Dihedral angle

About: Dihedral angle is a research topic. Over the lifetime, 15718 publications have been published within this topic receiving 174904 citations.

The shape and stability of liquid menisci at solid edges

Abstract: The shape and stability of liquid menisci attached to a solid edge with dihedral angle 2α is investigated. It is shown that in addition to the family of cylindrical menisci a family of azimuthally modified unduloids exists. A double Fourier series of the latter with respect to their axis (parallel to the extension of the edge) and with respect to the azimuth is derived. The dispersion relation between the axial wavenumber q, the azimuthal wavenumber s and the waviness parameter d is calculated. When the condition of constant contact angle γ along the contact lines with the solid is applied, a one-dimensional family of modified unduloids fitting to the edge is obtained. Their axial wavenumber q becomes independent of the waviness d at the bifurcation with the family of cylindrical menisci, such that this bifurcation limits the stability. The respective stability criteria are derived and evaluated. For α + γ > ½π the cylindrical menisci are convex. They reveal a maximum stable length, which quadratically tends to infinity when α + γ = ½π is approached. The smallest stable extension arises for the free cylindrical column (the Rayleigh jet), which is covered by the present investigations by assuming α = π, γ = ½π. For α + γ < ½π the cylindrical menisci are concave and stable: no bifurcation with the family of modified unduloids arises.

Conformational control of TT dimerization in DNA conjugates. A molecular dynamics study.

Abstract: The paper presents quantum yield results for the [2+2] and 6-4 photodimerization of TT steps in several DNA structures, including hairpins where the context dependence of the photodimerization yield is determined, and it develops a theoretical model that correctly describes the trends in dimerization yield with DNA structure. The DNA conjugates considered include dT(20), dA(20)dT(20), and three alkane-linked hairpins that contain a single TT step. The theoretical modeling of the [2+2] process is based on CASSCF electronic structure calculations for ethylene + ethylene, which show that photoexcitation of low-lying excited states leads to potential surfaces that correlate without significant barriers to a conical intersection with the ground state surface at geometries close to the dimer structure. The primary constraint on dimerization is the distance d between the two double bonds, and it is found that d < 3.52 A leads to quantum yield trends that match the observed trends within a factor of 3. Constraints on the dihedral angle between the two double bonds are not as important, and although it is possible to generate better dimerization yield predictions for some structures by including these constraints, the best overall picture is obtained with no constraint. For 6-4 dimerization, a distance g < 2.87 A and no constraint on dihedral angle provide an accurate description of the yield.

Synthesis, Spectroscopic, Electrochemical and Pb2+-Binding Studies of Tetrathiafulvalene Acetylene Derivatives

Abstract: A series of tetrathiafulvalene acetylene derivatives, [TTF−C⋮C−A] [A = C6H4N(CH3)2-4 (1), C6H4OCH3-4 (2), C6H5 (3), C6H4F-4 (4), C6H4NO2-4 (5), C5H4N-2 (6), C5H4N-3 (7), and C5H4N-4 (8)], have been designed and synthesized to provide insight into the nature of the donor−acceptor interaction via a π-conjugated triple bond. The X-ray crystal structure of [TTF−(C⋮C)−C6H4OCH3-4] (2) reveals that the phenyl ring linked by acetylene is almost coplanar to the plane of TTF with a dihedral angle of 3.6°. The strong intermolecular C−H···O hydrogen bonding was found to direct the molecular helical assemblies with a screw pitch of 5.148 A when viewed along the a-axis. Spectroscopic and electrochemical behaviors of the tetrathiafulvalene acetylene derivatives demonstrate that the TTF unit interacts with the electron-accepting group through the triple bond, thus leading to the intramolecular charge transfer. The pyridine-substituted TTF compounds 6−8 show remarkable sensing and coordinating properties toward Pb2+. Comp...

Trinuclear copper(II) complexes derived from Schiff-base ligands based on a 6-amino-6-deoxyglucopyranoside: structural and magnetic characterization.

Abstract: The trinuclear copper(II) complexes ([CuL1)(mu-ac)Cu(mu-ac)CuL1) (1) and ([CuL2)(mu-ac)Cu(mu-ac)CuL2) (2) of the tridentate aminosaccharide-derived Schiff-base ligands H2L1 [6-N-(salicylidene)amino-6-deoxy-1,2,3-tri-O-methyl-alpha-D-glucopyranoside] and H2L2 [6-N-(3,5-di-tert-butylsalicylidene)amino-6-deoxy-1,2,3-tri-O-methyl-alpha-D-glucopyranoside] were synthesized and structurally characterized. The trinuclear complex units can be described as two terminal copper-ligand moieties bridged by a central copper acetate moiety, with the Cu centers arranged in a triangular fashion. IR and UV/vis spectroscopic studies strongly indicate that the trinuclear structure is maintained in a methanolic solution. The temperature dependence of the magnetic susceptibility of both complexes shows a moderate antiferromagnetic coupling and can be well interpreted by applying a symmetric Cua-Cub-Cua' model with linear spin topology. The fit of the magnetic data affords coupling constants J of -34 and -24 cm(-1) for 1 and 2, respectively [H = -J(SaSb + SbSa')]. For mu-alkoxo-mu-acetato-bridged copper(II) complexes with a large dihedral angle between the adjacent coordination planes, as found in 1 and 2, such an antiferromagnetic coupling is unusual. However, density functional theory calculations of 2 using BP86, B3LYP*, and B3LYP density functionals confirmed a symmetric doublet ground state.