized and the metal f orbital is destabilized by ∼1.0 V each upon complexation for both bpy and phen adducts. The separation between the ligand-based oxidation wave (L0/-) and the metal-based reduction wave (Yb3+/2+) in the ytterbocene adducts is 0.79 V for both bpy and phen complexes. The UV−vis−near-IR absorption spectroscopic data for both the neutral adducts and the one-electron-oxidized complexes are consistent with those reported recently, but previously unreported bands in the near-IR have been recorded and assigned to ligand (π*)-to-metal (f orbital) charge-transfer (LMCT) transitions. These optical electronic excited states are the converse of the ground-state charge-transfer process (e.g., f13−L•- ↔ f14−L0). These new bands occur at ∼5000 cm-1 in both adducts, consistent with predictions from electrochemical data, and the spacings of the resolved vibronic bands in these transitions are consistent with the removal of an electron from the ligand π* orbital. The unusually large intensity observed in the f → f intraconfiguration transitions for the neutral phenanthroline adduct is discussed in terms of an intensity-borrowing mechanism involving the low-energy LMCT states. Raman vibrational data clearly reveal resonance enhancement for excitation into the low-lying π* → π* ligand-localized excited states, and comparison of the vibrational energies with those reported for alkali-metal-reduced diimine ligands confirms that the ligands in the adducts are reduced radical anions. Differences in the resonance enhancement pattern for the modes in the bipyridine adduct with excitation into different π* → π* levels illustrate the different nodal structures that exist in the various low-lying π* orbitals.
Resonance Raman spectroscopy, performed using spectroelectrochemistry and with excitation in the intervalence bands of three pyrazine-bridged, mixed-valence dimers of trinuclear ruthenium clusters, ...shows resonant enhancement of symmetric bridging ligand modes. The resonant enhancements and frequency shifts of these bridging ligand modes are observed as a function of varying electronic communication between charge sites, and they show that a three-state vibronic model which explicitly includes the participation of the bridging ligand is needed to explain the spectroscopic behavior of these near-delocalized complexes.
We report the first observation of five water trimer isotopomers using terahertz laser spectroscopy coupled with a pulsed slit jet expansion technique. A single c-type vibration−rotation-tunneling ...(VRT) band has been observed for each isotopomer between 40 and 50 cm-1. By considering all the experimental data, including results from isotopic substitution experiments, analyses of bifurcation tunneling splittings, and rotational analyses of the VRT bands, it has been possible to determine unambiguously the isotopic composition and structure of each isotopomer. We have also extended the measurements of the 41.1 cm-1 band in (D2O)3, first reported by Suzuki and Blake Suzuki, S.; Blake, G. A. Chem. Phys. Lett. 1994, 229, 499. All six VRT bands have been assigned to pseudorotational transitions, which principally involve flipping of the “free” hydrogen or deuterium atoms in the trimer from above to below the plane of the OOO ring. Interestingly, four of the six bands have been assigned to the same transition. The four trimers responsible for these bands, designated d6, d5a, d4a, and d3a, form a series in which the three deuterium atoms in the ring are sequentially substituted by hydrogen atoms. The corresponding experimental band origins for d6 to d3a show a 1−2 cm-1 blue shift upon each substitution. Existing pseudorotation models are unable to satisfactorily account for this result. We propose that, if the hydrogen or deuterium atoms within the ring are allowed to participate in the flipping motion, this band origin shift can be explained.
The effect of exciting each of the three classes of intermolecular vibrations on the hydrogen bond lifetime (τH) of the isolated water trimer is investigated by far-infrared laser spectroscopy. ...Single excitation of a librational vibration decreases τH by 3 orders of magnitude to τH = 1−6 ps, comparable to the time scale of a number of important bulk water dynamical relaxation processes. In contrast, excitation of translational or torsional vibrations has no significant effect (τH = 1−2 ns). Although such a dependence of τH on intermolecular motions has also been proposed for liquid water via computer simulations, these are the first experiments that provide a detailed molecular picture of the respective motions without extensive interpretation.
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