Mobilité conformationnelle et migration des liaisons π dans le [24]annulène

Conformational Mobility and Migration of the π Bonds of the [24]annulene. The configuration and the conformation of [24]annulene have been determined after a detailed analysis of its ¹H-NMR spectrum recorded at −95°. At this temperature, molecular dynamics is practically frozen, and the spectrum can be correctly simulated considering eight magnetic sites with the relevant couplings. The [24]annulene exhibits alternation of the double and the single bonds with the CTTTCTTTCTTT sequence (C=cis, T=trans) expressing the connectivity of the double bonds. The signal of the 9 protons pointing inside the ring is 7.72 ppm at lower field than the signal of the 15 outer protons; this indicates a marked paramagnetic ring current. Molecular dynamics is revealed by the dependence of the spectrum upon the temperature; the simulation of the line shape of these spectra indicates that the [24]annulene in solution exists as an equilibrium of two conformers A and B ( B / A ≤0.05), both having the same configuration. Each of these conformers undergoes two isodynamic processes: a migration of the π bonds on the adjacent single bonds (bond shift) described by V and a conformational mobility described by K. The two conformers interconvert extremely rapidely. Conformer A complies with C_3h symmetry, conformer B with C₃ symmetry. The enthalpy, entropy, and free energy of activation for the processes described by V and K in the major conformer A have been determined: these processes are slower than those observed in [16]annulene. From their values, we could deduce that the resonance energy in the [24]-7³annulene is negative and of the order of −9 to −10 kcal⋅mol⁻¹.     

Kinetics of diffusion-limited catalytically activated reactions: an extension of the Wilemski-Fixman approach

We study the kinetics of diffusion-limited catalytically activated A+B-->B reactions taking place in three-dimensional systems, in which an annihilation of diffusive A particles by diffusive traps B may happen only if the encounter of an A with any of the Bs happens within a special catalytic subvolumen: these subvolumens being immobile and uniformly distributed within the reaction bath. Suitably extending the classical approach of Wilemski and Fixman [J. Chem. Phys. 58, 4009 (1973)] to such three-molecular diffusion-limited reactions, we calculate analytically an effective reaction constant and show that it comprises several terms associated with the residence and joint residence times of Brownian paths in finite domains. The effective reaction constant exhibits a nontrivial dependence on the reaction radii, the mean density of catalytic subvolumens, and particles' diffusion coefficients. Finally, we discuss the fluctuation-induced kinetic behavior in such systems.     

Photoinduced morphism of gemini surfactant aggregates

The photochemical behaviour of an azobenzene chromophore inserted in a gemini surfactant imparts photocontrol to the resulting amphiphile assemblies, including the collapse, upon irradiation, of the multi lamellar vesicles formed in aqueous solution.     

Synthese et reactivite d''organometalliques perfluores. VII [1] reactivite comparee de derives perfluoroalkyles satures et insatures de l''etain et du mercure vis-a-vis d''electrophiles.

The reactivity of some derivatives of perfluorooctenyl-tin, perfluorooctenyl-mercury and perfluorooctyl-mercury towards protic acids, especially hydrogen chloride, is studied.

The easy cleavage of the tin - sp² carbon bond is painted out, and used to study possibilities of transferring the perfluorooctenyl radical on mercury, upon reaction with mercuric salts.

The mercury-perfluoroalkyl radical bond is shown to be, on the contrary, very resistant to an electrophilic attack.

These results are compared to those concerning the hydrocarbon analogs, and an interpretation, relied to the electronic density on the carbon atom bound to the metallic atom, is given.     

Blinded predictions of host-guest standard free energies of binding in the SAMPL5 challenge

In the context of the SAMPL5 blinded challenge standard free energies of binding were predicted for a dataset of 22 small guest molecules and three different host molecules octa-acids (OAH and OAMe) and a cucurbituril (CBC). Three sets of predictions were submitted, each based on different variations of classical molecular dynamics alchemical free energy calculation protocols based on the double annihilation method. The first model (model A) yields a free energy of binding based on computed free energy changes in solvated and host-guest complex phases; the second (model B) adds long range dispersion corrections to the previous result; the third (model C) uses an additional standard state correction term to account for the use of distance restraints during the molecular dynamics simulations. Model C performs the best in terms of mean unsigned error for all guests (MUE \(3.2\,<\,3.4\,<\,3.6\,\text{kcal}\,\text{mol}^{-1}\)—95 % confidence interval) for the whole data set and in particular for the octa-acid systems (MUE \(1.7\,<\,1.9\,<\,2.1\,\text{kcal}\,\text{mol}^{-1}\)). The overall correlation with experimental data for all models is encouraging (\(R^2\, 0.65\,<\,0.70<0.75\)). The correlation between experimental and computational free energy of binding ranks as one of the highest with respect to other entries in the challenge. Nonetheless the large MUE for the best performing model highlights systematic errors, and submissions from other groups fared better with respect to this metric.     

Influence of Equilibration Time in Solution on the Inclusion/Exclusion Topology Ratio of Host–Guest Complexes Probed by Ion Mobility and Collision‐Induced Dissociation

Host–guest complexes are formed by the creation of multiple noncovalent bonds between a large molecule (the host) and smaller molecule(s) or ion(s) (the guest(s)). Ion‐mobility separation coupled with mass spectrometry nowadays represents an ideal tool to assess whether the host–guest complexes, when transferred to the gas phase upon electrospray ionization, possess an exclusion or inclusion nature. Nevertheless, the influence of the solution conditions on the nature of the observed gas‐phase ions is often not considered. In the specific case of inclusion complexes, kinetic considerations must be taken into account beside thermodynamics; the guest ingression within the host cavity can be characterized by slow kinetics, which makes the complexation reaction kinetically driven on the timescale of the experiment. This is particularly the case for the cucurbituril family of macrocyclic host molecules. Herein, we selected para‐phenylenediamine and cucurbit[6]uril as a model system to demonstrate, by means of ion mobility and collision‐induced dissociation measurements, that the inclusion/exclusion topology ratio varies as a function of the equilibration time in solution prior to the electrospray process.     

Highly Phosphorescent Crystals of Square‐Planar Platinum Complexes with Chiral Organometallic Linkers: Homochiral versus Heterochiral Arrangements,Induced Circular Dichroism,and TD‐DFT Calculations

A novel class of chiral luminescent square‐planar platinum complexes with a π‐bonded chiral thioquinonoid ligand is described. Remarkably the presence of this chiral organometallic ligand controls the aggregation of this square planar luminophor and imposes a homo‐ or hetero‐chiral arrangement at the supramolecular level, displaying non‐covalent Pt–Pt and π–π interactions. Interestingly these complexes are highly luminescent in the crystalline state and their photophysical properties can be traced to their aggregation in the solid state. A TD‐DFT calculation is obtained to rationalize this unique behavior.