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Bond System (bond + system)
Selected AbstractsOn the Importance of CP-corrected Gradient Optimization in the Study of Hydrogen Bonded SystemsCHINESE JOURNAL OF CHEMISTRY, Issue 12 2003Wei-Zhou Wang Abstract Geometries, harmonic vibrational frequencies and interaction energies of the water-hydrogen sulfide dimer, hydrogen fluoride dimer and glycine zwitterion-water dimer were determined by the counterpoise-corrected (CP-corrected) gradient optimization that explicitly corrects for the basis set superposition error (BSSE) and CP-uncorrected (normal) gradient optimization respectively at the B3LYP and MP2 levels of theory, employing the popular Pople's standard 6,31G(d), 6,31G(d, p) and 6,311 + + G(d, p) basis sets in order to assess the importance of CP-corrected gradient optimization in the study of hydrogen bonded systems. The normal optimization of these three H-bonded systems obtained using these popular basis sets all yielded erratic results, whereas use of CP-corrected gradient optimization led to consistent results with those from larger basis sets. So this CP receipt becomes useful and necessary to correctly describe large systems, where the use of small basis sets may be necessary. [source] Analysis of the interaction of substituted coumarins with the DPPH free radical by means of multivariate statisticsJOURNAL OF PHARMACY AND PHARMACOLOGY: AN INTERNATI ONAL JOURNAL OF PHARMACEUTICAL SCIENCE, Issue 9 2004Demetris Vrakas The interaction of some substituted coumarin derivatives with the stable 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical was analysed by means of multivariate statistics using a variety of molecular descriptors. The compounds contain a conjugated double bond system, which was considered to be an essential structural characteristic for the free-radical scavenging activity. Partial least-square analysis led to an adequate two-component model based on bulk descriptors and the electronic properties concerning atoms involved or next to the double-bond system. [source] Interplay between dipolar, stacking and hydrogen-bond interactions in the crystal structures of unsymmetrically substituted esters, amides and nitriles of (R,R)- O,O,-dibenzoyltartaric acidACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2001Urszula Rychlewska The compounds analysed are the O,O,-dibenzoyl derivatives of (R,R)-tartaric acid, asymmetrically substituted by ester, amide and nitrile groups. Benzoylation does not introduce drastic changes to the molecular conformation. All investigated molecules adopt the planar T conformation of the four-carbon chain with noticeably smaller departures from the ideal conformation than observed in the nonbenzoylated analogs. Primary and secondary amides always orient the C=O bond antiperiplanar (a) with respect to the nearest C*,O bond, while methylester groups adjust their conformation to that adopted by the amide substituent situated at the other end of the molecule. Tertiary amides and carboxyl groups place their carbonyls at the same side as the nearest C*,O bond (the s form), but often deviations from coplanarity of the two bonds are significant and higher than those observed in the nonbenzoylated series. The results presented demonstrate the importance of dipole/dipole interactions between CO and ,C*H groups in stabilizing the molecular conformation, and between carbonyl groups in stabilizing crystal packing of the molecules that lack classical hydrogen-bond donor groups. An illustration is provided as to how a small change in mutual orientation of molecules arranged in a close-packed fashion causes a change in the character of intermolecular interactions from van der Waals to sandwich stacking between the benzoyloxy phenyls, and to dipolar between the benzoyloxy carbonyls. Hydrogen-bonded molecules tend to orient in a head-to-tail mode; the head-to-head arrangement being limited to cases in which terminal carbonyl groups are situated at one side of the molecule. The orientation of the benzoyloxy substituents with respect to the carbon main chain is such that the (O=)C,O,C,H bond system often deviates significantly from planarity. [source] How the CO in myoglobin acquired its bend: lessons in interpretation of crystallographic dataACTA CRYSTALLOGRAPHICA SECTION D, Issue 5 2001Boguslaw Stec Contrary to the expectation of chemists, the first X-ray structures of carbon monoxide bound to myoglobin (Mb) showed a highly distorted Fe,C,O bond system. These results appeared to support the idea of a largely steric mechanism for discrimination by the protein against CO binding, a lethal act for the protein in terms of its physiological function. The most recent independently determined high-resolution structures of Mb,CO have allowed the 25,year old controversy concerning the mode of CO binding to be resolved. The CO is now seen to bind in a roughly linear fashion without substantial bending, consistent with chemical expectations and spectroscopic measurements. Access to deposited diffraction data prompted a reevaluation of the sources of the original misinterpretation. A series of careful refinements of models against the data at high (1.1,Å) and modest resolutions (1.5,Å) have been performed in anisotropic versus isotropic modes. The results suggest that the original artifact was a result of lower quality crystals combined with anisotropic motion and limited resolution of the diffraction data sets. This retrospective analysis should serve as a caution for all researchers using structural tools to draw far-reaching biochemical conclusions. [source] Influence of Grain Phase on Slag Corrosion of Low-Cement Castable RefractoriesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2001Hossain Sarpoolaky The grain phase has a significant influence on the extent and mechanism of attack of low-cement castables with similar bond systems in a crucible corrosion test. The general corrosion mechanism, as determined by post mortem scanning electron microscopic examination of corroded samples and thermodynamic calculations for all four grain types examined (white-fused alumina (WFA), tabular alumina (TA), brown-fused alumina (BFA), and alumina-rich spinel (S)), involves initial reaction of the most penetrating slag (enriched in calcium, manganese, and iron, because these elements diffuse rapidly) with the fine alumina and calcium aluminates of the matrix. This reaction gives a CaO-rich local liquid, which can then react with each grain predominantly to form calcium hexaluminate (CA6) and hercynitic spinel. In the WFA system, a complete CA6 layer forms around the grain, whereas in the TA system, this layer is incomplete. In both systems, extensive penetration occurs, although corrosion is low. In the BFA system, titanates are released from the grain into the bond, leading to increased densification of the refractory, via liquid-phase sintering, and consequent low penetration. However, the resulting fluid liquid dissolves easily in the slag, so that corrosive wear is high, even though a CA6 layer forms around the grain. In the S grain system, uptake of the rapidly diffusing cations into the spinel crystal structure leads to silica-rich and viscous local liquid, which leads to low penetration and corrosion. [source] | ||||||||||