Molecular Geometry as a Source of Chemical Information: Applications of the Bond Valence Bond Number Models

ChemInform ◽  
2003 ◽  
Vol 34 (22) ◽  
Author(s):  
Slawomir J. Grabowski ◽  
Tadeusz M. Krygowski
Keyword(s):  
Valence Bond ◽  
Molecular Geometry ◽  
Bond Number ◽  
Bond Valence ◽  
Chemical Information

Tetrachlorodibenzo-p-Dioxin Isomer Differentiation by Capillary Gas Chromatography Fourier Transform-Infrared Spectroscopy

1987 ◽  
Vol 41 (5) ◽  
pp. 809-820 ◽  
Author(s):  
James Grainger ◽  
Leslie T. Gelbaum
Keyword(s):  
Gas Chromatography ◽  
Fourier Transform ◽  
Vapor Phase ◽  
Valence Bond ◽  
Fourier Transform Infrared ◽  
Molecular Geometry ◽  
Ether Linkage ◽  
Relative Electron ◽  
Ft Ir ◽  
Infrared Frequencies

Reference infrared vapor-phase spectra of the 22 tetrachlorodibenzo-dioxin (TCDD) isomers were recorded at low microgram concentrations. These reference spectra of synthetic mixture components separated chromatographically or by spectral subtraction exhibit distinct infrared spectra for each isomer. The infrared frequencies are delineated in correlation tables and are interpreted in terms of substitution patterns which determine the strength of the ether linkage. Absorbance values in the 1330–1280 cm−1 (C-O-C asymmetric stretch) region correlate with specific substitution patterns and molecular geometry. Relative electron-withdrawing capacities for chlorinated aromatic rings in TCDD isomers were estimated on the basis of relative capacities determined for model compounds. Qualitative correlations were established between electron-withdrawing capacities and the effects of resonance and field interactions on the ether linkage absorption frequencies of individual TCDD isomers. Gas chromatography Fourier transform infrared (GC/FT-IR) isomer assignments are generally consistent with those obtained by proton Fourier transform nuclear magnetic resonance (1H FT/NMR) and flame ionization gas chromatography (GC/FID). A chromatographically independent method of assigning TCDD isomer structures on the basis of ether linkage asymmetric stretching frequencies was established by utilization of valence-bond approximations. GC/FT-IR assignments for several TCDD isomers differ from isomer assignments in previously published results. A user-generated, vapor-phase reference library, containing individual TCDD spectra and spectra of isomer pairs that are incompletely resolved on chromatographic columns, correctly identified each isomer in variety of mixtures by means of a software algorithm.

Solution of organic crystal structures from powder diffraction by combining simulated annealing and direct methods

2003 ◽  
Vol 36 (2) ◽  
pp. 230-238 ◽  
Author(s):  
Angela Altomare ◽  
Rocco Caliandro ◽  
Carmelo Giacovazzo ◽  
Anna Grazia Giuseppina Moliterni ◽  
Rosanna Rizzi
Keyword(s):  
Monte Carlo ◽  
Simulated Annealing ◽  
Electron Density ◽  
Powder Diffraction ◽  
Direct Methods ◽  
Molecular Geometry ◽  
Molecular Structures ◽  
Chemical Information ◽  
Density Map ◽  
Electron Density Map

Theab initiocrystal structure solution from powder diffraction data can be attemptedviadirect methods. If heavy atoms are present, they are usually correctly located; then some crystal chemical information can be exploited to complete the partial structure model. Organic structures are more resistant to direct methods; as an alternative, their molecular geometry is used as prior information for Monte Carlo methods. In this paper, a new procedure is described which combines the information contained in the electron density map provided by direct methods with a Monte Carlo method which uses simulated annealing as a minimization algorithm. A figure of merit has been designed based on the agreement between the experimental and calculated profiles, and on the positions of the peaks in the electron density map. The procedure is completely automatic and has been included inEXPO; its performance has been validated and tested for a set of known molecular structures.

