Axial Ring Research Articles

Guest‐Induced Supramolecular Isomerism in Inclusion Complexes of T‐Shaped Host 4,4‐Bis(4′‐hydroxyphenyl)cyclohexanone

The T-shaped host molecule 4,4-bis(4'-hydroxyphenyl)cyclohexanone (1) has an equatorial phenol group and a cyclohexanone group along the arms and an axial phenol ring as the stem. The equatorial phenyl ring adopts a "shut" or "open" conformation, like a windowpane, depending on the size of the guest (phenol or o/m-cresol), for the rectangular voids of the hydrogen-bonded ladder host framework. The adaptable cavity of host 1 expands to 11x15-18 A through the inclusion of water with the larger cresol and halophenol guests (o-cresol, m-cresol, o-chlorophenol, and m-bromophenol) compared with a size of 10x13 A for phenol and aniline inclusion. The ladder host framework of 1 is chiral (P2(1)) with phenol, whereas the inclusion of isosteric o- and m-fluorophenol results in a novel polar brick-wall assembly (7x11 A voids) as a result of auxiliary C-H...F interactions. The conformational flexibility of strong O-H...O hydrogen-bonding groups (host 1, phenol guest), the role of guest size (phenol versus cresol), and weak but specific intermolecular interactions (herringbone T-motif, C-H...F interactions) drive the crystallization of T-host 1 towards 1D ladder and 2D brick-wall structures, that is, supramolecular isomerism. Host 1 exhibits selectivity for the inclusion of aniline in preference to phenol as confirmed by X-ray diffraction, 1H NMR spectroscopy, and thermogravimetry-infrared (TG-IR) analysis. The T(onset) value (140 degrees C) of aniline in the TGA is higher than those of phenol and the higher-boiling cresol guests (T(onset)=90-110 degrees C) because the former structure has more O-H...N/N-H...O hydrogen bonds than the clathrate of 1 with phenol which has O-H...O hydrogen bonds. Guest-binding selectivity for same-sized phenol/aniline molecules as a result of differences in hydrogen-bonding motifs is a notable property of host 1. Host-guest clathrates of 1 provide an example of spontaneous chirality evolution during crystallization and a two-in-one host-guest crystal (phenol and aniline), and show how weak C-H...F interactions (o- and m-fluorophenol) can change the molecular arrangement in strongly hydrogen-bonded crystal structures.

Investigations into the Selective Oxidation of Vicinal Diols to α‐Hydroxy Ketones with the NaBrO3/NaHSO3 Reagent: pH Dependence, Stoichiometry, Substrates and Origin of Selectivity.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Pyrazine bridged benzyl dicobaloximes: Competition between π-interaction and steric crowding in crystal structure

Pyrazine (Pz) bridged benzyl dicobaloximes [ArCH 2Co(dioxime) 2] 2-μ-Pz [dioxime = dmgH, dpgH] have been synthesized and characterized with 1H and 13C NMR. The complexes have been synthesized by a simple procedure in one-pot directly from the corresponding benzyl aqua cobaloxime. In the crystal structure of [PhCH 2Co(dpgH) 2] 2-μ-Pz, two cobaloxime units are in eclipsed form whereas they were completely staggered in the reported [EtCo(dpgH) 2] 2-μ-Pz. This is due to the π–π interaction between the axial benzyl group and phenyl ring of the equatorial dpgH group. Both cis and trans isomer crystallized together in the crystal structure of [PhCH 2Co(dpgH)(dmgH)] 2-μ-Pz. The cyclic voltammetry study in [PhCH 2Co(dioxime) 2] 2-μ-Pz [dioxime = dmgH, dpgH] shows two cobalt center mixed together due to electron delocalization through pyrazine and behaves like a monocobaloxime and the reduction potentials are much higher than the monocobaloximes.

