Determination of pyrethrin and pyrethroid pesticides in urine and water matrixes by liquid chromatography with diode array detection

The following pyrethrin and pyrethroid pesticides were determined in urine and water matrixes by liquid chromatography with diode array detection (LC-DAD): pyrethrin I, pyrethrin II, tetramethrin, baythroid, bifenthrin, fenvalerate, phenothrin, allethrin, resmethrin, cis-permethrin, and trans-permethrin. In addition, 3-phenoxybenzyl alcohol, a metabolite of various pyrethroids, was also successfully determined by the analytical method. The matrix extraction was simple, inexpensive, and fast, using only sodium chloride and acetonitrile. The acetonitrile extract was filtered and analyzed by LC-DAD. The method detection limits for the pyrethrin pesticides in 5 mL urine were determined to range from 0.002 to 0.04 microg/mL, depending on the individual pyrethrin. Recoveries from spiked tap water ranged from 77 to 96%; recoveries from urine ranged from 80 to 117%. This method is especially well-suited to clinical investigations, in which rapid analysis of forensic samples is often required.     

Stabilised phosphorus(I) and arsenic(I) iodide: readily-synthesised reagents for low oxidation state main group chemistry

Herein we report a surprisingly facile and clean synthesis of base-stabilised phosphorus(I) and arsenic(I) iodide salts, which are reagents that provide convenient access to new low oxidation state main group compounds.     

Electronic structure and properties of isoreticular metal-organic frameworks: the case of M-IRMOF1 (M = Zn, Cd, Be, Mg, and Ca)

We investigate the possibility of tailoring the electronic properties of isoreticular metal-organic materials by replacing the metal atom in the metal-organic cluster and by doping. The electronic structure of M-IRMOF1, where IRMOF1 stands for isoreticular metal-organic framework 1 and M = Be, Mg, Ca, Zn, and Cd, was examined using density-functional theory. The results show that these materials have similar band gaps (ca. 3.5 eV) and a conduction band that is split into two bands, the lower of which has a width that varies with metal substitution. This variation prompted us to investigate whether doping with Al or Li could be used to tailor the electronic properties of the Zn-IRMOF1 and Be-IRMOF1 materials. It is shown that replacing one metal atom with Al can effectively be used to create IRMOFs with different metallic properties. On the other hand, adding Li produces structural changes that render this approach less suitable.     

Prediction of Complexation Properties of Crown Ethers Using Computational Neural Networks

A computational neural network method was used for the prediction of stability constants of simple crown ether complexes. The essence of the method lies in the ability of a computer neural network to recognize the structure-property relationships in these host-guest systems. Testing of the computational method has demonstrated that stability constants of alkali metal cation (Na+, K+, Cs+)-crown ether complexes in methanol at 25 °C can be predicted with an average error of ±0.3 log K units based on the chemical structure of the crown ethers alone. The computer model was then used for the preliminary analysis of trends in the stabilities of the above complexes.     

A field electrode method for the determination of sulfide

This paper reports the development of a field electrode method for the determination of total sulfide in water. The method involves the use of preweighed sodium sulfide crystals in the standardization process. Sodium sulfide crystals were weighed and sealed in air-tight plastic volumetric flasks. Standards were prepared in the field by adding a sulfide antioxidant buffer to the flasks containing the sulfide crystals and diluting it to mark with deionized deaerated water. Standards of lower concentration were prepared by serial dilutions of first standards.The results of the reproducibility determination revealed that sulfide concentrations as low as 6 ppb could be measured with a reproducibility of better than ±10%.Water samples were collected from a series of lakes in Fort Bend County, Texas (near Houston) and analyzed for sulfide content. The sulfide ion concentrations of these samples were determined directly in the field by use of an Orion Model 407A/F specific meter equipped with a silver/sulfide ion selective electrode in conjunction with a double junction reference electrode.     

LOW OXIDATION STATE GROUP 15 ELEMENTS AS PNICTA-WITTIG REAGENTS

We have synthesized a new As(I) salt and performed a Density Functional Theory investigation into the use of such compounds as pnicta-Wittig reagents.     

Nonvolatile memory elements based on the intercalation of organic molecules inside carbon nanotubes

We propose a novel class of nonvolatile memory elements based on the modification of the transport properties of a conducting carbon nanotube by the presence of an encapsulated molecule. The guest molecule has two stable orientational positions relative to the nanotube that correspond to conducting and nonconducting states. The mechanism, governed by a local gating effect of the molecule on the electronic properties of the nanotube host, is studied using density functional theory. The mechanisms of reversible reading and writing of information are illustrated with a F4TCNQ molecule encapsulated inside a metallic carbon nanotube. Our results suggest that this new type of nonvolatile memory element is robust, fatigue-free, and can operate at room temperature.     

Efficient computation of potential energy first and second derivatives for molecular dynamics,normal coordinate analysis,and molecular mechanics calculations

By using two- and three-body internal coordinates and their derivatives as intermediates, it is possible to enormously simplify formulas for three- and four-body internal coordinates and their derivatives. Using this approach, simple formulas are presented for stretch (two-body), two types of bend (three-body), and wag and two types of torsion (four-body) internal coordinates and their first and second derivatives. The formulas are eminently suitable for economizing molecular dynamics and molecular mechanics calculations and normal coordinate analysis of complicated potential energy surfaces. Efficient methods for computing derivatives of entire potential energy terms, and in particular cross terms with switching functions, are presented.     

The origins of avoided crossings in polymer dispersion curves

A purely mathematical heuristic model is examined in order to explain the existence of an avoided crossing between two dispersion curves calculated for a certain potential energy surface designed to simulate the dynamics of a linear polyethylene chain. The model shows that a third mode coupled to a two-mode Fermi-resonant system is able to create a marked instability in a vibrational spectrum. It is noted that such an explanation would require a fourth-order coupling term, which is the reason given for the failure of normal mode analysis to properly identify this dynamical effect.