RESUMO
The number of techniques to measure number concentrations and size distributions of submicrometer particles has recently increased. Submicrometer particle standards are needed to improve the accuracy and reproducibility of these techniques. The number concentrations of fluorescently labeled polystyrene submicrometer sphere suspensions with nominal 100 nm, 200 nm and 500 nm diameters were measured using seven different techniques. Diameter values were also measured where possible. The diameter values were found to agree within 20%, but the number concentration values differed by as much as a factor of two. Accuracy and reproducibility related with the different techniques are discussed with the goal of using number concentration standards for instrument calibration. Three of the techniques were used to determine SI-traceable number concentration values, and the three independent values were averaged to give consensus values. This consensus approach is proposed as a protocol for certifying SI-traceable number concentration standards.
RESUMO
Simultaneous reduction of WCl6 and CCl4 with bismuth metal at 400 degrees C affords a black solid, from which the new cluster [W6CCl18]2- is extracted into aqueous HCl. The cluster is isolated in 34% yield as (Bu4N)2[W6CCl18] and features a metal-metal bonded W6 trigonal prism centered by a carbon atom and surrounded by 12 edge-bridging and 6 terminal chloride ligands. A cyclic voltammogram of [W6CCl18]2- in DMF shows the cluster undergoes two reduction and two oxidation processes, suggesting five chemically accessible redox states. Consistent with this extensive electrochemistry, DFT calculations on the diamagnetic [W6CCl18]2- species reveal closely spaced frontier orbitals, with an a2' ' HOMO situated 0.61 and 0.71 eV below unoccupied e' ' and e' orbitals, respectively. Oxidation of the cluster by a single equivalent of NO+ gives [W6CCl18]1-, which, as expected on the basis of the [W6CCl18]2- HOMO character, possesses a less elongated W6 trigonal prism. Reduction of [W6CCl18]2- with a single equivalent of cobaltocene affords [W6CCl18]3-, wherein population of a low-lying e' orbital leads to a significant Jahn-Teller distortion.