Optimized bond lengths and electron densities on atoms in heterosubstituted polyenes

1993 ◽  
Vol 28 (4) ◽  
pp. 246-250
Author(s):  
A. D. Kachkovskii ◽  
M. L. Dekhtyar'
Keyword(s):  
Electron Densities ◽  
Bond Lengths

scholarly journals Atomic radii from electron densities.

2000 ◽  
Vol 53 (2) ◽  
pp. 317 ◽  
Author(s):  
B. E. Etschmann ◽  
E. N. Maslen
Keyword(s):  
Angular Momentum ◽  
Bond Order ◽  
Free Atom ◽  
Nuclear Region ◽  
Electron Densities ◽  
Bond Lengths ◽  
Valence Electrons ◽  
Exchange Repulsion ◽  
Predicted Values ◽  
Atomic Radii

Bond lengths for diatomic molecules are predicted from atomic radii derived from free atom one-electron densities by postulating shielding factors for their valence electrons that depend on orbital angular momentum and on the bond order. The predicted values are closer to spectroscopically measured bond lengths than those based on earlier atomic radii inferred from a wider range of structural evidence. The bond lengths predicted by the sum of the atomic radii are corrected by a reduction that allows for charge transfer and by an extension associated with exchange repulsion of the overlapping electrons in the inter-nuclear region. Both corrections are related to free atom one-electron densities.

scholarly journals Hirshfeld atom refinement

IUCrJ ◽  
2014 ◽  
Vol 1 (5) ◽  
pp. 361-379 ◽  
Author(s):  
Silvia C. Capelli ◽  
Hans-Beat Bürgi ◽  
Birger Dittrich ◽  
Simon Grabowsky ◽  
Dylan Jayatilaka
Keyword(s):  
Hydrogen Atom ◽  
Diffraction Data ◽  
Iterative Procedure ◽  
Structural Parameters ◽  
X Ray Diffraction ◽  
Hydrogen Atoms ◽  
Electron Densities ◽  
X Ray ◽  
Bond Lengths ◽  
Better Than

Hirshfeld atom refinement (HAR) is a method which determines structural parameters from single-crystal X-ray diffraction data by using an aspherical atom partitioning of tailor-madeab initioquantum mechanical molecular electron densities without any further approximation. Here the original HAR method is extended by implementing an iterative procedure of successive cycles of electron density calculations, Hirshfeld atom scattering factor calculations and structural least-squares refinements, repeated until convergence. The importance of this iterative procedure is illustratedviathe example of crystalline ammonia. The new HAR method is then applied to X-ray diffraction data of the dipeptide Gly–L-Ala measured at 12, 50, 100, 150, 220 and 295 K, using Hartree–Fock and BLYP density functional theory electron densities and three different basis sets. All positions and anisotropic displacement parameters (ADPs) are freely refined without constraints or restraints – even those for hydrogen atoms. The results are systematically compared with those from neutron diffraction experiments at the temperatures 12, 50, 150 and 295 K. Although non-hydrogen-atom ADPs differ by up to three combined standard uncertainties (csu's), all other structural parameters agree within less than 2 csu's. Using our best calculations (BLYP/cc-pVTZ, recommended for organic molecules), the accuracy of determining bond lengths involving hydrogen atoms from HAR is better than 0.009 Å for temperatures of 150 K or below; for hydrogen-atom ADPs it is better than 0.006 Å2as judged from the mean absolute X-ray minus neutron differences. These results are among the best ever obtained. Remarkably, the precision of determining bond lengths and ADPs for the hydrogen atoms from the HAR procedure is comparable with that from the neutron measurements – an outcome which is obtained with a routinely achievable resolution of the X-ray data of 0.65 Å.

