Showing papers by "Richard J. Saykally published in 2006"
TL;DR: An overview of the nature of the interfacial structure of electrolyte solutions is presented and a detailed description of the new picture that is emerging is given.
Abstract: A qualitatively new understanding of the nature of ions at the liquid water surface is emerging Traditionally, the characterization of liquid surfaces has been limited to macroscopic experimental techniques such as surface tension and electrostatic potential measurements, wherein the microscopic picture then has to be inferred by applying theoretical models Because the surface tension of electrolyte solutions generally increases with ion concentration, all inorganic ions have been thought to be repelled from the air-water interface, leaving the outermost surface layer essentially devoid of ions This oversimplified picture has recently been challenged: first by chemical kinetics measurements, then by theoretical molecular dynamics simulations using polarizable models, and most recently by new surface sensitive experimental observations Here we present an overview of the nature of the interfacial structure of electrolyte solutions and give a detailed description of the new picture that is emerging
419 citations
TL;DR: The evaporative cooling rates of a droplet train of liquid water injected into vacuum have been studied via Raman thermometry and it is determined that the value of the evaporation coefficient (gamma(e)) ofLiquid water is 0.62 +/- 0.09, confirming that a rate-limiting barrier impedes the evAPoration rate.
Abstract: Recent theoretical and experimental studies of evaporation have suggested that on average, molecules in the higher-energy tail of the Boltzmann distribution are more readily transferred into the vapor during evaporation. To test these conclusions, the evaporative cooling rates of a droplet train of liquid water injected into vacuum have been studied via Raman thermometry. The resulting cooling rates are fit to an evaporative cooling model based on Knudsen's maximum rate of evaporation, in which we explicitly account for surface cooling. We have determined that the value of the evaporation coefficient (gamma(e)) of liquid water is 0.62 +/- 0.09, confirming that a rate-limiting barrier impedes the evaporation rate. Such insight will facilitate the formulation of a microscopic mechanism for the evaporation of liquid water.
152 citations
TL;DR: A detailed description of the experimental design and methodology used in these experiments, as well as a proof of principle experiment with the known surfactant tetrabutylammonium iodide, yielding surface enhancements in excellent agreement with surface tension measurements are presented.
Abstract: The existence of polarizable anions at the outermost layer of electrolyte solutions has received much recent attention from both theory and experiment, but remains controversial. Anions can be probed directly in the UV via their strong charge-transfer-to-solvent (CTTS) transitions. We have recently described experimental characterizations of enhanced concentrations of several anions at the air−water interface, using the surface-specific technique of second harmonic generation. Here we present a detailed description of the experimental design and methodology used in these experiments, as well as a proof of principle experiment with the known surfactant tetrabutylammonium iodide (TBAI), yielding surface enhancements in excellent agreement with surface tension measurements. Furthermore, we analyze the observed increase in the nonresonant contribution to the SHG response from the water background of alkali halide solutions. The observed change in the water structure of alkali halide (except iodide) solutions ...
149 citations
TL;DR: New results for the water dimer equilibrium constant K(p)(T) are presented, using a flexible potential energy surface fitted to spectroscopical data and an analysis of the convergence of the partition function reveals that quasi-bound states are likely to contribute to the equilibrium constant.
Abstract: We present new results for the water dimer equilibrium constant Kp(T) in the range 190−390 K, using a flexible potential energy surface fitted to spectroscopical data. The increased numerical complexity due to explicit consideration of the monomer vibrations is handled via an adiabatic (6 + 6)d decoupling between intra- and intermolecular modes. The convergence of the canonical partition function of the dimer is ensured by computing all energy levels up to dissociation for total angular momentum values J = 0−5 and using an extrapolation scheme to higher values. The newly calculated values for Kp(T) are in very good agreement with available experimental data at room temperature. At higher temperatures, an analysis of the convergence of the partition function reveals that quasi-bound states are likely to contribute to the equilibrium constant. Additional thermodynamical quantities (ΔG, ΔH, ΔS, and Cp) have also been determined and fit to quadratic expressions a + bT + cT2.
123 citations
TL;DR: Marked changes in the liquid water XAS are observed, indicating that interactions with the dissolved monovalent cations do not significantly perturb the unoccupied molecular orbitals of water molecules in the vicinity of the cations and that water-chloride interactions are primarily responsible for the observed spectral changes.
