Catherine E. Check

The thermochemistry of Group 15 tetrachloride anions

Bond strengths for a series of Group 15 tetrachloride anions AC14− (A = P, As, Sb, and Bi) have been determined by measuring thresholds for collision-induced dissociation of the anions in a flowing afterglow-tandem mass spectrometer. The central atoms in these systems have ten electrons, which violates the octet rule: the bond dissociation energies for ACl4− help to clarify the effect of the central atom on hypervalent bond strengths. The 0 K bond energies in kJ mol−1 are D(Cl3A-Cr−1) = 90 ± 7, 115 ± 7, 161 ± 8, and 154 ± 15, respectively. Computational results using the B3LYP/LANL2DZpd level of theory are higher than the experimental bond energies. Calculations give a geometry for BiCl4− that is essentially tetrahedral rather than the see-saw observed for the other tetrachlorides. NBO calculations predict that the phosphorus and arsenic systems have 3C-4E bonds, while the antimony and bismuth systems are more ionic.

Single crystal growth, bonding analysis and superconductivity of V2Ga5

Synthesis of the superconducting V2Ga5 compound was achieved using molten Ga as the flux material. Large single crystals of 30 mm in length can be obtained with this method. Single crystal X-ray diffraction and computational studies were carried out for this compound. V2Ga5 crystallizes in the tetragonal lattice P4/mbm (No. 127) with lattice constants a=8.936(3) A, c=2.683(2) A, V=214.3(2) A3, and Z=2. Computational study shows that bonding in this compound is delocalized, and the majority states at the Fermi level are the vanadium d-band. Resistivity and magnetization measurements have confirmed that the solid is a bulk type II superconductor with TC=3.59 K.

Testing Trends in Environmental Compliance

Many environmentalregulations are based on threshold levels. Such a level is generally set such that exposure to a toxin above the level is considered a hazard to health. In many instances, the level may be exceeded a certain number of times before regulatory action is taken. Such is the case with the National Ambient Air Quality Standards (NAAQS) set by the Environmental Protection Agency (EPA) in accordance with the 1990 amendment of the Clean Air Act of 1970. This set of standards sets threshold levels for several pollutants including carbon monoxide, nitrogen dioxide, ground-level ozone, and lead. Many areas of the United States, which the EPA designates as nonattainment areas, persistently exceed the air pollution levels set by the NAAQS. Progress in these areas is of ten slow but can be studied through data collected on the elapsed time between violations. The purpose of this article is to present a relatively easy but flexible statistical method for assessing whether an area is making progress with compliance to the NAAQS. The method presented in this article is based on a randomization test of a general model for the mean time between violations of the standard. The modeling-type approach of this method makes it flexible enough to study many types of trends. The use of the randomization test makes the method simple to apply and requires fewer assumptions than many other proposed methods. We discuss some general theoretical conditions for which the method is well suited and study the performance of this method empirically, Finally, we apply the method to a well-known set of interviolation times corresponding to data collected on ground-level ozone in Houston, Texas.