[Show abstract][Hide abstract] ABSTRACT: X-ray absorption spectra of aqueous 4 and 6 M potassium hydroxide solutions have been measured near the oxygen K edge. Upon addition of KOH to water, a new spectral feature (532.5 eV) emerges at energies well below the liquid water pre-edge feature (535 eV) and is attributed to OH- ions. In addition to spectral changes explicitly due to absorption by solvated OH- ions, calculated XA spectra indicate that first-solvation-shell water molecules exhibit an absorption spectrum that is unique from that of bulk liquid water. It is suggested that this spectral change results primarily from direct electronic perturbation of the unoccupied molecular orbitals of first-shell water molecules and only secondarily from geometric distortion of the local hydrogen bond network within the first hydration shell. Both the experimental and the calculated XA spectra indicate that the nature of the interaction between the OH- ion and the solvating water molecules is fundamentally different than the corresponding interactions of aqueous halide anions with respect to this direct orbital distortion. Analysis of the Mulliken charge populations suggests that the origin of this difference is a disparity in the charge asymmetry between the hydrogen atoms of the solvating water molecules. The charge asymmetry is induced both by electric field effects due to the presence of the anion and by charge transfer from the respective ions. The computational results also indicate that the OH- ion exists with a predominately "hyper-coordinated" solvation shell and that the OH- ion does not readily donate hydrogen bonds to the surrounding water molecules.
Full-text · Article · Jul 2007 · The Journal of Physical Chemistry A
[Show abstract][Hide abstract] 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.
Full-text · Article · Nov 2006 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] 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.
Full-text · Article · Apr 2006 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: Oxygen K-edge X-ray absorption spectra (XAS) of aqueous chloride solutions have been measured for Li(+), Na(+), K(+), NH(4)(+), C(NH(2))(3)(+), Mg(2+), and Ca(2+) 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 transfer from the water molecules onto the divalent cations.
Full-text · Article · Apr 2006 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] 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.
Full-text · Article · Feb 2006 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: The nitrogen K-edge spectra of aqueous proline and diglycine solutions have been measured by total electron yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at neutral and high pH. All observed spectral features have been assigned by comparison to the recently reported spectrum of aqueous glycine and calculated spectra of isolated amino acids and hydrated amino acid clusters. The sharp preedge resonances at 401.3 and 402.6 eV observed in the spectrum of anionic glycine indicate that the nitrogen terminus is in an "acceptor-only" configuration, wherein neither amine proton is involved in hydrogen bonding to the solvent, at high pH. The analogous 1s --> sigma(NH) preedge transitions are absent in the NEXAFS spectrum of anionic proline, implying that the acceptor-only conformation observed in anionic glycine arises from steric shielding induced by free rotation of the amine terminus about the glycine CN bond. Anionic diglycine solutions exhibit a broadened 1s --> pi(CN) resonance at 401.2 eV and a broad shoulder resonance at 403 eV, also suggesting the presence of an acceptor-only species. Although this assignment is not as unambiguous as for glycine, it implies that the nitrogen terminus of most proteins is capable of existing in an acceptor-only conformation at high pH. The NEXAFS spectrum of zwitterionic lysine solution was also measured, exhibiting features similar to those of both anionic and zwitterionic glycine, and leading us to conclude that the alpha amine group is present in an acceptor-only configuration, while the end of the butylammonium side chain is fully solvated.
Full-text · Article · Nov 2005 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: Measurements of the oxygen K-edge X-ray absorption spectrum (XAS) of aqueous sodium halide solutions demonstrate that ions significantly perturb the electronic structure of adjacent water molecules. The addition of halide salts to water engenders an increase in the preedge intensity and a decrease in the postedge intensity of the XAS, analogous to those observed when increasing the temperature of pure water. The main-edge feature exhibits unique behavior and becomes more intense when salt is added. Density functional theory calculations of the XAS indicate that the observed red shift of the water transitions as a function of salt concentration arises from a strong, direct perturbation of the unoccupied molecular orbitals on water by anions, and does not require significant distortion of the hydrogen bond network beyond the first solvation shell. This contrasts the temperature-dependent spectral variations, which result primarily from intensity changes of specific transitions due to geometric rearrangement of the hydrogen bond network.
Full-text · Article · May 2005 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: The carbon, nitrogen, and oxygen K-edge spectra were measured for aqueous solutions of glycine by total electron yield near-edge X-ray absorption fine structure (TEY NEXAFS) spectroscopy. The bulk solution pH was systematically varied while maintaining a constant amino acid concentration. Spectra were assigned through comparisons with both previous studies and ab initio computed spectra of isolated glycine molecules and hydrated glycine clusters. Nitrogen K-edge solution spectra recorded at low and moderate pH are nearly identical to those of solid glycine, whereas basic solution spectra strongly resemble those of the gas phase. The carbon 1s --> pi*(C=O) transition exhibits a 0.2 eV red shift at high pH due to the deprotonation of the amine terminus. This deprotonation also effects a 1.4 eV red shift in the nitrogen K-edge at high pH. Two sharp preedge features at 401.3 and 402.5 eV are also observed at high pH. These resonances, previously observed in the vapor-phase ISEELS spectrum of glycine, have been reassigned as transitions to sigma* bound states. The observation of these peaks indicates that the amine moiety is in an acceptor-only hydrogen bond configuration at high pH. At low pH, the oxygen 1s --> pi*(C=O) transition exhibits a 0.25-eV red shift due to the protonation of the carboxylic acid terminus. These spectral differences indicate that the variations in electronic structure observed in the NEXAFS spectra are determined by the internal charge state and hydration environment of the molecule in solution.