Abstract: Oxygen K-edge X-ray absorption spectra (XAS) of aqueous chloride solutions have been measured for Li+, Na+, K+, NH4+, C(NH2)3+, Mg2+, and Ca2+ at 2 and 4 M cation concentrations. Marked changes in the liquid water XAS are observed upon addition of the various monovalent cation chlorides that are nearly independent of the identity of the cation. This indicates that interactions with the dissolved monovalent cations do not significantly perturb the unoccupied molecular orbitals of water molecules in the vicinity of the cations and that water−chloride interactions are primarily responsible for the observed spectral changes. In contrast, the addition of the divalent cations engenders changes unique from the case of the monovalent cations, as well as from each other. Density functional theory calculations suggest that the ion-specific spectral variations arise primarily from direct electronic perturbation of the unoccupied orbitals due to the presence of the ions, probably as a result of differences in charge ...
123 citations
TL;DR: Calculations conclusively show that when hydrogen-bonding configurations are chosen randomly, the calculated XAS does not reproduce the experimental XAS regardless of the bonding model employed, thus casting serious doubt on the validity of the rings and chains model for liquid water.
Abstract: It was recently suggested that liquid water primarily comprises hydrogen-bonded rings and chains, as opposed to the traditionally accepted locally tetrahedral structure (Wernet et al. Science 2004, 304, 995). This controversial conclusion was primarily based on comparison between experimental and calculated X-ray absorption spectra (XAS) using computer-generated ice-like 11-molecule clusters. Here we present calculations which conclusively show that when hydrogen-bonding configurations are chosen randomly, the calculated XAS does not reproduce the experimental XAS regardless of the bonding model employed (i.e., rings and chains vs tetrahedral). Furthermore, we also present an analysis of a recently introduced asymmetric water potential (Soper, A. K. J. Phys.: Condens. Matter 2005, 17, S3273), which is representative of the rings and chains structure, and make comparisons with the standard SPC/E potential, which represents the locally tetrahedral structure. We find that the calculated XAS from both potentials is inconsistent with the experimental XAS. However, we also show the calculated electric field distribution from the rings and chains structure is strongly bimodal and highly inconsistent with the experimental Raman spectrum, thus casting serious doubt on the validity of the rings and chains model for liquid water.
100 citations
TL;DR: In this article, the authors describe a simple multiplex vibrational spectroscopic imaging technique based on employing chirped femtosecond pulses in a coherent anti-Stokes Raman scattering (CARS) scheme.
Abstract: We describe a simple multiplex vibrational spectroscopic imaging technique based on employing chirped femtosecond pulses in a coherent anti-Stokes Raman scattering (CARS) scheme. Overlap of a femtosecond Stokes pulse with chirped pump/probe pulses introduces a temporal gate that defines the spectral resolution of the technique, allowing single-shot acquisition of high spectral resolution CARS spectra over a several hundred wavenumber bandwidth. Simulated chirped (c-) CARS spectra match the experimental results, quantifying the dependence of the high spectral resolution on the properties of the chirped pulse. c-CARS spectromicroscopy offers promise as a simple and generally applicable high spatial resolution, chemically specific imaging technique for studying complex biological and materials samples.
60 citations
TL;DR: Photodissociation experiments on extensively hydrated clusters of trivalent lanthanum using resonant infrared radiation indicate that a minimum droplet size of approximately a nanometer is necessary for these trivalents species to survive intact, suggesting that elemental speciation of triValent metal ions from aqueous solutions should be possible using nanodrop mass spectrometry.
51 citations
TL;DR: The oxygen K edge X-ray absorption spectra of aqueous HCl and NaCl solutions reveal distinct perturbations of the local water molecules by the respective solutes, and indicate that the effect of monovalent cations on the nature of the unoccupied orbitals of water molecules in the first solvation shell is negligible, in contrast to the large effects ofmonovalent anions.
Abstract: The oxygen K edge X-ray absorption spectra of aqueous HCl and NaCl solutions reveal distinct perturbations of the local water molecules by the respective solutes. While the addition of NaCl leads to large spectral changes, the effect of HCl on the observed X-ray absorption spectrum is surprisingly small. Density functional theory calculations suggest that this difference primarily reflects a strong blue shift of the hydrated proton (in either the Eigen (H9O4+) or Zundel (H2O5+) forms) spectrum relative to that of H2O, indicating the tighter binding of electrons in H3O+. This spectral shift counteracts the spectral changes that arise from direct electrostatic perturbation of water molecules in the first solvation shell of Cl-. Consequently, the observed spectral changes effected by HCl addition are minimal compared to those engendered by NaCl. Additionally, these results indicate that the effect of monovalent cations on the nature of the unoccupied orbitals of water molecules in the first solvation shell is negligible, in contrast to the large effects of monovalent anions.