Bond-valence methods for pKa prediction. II. Bond-valence, electrostatic, molecular geometry, and solvation effects

2006 ◽  
Vol 70 (16) ◽  
pp. 4057-4071 ◽  
Author(s):  
Barry R. Bickmore ◽  
Kevin M. Rosso ◽  
Christopher J. Tadanier ◽  
Eric J. Bylaska ◽  
Darrin Doud
Keyword(s):  
Molecular Geometry ◽  
Bond Valence ◽  
Pka Prediction ◽  
Solvation Effects

Bond valence sum: a new soft chemical constraint forRMCProfile

2009 ◽  
Vol 42 (2) ◽  
pp. 179-184 ◽  
Author(s):  
Stefan T. Norberg ◽  
Matthew G. Tucker ◽  
Stephen Hull
Keyword(s):  
Neutron Scattering ◽  
Bond Valence ◽  
Scattering Data ◽  
Chemical Information ◽  
Ion Conductor ◽  
Oxide Ion Conductor ◽  
Fitting Procedure ◽  
Bond Valence Sum ◽  
Scattering Lengths ◽  
Oxide Ion

The implementation of a new soft chemical constraint for the reverse Monte Carlo (RMC) programRMCProfile, based on bond valence sum (BVS) calculations, is described and its advantages for the analysis of `total scattering' diffraction data collected from disordered crystalline systems discussed. The inclusion of the BVS formalism proves particularly valuable in the early stages of the RMC fitting procedure, by avoiding the formation of regions containing chemically unreasonable local configurations which can become frozen in. Furthermore, this approach provides the fitting procedure with additional chemical information to differentiate between cation species that share the same crystallographic sites within the averaged unit cell and possess similar neutron scattering lengths. These issues are illustrated using total neutron scattering data collected at room temperature on the oxide-ion conductor Zr2Y2O7and the nonlinear optical material KTiOPO4.

scholarly journals Comprehensive derivation of bond-valence parameters for ion pairs involving oxygen

2015 ◽  
Vol 71 (5) ◽  
pp. 562-578 ◽  
Author(s):  
Olivier Charles Gagné ◽  
Frank Christopher Hawthorne
Keyword(s):  
Ion Pairs ◽  
Valence Bond ◽  
Bond Valence ◽  
Data Set ◽  
Coordination Polyhedra ◽  
Reduced Gradient Method ◽  
Advantages And Disadvantages ◽  
Reduced Gradient ◽  
Generalized Reduced Gradient ◽  
Two Parameter

Published two-body bond-valence parameters for cation–oxygen bonds have been evaluatedviathe root mean-square deviation (RMSD) from the valence-sum rule for 128 cations, using 180 194 filtered bond lengths from 31 489 coordination polyhedra. Values of the RMSD range from 0.033–2.451 v.u. (1.1–40.9% per unit of charge) with a weighted mean of 0.174 v.u. (7.34% per unit of charge). The set of best published parameters has been determined for 128 ions and used as a benchmark for the determination of new bond-valence parameters in this paper. Two common methods for the derivation of bond-valence parameters have been evaluated: (1) fixingBand solving forRo; (2) the graphical method. On a subset of 90 ions observed in more than one coordination, fixingBat 0.37 Å leads to a mean weighted-RMSD of 0.139 v.u. (6.7% per unit of charge), while graphical derivation gives 0.161 v.u. (8.0% per unit of charge). The advantages and disadvantages of these (and other) methods of derivation have been considered, leading to the conclusion that current methods of derivation of bond-valence parameters are not satisfactory. A new method of derivation is introduced, the GRG (generalized reduced gradient) method, which leads to a mean weighted-RMSD of 0.128 v.u. (6.1% per unit of charge) over the same sample of 90 multiple-coordination ions. The evaluation of 19 two-parameter equations and 7 three-parameter equations to model the bond-valence–bond-length relation indicates that: (1) many equations can adequately describe the relation; (2) a plateau has been reached in the fit for two-parameter equations; (3) the equation of Brown & Altermatt (1985) is sufficiently good that use of any of the other equations tested is not warranted. Improved bond-valence parameters have been derived for 135 ions for the equation of Brown & Altermatt (1985) in terms of both the cation and anion bond-valence sums using the GRG method and our complete data set.

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