Full-text · Article · Mar 2005 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: A strong temperature dependence of oxygen K-edge x-ray absorption fine structure features was observed for supercooled and normal liquid water droplets prepared from the breakup of a liquid microjet. Analysis of the data over the temperature range 251 to 288 kelvin (-22 degrees to +15 degrees C) yields a value of 1.5 +/- 0.5 kilocalories per mole for the average thermal energy required to effect an observable rearrangement between the fully coordinated ("ice-like") and distorted ("broken-donor") local hydrogen-bonding configurations responsible for the pre-edge and post-edge features, respectively. This energy equals the latent heat of melting of ice with hexagonal symmetry (ice Ih) and is consistent with the distribution of hydrogen bond strengths obtained for the "overstructured" ST2 model of water.
[Show abstract][Hide abstract] ABSTRACT: radiation by cloud droplets that is several W/m2 above the linear direct absorption process. Mie scattering calculations performed at the D x=0 .1 (x =2 pr/l) resolution typically implemented in cloudy sky radiative transfer models are shown to be insufficient for accurate determination of the attenuation of SW radiation when considered over relatively narrow wavelength ranges, consistent with the recent finding of Nussenzveig (2003). However, for broadband calculations we find positive and negative errors in Mie calculations at D x=0 .1 nearly cancel resulting in reasonable estimates of SW attenuation. INDEX TERMS: 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 3359 Meteorology and Atmospheric Dynamics: Radiative processes. Citation: Cappa, C. D., K. R. Wilson, B. M. Messer, R. J. Saykally, and R. C. Cohen (2004), Optical cavity resonances in water micro-droplets: Implications for shortwave cloud forcing, Geophys. Res. Lett., 31, L10205, doi:10.1029/2004GL019593.
Full-text · Article · May 2004 · Geophysical Research Letters
[Show abstract][Hide abstract] ABSTRACT: The influence of narrow optical resonances, which result from trapping of light rays via total internal reflection in water droplets, on the absorption of shortwave (SW) solar radiation has been characterized through high resolution Mie scattering calculations. Our results indicate that these resonances engender an increase in absorption of solar radiation by cloud droplets by several W/m2 above the linear direct absorption process in the range 0.3-1.1 mu m. This work suggests that Mie scattering calculations performed at the 0.1x (x = 2pi r/lambda ) resolution typically implemented in cloudy sky radiative transfer models may not be sufficient to correctly model SW absorption, consistent with the recent finding of Nussenveig . Stimulated Raman scattering experiments on pure water microdroplets yield cavity enhancements in general agreement with the Mie theory results.
[Show abstract][Hide abstract] ABSTRACT: We report the results of computational and experimental studies concerning the chemical ionization mass spectrometric detection of hydrogen peroxide (HOOH) and methyl hydroperoxide (CH3OOH). GAUSSIAN2 (G2) electronic structure calculations are used to predict structures, natural charges of the atoms and energies for the neutral species, as well as for the cation, anion, and the proton and fluoride adduct species. These calculations are used to predict ion–molecule reaction thermodynamics as a guide to the experimental development of chemical ionization mass spectrometric detection methods. Both HOOH and CH3OOH are predicted to react exothermically with O2+ and F− to yield the cationic and fluoride adduct species, respectively. In addition, CH3OOH is predicted to react exothermically with H3O+ to yield the proton adduct species. The feasibility of F− chemical ionization mass spectrometric detection of peroxides was experimentally explored through kinetic studies. The fluoride adduct formation reactions for both HOOH and CH3OOH were found to proceed at or near collision-limited rates.
Preview · Article · Feb 2000 · International Journal of Mass Spectrometry
[Show abstract][Hide abstract] ABSTRACT: The overall rate constant and an upper limit for the CH 3 ONO 2 product channel for the CH 3 O 2 + NO reaction have been measured using the turbulent flow technique with high-pressure chemical ionization mass spectrometry for the detection of reactants and products. At room temperature and 100 Torr pressure, the rate constant (and the two standard deviation error limit) was determined to be (7.8 (2.2) × 10 -12 cm 3 molecule -1 s -1 . The temperature dependence of the rate constant was investigated between 295 and 203 K at pressures of either 100 or 200 Torr, and the data was fit to the following Arrhenius expression: (9.2 -3.9 +6.0 × 10 -13) exp[(600 (140)/T] cm 3 molecule -1 s -1 . Although the room-temperature rate constant value agrees well with the current recommendation for atmospheric modeling, our values for the rate constant at the lowest temperatures accessed in this study (203 K) are about 50% higher than the same recommendation. No CH 3 -ONO 2 product was detected from the CH 3 O 2 + NO reaction (using direct CH 3 ONO 2 detection methods for the first time), but an improved upper limit of 0.03 (at 295 K and 100 torr) for this branching channel was determined.
Preview · Article · Jun 1999 · The Journal of Physical Chemistry A
[Show abstract][Hide abstract] ABSTRACT: We report the results of a systematic Gaussian2 ab initio study of the ROX (R=H, CH3; X=F, Cl, Br) series. The calculated standard enthalpies of formation (ΔHf298K) provide the following estimates for the previously undetermined R=CH3 series; ΔHf=−94.9, −74.0, and −57.0 kJ mol−1 for X=F, Cl, and Br, respectively. The calculated ionization potentials (IP) provide an estimate of 10.24 eV for the previously undetermined IP of CH3OBr. The first determination of fluoride affinities for ROX species are presented and are shown to depend strongly on the orientation of the F−+ROX complex and on the identity of the halogen substituent.
Preview · Article · Feb 1999 · Chemical Physics Letters