Search results for "bond"
showing 10 items of 3527 documents
Effect of endodontic treatment on fatigue resistance of fiber posts bonding
2003
Objectives: The aim of this study is to evaluate the effect of the endodontic treatment on the fatigue resistance of endodontic post adhesive interfaces. Materials and methods: 50 single-rooted human teeth have been severed at the cement-enamel junction and randomly assigned to 5 groups receiving different endodontic treatments as follows: 1) distilled water + gutta-percha (control); 2) NaOCl 5% + gutta-percha and Pulp Canal Sealer EWT (Kerr); 3) NaOCl 5% + gutta-percha and Top Seal (Dentsply-Maillefer); 4) NaOCl 5% and EDTA 10% (alternatively) + gutta-percha and Pulp Canal Sealer EWT; 5) NaOCl 5% and EDTA 10% (alternatively) + gutta-percha and Top Seal. Subsequently, Light-Post DT #2 quart…
Solubility and solvation features of native cyclodextrins in 1-ethyl-3-methylimidazolium acetate
2022
The comprehension of the mechanism entailing efficient solvation of cyclodextrins (CD) by green solvents is of great relevance to boost environmentally sustainable usages of smart supramolecular systems. Here, 1-ethyl-3- methylimidazolium acetate, an ecofriendly ionic liquid (IL), is considered as an excellent solvent for native CDs. This IL efficiently dissolves up to 40 wt.% β- and γ-CD already at ambient temperature and X-ray scattering indicates that CDs do not tend to detrimental flocculation under these drastic concentration conditions. Simu- lation techniques reveal the intimate mechanism of CD solvation by the ionic species: while the strong hydrogen bonding acceptor acetate anion i…
Comparison of some dispersion-corrected and traditional functionals as applied to peptides and conformations of cyclohexane derivatives.
2012
We compare the energetic and structural properties of fully optimized α-helical and antiparallel β-sheet polyalanines and the energetic differences between axial and equatorial conformations of three cyclohexane derivatives (methyl, fluoro, and chloro) as calculated using several functionals designed to treat dispersion (B97-D, ωB97x-D, M06, M06L, and M06-2X) with other traditional functionals not specifically parametrized to treat dispersion (B3LYP, X3LYP, and PBE1PBE) and with experimental results. Those functionals developed to treat dispersion significantly overestimate interaction enthalpies of folding for the α-helix and predict unreasonable structures that contain Ramachandran φ and …
New Molecular Topologies by Fourfold Metathesis Reactions within a Hydrogen-Bonded Calix[4]arene Dimer
2003
A calix[4]arene tetrapentyl ether in the cone conformation substituted at its wide rim by four m-(omega-octenyloxy)phenyl urea groups forms hydrogen-bonded dimeric capsules in dichloromethane/benzene (95:5). Metathesis reaction with Grubbs' catalyst under high-dilution conditions (1.1 x 10(-4) M) followed by hydrogenation leads to a covalent connection of all the urea groups within a dimer. Three topologically different products may be expected in such a reaction: a bis[2]catenane, a doubly bridged monocatenane and a tetrabridged capsule. All three possible reaction products could be isolated in an overall yield up to 60 % for the separated and purified compounds. Their identification was b…
Complementarity of reaction force and electron localization function analyses of asynchronicity in bond formation in diels-alder reactions
2014
We have computationally compared three Diels-Alder cycloadditions involving cyclopentadiene and substituted ethylenes; one of the reactions is synchronous, while the others are slightly or highly asynchronous. Synchronicity and weak asynchronicity are characterized by the reaction force constant κ(ξ) having just a single minimum in the transition region along the intrinsic reaction coordinate ξ, while for high asynchronicity κ(ξ) has a negative maximum with minima on both sides. The electron localization function (ELF) shows that the features of κ(ξ) can be directly related to the formation of the new C-C bonds between the diene and the dienophile. There is thus a striking complementarity b…
Phosphine Ligands Based on the Ferrocenyl Platform: Advances in Catalytic Cross-Couplings
2023
Ferrocenyl skeletons tagged with a variety of donor atoms (metalloligands) have become popular in modern metal-catalyzed organic transformations, broadening the applications. Ferrocenylphosphines, such as 1′-bis(diphenylphosphino)ferrocene (dppf), have been widely used as metalloligands in academic and industrial research, motivating further investigation into novel ferrocenylphosphine ligands. The current review concentrated non-exhaustively on emblematic applications of different ferrocenylphosphine ligands in transition metal-catalyzed cross-coupling strategies. Initially, the stereochemical aspects of the ferrocenyl skeleton, nomenclature, and applications of chiral ferrocenyl phosphine…
Relaxation phenomena in mixed isomeric alcohols by Mandelstam-Brillouin scattering
1991
Mandelstam-Brillouin scattering data in mixed isomeric alcohols n-pentanol (nPe-OH) and 2-methyl-2-butanol (2Me-2BuOH) are presented. The hypersonic velocity and normalized absorption are measured as a function of the scattering angle, in the temperature range from - 15-degrees-C to + 45-degrees-C, and as a function of n-PeOH molar fraction going from the pure n-PeOH to the pure 2Me-2BuOH. The experimental results confirm the existence of a shear relaxation phenomenon in the GHz region, that has been previously detected in pure liquids. The temperature dependence of the relaxation time tau-s and of the shear modulus G-infinity evaluated within viscoelastic liquid models, support the existen…
Three-dimensional solvation structure of ethanol on carbonate minerals
2020
Calcite and magnesite are important mineral constituents of the earth’s crust. In aqueous environments, these carbonates typically expose their most stable cleavage plane, the (10.4) surface. It is known that these surfaces interact with a large variety of organic molecules, which can result in surface restructuring. This process is decisive for the formation of biominerals. With the development of 3D atomic force microscopy (AFM) it is now possible to image solid–liquid interfaces with unprecedented molecular resolution. However, the majority of 3D AFM studies have been focused on the arrangement of water at carbonate surfaces. Here, we present an analysis of the assembly of ethanol – an o…
Impact of structural features of odorant molecules on their retention/release behaviours in dairy and pectin gels
2014
International audience; Reducing of fat content in food requires a reformulation that may cause a different perception of aroma. Maintaining an adequate level of acceptability of these reformulated products for consumers requires a better understanding of the mechanisms that control the retention of odorant molecules in food matrices. Although pectins are commonly employed as thickeners, their effect on the retention of odorant molecules in nonhomogeneous products has been examined more frequently than their effect on the retention of odorant molecules in simple gels models.The purpose of this study was to explore and compare the respective effects of pectin in simple model systems. The rel…
Calculation of proton activity in aqueous protolyte solutions
1980
Abstract A general and thermodynamically exact equation for the calculation of the proton (hydroxide ion) activity of aqueous solutions of monoprotic acidic (and basic) species is given. Under certain conditions, various approximate equations can be derived from the exact form; the applicability of these depends mainly on both the thermodynamic equilibrium constant, K p , of the protolytic reaction, and the stoichiometric concentration, C o , of the protolyte. Taking into account the mean ionic activity coefficients which are calculated by means of the Davies equation, diagrams are constructed exhibiting those combinations of K p and C o for which the respective approximations can be applie…