On three-dimensional stability of a uniform, rigidly rotating film on a rotating cylinder

We study the linear stability of rigidly rotating films on a rotating circular cylinder in the absence of gravity. Three-dimensional disturbances are examined, particularly in the limit in which the liquid volume fraction is small, it being assumed throughout that there is a continuous film coating the surface of the cylinder. The most unstable mode for thin film flows on the inside surface of a cylinder, that is, rimming flows, is shown to be purely axial (ring instability), and the critical wave number is identified for given values of the reduced Reynolds number and reciprocal Weber number S. For thin film flows on the outside surface of a cylinder, that is, coating flows, the critical wave number (that with maximal growth rate) is also identified for small reduced Reynolds numbers. For S > 1, the disturbance is purely axial (ring instability). For 0 < S < 1, however, the most unstable disturbance is shown to be either a purely axial or a purely azimuthal mode. Numerical results suggest that a stripe instability (azimuthal mode) is likely to occur. While agreeing with some experimental evidence, this result conflicts with results from others, suggesting that effects ignored in this study, gravity for example, may play a significant role in mode selection.

Investigations into the selective oxidation of vicinal diols to α-hydroxy ketones with the NaBrO 3/NaHSO 3 reagent: pH dependence, stoichiometry, substrates and origin of selectivity

The NaBrO 3/NaHSO 3 reagent is one of the few oxidizing agents that chemoselectively oxidizes vicinal diols to α-hydroxy ketones with little overoxidation to the corresponding vicinal-dione or dicarboxylic acid. Oxidation reactions performed with this reagent showed strong pH dependence. cis-Vicinal diols reacted faster than trans-vicinal diols to the α-hydroxy ketone product. Hydroxy functional groups at axial ring positions were more readily oxidized than equatorial hydroxy groups. The application of the NaBrO 3/NaHSO 3 reagent for the chemoselective oxidation of vicinal diols was limited to simple systems and failed with more complex monosaccharide compounds probably due to acid catalyzed dehydrogenation reactions. Despite the simple reaction set-up and good selectivity towards the α-hydroxy ketone product, the actual oxidation reaction mechanism is highly complex and postulated to involve at least six different equilibria with a plethora of bromine containing species. A possible oxidation reaction mechanism is discussed.

Ring Rolling Research at the Dresden University of Technology – its History from the Beginning in the 70s to the Present

The paper presents a brief history of ring rolling development at the Dresden University of Technology (DUT) from the 70s to the present. The technological scope covers the techniques explored at the DUT: Tangential profile ring rolling, axial profile tube rolling/roll plunging and the TRENPRO® method to produce shaped rings. Starting from the state of the art, technological principle, range of application and machine‐oriented implementation of these techniques are described. Furthermore, other common ring rolling variants, such as axial profile ring rolling, skew‐rolling and radial‐axial ring rolling, are taken into consideration. Finally, the authors provide a summary of problems still to be solved and ways to refine and disseminate the “ring rolling” discipline, assessing the rolling methods’ potential as extremely forward‐looking.

Crystal structure of cis-polar, trans(Cl,O 5)-Na 2[Rh(1,3-pddadp)Cl] · 7H 2O and structural correlations between octahedral pentadentate metal(III) complexes with diaminopolycarboxylato-type ligands

The pentadentate complex cis-polar, trans(Cl,O 5)-Na 2[Rh(1,3-pddadp)Cl] · 7H 2O (where 1,3-pddadp is the 1,3-propanediamine- N, N ′-diacetate- N, N ′-di-3-propionate ion) was prepared and characterized. The structure of the complex was determined by X-ray analysis. The rhodium(III) ion is surrounded octahedrally by the two N and three O atoms of the 1,3-pddadp ligand and one chloride anion. The chloride anion occupies the trans position with respect to the oxygen atom of the axial glycinate ring and the cis-polar orientation with respect to the tertiary nitrogen atom bonded to the uncoordinated 3-propionate group. The investigated pentadentate complex derives directly from the trans(O 5O 6) isomer of the corresponding hexadentate [Rh(1,3-pddadp)] − complex. The conformations of the chelate rings were found to be envelope and flat for in-plane (G ′′) and axial (R ′) glycinate rings, and sofa and chair for in-plane β-alaninate (G ′) and 1,3-propanediamine (T) rings, respectively. The electronic absorption spectrum of cis-polar, trans(Cl,O 5)-[Rh(1,3-pddadp)Cl] 2 − is also given and compared with those of hexadentate trans(O 5O 6)-[Rh(1,3-pddadp)] − and trans(O 5)-[Rh(1,3-pddadp)] − complexes.

