Charge calculations in molecular mechanics. Part 8 Partial atomic charges from classical calculations

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.     

Ionisation et Fragmentation en Spectrométrie de Masse. VI—Energie d'Activation de la Fragmentation Principale des Monohalogénobenzènes

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.     

High Performance Varistor Discs Obtained from Chemically Synthesized Doped Zinc Oxide Powder

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.     

Ruthenium(II) complexes of monodonor ligands: efficient reagents for asymmetric ketone hydrogenation

[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%.     

Thermal degradation of a series of polyester polyurethanes

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.     

Comparison of analytical techniques to quantify malondialdehyde in milk powders

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.     

Chemical Synthesis of Oligonucleotides Containing N6-Methyladenine Residues in the GATC Site

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.     

Irreversible cathodic behavior of graphite in the presence of some mixtures of onium salts

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.     

Le diptérocarpol—II Stéréochimie en C-13 et en C-17

Various reactions of ring D of dipterocarpol (hydroxy-dammarenone-II) show this triterpene to have the configuration 13β, 17H, in accordance with biogenetic speculations.     

Structural and energetic aspects of the protonation of phenol,catechol, resorcinol,and hydroquinone

The various protonated forms of phenol (1), catechol (2), resorcinol (3), and hydroquinone (4) were explored by ab initio quantum chemical calculations at the MP2/6-31G(d) and B3LYP/6-31G(d) levels. Proton affinities (PA) of 1-4 were calculated by the combined G2(MP2,SVP) method, and their gas-phase basicities were estimated after calculation of the change in entropy on protonation. These theoretical data were compared with the corresponding experimental values determined in a high-pressure mass spectrometer. This comparison confirmed that phenols are essentially carbon bases and that protonation generally occurs in a position para to the hydroxyl group. Resorcinol is the most effective base (PA = 856 kJ mol-1) due to the participation of both oxygen atoms in the stabilization of the protonated form. Since protonation is accompanied by a freezing of the two internal rotations, a significant decrease in entropy is observed. The basicity of catechol (PA = 823 kJ mol-1) is due to the existence of an intramolecular hydrogen bond, which is strengthened upon protonation. The lower basicity of hydroquinone (PA = 808 kJ mol-1) is a consequence of the fact that protonation necessarily occurs in a position ortho to the hydroxyl group. When the previously published data are reconsidered and a corrected protonation entropy is used, a proton affinity value of 820 kJ mol-1 is obtained for phenol.