High‐Temperature Chlorination‐Reduction Sequence for the Preparation of Silicon Hydride Modified Silica Surfaces

A general method for the functionalization of silica surfaces with silicon hydride (Si–H) groups is described for four different preparations of silica. The silica surface is reduced in a two‐step chlorination–reduction procedure within a simple gas‐flow system at high temperatures. After initial dehydroxylation of the silica surface, silicon chloride groups are formed by the reaction with thionyl chloride. The chlorination activates otherwise inaccessible surface siloxane moieties. A high silicon–hydride surface concentration results from the subsequent reduction of the chlorinated surface with hydrogen. The physical properties of the resulting silica are analyzed using scanning electron microscopy, as well as dynamic light scattering and Brunauer–Emmet–Teller measurements. The chlorination–reduction sequence has no significant impact on the structure, surface area and mesopore size of the silica materials used. The surface of the materials is characterized by diffuse reflectance infrared Fourier transform (DRIFT) and ²⁹Si CP/MAS NMR spectroscopy. The silicon–hydride groups are mostly of the ‐type. The use of high temperatures (>800 °C) results in the condensation of internal and surface silanol groups. Therefore, materials with both a fully condensed silica matrix as well as a surface free of silanol groups are obtained. The materials are ideal precursors for further molecular silica surface modification, as demonstrated with a ferrocene derivative.     

Photoquenching effects and S1 absorption in cryptocyanine

Photoquenching methods are employed to obtain excited state molecular parameters for cryptocyanine. The results are in good agreement with previously reported data obtained by ps and flash spectroscopy methods.     

Nickel complexes with new bidentate P,N phosphinitooxazoline and -pyridine ligands: application for the catalytic oligomerization of ethylene

The phosphinitooxazoline 4,4-dimethyl-2-[1-oxy(diphenylphosphine)-1-methylethyl]-4,5-dihydrooxazole (9), the corresponding phosphinitopyridine ligands 2-ethyl-[1'-methyl-1'-oxy(diphenylphosphino)]pyridine (11) and 2-ethyl-6-methyl-[1'-methyl-1'-oxy(diphenylphosphino)]pyridine (12), which have a one-carbon spacer between the phosphinite oxygen and the heterocycle, and the homologous ligand 2-propyl-[2'-methyl-2'-oxy(diphenylphosphino)]pyridine (13), with a two-carbon spacer, were prepared in good yields. The corresponding mononuclear [NiCl(2)(P,N)] complexes 14 (P,N = 9), 15 (P,N = 11), and 16 (P,N = 12) and the dinuclear [NiCl(micro-Cl)(P,N)](2) 17 (P,N = 13) Ni(II) complex were evaluated in the catalytic oligomerization of ethylene. These four complexes were characterized by single-crystal X-ray diffraction in the solid state and in solution with the help of the Evans method, which indicated differences between the coordination spheres in the solution and the solid state. In the presence of methylalumoxane (MAO) or AlEt(3), only the decomposition of the Ni complexes was observed. However, complexes 14-17 provided activities up to 50000 mol C(2)H(4)/(mol Ni).h (16 and 17) in the presence of only 6 equiv of AlEtCl(2). The observed selectivities for ethylene dimers were higher than 91% (for 14 or 15 in the presence of only 1.3 equiv of AlEtCl(2)). The activities for 14-17 were superior to that of [NiCl(2)(PCy(3))(2)], a typical dimerization catalyst taken as a reference. The selectivities of the complexes 14-17 for ethylene dimers and alpha-olefins were the same order of magnitude. From the study of the phosphinite 9/AlEtCl(2) system, we concluded that in our case ligand transfer from the nickel atom to the aluminum cocatalyst is unlikely to represent an activation mechanism.     

Non-exponential fluorescence decay of gas phase acetaldehyde

Non-exponential fluorescence decay with “fast” and “slow” components was observed for acetaldehyde samples in the pressure range 25–400 m Torr using laser excitation at 320.0 nm. Both fluorescence lifetimes and quantum yields for the “slow” component were found to obey Stern-Volmer relationships. Above 5 Torr, lifetimes and yields become independent of pressure. These data are interpreted in terms of intermediate-type, singlet-triplet coupling and reversible intersystem crossing.     