The design of bells with harmonic overtones.

Musical bells have had limited application due to the presence of inharmonic partials in the lower part of their acoustic spectra. A series of bells has been designed that contains up to seven partial frequencies in the harmonic series beginning at the fundamental frequency. This was achieved by choosing geometries for finite-element analysis models in which as many purely circumferential bending modes as possible occurred at frequencies below any mode with an axial ring node. The bell models were then fine tuned using gradient projection method shape optimization and the resulting profiles were cast in silicon bronze. A range of bell geometries and timbres is analyzed using psycho-acoustic models and is discussed in relation to European carillon bells.

1H NMR study of the molecular structure and magnetic properties of the active site for the cyanomet complex of O2-avid hemoglobin from the trematode Paramphistomum epiclitum.

The solution molecular and electronic structures of the active site in the extremely O2-avid hemoglobin from the trematode Paramphistomum epiclitum have been investigated by 1H NMR on the cyanomet form in order to elucidate the distal hydrogen-bonding to a ligated H-bond acceptor ligand. Comparison of the strengths of dipolar interactions in solution with the alternate crystal structures of methemoglobin establish that the solution structure of wild-type Hb more closely resembles the crystal structure of the recombinant wild-type than the true wild-type met-hemoglobin. The distal Tyr66(E7) is found oriented out of the heme pocket in solution as found in both crystal structures. Analysis of dipolar contacts, dipolar shift and paramagnetic relaxation establishes that the Tyr32(B10) hydrogen proton adopts an orientation that allows it to make a strong H-bond to the bound cyanide. The observation of a significant isotope effect on the heme methyl contact shifts confirms a strong contact between the Tyr32(B10) OH and the ligated cyanide. The quantitative determination of the orientation and anisotropies of the paramagnetic susceptibility tensor reveal that the cyanide is tilted approximately 10 degrees from the heme normal so as to avoid van der Waals overlap with the Tyr32(B10) Oeta. The pattern of heme contact shifts with large low-field shifts for 7-CH3 and 18-CH3 is shown to arise not from the 180 degrees rotation about the alpha-gamma-meso axis, but due to the approximately 45 degrees rotation of the axial His imidazole ring, relative to that in mammalian globins.

Unique Three-dimensionally Expanded Nanoporous Structure Constructed with a Cu(I) and cis,cis-1,3,5-Triaminocyclohexane Having a 3-Fold Axial Symmetry

Abstract Unique supramolecular crystal with a large macrocyclic channel structure has been prepared from the reaction of a [CuI(MeCN)4]SbF6 and cis,cis-1,3,5-triaminocyclohexane with a three-fold axial symmetry, whose channel is constructed from a distorted pseudofive-fold axial symmetric ring with a pore size of 5×12 Å2.

Influence of the Distal His in Imparting Imidazolate Character to the Proximal His in Heme Peroxidase: 1H NMR Spectroscopic Study of Cyanide-Inhibited His42→Ala Horseradish Peroxidase

The functional higher oxidation states of heme peroxidases have been proposed to be stabilized by the significant imidazolate character of the proximal His. This is induced by a "push-pull" combination effect produced by the proximal Asp that abstracts ("pulls") the axial His ring N(delta)H, along with the distal protonated His that contributes ("pushes") a strong hydrogen bond to the distal ligand. The molecular and electronic structure of the distal His mutant of cyanide-inhibited horseradish peroxidase, H42A-HRPCN, has been investigated by NMR. This complex is a valid model for the active site hydrogen-bonding network of HRP compound II. The (1)H and (15)N NMR spectral parameters characterize the relative roles of the distal His42 and proximal Asp247 in imparting imidazolate character to the axial His. 1D/2D spectra reveal a heme pocket molecular structure that is highly conserved in the mutant, except for residues in the immediate proximity of the mutation. This conserved structure, together with the observed dipolar shifts of numerous active site residue protons, allowed a quantitative determination of the orientation and anisotropies of the paramagnetic susceptibility tensor, both of which are only minimally perturbed relative to wild-type HRPCN. The quantitated dipolar shifts allowed the factoring of the hyperfine shifts to reveal that the significant changes in hyperfine shifts for the axial His and ligated (15)N-cyanide result primarily from changes in contact shifts that reflect an approximately one-third reduction in the axial His imidazolate character upon abolishing the distal hydrogen-bond to the ligated cyanide. Significant changes in side chain orientation were found for the distal Arg38, whose terminus reorients to partially fill the void left by the substituted His42 side chain. It is concluded that 1D/2D NMR can quantitate both molecular and electronic structural changes in cyanide-inhibited heme peroxidase and that, while both residues contribute, the proximal Asp247 is more important than the distal His42 in imparting imidazole character to the axial His 170.

