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31.
Charge calculations in molecular mechanics. Part 8 Partial atomic charges from classical calculations 总被引:1,自引:0,他引:1
Raymond J. Abraham Guy H. Grant Ian S. Haworth Paul E. Smith 《Journal of computer-aided molecular design》1991,5(1):21-39
Summary The CHARGE2 programme, which involves the classical calculation of both the inductive and resonance contributions to the partial atomic charges in molecules is described, and the charges and electrostatic potentials obtained presented for some illustrative examples.In substituted methanes (CH3X, CF3X, CCl3X) the effects of varying the electronegativity of the substituents and the - and -substituent contributions are clearly illustrated for a variety of substituent groups X.The problems involved in the inclusion of silicon into this scheme are detailed, together with the methods of overcoming them. The partial atomic charges ( and contributions) and electrostatic potentials for some silicon oxygen compounds are presented and discussed.The partial atomic charges from CHARGE2 for all the natural amino acids as their N-acetyl, N-methyl-amides are given and compared with those obtained from the AMBER and ECEPP/2 force fields. Considerable differences in these figures are observed, with the AMBER charges consistently much larger than those from the other two methods.The CHARGE2 partial atomic charges and electrostatic potentials for the four common nucleic acids, adenine, cytosine, guanine and thymine, are given and compared with those derived from other calculations. Again there is general similarity but also there are considerable differences, with those from the AMBER force field somewhat larger than the other methods.For previous parts in this series, see Refs. 1-7. 相似文献
32.
Guy Bouchoux 《Journal of mass spectrometry : JMS》1977,12(11):681-684
The activation energy of the fragmentation [C6H5X?]+ → [C6H5]+ + X? (X = Cl, Br, I) is calculated by various methods. These results are compared with determinations of kinetic energy release and with rate constant values available in the literature. 相似文献
33.
Ramón Puyané Isabelle Guy Renaud Metz 《Journal of Sol-Gel Science and Technology》1998,13(1-3):575-578
The ceramic microstructure, the chemical homogeneity of specific dopants and the mechanical integrity of a varistor disc are critical parameters in determining the transient voltage suppression features of these devices. The material properties and overall quality of the starting ceramic powders used to produce such components are essential in achieving the desired properties. The present work describes a novel chemical method developed to produce doped zinc oxide powders and an industrial scale manufacturing process for the production of final varistor blocks for surge arrester applications. The results are compared with those obtained when using standard varistor powder made by the mixed oxide route is used. All the fundamental electrical properties of the discs have been determined and correlated with the relevant manufacturing steps. 相似文献
34.
Ruthenium(II) complexes of monodonor ligands: efficient reagents for asymmetric ketone hydrogenation
Xu YA Clarkson GC Docherty G North CL Woodward G Wills M 《The Journal of organic chemistry》2005,70(20):8079-8087
[Chemical reaction: See text] A series of BINOL-derived ligands have been prepared and incorporated into ruthenium(II) complexes containing a diamine ligand. The complexes have proven to be excellent catalysts for the asymmetric hydrogenation of ketones, giving reduction products with enantiomeric excesses of up to 99%. 相似文献
35.
N. Grassie M. Zulfiqar M. I. Guy 《Journal of polymer science. Part A, Polymer chemistry》1980,18(1):265-274
A series of polyester polyurethanes with a range of polyester contents was prepared from an ethylene glycol/propylene glycol/adipic acid polyster, butane diol, and methylene bis (4-phenyl isocyanate) (MBPI). They were thermally degraded under vacuum and the products of degradation were identified. The urethane linkages decompose first as the temperature is increased and the stability increases with polyester content. Reaction mechanisms were proposed which account for the principal features of the reaction and the products of degradation. 相似文献
36.
Single-photon ionization dynamics of two conformers of glycine is studied by classical trajectory simulations using the semiempirical PM3 potential surface in "on the fly" calculations. Initial conditions for the trajectories are weighted according to the Wigner distribution function computed for the initial vibrational ground state. Vertical ionization in the spirit of the classical Franck-Condon principle is assumed. The dynamics of the two conformers are compared during the first 10 ps. The comparison shows very different dynamical behavior for the two conformers. In particular, the chemical fragmentation pathways differ in part. Also, one of the conformers gives much higher rates of conformational transitions, while the other conformer gives larger chemical fragmentation yields. The example shows significantly different chemical dynamics for two conformers close in energy and separated by a low barrier. 相似文献
37.
Several analytical methods were compared to quantify malondialdehyde (MDA) in milk powders. Modified thiobarbituric acid (TBA) methods, using either visible spectrophotometry (direct absorbance reading or after third derivative transformation of the spectrum) or HPLC, required derivatisation at elevated temperature, which appeared to catalyse artefactual MDA formation and thus overestimate the MDA content. In contrast to the TBA derivatisation method, the measurement of MDA as the dinitrophenylhydrazone derivative by HPLC or as the phenylhydrazone product by GC-MS with a deuterated internal standard resulted in lower estimates in the ranges of 2-17- and 3-30-fold, respectively; apparently due to the milder derivatisation conditions. The estimates of MDA determined by both HPLC-UV and GC-MS techniques result in lower values which are similar in magnitude even though the GC-MS technique is more sensitive. 相似文献
38.
To investigate by NMR the effects of adenine methylation, a set of oligodeoxynucleotides has been synthesized by phosphotriester or phosphoramidite procedures on silica-gel support. The preparation of fully protected 2′-deoxy-N6-methyladenosine 3′-phosphate 4 and (2′-deoxy-N6-methyladenosin-3′-O-yl)(methoxy)morpholinophosphine 5 is described. The large-scale chemical synthesis, purification by HPLC, and characterization by MS of these molecules containing N6-methyladenine is reported. 相似文献
39.
The electrochemically induced dissolution of ferrocene in a carbon paste electrode is investigated by using chronoamperometry at constant potential. Two types of behaviour are observed, according to the initial form of ferrocene: particles or deposit. The experimental i-t curves are respectively in agreement with the theoretical curves, derived earlier, in the case of spherical particles or planar slabs of solid. The analysis of the curve shows that the process is diffusion controlled, so that the dissolution rate of ferrocene cannot be measured. The parameters calculated from the curves (thickness of the deposit, initial distance from the electrode, area) have a reasonable order of magnitude. 相似文献
40.
Various reactions of ring D of dipterocarpol (hydroxy-dammarenone-II) show this triterpene to have the configuration 13β, 17H, in accordance with biogenetic speculations. 相似文献