Monte Carlo simulation results are reported for clusters consisting of 250 water molecules surrounding serine, both in the neutral form and in two zwitterionic conformers (in order to gain some insight into conformational effects). Calculations were carried out at 300 K and using two-body potentials obtained by means of quantum-mechanical calculations. The spatial dependence of the average interaction energies was investigated. The solvation structure was investigated by means of radial distribution functions and probability density maps, which showed a few water molecules directly solvating the hydrophilic groups and, beyond them, a more or less rich and complex hydrogen-bonded network of solvent molecules. 相似文献
Zusammenfassung Darstellung und Eigenschaften von Ammonium-, Kalium-, Silber- und Thalliumsalzen des Triselenimides und der Diimidotriselensäure werden beschrieben, Infrarot-Spektren sowieDebye-Scherrer-Diagramme einiger dieser erstmalig dargestellten Verbindungen werden zu ihrer Charakterisierung mitgeteilt.Mit 3 Abbildungen1. Mitt.:A. Engelbrecht undF. Clementi, Mh. Chem.92, 555 (1961). 相似文献
The energy of (H2O)3 has been calculated for 29 geometrical configurations of the trimer using the SCF LCAO MO method and extended as well as minimal basis sets of Gaussian functions. For two configurations two intermediate basis sets have also been tested. The results show the nonadditive component of the interaction energy to be small. They also indicate that fairly reliable results for the trimer can be obtained using minimal basis sets and the counterpoise method to eliminate the basis set superposition error. The nonadditive contribution to the interaction energy is shown to be mainly due to the long-range induction interaction. 相似文献
Deuteriodeprotonation of some substituted 4,5-borazarothieno[2,3-c]pyridines (I) and 7,6-borazarothieno[3,2-c]pyridines (II) has been studied by the nmr technique. Exchange occurred predominantly in the 3-position, and the effect of methyl substitution on rate is discussed. The rates of exchange in some derivatives of I and II were compared with those of the isoelectronic thieno[2,3-c]pyridines (III) and thieno[3,2-c]pyridines (IV). Similar rates were obtained, confirming the aromatic nature of I and II. The deuteriodeprotonation of 4-methyl-4,5-borazaro-thieno[2,3-c]pyridine (Ie), 7-methyl-7,6-borazarothieno[3,2-c]pyridine (IIe), 4-methylthieno-[2,3-c]pyridine (IIIe), and 7-methylthieno[3,2-c]pyridine (IVe) were measured at different concentrations of deuteriosulfuric acid and different temperatures, showing that the protonated heterocycles are substrates in the deuteriodeprotonation reaction. Standard rates at p0 H and 100° were calculated for these systems. 相似文献
A procedure for elemental composition determination of water-borne river particles (Po River) on both size-fractionated and unfractionated submicron particles (0.1–1 μm) by graphite furnace atomic absorption spectroscopy (GFAAS) and inductively coupled plasma-mass spectrometry (ICP-MS) is reported. Sample fractionation was performed using sedimentation field-flow fractionation (SdFFF). The distribution of relative mass vs. particle size was determined using UV detection. Fractions were collected over a narrow size range for scanning electron microscopy. With this combination of techniques the mass, elemental composition, and shape distributions can be obtained across the size spectrum of the sample.
The size distributions of the major elements (Al, Fe) were determined by coupling both GFAAS and ICP-MS techniques to the SdFFF. The procedure was validated using a reference clay sample. Satisfactory agreement was found between both the GFAAS and ICP-MS aluminium signal and the UV detector signal. Some discrepancies were observed in the Fe/Al ratios when comparing GFAAS and ICP-MS. Thus further investigation is in order to fully assess the role of SdFFF-ICP-MS and SdFFF-GFAAS techniques for elemental characterisation of aquatic colloids. Both GFAAS and ICP-MS signals unambiguously indicate a significantly higher Fe content in the lower size range, which is consistent with previous investigations.
Trace element levels in unfractionated Po River particles, determined by both GFAAS and ICP-MS, show good agreement. The high levels of Cu, Pb, Cr and Cd found associated with the colloidal particles underlines the significance of the environmental role played by the suspended matter in rivers in both highly industrialised and intensively cultivated areas. 相似文献
We have extended to molecules a density functional previously parametrized for atomic computations. The Coulombhole-Hartree-Fock functional, introduced by Clementi in 1963, estimated the dynamic correlation energy by the computation of a Hartree-Fock type single-determinant wavefunction, where the Hartree-Fock potential was augmented with an effective potential term, related to a hard Coulomb hole enclosing each electron. The method was later revised by S. Chakravorty and E. Clementi, Phys. Rev. A, 38 (1989) 2290, so that a Yukawa-type soft Coulomb hole replaced the previous hard hole. Atomic correlation energies, computed for atoms with Z = 2 to 54, as well as for a number of excited states, validated the method. In this work we have parametrized for molecules a function which controls the width of the soft Coulomb hole by fitting the first and second atomic ionization potentials of atoms with 1 Z 18 and the binding energies of a few diatomic molecules. The parametrization was successfully validated by computing the dissociation energy for a number of molecules. A few-determinant version of the Coulomb-Hartree-Fock method (CHF-N) necessary to account for the non-dynamic correlation correction and to ensure proper dissociation products, is briefly discussed with reference to a previous proposal by G.C. Lie and E. Clementi, J. Chem. Phys., 60 (1974) 1275 and 60 (1974) 1288. 相似文献