The practicalities of ring rolling simulation for profiled rings

Ring rolling finite element simulation for profiled rings is currently not practicable with existing Lagrangian codes and on any realistic computational platform. The main reason for this is that a large number of incremental stages are typically required for completion of a full simulation and the computational cost per increment is high. The nature of ring rolling means that the amount of deformation that takes place in a given increment is relatively small when compared with typical metal forming processes. Small errors generated over each increment can grow to give highly inaccurate predictions after not so many ring revolutions. Inaccuracies manifest themselves in volume growth, incomplete profile filling, overall dimensional inaccuracy and inaccurate force and stress level predictions. This paper describes measures that make the analysis of profiled ring rolling a more practicable proposition. A model has been developed comprising the finite element flow formulation, an arbitrary Lagrangian–Eulerian update strategy, and a novel iterative solution scheme called the successive preconditioned conjugate gradient method. The focus of the paper is on methods developed to produce accurate predictions and that ensure numerical stability for an arbitrary high number of ring revolutions. The approach adopted includes methods for volume control, circular movement stability, circular interpolation techniques for accurate transfer of state variables, contact evolution, etc. The methods are validated by comparison with earlier experimental work and previously developed models for both pure radial, and radial–axial ring rolling. In addition, some results are presented for the analysis of commercially produced railway wheels and rings.

A practical method for finite element ring rolling simulation using the ALE flow formulation

Finite element ring rolling simulation by conventional Lagrangian codes carries an excessive computational cost. The main reason for this is the large number of incremental stages typically required to complete a full simulation. The nature of ring rolling however means that the amount of deformation taking place in a given increment is relatively small compared with typical metal forming processes. This paper describes measures that make the analysis of ring rolling a practicable proposition. The resulting model is based on a threefold approach, comprising the finite element flow formulation, an arbitrary Lagrangian Eulerian update strategy, and a novel iterative solution scheme called the successive preconditioned conjugate gradient method. The approach exploits the slowly evolving nature of the problem with the effect of reducing the time penalty for each deformation increment. In addition, a number of issues specific to ring rolling have been addressed including the problem of how the mandrel interface is dealt with for arbitrarily shaped rollers. The importance of addressing this particular issue is also illustrated. The method is validated by comparison with earlier experimental work and previously developed models for both pure radial, and radial–axial ring rolling.

Stereoselective synthesis of the axially chiral A-B ring system of vancomycin utilizing a planar chiral arene chromium complex.

[reaction: see text] The axial biaryl ring system of vancomycin was stereoselectively synthesized by utilizing a planar chiral tricarbonyl(arylhalide)chromium complex. Both enantiomers of the planar chiral (arylbromide)chromium complexes, (+)-9 and ent-(-)-9, can be stereoselectively transferred to an absolutely identical key intermediate 23 for the vancomycin A-B ring system by the diastereoselective Suzuki-Miyaura cross-coupling reaction as key step.

The Full-Space Elastodynamic Green’s Functions for Time-Harmonic Radial and Axial Ring Sources in a Homogeneous Isotropic Medium

The elastodynamic Green’s functions for time-harmonic radial and axial ring sources in a homogeneous, isotropic, linear elastic full-space medium are derived using the Fourier-Hankel transform. The Green’s functions are found to have the same logarithmic singularities as the Legendre functions of positive and negative half-degree of the second kind. As the frequency approaches zero, the Green’s functions approach the corresponding elastostatic Green’s functions. The far-field displacement and stress components are also derived.