Pyridopyridazines. II. Some reactions of Pyrido[2,3-d]- and Pyrido[3,4-d]-pyridazine

1969 ◽  
Vol 22 (8) ◽  
pp. 1745 ◽  
Author(s):  
DB Paul ◽  
HJ Rodda
Keyword(s):  
Electron Densities ◽  
Charge Densities ◽  
Bond Lengths ◽  
Alkaline Conditions ◽  
Chlorous Acid

Pyridopyridazines have been oxidized under acid and alkaline conditions and the sites of oxidation have been rationalized on the basis of charge densities. Pyrido-[2,3-d]pyridazine has been brominated, aminated, and made to react with hypo-chlorous acid and with peracids. The positions of substitution and addition are discussed in relation to calculated π-electron densities and bond lengths. Attention is drawn to some unusual features of the N.M.R. spectra of 1,2-diazine N-oxides.

The cytotoxicity of ortho alkyl substituted 4-X-phenols: A QSAR based on theoretical bond lengths and electron densities

2006 ◽  
Vol 16 (5) ◽  
pp. 1249-1254 ◽  
Author(s):  
R.J. Loader ◽  
N. Singh ◽  
P.J. O’Malley ◽  
P.L.A. Popelier
Keyword(s):  
Electron Densities ◽  
Bond Lengths

Vibrational spectra of aquadioxotetra; peroxodivanadates(V) M2[V2O2(O2)4(H2O)]·xH2O (M = N(CH3)4, Cs)

1990 ◽  
Vol 55 (6) ◽  
pp. 1485-1490 ◽  
Author(s):  
Peter Schwendt ◽  
Milan Sýkora
Keyword(s):  
Raman Spectra ◽  
Vibrational Spectra ◽  
Normal Coordinate Analysis ◽  
Force Constants ◽  
Infrared And Raman Spectra ◽  
Empirical Correlations ◽  
Normal Coordinate ◽  
Bond Lengths ◽  
Stretching Vibrations

The infrared and Raman spectra of M2[V2O2(O2)4(H2O)]·xH2O and M2[V2O2(O2)4(D2O)]·xD2O (M = N(CH3)4, Cs) were measured. In the region of the vanadium-oxygen stretching vibrations, the spectra were interpreted based on normal coordinate analysis, employing empirical correlations between the bond lengths and force constants.

scholarly journals Crystal structure of K[Hg(SCN)3] – a redetermination

2014 ◽  
Vol 70 (9) ◽  
pp. i46-i46 ◽  
Author(s):  
Matthias Weil ◽  
Thomas Häusler
Keyword(s):  
Crystal Structure ◽  
Room Temperature ◽  
Three Dimensional ◽  
Lattice Parameters ◽  
Bond Lengths ◽  
Dimensional Network ◽  
Anisotropic Displacement Parameters ◽  
Precision And Accuracy ◽  
Standard Deviations ◽  
Atomic Coordinates

The crystal structure of the room-temperature modification of K[Hg(SCN)3], potassium trithiocyanatomercurate(II), was redetermined based on modern CCD data. In comparison with the previous report [Zhdanov & Sanadze (1952).Zh. Fiz. Khim.26, 469–478], reliability factors, standard deviations of lattice parameters and atomic coordinates, as well as anisotropic displacement parameters, were revealed for all atoms. The higher precision and accuracy of the model is, for example, reflected by the Hg—S bond lengths of 2.3954 (11), 2.4481 (8) and 2.7653 (6) Å in comparison with values of 2.24, 2.43 and 2.77 Å. All atoms in the crystal structure are located on mirror planes. The Hg2+cation is surrounded by four S atoms in a seesaw shape [S—Hg—S angles range from 94.65 (2) to 154.06 (3)°]. The HgS4polyhedra share a common S atom, building up chains extending parallel to [010]. All S atoms of the resulting1∞[HgS2/1S2/2] chains are also part of SCN−anions that link these chains with the K+cations into a three-dimensional network. The K—N bond lengths of the distorted KN7polyhedra lie between 2.926 (2) and 3.051 (3) Å.

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