Anthony S. Brown

The electron distribution in corundum. A study of the utility of merging single-crystal and powder diffraction data

Powder X-ray diffraction data for corundum were collected by a variety of methods and reduced to structure amplitudes by two profile-fitting techniques. The resulting averaged powder-data set was merged with three different single-crystal data sets to assess the improvements possible over least-squares modelling of extinction for accurate electron density analysis of minerals. With reference to the deformation electron density derived from multipole refinements, it is concluded that this strategy offers advantages over the post facto modelling of severe extinction effects commonly observed in such systems. The deformation electron density is found to be in quantitative agreement with the results of recent ab initio calculations on clusters and the bulk.

The determination of electric field gradients from X-ray diffraction data

The electric field gradient (EFG) tensors for Al and O atoms in corundum and for C and D(2H) in hexadeuterobenzene (C6D6) are determined from accurate X-ray diffraction data using four independent computational strategies. Two of the strategies capitalize on the fact that the electric field gradient is the second derivative of the electrostatic potential, a simple relationship which has not been exploited previously in analyses of diffraction data. The convergence behaviour of the tensor components calculated by each strategy is examined for both systems, and the fully converged results obtained with each computational strategy are shown to be identical. It is found that the orientation of the tensors and the signs of their components are well determined from diffraction data, but the magnitudes and asymmetry of the components are not. Discrepancies in previous analyses of the corundum data are resolved, and it is shown that the origin of the EFG at hydrogen is quite different from that at heavier nuclei....

Overall rate of alternating copolymerization of vinyl acetate with maleic anhydride in methyl ethyl ketone

SummaryThe initial rate of radical copolymerization of vinyl acetate(D) and maleic anhydride(A) was measured dilatometrically in methyl ethyl ketone at 50°C. The rate was found to be maximum at the feed maleic anhydride mole fraction of 0.40,0.44 and 0.47 when the total monomer concentration was 0.75 M.1.50 M and 2.25 M, respectively. Participation of both free monomers and the complex was considered. Georgiev and Zubovs method gave α=kAD/kDA=0.24, βA=kAC/kAC=35,βD=kDC/kDA =2.2 and kAC/kDC=3.9. The equilibrium constant of donor-acceptor complexation between vinyl acetate and maleic anhydride was determined to be 0.030 in acetonitrile at 27°C.

A method for the determination of maleic anhydride content in copolymers

SummaryA simple and reliable non-aqueous direct titration method for the determination of anhydride content in maleic anhydride-vinyl acetate copolymers is described. The procedure involves reaction of the anhydride with a large excess of aniline followed by direct titration of the resulting monoacid with ethanolic NaOH using potentiometric or thymol blue end point detection. The method overcomes many of the disadvantages of previously published methods, and its application to other copolymers is considered.

An electron-density study of germanium: evaluation of the available experimental data

Deformation and valence electron densities in germanium are derived via Fourier summation and multipole refinement of a selectively merged set of X-ray structure factors. The deformation density for germanium appears to be qualitatively different from that in silicon and diamond. The available experimental data are evaluated in the light of problems encountered in the electron-density analysis. In particular, the uncertainties associated with the experimental measurements are compared with the valence-electron contribution to the structure factors. The need for a better and more extensive data set for germanium is highlighted.

Rate of radical polymerization of N,N-dimethylaminoethyl methacrylate in cyclohexane

SummaryDilatometric measurements of the rate of polymerization of N,N-dimethylaminoethyl methacrylate(DMAEMA) gave kp/k21/2=7.67×10−2 (mole. sec/l)1/2 at [DMAEMA]0 =2.000 M in cyclohexane at 60°C. The rate was proportional to the first order and 0.5 order of DMAEME and AIBN concentrations, respectively, in cyclohexane. The initial rate of polymerization varied little in organic solvents.