Nickel and iron complexes with oxazoline- or pyridine-phosphonite ligands; synthesis, structure and application for the catalytic oligomerisation of ethylene

The bis(oxazolinyl)phenylphosphonite ligand (bis(4,4-dimethyl-2-(1-hydroxy-1-methylethyl)-4,5-dihydrooxazole)phenylphosphonite, NOPONMe2)) and the new pyridine-phosphonite ligand (2-ethyl(1'-methyl-1-hydroxy)pyridine-6H-dibenz[c,e][1,2]oxaphosphorin) have been used for the preparation of the mononuclear complexes [NiCl2(NOPONMe2)] 18 and [NiCl2(6)2] 19, respectively, which catalyze the oligomerisation of ethylene with activities up to 57300 mol C2H4 mol Ni(-1) h(-1) (19 in the presence of only 6 equivalents of AlEtCl2). The selectivities for C4 dimers were as high as 90% (18 in the presence of only 2 equivalents of AlEtCl2) with selectivities for 1-butene of 21-22% of the C4 fraction. In the presence of 400 or 800 equivalents of MAO as cocatalyst, complex 19 yielded turnover frequencies of 7400 mol C2H4 mol Ni(-1) h(-1) and 13200 mol C2H4 mol Ni(-1) h(-1), respectively. The selectivities for 1-butene and ethylene dimers were similar to those obtained with AlEtCl2. The fact that 19 with a cyclic phosphonite moiety leads to higher activities and selectivities than 18 which contains an acyclic phosphonite group underlines the importance of the ligand on the catalytic properties of its metal complex. An unprecedented dinuclear iron complex [FeCl2(4,4-dimethyl-2-[(1-hydroxy-1-methyl)ethyl]-4,5-dihydrooxazolate)]2 20 was also obtained which contains two pentacoordinated metal centers coordinated by a bridging-chelating oxazoline-alcoholate. Complexes 18-20 are paramagnetic in solution, as determined by the Evans method.     

Electroanalytical simulations. Orthogonal collocation simulation of fast second-order chemical reactions coupled to an electron transfer with a heterogeneous equivalent formulation

The combination of orthogonal collocation and the heterogeneous equivalent technique is extended to simulate cyclic voltammograms of fast second-order follow-up reactions coupled to an electron transfer at an electrode surface. The $\text{ED}$ (reversible electron transfer with irreversible follow-up dimerization) and $\text{EC}$₂ (reversible electron transfer with irreversible second-order follow-up reaction) models are considered. The non-linear boundary equations are solved numerically. No linear approximation of the concentration profiles is required. The use of non-linear space coordinate transformations is described. Peak potential and peak current function results are compared with literature values and agreement is found. The transition between the second-order $\text{EC}$₂ and the corresponding first-order mechanisms is discussed.     

Electrochemical oxidations: Part IV. Electrochemical investigations into the behaviour of 2,6-di-tert-butyl-4-(4-dimethylaminophenyl)-phenol part 1. Phenol and the species derived from it: Phenoxy radical,phenolate anion and phenoxenium cation

The anodic oxidation of 2,6-di-tert-butyl-4-(4-dimethylaminophenyl)phenol (IIa) was investigated by cyclic voltammetry. The stable products resulting from the oxidation are phenoxenium cation (IId) (which itself is reduced in two one-electron steps to the phenolate anion (IIc) via the phenoxy radical, IIb) and the “protonated phenol (III) (which is oxidized to (IId) in a second peak at higher potential). A mechanism for the electrochemical oxidation of phenol (IIa) is suggested.     

Transition probabilities for coherent multiphoton absorption processes

The feasibility of coherent multiphoton propagation effects such as two-photon self-induced transparency is examined by calculating the coherent transition probabilities for a multiphoton process. For a two-photon excitation of a three-level system, periodic probability functions are obtained which increase smoothly as the intermediate state approaches resonance with the radiation field. The results for a coherent multiphoton excitation of a multilevel system are an extension of Rabi's “strong signal theory” for a one-photon excitation of a two-level system.