Translational and rotational diffusion of an anisotropic particle in a molecular liquid: Long-time tails and Brownian limit

Formally exact equations are written down, describing the translational and rotational diffusion of an anisotropic tagged particle in a fluid of anisotropic particles. These equations are tractable in the long-time limit, and reduce to the solution of ordinary hydrodynamic equations supplemented by slip boundary conditions in the Brownian limit for a smooth tagged particle. No rotational viscosities or spin-diffusion constants appear in these results. The relation to other work is discussed.     

Heterodyne detection of CO2 laser radiation scattered from a plasma arc jet

Observations have been made of CO₂ laser radiation scattered incoherently from an argon arc jet plasma. The heterodyne detection technique employed is discussed.     

The acidity of tert-butyl alcohol in near- and supercritical water: a polarizable continuum approach

We use a polarizable continuum approach to study the acidity of tert-butyl alcohol in water at ambient, near-critical, and supercritical conditions. In the most straightforward calculation, the bare ionic species (the tert-butoxy anion and the hydronium cation) are placed in cavities surrounded by a dielectric continuum, using a dielectric constant corresponding to the state point. A second method is first to solvate these ions with a small number of explicit water molecules and then surround this cluster with the dielectric. This is the cluster-continuum approach. We discuss the advantages and disadvantages of these two schemes and we also discuss various ways in which the second method can be implemented. No method showed quantitative agreement with all available experimental results but the first, straightforward method was the most successful in predicting the correct trends. From both a numerical and a theoretical point of view, we believe this is the method of choice.     

Alkynyliodonium salts in organic synthesis. Application to the preparation of the tricyclic core of (+/-)-halichlorine

The tricyclic core of halichlorine has been synthesized through the use of an alkynyliodonium salt/alkylidenecarbene/1,5 C-H insertion sequence that sets the pivotal quaternary center in the target.     

Influence of oxygen and sulphur donor atoms on the electrochemistry of bis-chelates of nickel(II)

The comparative effects of oxygen and sulphur donor atoms upon the electrochemical behaviour of isostructural bis-chelates of nickel(II) has been investigated by d.c. and a.c. polarography in acetone. Substitution of oxygen by sulphur as a donor atom has been found to affect the redox processes both thermodynamically and kinetically. Thus, reductions of the oxygen based compounds occur at more negative potentials than do those of their sulphur analogues and are characterised by slower rates of heterogeneous charge transfer (k_s). Uncertainties in the relative magnitudes of k_s values due to double layer effects are minimised in this system as a result of the small potential range within which the E_1/2-values fall. Brief comments on a related NiS₂N₂ complex and on [Ni(mnt)₂]^2? are also included for comparison purposes.     

Partial oxidation of 4-tert-butyltoluene catalyzed by homogeneous cobalt and cerium acetate catalysts in the Br-/H2O2/acetic acid system: insights into selectivity and mechanism

The partial oxidation of 4-tert-butyltoluene to 4-tert-butylbenzaldehyde by hydrogen peroxide in glacial acetic acid, catalyzed by bromide ions in combination with cobalt(II) acetate or cerium(III) acetate, has been studied in detail. Based on the observed differences in reaction rates and product distributions for the different catalysts, a reaction mechanism involving two independent pathways is proposed. After the initial formation of a benzylic radical species, either oxidation of this intermediate by the metal catalyst or reaction with bromine generated in situ occurs, depending on which catalyst is used. The first pathway leads to the exclusive formation of 4-tert-butylbenzaldehyde, whereas reaction of the radical intermediate with bromine leads to formation of the observed side products 4-tert-butylbenzyl bromide and its hydrolysis and solvolysis products 4-tert-butylbenzyl alcohol and 4-tert-butylbenzyl acetate, respectively. The cobalt(II) catalysts Co(OAc)(2) and Co(acac)(2) are able to quickly oxidize the radical intermediate, thereby largely preventing the bromination reaction (i.e., side-product formation) from occurring, and yield the aldehyde product with 75-80 % selectivity. In contrast, the cerium catalyst studied here exhibits an aldehyde selectivity of around 50 % due to the competing bromination reaction. Addition of extra hydrogen peroxide leads to an increased product yield of 72 % (cerium(III) acetate) or 58 % (cobalt(II) acetate). Product inhibition and the presence of increasing amounts of water in the reaction mixture do not play a role in the observed low incremental yields.     

Additivity of ring distortions in halogen-substituted aromatics: a gas-phase electron diffraction and computational study

The gas-phase structures of 1,2,3-trifluorobenzene, 1,3,5-trifluorobenzene, 2,6-difluoropyridine, and 2,6-dichloropyridine have been determined using electron diffraction and computational methods. The accuracy afforded by modern experimental methods has allowed subtle trends to be identified in the geometrical parameters of the rings. Comparisons have also been made between experimental and theoretical structures. Although the effects of multiple fluorine substituents on a benzene ring are shown to be more or less additive, distortions caused by replacement of a ring carbon atom by nitrogen and by halogen substituents are not.     

Length multiplicities of hyperbolic 3-manifolds

LetM = ℍ³/Γ be a hyperbolic 3-manifold, where Γ is a non-elementary Kleinian group. It is shown that the length spectrum ofM is of unbounded multiplicity.     

Structural and thermochemical studies of pyrrolidine borane and piperidine borane by gas electron diffraction and quantum chemical calculations

Structural Chemistry - The gaseous structures, thermochemical properties and dehydrogenation reaction energy profiles of the borane complexes of pyrrolidine and piperidine have been investigated...     

Isopropenyl Esters (iPEs) in Green Organic Synthesis

The need for greener compounds able to replace conventional ones with similar reactivity is crucial for the development of sustainable chemistry. Isopropenyl esters (iPEs) represent one eco-friendly alternative to acyl halides and anhydrides. This review provides a comprehensive overview of the preparation methodologies and reported synthetic applications of iPEs and, in particular, of isopropenyl acetate (iPAc). Intriguingly, the presence of a C=C double bond adjacent to the ester functionality makes iPEs appealing in different chemoselective organic synthesis transformations. For instance, the acyl moiety is suitable for transesterification reactions in presence of different heteroatom-based nucleophiles (C−, O−, N−, S−, Se−); these reactions are irreversible thanks to the formation of acetone, obtained upon keto-enol tautomerization of the prop-1-en-2-ol (isopropenyl) leaving group. Similarly, the unsaturation contained in the isopropenyl synthon could be selectively exploited in organic synthesis for electrophilic and/or radical additions as well as in metal-catalyzed cross-coupling reactions. To conclude, iPEs recently found major interest in the direct modification of biomass (i.e. lignin or cellulose) and in the implementation of tandem reactions of esterification-acetalization by exploiting the co-formation of acetone during the reaction.