Ring strain and its effect on the rate of the general-base catalyzed enolization of cyclobutanone

[reaction: see text] The 3-quinuclidinone-catalyzed (pK(BH) = 7.5) enolization of cyclobutanone (1) in D(2)O at 25 degrees C, I = 1.0 (KCl) was followed by deuterium incorporation, which was determined by (1)H NMR. The second-order rate constant for the buffer-catalyzed deprotonation of 1 was found to be k(B) = 3.3 x 10(-4) M(-1) s(-1), which is compared to rates for acetone and 2-(2'-oxopropyl)benzaldehyde under similar conditions. The data shows that ring strain has very little effect on the energy barrier to deprotonation of 1 vs the unstrained systems.     

The viscoelasticity and structure of fibrous proteins

Summary DynamicYoung's moduli of wool fibres in water have been measured over a range of two decades in rate of extension and at temperatures from 0 to 90 C. Small displacements were used, corresponding to extensions of 0.1%. These were imposed upon mean extensions of (a) 1%, (b) 15% and (c) 40%, representative of the three main extension regions of-keratin. The results obtained show great differences from the slopes of the corresponding regions of the force-extension curve carried out at the same temperature and rate of extension.Comparison of the small-strain dynamic moduli in the three regions (a), (b) and (c) indicate that there is a component of the modulus in (a) amounting to 1.3×10¹⁰ dynes/cm² which is associated with intact-helical filaments, and which is substantially constant up to at least 70 C.Approximate superposition of the dynamic moduli at different rates was achieved using theWilliams-Landel-Ferry (WFL) equation with a reference temperature of 35 C. It is thereby concluded that under the present conditions-keratin has an effective glasstransition in the vicinity of –15 C. Above 60–70 C the WLF relation was not obeyed, indicating the presence of a relaxing process with a different temperature-dependence from that of the normal viscoelastic mechanisms; it is suggested that this process may consist in the fission and re-formation of cystine groups.

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Zusammenfassung Die dynamischenYoung-Moduln von Wollfasern in Wasser wurden über einen Bereich von zwei Dekaden der Streckgeschwindigkeit und Temperaturen zwischen 0 und 90 gemessen. Die maximal verwendeten Auslenkungen entsprechen einer Ausdehnung von 0,1%. Diese wurden den mittleren Dehnungen von (a) 1%, (b) 15% und (c) 40% überlagert entsprechend den drei Hauptbereichen der Dehnung von-Keratin. Die erhaltenen Ergebnisse zeigen gro\e Differenzen für die Steigungen der entsprechenden Bereiche Kraft-Dehnungs-Diagramm für gleiche Temperaturen und gleiche Dehnungsgeschwindigkeit.Ein Vergleich der Dehn-Moduln für kleine Dehnungen in den drei Bereichen (a), (b) und (c) zeigen, da\ in (a) eine Komponente des Moduls von etwa 1,3×10¹⁰dyn/ cm² enthalten ist, verbunden mit den intakten-Helices der Moleküle. Und diese bleiben im wesentlichen unverändert bis zu Temperaturen von wenigstens 70 C.Näherungsweise Superposition der dynamischen Moduln bei verschiedenen Dehngeschwindigkeiten konnte mit Hilfe derWilliams-Landel-Ferry (WLF)-Gleichung hinsichtlich einer Referenz-Temperatur von 35 C erreicht werden. Hieraus folgt, da\ unter den angewandten Bedingungen das-Keratin eine effektive Glasübergangstemperatur in der Nachbarschaft von –15C besitzt. Oberhalb von 60 bis 70 C wird die WLP-Gleichung nicht befolgt. Das zeigt die Mitwirkung von Relaxations-Prozessen mit einer unterschiedlichen Temperatur-Abhängigkeit von der der normalen viscoelastischen Mechanismen an. Es ist zu vermuten, da\ dieser Proze\ auf der Aufspaltung und Wiederbildung von Cystin-Gruppen beruht.

