1,124 research outputs found
Redetermination of trans-cyclohexane-1,4-diammonium dichloride
A redetermination of the crystal structure of the title compound, C6H16N2
2+·2Cl−, was undertaken. All atomic coordinates including those of the H atoms were refined freely. The cation is located on a centre of symmetry. Important for the crystal structure are wavy hydrogen-bonded layers that are formed by ammonium groups and chloride anions, giving hydrogen-bonded rings
The Molecular Structure of Cyclobutane
The cyclobutane molecule has been found by electron diffraction to have the following bond distances and bond angles: C–C, 1.568±0.02A; C–H, 1.098±0.04A; ∠HCH, 114±8°. On the average the ring is nonplanar, with dihedral angle 20° (+10°, −20°), but the equilibrium symmetry may be either D_(2d) (puckered ring) or D_(4h) (planar ring with low rigidity leading to large amplitude of out‐of‐plane bending). This point is discussed in connection with earlier spectroscopic work. The long bond distances found in four‐membered rings are contrasted against the short distances in three‐membered rings, and the strain energies, bond distances, and HCH angles of cycloalkanes are discussed in terms of modern valence concepts. It is suggested that the potential energy arising from a repulsion of the nonbonded carbon atoms may contribute significantly to the apparently anomalously high strain energy of cyclobutane. The repulsive force associated with such a potential is shown to account satisfactorily for the long C–C distances
The Molecular Structure of Cyclopropene, C_3C_4
Cyclopropene, C_3H_4, has been investigated by the electron diffraction method. The cyclic structure is confirmed and the following interatomic distances and angles are found: C–C, 1.525±0.02 A; C=C, 1.286±0.04 A; 〈C–H〉, 1.087±0.04 A; ∠H–C–H, 118° (assumed); ∠C=C–H, 152±12°
The Molecular Structure of Cyclobutane
The cyclobutane molecule has been found by electron diffraction to have the following bond distances and bond angles: C–C, 1.568±0.02A; C–H, 1.098±0.04A; ∠HCH, 114±8°. On the average the ring is nonplanar, with dihedral angle 20° (+10°, −20°), but the equilibrium symmetry may be either D_(2d) (puckered ring) or D_(4h) (planar ring with low rigidity leading to large amplitude of out‐of‐plane bending). This point is discussed in connection with earlier spectroscopic work. The long bond distances found in four‐membered rings are contrasted against the short distances in three‐membered rings, and the strain energies, bond distances, and HCH angles of cycloalkanes are discussed in terms of modern valence concepts. It is suggested that the potential energy arising from a repulsion of the nonbonded carbon atoms may contribute significantly to the apparently anomalously high strain energy of cyclobutane. The repulsive force associated with such a potential is shown to account satisfactorily for the long C–C distances
Lung transplant referral for individuals with cystic fibrosis: Cystic Fibrosis Foundation consensus guidelines
The Molecular Structure of Cyclopropene, C_3C_4
Cyclopropene, C_3H_4, has been investigated by the electron diffraction method. The cyclic structure is confirmed and the following interatomic distances and angles are found: C–C, 1.525±0.02 A; C=C, 1.286±0.04 A; 〈C–H〉, 1.087±0.04 A; ∠H–C–H, 118° (assumed); ∠C=C–H, 152±12°
Peak positions and shapes in neutron pair correlation functions from powders of highly anisotropic crystals
The effect of the powder average on the peak shapes and positions in neutron
pair distribution functions of polycrystalline materials is examined. It is
shown that for highly anisotropic crystals, the powder average leads to shifts
in peak positions and to non-Gaussian peak shapes. The peak shifts can be as
large as several percent of the lattice spacing
Hydration dynamics at fluorinated protein surfaces
Water-protein interactions dictate many processes crucial to protein function including folding, dynamics, interactions with other biomolecules, and enzymatic catalysis. Here we examine the effect of surface fluorination on water-protein interactions. Modification of designed coiled-coil proteins by incorporation of 5,5,5-trifluoroleucine or (4S)-2-amino-4-methylhexanoic acid enables systematic examination of the effects of side-chain volume and fluorination on solvation dynamics. Using ultrafast fluorescence spectroscopy, we find that fluorinated side chains exert electrostatic drag on neighboring water molecules, slowing water motion at the protein surface
Chiral Polymerization in Open Systems From Chiral-Selective Reaction Rates
We investigate the possibility that prebiotic homochirality can be achieved
exclusively through chiral-selective reaction rate parameters without any other
explicit mechanism for chiral bias. Specifically, we examine an open network of
polymerization reactions, where the reaction rates can have chiral-selective
values. The reactions are neither autocatalytic nor do they contain explicit
enantiomeric cross-inhibition terms. We are thus investigating how rare a set
of chiral-selective reaction rates needs to be in order to generate a
reasonable amount of chiral bias. We quantify our results adopting a
statistical approach: varying both the mean value and the rms dispersion of the
relevant reaction rates, we show that moderate to high levels of chiral excess
can be achieved with fairly small chiral bias, below 10%. Considering the
various unknowns related to prebiotic chemical networks in early Earth and the
dependence of reaction rates to environmental properties such as temperature
and pressure variations, we argue that homochirality could have been achieved
from moderate amounts of chiral selectivity in the reaction rates.Comment: 15 pages, 6 figures, accepted for publication in Origins of Life and
Evolution of Biosphere
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