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71.
Conductance measurements of 12 quaternary ammonium salts in propylene carbonate (PC) have been made at 25°C. The cations were either tetramethylammonium, tetraethylammonium, or tetra-n-butylammonium, and the concentrations of salt varied from about 2×10–4 to 5×10–3
M. The data were analyzed by the equation of Pitts. The results showed that the benzoate, nitrobenzoate, and pentachlorophenolate salts are completely dissociated. The nitrophenolate, chlorophenolate, methylsulfonate, and nitrate salts are only slightly associated (K
A
from 2.5 to 6.5), while the acetate, phenylacetate, and nitrophenolate salts display somewhat more extensive association, with ionpair association constants from 17 to 45. Limiting molar conductances for the anions were derived. The factors affecting ionic mobilities in this dipolar aprotic solvent are discussed.On leave 1973–1975 from the University of Gdask, Poland. 相似文献
72.
73.
A J Bates I J Galpin A Hallett D Hudson G W Kenner R Ramage R C Sheppard 《Helvetica chimica acta》1975,58(3):688-696
The products 1 and 2 (X = OTs) have been isolated from the reaction between hexamethylphosphortriamide and p-toluene sulfonic anhydride and the latter converted into μ-oxo-bis-[tris-(dimethylamino)-phosphonium]-bis-tetrafluoroborate 2 (X = BF4). This is a practical reagent for the formation of the peptide link. Where racemisation is possible via oxazolone formation this can be decreased by the addition of 1-hydroxybenzotriazole or N-hydroxysuccinimide. These additives may also increase the efficiency of condensation at glycine and proline residues. 相似文献
74.
75.
Resonance Raman scattering has been observed from metastable O2 molecules produced in single crystals of NaClO3 by γ-irradiation at 300 K. Evidence that the observed bands are due to O2 is provided by the Raman spectrum of irradiated 18O enriched NaClO3 in which bands due to 16O2, 16O 18O, and 18O2 were identified. The Raman band at 1544 cm?1 ascribed to metastable O2 disappears on bleaching with intense 4880 Å radiation enabling the identification of a weaker band at 1557 cm?1 that is assigned to the stable form of O2. 相似文献
76.
77.
Thai A Skrobol C Bates PK Arisholm G Major Z Krausz F Karsch S Biegert J 《Optics letters》2010,35(20):3471-3473
We present three-dimensional simulations of optical-parametric chirped-pulse amplification stages for a few-cycle petawatt-class laser. The simulations take into account the effects of depletion, diffraction, walk-off, quantum noise, and the nonlinear refractive index (n(2)). In the absence of n(2) effects, we show these stages can generate 3.67J pulses supporting 4fs transform-limited pulse durations. Adding the nonlinear refractive index to the simulation, the energy output is reduced by ~11% and the bandwidth narrows by ~129nm, increasing the Fourier limit by ~17.5%. 相似文献
78.
Hong Lu Peter W. Bates Wenping Chen Mingji Zhang 《Advances in Computational Mathematics》2018,44(3):861-878
We derive a spectral collocation approximation to the fractional Laplacian operator based on the Riemann-Liouville fractional derivative operators on a bounded domain Ω = [a, b]. Corresponding matrix representations of (?△) α/2 for α ∈ (0,1) and α ∈ (1,2) are obtained. A space-fractional advection-dispersion equation is then solved to investigate the numerical performance of this method under various choices of parameters. It turns out that the proposed method has high accuracy and is efficient for solving these space-fractional advection-dispersion equations when the forcing term is smooth. 相似文献
79.
Whitfield TW Martyna GJ Allison S Bates SP Vass H Crain J 《The journal of physical chemistry. B》2006,110(8):3624-3637
The results of classical molecular dynamics (MD) simulations and Raman spectroscopy studies of neat liquid N-methylacetamide (NMA), the simplest model system relevant to the peptides, are reported as a function of temperature and pressure. The MD simulations predict that near ambient conditions, the molecules form a hydrogen bond network consisting primarily of linear chains. Both the links between molecules within the hydrogen-bonded chains and the associations between chains are stabilized by weak methyl-donated "improper" hydrogen bonds. The three-dimensional structural motifs observed in the liquid show some similarity to protein beta-sheets. The temperature and pressure dependence of the hydrogen bond network, as probed by the mode frequency of the experimentally determined amide-I Raman band, blue shifts on heating and red shifts under compression, respectively, suggesting weakened and enhanced hydrogen bonding in response to temperature and pressure increases. Disruption of the hydrogen-bonding network is clearly observed in the simulation data as temperature is increased, whereas the improper hydrogen bonding is enhanced under compression to reduce the energetic cost of increasing the packing fraction. Because of the neglect of polarizability in the molecular model, the computed dielectric constant is underestimated compared to the experimental value, indicating that the simulation may underestimate dipolar coupling in the liquid. 相似文献
80.
We present the results of ab initio calculations of the structural, electronic and lattice dynamical properties of the solid-state crystal of the glycyl-l-alanine dipeptide. Intramolecular bond lengths are found to be in good agreement with experimental values; lattice constants are in reasonable agreement, although it is found that discrepancies do exist. A hierarchy of hydrogen bond strengths is found, with those between (oppositely-charged) amine and carboxy functional groups being strongest. The crystal is found to be an indirect-bandgap material, with indirect bandgaps ≈4.95 eV, compared to a direct bandgap of 5.00 eV. Analysis of the electronic structure reveals that the electronic states in the near vicinity of the energy gap arise from carboxylate and amide oxygen atoms. The arrangement of both molecules and hydrogen bonds in the unit cell is found to manifest itself in increased bandwidth along specific reciprocal space directions, reflecting coupling brought about by hydrogen bonds. Determination of the zone-centre lattice dynamical behaviour permits the IR absorption spectrum to be explained. Intermolecular hydrogen bonds are found to couple intramolecular motions in adjacent moelcules, revealing the importance of an accurate treatment of intermolecular interactions, even for high-frequency vibronic modes. 相似文献