Conformational Studies in the Cyclohexane Series. 3. The Dihalocyclohexanes

The energies of axial and equatorial fluoro-, chloro-, and bromocyclohexanes as well as diaxial, axial−equatorial, and diequatorial 1,2-, 1,3-, and 1,4-dihalocyclohexanes were calculated using the hybrid density functional methods B3LYP and B3P86 as well as MP2 and QCISD and the 6-311G* and 6-311+G(2df,p) basis sets. The best agreement with experimental data was found with QCISD/6-311+G(2df,p). Solvent effects on the relative energies were calculated using the SCIPCM reaction field model. The effect of the halo substituents on the geometry of the cyclohexane ring was examined, and it was found that the effect of an axial substituent was local flattening of the cyclohexane ring, but no effect was found at the remote ring carbons, and no evidence of 1,3-diaxial interactions between the halogen and the axial ring hydrogens was found. In the case of the 1,2-dihalides, the calculations reproduce the preference for the diaxial form with X = Cl, but also predict that the energy difference between diaxial and diequatorial will be quite small when X = F. This appears to be related to the preference for the gauche form of 1,2-difluoroethane. The calculated relative energies of the 1,4-dihalocyclohexanes are in good agreement with electron diffraction data. A calculation of the electrostatic effects in the 1,4-dichlorocyclohexane reproduced the observed preference for the aa conformer.

Morphology of the 12 Micron Seyfert Galaxies. I. Hubble Types, Axial Ratios, Bars, and Rings

We have compared the morphological characteristics of the 891 galaxies in the Extended 12-micron Sample (E12GS), and assessed the effect of the 12-micron selection criterion on galaxy properties. The normal spirals in the E12GS have the same axial ratios, morphological types, and bar and ring fractions as other normal spirals. The HII/starburst galaxies have a higher incidence of bars, and more than twice the normal rate of "peculiar" morphologies, both of which are attributable to relatively recent disturbances. The galaxies with active nuclei (AGNs: Seyferts and LINERs) have the same incidence of bars as normal spirals, but show rings significantly more often than normal galaxies or starbursts. The LINERs have elevated rates of inner rings, while the Seyferts have outer ring fractions several times those in normal galaxies. The different formation times of bars and rings suggest an interpretation of these differences. Bars form relatively quickly, and indicate that material is recently being transported (by redistribution of angular momentum) to the center of the galaxy, where it is likely to trigger a short (e.g., $\lesssim 10^8$ yrs) burst of star formation. Outer rings may result from similar disturbances, but require much more time to form, while inner rings form in a shorter time, more comparable to bars. Thus it may be that after an interaction or instability triggers an infall of gas, the galaxy in the earliest stage is likely to show enhanced star formation in its center, while later it is more likely to show LINER activity, and still later likely to be a Seyfert.

Solution NMR Determination of the Anisotropy and Orientation of the Paramagnetic Susceptibility Tensor as a Function of Temperature for Metmyoglobin Cyanide: Implications for the Population of Excited Electronic States

Comprehensive 1H NMR assignments of the heme cavity proton resonances of sperm whale metmyoglobin cyanide have provided the dipolar shifts for nonligated residues which, together with the crystal coordinates of carbonyl myoglobin, allow accurate determination of both the anisotropies and orientation of the paramagnetic susceptibility tensor, χ, in the molecular framework. The resulting axial, Δχax = 2.48 × 10-8 m3/mol, and rhombic anisotropy, Δχrh = −0.58 × 10-8 m3/mol, values at 25 °C determined from the most complete set of dipolar shifts are determined to 2% and 6% uncertainty, respectively, and agree well with theoretical estimates (Horrocks, W. D., Jr. and Greenberg, E. S. Mol. Phys. 1974, 27, 993−999). Numerically and spatially restricted input data sets lead to larger uncertainties in Δχax and Δχrh, but do not systematically bias the orientation of the tensor. Determination of the anisotropies and orientation over the temperature range 5−50 °C shows that the susceptibility tensor orientation is minimally influenced, with both anisotropies well-behaved, and with Δχax exhibiting a temperature behavior close to that predicted for the system. The quantitative determination of the magnetic anisotropies over temperature allows the quantitative separation of contact and dipolar shifts for the iron ligands. The heme contact shifts reflect the expected dominant π spin density at pyrrole positions, but the meso-protons exhibit low-field contact shifts indicative of unpaired spin in a σ orbital. Such delocalized σ spin density could arise from either deformation of the heme from planarity or the loss of σ/π separation for the dxz, dyz orbitals when the major magnetic axis is tilted strongly from the heme normal as is experimentally observed. The observed anomalous temperature dependencies of the heme methyl and axial His ring contact shifts, as well as that of the rhombic anisotropy, are all consistent with thermal population of the excited orbital state. The limitations for quanitatively determining the excited orbital state energy separation from the available NMR data are discussed.

