Molecular mechanics (MM2) calculations and cone angles of phosphine ligands

Molecular mechanics (MM2) calculations were performed on 54 conformations of 18 phosphines (PH₃; PH_3−nR_n, where n = 1,…3, and R = Me and Et, n = 1 or 2 and R =ⁱPr, and n = 1 and R =^tBu, PMe₂Et, PMeEt₂, and PPhMe₂, and PPh₂R where R = Me, Et, ⁱPr, ^tBu and Ph). The results are compared to those previously obtained from MINDO/3 and MNDO calculations, and to experimental data. Single conformer cone angles and weighted average cone angles were calculated from MM2 optimized geometries employing Tolman's general definition, and they are compared to Tolman's values, MINDO/3 results, and T.L. Brown's E_R values. Of the cone angle definitions used, the weighted average values are suggested as the best single representation of phosphine ligand sizes. The steric parameters (cone angle and E_R values) alone, and in conjunction with electronic parameters, are correlated with experimental data.     

Meeting Report: Ten Years of Atomic and Molecular Science at Elettra

On the 31^st of May and the 1^st of June, 2007, a workshop to celebrate 10 years of atomic and molecular science was held at Elettra. The occasion marked the 10^th anniversary since the first beamtime was officially allocated to a user at the Gas Phase beamline, namely George King of Manchester University. As a satisfied user, King was not only present, but had beamtime during the workshop.     

Strain relaxation in the epitaxy of La2/3Sr1/3MnO3 grown by pulsed-laser deposition on SrTiO3(001)

With a Curie point at 370?K, the half-metal (La_0.7Sr_0.3)MnO₃ (LSMO) is one of the most interesting candidates for electronic devices based on tunnel magnetoresistance. SrTiO₃ (STO) is up to now the best substrate for the epitaxy of suitable thin films of LSMO. The pseudocubic unit cell of rhombohedral LSMO has a parameter a _LSMO such that (a _STO ? a _LSMO)/a _LSMO = +?0.83% (where a _STO is the parameter of cubic STO) and an angle of 90.26°. As strained growth is tetragonal, relaxation implies recovery of both the pseudocubic parameter and of the original angle. In the LSMO layers that we prepare by pulsed-laser deposition, we show that these two processes are quite independent. The angular distortion is partially recovered by twinning in films 25?nm thick, while recovery of the parameter never occurs in the thickness range that we explored (up to 432?nm). A relaxation, however, takes place above a thickness of 100?nm, associated with a transition from two-dimensional to three-dimensional columnar growth. It is accompanied by chemical fluctuations. Our magnetic measurements exhibit Curie temperatures and magnetic moments very close to the bulk values in those layers where the crystal parameter is strained but the angle partially relaxed.     

Use of Extraction Disks for Trace Enrichment of Various Pesticides from River and Sea Water Samples

Abstract

C-18 Empore extraction disks were used for the isolation and trace enrichment of different groups of pesticides from river water and artificial sea water at concentration levels of 0.2, 5 and 20 μg/l [chlorotriazines, (atrazine and simazine), their dealkylated metabolites, (deethyl- and deisopropylatrazine), organophosphorus (parathion-ethyl), phenylurea (linuron), anilide (propanil), carbamate (aldicarb and carbofuran) and carbamate transformation products (aldicarb sulfoxide, aldicarb sulfone and 3-hydroxy-7-phenol carbofuran]. The extraction disks allowed high flow rates thus 51 samples could be processed within 2h. 30 min.

For most of the pesticides the recoveries, as determined by liquid chromatography with diode array detection (LC-DAD), varied from 74 up to 125% with coefficients of variations (CV) of 5-10%, whereas for the carbamate transformation products the recoveries were in the range of 30-35% having a CV of 17-21%. At spiking level of 0.2 μg/l the dealkylated triazine metabolites and the carbamate transformation products were not detected at all.