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[en] The PH and CH tautomers of mono-, di-, tri-, tetra-, and pentaphospholes were calculated at the B3LYP/aug-cc-pVTZ level. Except for 1,2,3-triphosphole and pentaphosphole, the most stable phosphole form is one of the CH tautomers. For 1,2-diphosphole, 1,3-diphosphole, and 1,2,4-triphosphole, the energy difference between the tautomers is large and only one tautomer may be observed. For other phospholes, especially tetraphosphole, two tautomers may coexist. Non-planarity of the five membered ring is present only in the σ3λ3-P atom containing molecules, i.e., PH type of tautomers, while in the CH type of tautomers the ring is planar. As earlier reported by Nyulaszi, the PH tautomers flatten, and the energy difference between PH phosphole and the planar transition state structure decreases, as the number of P atoms in the phosphole molecule is increased. Bond length changes were discussed in terms of configuration of the atoms attached to the bond, and the molecular frequency variations were examined according to the different surroundings of the appropriate vibrators
[en] Close-packed clusters of vacancies are those in which every vacancy has at least one other vacancy in a nearest neighbour position. Recently a study has been carried out of the possible configurations of small clusters of this type in all single lattice structures [see Phil. Mag., vol. 32, p. 379 (1975)]. This work is now being extended to consider the migration, growth and contraction of clusters and results for the face centred cubic structure are presented. (Auth.)
[en] The crystal structure of W18O49, has been refined from an untwinned sample and registered by means of X-rays diffractometric techniques. The deformation of the polyhedra is described from the dispersion of the O-O distances, from the shape of the windows of the perovskite cages and from the location of the W inside their octahedra. A metallic cluster is observed, according to the weak value of the mean oxydation state of the W atoms. Calculations of sites potential are carried inside the large hexagonal tunnels which are empty. They suggest that the existence of a large cation as Cs+ should be possible in the tunnels of such structures
[fr]La structure cristalline de W18O49, est reaffinee a partir du spectre de diffraction X d'un cristal non macle. La deformation des polyedres est interpretee par la dispersion des distances O-O, par la forme des fenetres des cages de type perovskite et par la situation des atomes W au sein de leurs octaedres WO6. L'observation d'un cluster metallique confirme la faible valeur de l'etat d'oxydation moyen des atomes W. L'insertion de cations de grande taille tel que Cs+ n'est pas theoriquement exclue dans ce type structural
[en] The extended variant of the bond valence (s)-bond length (r) correlation curve for boron(III)-oxygen bonds has been closely approximated using the three-parameter function s = [k/(r - l)] - m, where s is measured in valence units (vu), r is measured in Aa, k = 0.53 Aa.vu, l = 0.975(1) Aa and m = 0.32 vu. The function s = exp[(r0 - r)/b] traditionally used in the modern bond valence model requires the separate set of the bond valence parameters (r0 = 1.362 Aa; b = 0.23 Aa) in order to approximate the above s-r curve for the bonds shorter than ∝1.3 Aa.
[en] Highlights: • Wave propagation in nanorods and nanobeams are studied by using Doublet Mechanics. • Wave characteristic of double-walled nanotubes are investigated. • Classical rod theory and Euler –Bernoulli beam theory is used. • Bond length of atoms is intrinsic length scale of the structure in doublet mechanics. Flexural and axial wave propagation in double walled carbon nanotubes embedded in an elastic medium and axial wave propagation in single walled carbon nanotubes are investigated. A length scale dependent theory which is called doublet mechanics is used in the analysis. Governing equations are obtained by using Hamilton principle. Doublet mechanics results are compared with classical elasticity and other size dependent continuum theories such as strain gradient theory, nonlocal theory and lattice dynamics. In addition, experimental wave frequencies of graphite are compared with the doublet mechanics theory. It is obtained that doublet mechanics gives accurate results for flexural and axial wave propagation in nanotubes. Thus, doublet mechanics can be used for the design of electro-mechanical nano-devices such as nanomotors, nanosensors and oscillators.
[en] Calculations for the reactive potential energy surfaces, which are relevant for examining the low-energy channels connected with the LiH2+ three atom ionic system, are carried out using a multireference valence bond (MRVB) approach. More than 11.000 points have been computed over the grid of the three relevant coordinates (two bond distances and the enclosed angle) and the lowest three electronic states have been followed over the spatial configuration of the above coordinates. Several aspects of the reactive behavior could be gleaned from an analysis of the computed surfaces and various features of the reactive outcomes are extracted from that analysis
[en] Highlights: • The smallest all-electron basis set of valence double zeta quality is reported. • DZP-DKH is a compact, but yet reliable, basis set to be used in place of ECP. • B3LYP/DZP-DKZ geometric parameters and atomization energy for lanthanide trifluorides. Segmented all-electron basis sets of valence double zeta quality plus polarization functions (DZP) for the elements from Ce to Lu are generated to be used with the non-relativistic and Douglas–Kroll–Hess (DKH) Hamiltonians. At the B3LYP level, the DZP-DKH atomic ionization energies and equilibrium bond lengths and atomization energies of the lanthanide trifluorides are evaluated and compared with benchmark theoretical and experimental data reported in the literature. In general, this compact size set shows to have a regular, efficient, and reliable performance. It can be particularly useful in molecular property calculations that require explicit treatment of the core electrons.
[en] A script was created to allow SHELXL to use the new CDL v.1.2 stereochemical library which defines the target values for main-chain bond lengths and angles as a function of the residue’s ϕ/ψ angles. Test refinements using this script show that the refinement behavior of structures at resolutions even better than 1 Å is substantially enhanced by the use of the new conformation-dependent ideal geometry paradigm. To utilize a new conformation-dependent backbone-geometry library (CDL) in protein refinements at atomic resolution, a script was written that creates a restraint file for the SHELXL refinement program. It was found that the use of this library allows models to be created that have a substantially better fit to main-chain bond angles and lengths without degrading their fit to the X-ray data even at resolutions near 1 Å. For models at much higher resolution (∼0.7 Å), the refined model for parts adopting single well occupied positions is largely independent of the restraints used, but these structures still showed much smaller r.m.s.d. residuals when assessed with the CDL. Examination of the refinement tests across a wide resolution range from 2.4 to 0.65 Å revealed consistent behavior supporting the use of the CDL as a next-generation restraint library to improve refinement. CDL restraints can be generated using the service at http://pgd.science.oregonstate.edu/cdl_shelxl/
[en] In the present work we have studied in detail the problem of the mean ''size'' of mixing of chain-propagating atoms and chain-terminating atoms on a one-dimensional lattice, with respect to the temperature and the interaction constants. Some comparisons of the average chain length of a symmetric system to that of an asymmetric one allows us to state that the asymmetry of bonds play an important role in determining the mean chain length or the mean ''size'' of boxes. (author). 4 refs, 6 figs