Influence of proximal side mutations on the molecular and electronic structure of cyanomet myoglobin: an 1H NMR study.

A series of proximal side mutants of sperm whale metmyoglobin (metMb) that involves residues which provide hydrogen bonds to the axial His and heme have been prepared, and the CO binding and solution molecular and electronic structure has been investigated by 1H NMR. These include Ser92(F7), whose O gamma serves as a hydrogen-bond acceptor to the axial His ring NdeltaH and whose O gamma H serves as hydrogen-bond donor to the 7-propionate carboxylate, and His97(FG3) whose ring provides the other hydrogen-bond donor to the 7-propionate carboxylate. 2D NMR data on the S92A-metMbCN, S92P-metMbCN and H97F-metMbCN show that the distal structure is completely conserved and that proximal side structural changes are highly localized. For the S92A-metMbCN, altered dipolar contacts to the F-helix backbone show that the axial His imidazole has rotated clockwise by approximately 10 degrees relative to a stationary heme, while in H97F-metMbCN, the altered heme-E helix backbone contacts reveal that the heme has rotated counterclockwise by approximately 3 degrees relative to a conserved axial His. The pattern of axial His rotation was qualitatively predicted by energy minimization calculations. The assignments and conserved structural elements allow the determination of a set of magnetic axes whose major magnetic axis is unchanged with respect to WT and confirms that local distal, and not proximal, interactions control the orientation of the major magnetic axis and, by inference, the degree and direction of tilt of the Fe-CN from the heme normal. The rhombic magnetic axes in S92A-metMbCN are rotated approximately 10 degrees in the opposite direction from the established approximately 10 degrees rotation for the axial His ring as expected. It is shown, moreover, that the pairwise alpha-, gamma-meso vs beta-, delta-meso-H hyperfine shift differences are well predicted by the change in the location of the rhombic magnetic axes. Carbon monoxide ligation rates experience minor but systematic perturbation for the S92A substitutions which confirms an influence (albeit very small) for axial His orientation on ligand affinity.

Illaenidae (Trilobita): Morphology of thorax, classification, and mode of life

The axial rings of the thorax of seven Ordovician species of Illaenidae, and of the type species of the SilurianCybantyx,all have the ring curved downward and forward, lacking the articulating furrow but traversed by a stout ventral ridge; the doublure similarly curves downward and forward. This type of axial ring is apparently unique among post-Cambrian trilobites. The axial ring in Scutelluidae is like that occurring in most post-Cambrian trilobites, having a convex ring, deep articulating furrow, and horizontal half ring and doublure. These and many other characters distinguish the Styginidae-Scutelluidae group from Illaenidae.Cybantyxis not an effaced scutelluid, but an illaenid. Upper Cambrian illaenid-like trilobites, which have the illaenid type of thoracic axial ring, deserve consideration as possible ancestors of post-Cambrian illaenids. The thorax is given considerable flexibility by the illaenid axial ring, enabling the body to have been flexed unusually strongly concavely dorsally, as well as to enroll. The musculature suggested omits dorsal longitudinal muscles. It is argued that many illaenids may have been vagrant benthos, particularly adapted for negotiating over irregular surfaces. The view that illaenids lived partly in a burrow is not supported by specimens preserved in such an attitude. The arrangement and varying strength of the terrace ridges on the exoskeleton ofCybantyxis consistent with their having had a sensory function; anastomosing caeca on the fixed cheek and the pleural region of the pygidium are described.