45 citations
TL;DR: In this paper, the authors used fluorescence correlation spectroscopy (FCS) to investigate the hydrodynamic and photophysical properties of PR1 (phytofluor red 1), an intensely red fluorescent biliprotein variant of the truncated cyanobacterial phytochrome 1 (Cph1Δ), which consists of the N-terminal 514 amino acids.
Abstract: Fluorescence correlation spectroscopy (FCS) was used to investigate the hydrodynamic and photophysical properties of PR1 (phytofluor red 1), an intensely red fluorescent biliprotein variant of the truncated cyanobacterial phytochrome 1 (Cph1Δ, which consists of the N-terminal 514 amino acids). Single-molecule diffusion measurements showed that PR1 has excellent fluorescence properties at the single-molecule level, making it an interesting candidate for red fluorescent protein fusions. FCS measurements for probing dimer formation in solution over a range of protein concentrations were enabled by addition of Cph1Δ apoprotein (apoCph1Δ) to nanomolar solutions of PR1. FCS brightness analysis showed that heterodimerization of PR1 with apoCph1Δ altered the chemical environment of the PR1 chromophore to further enhance its fluorescence emission. Fluorescence correlation measurements also revealed interactions between apoCph1Δ and the red fluorescent dyes Cy5.18 and Atto 655 but not Alexa Fluor 660. The concentration dependence of protein:dye complex formation indicated that Atto 655 interacted with, or influenced the formation of, the apoCph1 dimer. These studies presage the utility of phytofluor tags for probing single-molecule dynamics in living cells in which the fluorescence signal can be controlled by the addition of various chromophores that have different structures and photophysical properties, thereby imparting different types of information, such as dimer formation or the presence of open binding faces on a protein.
38 citations
TL;DR: In this article, the authors report the measurement and analysis of the complete perpendicular kn−21−21 −00 (D2O)3 torsional band (origin 2940.9376(3) GHz), the upper state of which is the highest energy (98.09912 ǫ cm−1) torsion state yet observed.
TL;DR: The terahertz vibration-rotation-tunneling (VRT) spectrum of the ammonia dimer (NH(3))(2) has been measured and transitions have been assigned for VRT states of the A-A (ortho-ortho) combinations of NH(3) monomer states.
Abstract: The terahertz vibration-rotation-tunneling (VRT) spectrum of the ammonia dimer (NH(3))(2) has been measured between ca. 78.5 and 91.9 cm(-1). The dipole-allowed transitions are separated into three groups that correspond to the 3-fold internal rotation of the NH(3) subunits. Transitions have been assigned for VRT states of the A-A (ortho-ortho) combinations of NH(3) monomer states. The spectrum is further complicated by strong Coriolis interactions. K = 0 <-- 0, K = 1 <-- 0, K = 0 <-- 1, and K = 1 <-- 1 progressions have been assigned. The band origins, rotational constants, asymmetry doubling, centrifugal distortion, and Coriolis coupling constant have been determined from the fit to an effective Hamiltonian. These VRT transitions are tentatively assigned to an out of plane vibration with a K = 0 state at 89.141305(47) cm(-1), and a K = 1 state at 86.77785(9) cm(-1).
TL;DR: The use of evanescent wave cavity ringdown spectroscopy (e-CRDS) to measure the pH of the silica -water interface using a molecular probe tethered to the surface using a large and charged organic probe is described.
