Search results for "Molecules"
showing 10 items of 1147 documents
Revisiting the charge density analysis of 2,5-dichloro-1,4-benzoquinone at 20 K
2017
A high-resolution X-ray diffraction measurement of 2,5-dichloro-1,4-benzoquinone (DCBQ) at 20 K was carried out. The experimental charge density was modeled using the Hansen–Coppens multipolar expansion and the topology of the electron density was analyzed in terms of the quantum theory of atoms in molecules (QTAIM). Two different multipole models, predominantly differentiated by the treatment of the chlorine atom, were obtained. The experimental results have been compared to theoretical results in the form of a multipolar refinement against theoretical structure factors and through direct topological analysis of the electron density obtained from the optimized periodic wavefunction. The si…
On the application of an experimental multipolar pseudo-atom library for accurate refinement of small-molecule and protein crystal structures
2007
International audience; With an increasing number of biomacromolecular crystal structures being measured to ultra-high resolution, it has become possible to extend to large systems experimental charge-density methods that are usually applied to small molecules. A library has been built of average multipole populations describing the electron density of chemical groups in all 20 amino acids found in proteins. The library uses the Hansen & Coppens multipolar pseudo-atom model to derive molecular electron density and electrostatic potential distributions. The library values are obtained from several small peptide or amino acid crystal structures refined against ultra-high-resolution X-ray diff…
Three‐Component Self‐Assembly Changes its Course: A Leap from Simple Polymers to 3D Networks of Spherical Host–Guest Assemblies
2021
Angewandte Chemie / International edition 60(21), 12132 - 12142 (2021). doi:10.1002/anie.202103178
Approaching a “naked” boryl anion: amide metathesis as a route to calcium, strontium, and potassium boryl complexes
2020
Abstract Amide metathesis has been used to generate the first structurally characterized boryl complexes of calcium and strontium, {(Me3Si)2N}M{B(NDippCH)2}(thf)n (M=Ca, n=2; M=Sr, n=3), through the reactions of the corresponding bis(amides), M{N(SiMe3)2}2(thf)2, with (thf)2Li‐ {B(NDippCH)2}. Most notably, this approach can also be applied to the analogous potassium amide K{N(SiMe3)2}, leading to the formation of the solvent‐free borylpotassium dimer [K{B(NDippCH)2}]2, which is stable in the solid state at room temperature for extended periods (48 h). A dimeric structure has been determined crystallographically in which the K+ cations interact weakly with both the ipso‐carbons of the flanki…
Calculation of size‐intensive transition moments from the coupled cluster singles and doubles linear response function
1994
Coupled cluster singles and doubles linear response (CCLR) calculations have been carried out for excitation energies and dipole transition strengths for the lowest excitations in LiH, CH+, and C4and the results compared with the results from a CI-like approach to equation of motion coupled cluster (EOMCC). The transition strengths are similar in the two approaches for single molecule calculations on small systems. However, the CCLR approach gives size-intensive dipole transition strengths, while title EOMCC formalism does not. Thus, EOMCC calculations can give unphysically dipole transition strengths, e.g., in EOMCC calculations on a sequence of noninteracting LiH systems we obtained a neg…
On n–Fold Blocking Sets
1986
An n-fold blocking set is a set of n-disjoint blocking sets. We shall prove upper and lower bounds for the number of components in an n-fold blocking set in projective and affine spaces.
Three cyclic branched covers suffice to determine hyperbolic knots.
2005
Let n > m > 2 be two fixed coprime integers. We prove that two Conway reducible, hyperbolic knots sharing the 2-fold, m-fold and n-fold cyclic branched covers are equivalent. Using previous results by Zimmermann we prove that this implies that a hyperbolic knot is determined by any three of its cyclic branched covers.
Theory of tailor automata
2019
Abstract In the paper, a fragment of the new theory of tailor automata is presented, within which a deterministic finite automaton was defined. The proposed automaton provides a theoretical model of an informally characterized biomolecular automaton. The idea of working of which is founded on the concept of alternating cut of some double-stranded fragments of DNA, with the use of a restriction enzyme and ligations of some double-stranded fragments of DNA, with the use of the ligase enzyme.
Tilted phases of fatty acid monolayers
1995
X‐ray diffraction data from water‐supported monolayers of fatty acids with chain lengths from 19 to 22 is presented. The structures of the tilted mesophases L2’, L2, and Ov are characterized in detail. The contributions to the unit cell distortion from the tilt and the ordering of the backbone planes of the molecules are separated. It is shown that at the swiveling transition L2’–L2, not only the tilt azimuth but also the packing of the backbone planes change discontinuously. We demonstrate that the tilting transition LS–L2 is accompanied by the ordering of the backbone planes and may be discontinuous. Evidence is presented for a herringbone ordering transition within the L2 region. The dis…
Molecular dynamics simulation of single DNA stretching reveals a novel structure
2005
Abstract MD simulation, to closely mimic a torsionally unconstrained single-molecule stretching experiment of dsDNA, uncovers three distinct force regimes, characterized by fast and slow elongation regions with a transition regime in between, where the α and γ backbone torsion angles of the elongated double-stranded DNA find rapidly new stationary values. In the slow elongation regime DNA gradually looses its twist, collectively breaks all base-pair H-bonds and develops a remarkable base-stacked structure with the bases strongly tilted, forming a zipper-like stack on the major groove side, stabilized by the narrowing distance between the elongated strands, and by specific water interactions.