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31.
32.
Deborah C. Bebout James F. Bush II Elizabeth M. Shumann Julie A. Viehweg Margaret E. Kastner Damon A. Parrish Steven M. Baldwin 《Journal of chemical crystallography》2003,33(5-6):457-463
The dimeric mercurous ion has been encapsulated by a pair of the tetradentate tripodal nitrogen ligands tris[(2-(6-methylpyridyl))methyl]amine (TLA). The complex [Hg2(TLA)2](ClO4)2 (1) was isolated directly from an acetonitrile solution of Hg(ClO4)2 3H2O and TLA. Complex 1 crystallizes in the triclinic space group
with a = 10.537(2) Å, b = 10.751(2) Å, c = 10.907(2) Å, = 75.20(3), = 73.73(3), = 75.73(3), and Z = 1. The cation is located an inversion center. The Hg–Hg and Hg–Namine bond distances are 2.5469(8) and 2.297(6) Å, respectively, and the average Hg–Npyridyl bond length is 2.75(7) Å. Complex 1 was stable indefinitely in acetonitrile-d
3 solution, permitting detection of 13 and 22 Hz heteronuclear couplings between the Hg(I) ions and the methylene protons of the ligand. Comparisons with the structures and spectroscopic properties of related mercuric and mercurous complexes are made. 相似文献
33.
The electron-spin resonance spectra of Mn2+ ions and nuclear-quadrupole resonance spectra of 175Lu are investigated to find out the possibility of implementing the technique of dynamic alignment of nuclei using LuNbO4 single crystals doped with Mn2+. An estimate for the electron-spin resonance frequency of Mn2+ ions is obtained, and the temperature dependences of the quadrupole coupling constant eQq and the anisotropy parameter that characterizes the asymmetry of the electric field gradient at 175Lu nuclei are studied. It is demonstrated that LuNbO4 single crystals doped with Mn2+ ions can be used as working media in experiments on dynamic alignment of nuclei. 相似文献
34.
35.
Bush MF Oomens J Saykally RJ Williams ER 《Journal of the American Chemical Society》2008,130(20):6463-6471
The structures of isolated alkaline earth metal cationized amino acids are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and theory. These results indicate that arginine, glutamine, proline, serine, and valine all adopt zwitterionic structures when complexed with divalent barium. The IRMPD spectra for these ions exhibit bands assigned to carboxylate stretching modes, spectral signatures for zwitterionic amino acids, and lack bands attributable to the carbonyl stretch of a carboxylic acid functional group. Structural and spectral assignments are strengthened through comparisons with absorbance spectra calculated for low-energy structures and the IRMPD spectra of analogous ions containing monovalent alkali metals. Many bands are significantly red-shifted from the corresponding bands for amino acids complexed with monovalent metal ions, owing to increased charge transfer to divalent metal ions. The IRMPD spectra of arginine complexed with divalent strontium and barium are very similar and indicate that arginine adopts a zwitterionic form in both ions. Calculations indicate that nonzwitterionic forms of arginine are lowest in free energy in complexes with smaller alkaline earth metal cations and that zwitterionic forms are preferentially stabilized with increasing metal ion size. B3LYP and MP2 calculations indicate that zwitterionic forms of arginine are lowest in free energy for M = Ca, Sr, and Ba. 相似文献
36.
Nonergodicity in electron capture dissociation investigated using hydrated ion nanocalorimetry 总被引:1,自引:0,他引:1
Leib RD Donald WA Bush MF O'Brien JT Williams ER 《Journal of the American Society for Mass Spectrometry》2007,18(7):1217-1231
Hydrated divalent magnesium and calcium clusters are used as nanocalorimeters to measure the internal energy deposited into size-selected clusters upon capture of a thermally generated electron. The infrared radiation emitted from the cell and vacuum chamber surfaces as well as from the heated cathode results in some activation of these clusters, but this activation is minimal. No measurable excitation due to inelastic collisions occurs with the low-energy electrons used under these conditions. Two different dissociation pathways are observed for the divalent clusters that capture an electron: loss of water molecules (Pathway I) and loss of an H atom and water molecules (Pathway II). For Ca(H(2)O)(n)(2+), Pathway I occurs exclusively for n >or= 30 whereas Pathway II occurs exclusively for n 相似文献
37.
The 57 Fe spin echo spectra were studied in local magnetic fields of a BiFeO3 ferroelectric antiferromagnet over the temperature range 77–304 K. The line shape analysis confirmed the presence of a spatially modulated, incommensurate cycloidal spin structure in BiFeO3 and allowed the actual spin distribution to be reproduced throughout the cycloid length. The distribution was found to be essentially anharmonic. The modulated structure is stable over the whole temperature range studied. The cycloid wave becomes more harmonic with temperature elevation. 相似文献
38.