     

Neutron diffraction study of [H(4)Co(4)(C(5)Me(4)Et)(4)], a tetrahedral metal cluster complex with four face-bridging hydride ligands

A single-crystal neutron diffraction analysis of the cluster complex [H(4)Co(4)(C(5)Me(4)Et)(4)] was carried out on the new quasi-Laue diffractometer VIVALDI at the Institut Laue-Langevin. The structure consists of four face-bridging hydrides attached to a tetrahedral cobalt metal core. Average distances and angles in the core of the molecule are as follows: Co-Co = 2.571(8), Co-C = 2.158(6), Co-H = 1.749(7), H.H = 2.366(9) A; Co-H-Co = 94.6(3), H-Co-H = 85.1(3) degrees. The hydride ligands are located off the Co-Co-Co planes by an average distance of 0.923(8) A. It is suggested that the dimensions of the HCo(3) fragments found in this molecule provide reasonable estimates for analogous distances and angles associated with chemisorbed H atoms situated on the 3-fold hollows of a cobalt surface. Crystallographic details: space group P2(1)/a (monoclinic); a = 21.979(2), b = 10.924(1), c = 34.406(2) A; beta = 90.81(1) degrees; Z = 8. Final agreement factor: R(F) = 0.099 for 3779 reflections [I > 2sigma(I)] collected at 20 K.     

Large-scale resonance amplification of optical sensing of volatile compounds with chemoresponsive visible-region diffraction gratings

Micropatterning of the vapochromic charge-transfer salt, [Pt(CNC6H4C10H21)4][Pd(CN)4], on transparent platforms yields transmissive chemoresponsive diffraction gratings. Exposure of the gratings to volatile organic compounds (VOCs) such as chloroform and methanol leads to VOC uptake by the porous material comprising the grating lattice or framework, and a change in the material's complex refractive index, ?. The index change is accompanied by a change in the degree of index contrast between the lattice and the surrounding medium (in this case, air), and a change in the diffraction efficiency of the grating. When a monochromatic light source that is not absorbed by the lattice material is employed as a probe beam, only changes in the real component of ? are sensed. Under these conditions, the grating behaves as a nonselective, but moderately sensitive, sensor for those VOCs capable of permeating the porous lattice material. When a probe color is shifted to a wavelength coincident with the vapochromic charge-transfer transition of the lattice material, the sensor response is selectively amplified by up to 3.5 orders of magnitude, resulting in greatly enhanced sensitivity and some degree of chemical specificity. On the basis of studies at four probe wavelengths, the amplification effect is dominated by resonant changes in the imaginary component of the refractive index. The observed wavelength- and analyte-dependent amplification effects are quantitatively well described by a model that combines a Kramers-Kronig analysis with an effective-medium treatment of dielectric effects.     

An electrochemical study of redox indicators : Part I. Substituted chrysoidines

The electrochemical oxidation of various substituted chrysoidines was studied by cyclic voltammetry, to determine which have stable oxidation products. Only 4-hydroxy-chrysoidine has a stable product; the apparent oxidation potential is 0.779 V vs. NHE. 4-Methoxy- and 4-ethoxy-chrysoidine rapidly lose methanol or ethanol, respectively, so that the 4-hydroxychrysoidine wave appears on subsequent scans. All the other chrysoidines studied are irreversible. The results indicate that a hydroxy group in the 4-position is necessary for stability; the 4-alkoxychrysoidines can achieve the stable quinoidal structure by cleavage of the alkoxy group after nucleophilic attack.     

