C-H Activation and C-C coupling of arenes by cationic Pt(II) complexes

The synthesis and characterization of cationic platinum complexes of the type [(R(2)PC(2)H(4)PR(2))PtMe(OEt(2))]BAr(F) (R = Cy, Et) are reported. These electrophilic platinum cations are found to react quantitatively with arenes (benzene, toluene) at room temperature by undergoing intermolecular C-H activation with concomitant C-C coupling to generate complexes of the type [[Pt(R(2)PC(2)H(4)PR(2))](2)(mu-eta(3):eta(3)-biaryl)][BAr(F)](2). The dianionic biaryl ligands in these compounds exhibit a rare mu-eta(3):eta(3)-bis-allyl bonding mode and can be removed from the complex with stoichiometric oxidants to generate the free biaryl and [(R(2)PC(2)H(4)PR(2))Pt(mu-X)](2)[BAr(F)](2) (R = Cy, Et; X = Cl, I). The cationic platinum complexes [(R(2)PC(2)H(4)PR(2))PtMe(OEt(2))]BAr(F) (R = Cy, Et) are also quite reactive with water, forming the bridging hydroxide complexes [(R(2)PC(2)H(4)PR(2))Pt(mu-OH)](2)[BAr(F)](2) (R = Cy, Et). A possible mechanism is proposed for the C-C coupling reaction based upon the structures of these bridging biphenyl complexes, which provides a new perspective for the related palladium-catalyzed oxidative coupling of arenes to form biaryls.     

Imposing curvature on a polyarene by intramolecular palladium-catalyzed arylation reactions: A simple synthesis of Dibenzo

Palladium catalysis has been found to offer an effective solution-phase alternative to gas-phase flash vacuum pyrolysis as a method for converting the planar ring system of 7, 10-di(2-bromophenyl)fluoranthene (4) to that of the C(28)H(14) bowl-shaped fullerene fragment dibenzo[a,g]corannulene (5).     

The effect of thermal processing on PVC—VI. The role of hydrogen chloride

The reaction between cumene hydroperoxide (CHP) and anhydrous hydrogen chloride under various conditions of temperature and concentration has been studied in an inert solvent (chlorobenzene) and in an oxidisable medium (cumene). The kinetics were first order with respect to each reactant; the energy of activation of the overall second-order reaction was 53.4 kJ mol^?1 K^?1. Competing ionic and free radical mechanisms were found to operate, the latter predominating at relatively high HCl concentrations, leading to rapid pro-oxidation in cumene. The effect of typical organo-tin PVC stabilizers on the pro-oxidant process were examined. It was found that dibutyltin maleate neutralized the HCl thus eliminating the pro-oxidant effect when used in stoichiometric proportions, but had little other effect. Dioctyltin bis(isol-octylthioglycollate) on the other hand not only neutralized the HCl stoichiometrically but gave additional oxidative stabilization over a wide concentration range.     

The determination of intrinsic reaction coordinates by density functional theory

The first implementation of the intrinsic reaction coordinate (IRC ) method within the density functional theory (DFT ) framework is presented. The implementation has been applied to four different types of chemical reactions represented by the isomerization process, HCN ? HNC (A); the S_N2 process, H^? + CH₄ ? CH₄ + H^? (B); the exchange process, H˙ + HX ? HX + H˙ (X ? F,Cl) (C); and the elimination process, C₂H₅Cl ? C₂H₄ + HCl (D). The present study presents for each process optimized structures and calculated harmonic vibrational frequencies for the reactant(s), the transition state, and the product(s) along with the IRC path connecting the stationary points. The calculations were carried out within the local density approximation (LDA ) as well as the LDA/NL scheme where the LDA energy expression is augmented by Perdew's and Becke's nonlocal (NL ) corrections. The LDA and LDA/NL results are compared with each other as well as the best available ab initio calculations and experimental data. For reaction (D), ab initio calculations based on MP 2 geometries and MP 4SDTQ energies have been added due to the lack of accurate published post-HF calculations on this process. A detailed discussion is provided on the efficiency of the IRC algorithms, the relative accuracy of the DFT and ab initio schemes, as well as the reaction mechanisms of the four reactions. It is concluded that the LDA/NL scheme affords the same accuracy as does the MP 4 method. The post-HF methods seem to overestimate activation energies, whereas the corresponding LDA/NL estimates are too low. The LDA activation energies are even lower than the LDA/NL counterparts. The incorporation of the IRC method into the DFT framework provides a promising and reliable tool for probing the chemical reaction path on the potential energy surfaces, even for large-size systems. IRC calculations by ab initio methods of an accuracy similar to the LDA/NL scheme, such as the MP 4 scheme, are not feasible. © John Wiley & Sons, Inc.     

Mechanisms of antioxidant action: The effect of thermal processing on the antioxidant activity of sulphur compounds

The nickel dithiophosphates and their corresponding disulphides are shown to be more effective uv stabilisers for LDPE than the closely related nickel xanthates and disulphides (dixanthogens). In general, the nickel complexes in both series are more effective uv stabilisers than the disulphides.Evidence is presented to show that the better performance of the dithiophosphates than the xanthates is associated with their higher solubility in the polymer. For the same reason, nickel dithiolates containing long alkyl substituents are more effective uv stabilisers than those containing shorter alkyl substituents.Thermal processing adversely affects the photo-stabilising performance of nickel dithiolates whereas the performance of the less efficient disulphides is enhanced under the same conditions. In the former case, the uv stability of the polymer is shown to be directly proportional to the amount of the nickel complex left after processing. In the case of the disulphides, on the other hand, the evidence suggests that increasing the severity of their processing increases the extent of oxidation to products which are more effective uv stabilisers than the parent disulphides.     

Recoil spectrometry: Ion accelerator based elemental characterisation of surface layers

Recoil Spectrometry covers a group of techniques that are very similar to the well known Rutherford backscattering Spectrometry technique, but with the important difference that one measures the recoiling target atom rather than the projectile ion. This makes it possible to determine both the identity of the recoil and its depth of origin from its energy and velocity, using a suitable detector system. The incident ion is typically high-energy (30–100MeV)³⁵C1,⁸¹Br or¹²⁷I. Low concentrations of light elements such as C, O and N can be profiled in a heavy matrix such as Fe or GaAs. Here we present an overview of mass and energy dispersive recoil Spectrometry and illustrate its successful use in some typical applications.     

The uniqueness of the adjoint operation I

Let H be a complex, infinite-dimensional Hilbert space. Let B(H) denote the set of bounded linear operators on H. This paper contains a nonlinear characterization of the adjoint operation on B(H). The statement of this result is:THEOREM:Let h: B(H) B(H)be a function such that h(I)0.Then h(ST)=h(T)h(S)and h(S)S0for all elements Sand Tof B(H)if and only if h(S)=S^* for all S B(H).