Bruce L. Bush Christopher I. Bayly Thomas A. Halgren 《Journal of computational chemistry》1999,20(14):1495-1516
Bond-charge increments (BCIs) are additive parameters used to assign atomic charges for the MMFF force field. BCI parameters are classified parsimoniously according to two atom types and the bond order. We show how BCIs may be fitted rapidly by linear least squares to the calculated ab initio electrostatic potential (ESP) or to the electrostatic field. When applied simultaneously to a set of compounds or conformations, the method yields consensus values of the BCIs. The method can also derive conventional “ESP-fit” atomic charges with improved numerical stability. The method may be generalized to determine atom multipoles, multicenter charge templates, or electronegativities, but not polarizability or hardness. We determine 65 potential-derived (PD) BCI parameters, which are classified as in MMFF, by fitting the 6-31G* ESP or the electrostatic field of the 45 compounds in the original MMFF94 training set. We compare the consensus BCIs with classified BCIs that were fit to each molecule individually and with “unique-bond” BCIs (ESP-derived atom charges). Consensus BCIs give a satisfactory representation for about half of the structures and are robust to the adjustment of the alkyl CH bond increment to the zero value employed in MMFF94. We highlight problems at three levels: Point approximation: the potential near lone pairs on sulfur and to some extent nitrogen cannot be represented just by atom charges. Bond classification: BCIs classified according to MMFF atom types cannot represent all delocalized electronic effects. The problem is especially severe for bonds between atoms of equivalent MMFF type, whose BCI must be taken as zero. Consensus: discrepancies that occur in forming the consensus across the training set indicate the need for a more detailed classification of BCIs. Contradictions are seen (e.g., between acetic acid and acetone and between guanidine and formaldehydeimine). We then test the three sets of PD-BCIs in energy minimizations of hydrogen-bonded dimers. Unique-bond BCIs used with the MMFF buffered 14–7 potential reproduce unscaled quantum chemical dimer interaction energies within 0.9 kcal/mol root mean square (or 0.5, omitting two N-oxides). These energies are on average 0.7 (or 0.5) kcal/mol too weak to reproduce the scaled quantum mechanical (SQM) results that are a benchmark for MMFF parameterization. Consensus BCIs tend to weaken the dimer energy by a further 0.4–0.6 kcal/mol. Thus, consensus PD-BCIs can serve as a starting point for MMFF parameterization, but they require both systematic and individual adjustments. Used with a “harder” AMBER-like Lennard–Jones potential, unique-bond PD-BCIs without systematic adjustment give dimer energies in fairly good agreement with SQM. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1495–1516, 1999 相似文献
39.
Tatjana Ovchinnikova Irina Zhuravleva Ljudmila Bush Nicolay Il'in 《Macromolecular Symposia》1999,144(1):303-311
The study of glycolide (GL) and ethylenoxalate (EO) copolymerization with the use of SnCl2 2 H2O as a catalyst was carried out by 1H NMR spectroscopy. Monomer reactivities are found to be close and equal to 1,1 and 1,11 for GL and 0,71 and 0,85 for EO at 150 and 170 °C, respectively. Kinetic data obtained show that the reaction proceeds without induction period up to entire consumption of every monomer. The detailed identification of 1H spectra enables to follow the polymer microstructure in the course of copolymerization. Thermal properties of copolymers and their biodegradation abilities have been studied. The biodegradation rate is shown to increase with growing EO proportion in the copolymer. The lactone biodegradation mechanism where the electrophility of carbon and nucleophility of OH group are playing the most important role, is suggested. The correlation between the biodegradation rate and the chemical shift in 13C NMR spectra of the carbon atom of the carbonyl group has been established. 相似文献
40.
V. A. Andrianov A. A. Bush A. L. Erzinkyan K. E. Kamentsev 《Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques》2017,11(4):704-709
The operating conditions of X-Ray sources based on LiNbO3 crystals are investigated during heating-cooling cycles. It is demonstrated that the radiation intensity is unstable. The radiation is accompanied by electrical breakdowns in the Z plane of the crystals and emission of photon packets not described by the Poisson distribution. Visualization of the electron beam through the grid electrode by a luminescent screen showed that the electron beam is not uniform in the Z-plane of the crystal and greatly changes with temperature. It is found that, under definite conditions, the numerous redistributions of the intense emission zones occurred between different areas at Z-surface of the crystal. Possible reasons for the observed effects are examined. The obtained data are important for creating pyroelectric X-ray and neutron sources presuming the usage of strong electric fields whose strength reaches 105 V/cm. 相似文献