Refinement of borate structures from 11B MAS NMR spectroscopy and density functional theory calculations of 11B electric field gradients

The refinement of borate structures using DFT calculations combined with experimental (11)B quadrupole coupling parameters from solid-state NMR spectroscopy is presented. The (11)B electric field gradient (EFG) tensors, calculated using the WIEN2k software for trigonal and tetrahedral boron sites in a series of model compounds, exhibit a convincing linear correlation with the quadrupole coupling tensor elements, determined from (11)B MAS NMR spectra of the central or satellite transitions. The model compounds include Li(2)B(4)O(7), Mg(2)B(2)O(5), Mg(3)B(2)O(6), NH(4)B(C(6)H(5))(4), and colemanite (CaB(3)O(4)(OH)(3).H(2)O). The (11)B quadrupole moment, Q = 0.0409 +/- 0.0002 barn, derived from the linear correlation, is in excellent agreement with the accepted value for Q((11)B). This demonstrates that DFT (WIEN2k) calculations can provide precise (11)B quadrupole coupling parameters on an absolute scale. On the other hand, DFT calculations based on the reported crystal structures for datolite (CaBSiO(4)(OH)) and danburite (CaB(2)Si(2)O(8)) cannot reproduce the experimental (11)B quadrupole coupling parameters to the same high precision. However, optimization of these structures by minimization of the forces between the atoms (obtained by DFT) results in a significant improvement between the calculated and experimental (11)B quadrupole coupling parameters, which indicates that reliable refinements of the borate structures are obtained by this method. Finally, the DFT calculations also provide important structural information about the sign and orientation of the EFG tensor elements in the crystal frame, a kind of information that cannot be achieved from (11)B NMR experiments on powdered samples.     

Optical activity in the biaryl series

The relationship between the CD spectra of the chiral biaryls and their stereochemical configuration, as a function of the dihedral angle between the molecular planes of the aromatic moieties, has been investigated for biphenyl, 1,1′-binaphthyl, 1,1′-bianthryl and 9,9′-bianthryl in the exciton approximation and, for the 1,1′-binaphthyls, in the π-SCF approximation. Both methods provide unambiguous assignments of absolute configuration except for biaryls with a critical dihedral angle of π/2 in those with effective D_data2 chromophoric symmetry, or 100–110° in the case of the 1,1′-binaphthyls.     

PHOTOREDUCTION OF SOME NITROBIPHENYL ETHER HERBICIDES TO NITRO RADICAL ANIONS BY β-CAROTENE AND RELATED COMPOUNDS

Abstract —The nitrobiphenyl ether herbicides; 4-nitrobiphenyl ether, bifenox, nitrofen, acifluorfen, acifluorfen-methyl, acifluorfen-ethyl, and oxyfluorfen were reduced to their corresponding nitro radical anions by visible light in the presence of β-carotene, lycopene, retinol, retinal, retinoic acid and retinyl acetate in anaerobic solutions at high pH. It was more difficult to obtain spectra of bifenox, nitrofen and oxyfluorfen than acifluorfen derivatives, probably due to their poor solubilities. In neutral solutions the steady-state concentration of the nitro radical anions was low due to their faster rate of dispropor-tionation and the poor solubility of β-carotene. In the presence of retinal, the nitro radical anion of acifluorfen was produced at pH 7.4. Compounds containing conjugated double bonds such as crotonaldehyde and 2.4-hexadienal also reduced acifluorfen and its derivatives to their respective nitro radical anions in the presence of light. Ubiquinone-50 which does not contain conjugated double bonds in the side chain did not reduce acifluorfen under similar conditions.     

Some electrohydrodynamic effects in fluid dispersions

Under the combined influence of applied shear and electric fields, the macroscopic behavior of suspensions reflects coupling of the two fields which produces stresses at the phase boundaries. These stresses in turn cause motion and/or deformation of the suspended phase. Electrohydrodynamic (EHD) factors which govern these motions and deformations are reviewed in detail. both in a general theoretical context and also in terms of extensive experimental observations. Throughout an attempt has been made to relate the microscopic behaviour (including interactions) of the suspended phase to macroscopically observable or bulk properties of the suspension as a whole.The phenomena reviewed here are important to a large number of colloidal problems of interest to the physical, biological and earth sciences.