Abstract: The recent Communication “Interfacial pH at an Isolated Silica-Water Surface” published in the Journal of the American Chemical Society(O’Reilly, J. P.; Butts, C. P.; I’Anson, I. A.; Shaw, A. M.J. Am. Chem. Soc. 2005, 127, 1632) describes the use of evanescent wave cavity ringdown spectroscopy (e-CRDS) to measure the pH of the silica -water interface using a molecular probe tethered to the surface. The communication reports two major findings: The observation of a surface pH of two units lower than the bulk at high pH and the existence of a stable charged near-surface layer that is not disrupted upon lowering the pH to 1. Although the experiment constitutes a clever approach to the problem, the authors neglect several aspects of the experiment and overlook inherent problems with the indirect approach of using a molecular probe to detect the interfacial pH. The authors wish to probe the pH at the neat water -silica interface and state that “the tethered chromophore is only sensitive to the pH in the local environment within 1 nm of the surface.” More precisely, the dye molecule is only sensitive to the local environment around the chromophore itself (note that the probe molecule is larger than 1 nm). This implies that the probe actually will not measure the properties of the neat surface, but rather the properties of the interface perturbed by the chromophore. Since this is a local effect, the perturbation is independent of the surface concentration of the chromophore. Depending on the nature of the interface and the specific probe used, the degree of perturbation of the local interfacial structure will vary. Whereas Steel and Walker have successfully used small uncharged molecular probes to probe the structure of liquid-liquid interfaces, 1 the large and charged organic probe employed in the present study would probably have a significant perturbing effect on the water-silica interface and is thus probing the local region around the silica -dye-water interface. The effect of the interface on the molecular probe itself is likewise important in these kinds of experiments and needs to be accounted for. Consider the generic acid -base equilibrium
01 Jul 2006
TL;DR: These studies presage the utility of phytofluor tags for probing single-molecule dynamics in living cells in which the fluorescence signal can be controlled by the addition of various chromophores that have different structures and photophysical properties, thereby imparting different types of information, such as dimer formation or the presence of open binding faces on a protein.
Abstract: Fluorescence correlation spectroscopy (FCS) was used to investigate the hydrodynamic and photophysical properties of PR1 (phytofluor red 1), an intensely red fluorescent biliprotein variant of the truncated cyanobacterial phytochrome 1 (Cph1 Delta, which consists of the N-terminal 514 amino acids). Single-molecule diffusion measurements showed that PR1 has excellent fluorescence properties at the single-molecule level, making it an interesting candidate for red fluorescent protein fusions. FCS measurements for probing dimer formation in solution over a range of protein concentrations were enabled by addition of Cph1 Delta apoprotein (apoCph1 Delta) to nanomolar solutions of PR1. FCS brightness analysis showed that heterodimerization of PR1 with apoCph1 Delta altered the chemical environment of the PR1 chromophore to further enhance its fluorescence emission. Fluorescence correlation measurements also revealed interactions between apoCph1 Delta and the red fluorescent dyes Cy5.18 and Atto 655 but not Alexa Fluor 660. The concentration dependence of protein:dye complex formation indicated that Atto 655 interacted with, or influenced the formation of, the apoCph1 dimer. These studies presage the utility of phytofluor tags for probing single-molecule dynamics in living cells in which the fluorescence signal can be controlled by the addition of various chromophores that have different structures and photophysical properties, thereby imparting different types of information, such as dimer formation or the presence of open binding faces on a protein.
01 Jan 2006
TL;DR: In this paper, the three-dimensional intermolecular potential energy surface (IPS) for Ar-NH was determined from a least-squares fit to 61 far infrared and microwave vibration-rotation-tunneling (VRT) measurements and to temperature-dependent second virial coefficients.
Abstract: The three-dimensional intermolecular potential energy surface (IPS) for Ar-NH, has been determined from a least-squares fit to 61 far infrared and microwave vibration-rotation-tunneling (VRT) measurements and to temperature-dependent second virial coefficients. The three intermolecular coordinates (R ,19,+) are treated without invoking any approximations regarding their separability, and the NH, inversion-tunneling motion is included adiabatically. A surface with 13 variable parameters has been optimized to accurately reproduce the spectroscopic observables, using the collocation method to treat the coupled multidimensional dynamics within a scattering formalism. Anisotropy in the IPS is found to significantly mix the free rotor basis functions. The 149.6 cm-’ global minimum on this surface occurs with the NH, symmetry axis nearly perpendicular to the van der Waals bond axis (@=96.6”), at a center-of-mass separation of 3.57 A, and with the Ar atom midway between two of the NH3 hydrogen atoms (+=60”). The position of the global minimum is very different from the center-of-mass distance extracted from microwave spectroscopic studies. Long-range (R > 3.8 A) attractive interactions are greatest when either a N-H bond or the NH, lone pair is directed toward the argon. Comparisons with ab initio surfaces for this molecule as well as the experimentally determined IPS for Ar-Hz0 are presented.