No abstract

Abstract-The U.S. Environmental Protection Agency (EPA) is reevaluating the risks associated with 2,3,7,8-tetrachlorodibenzop-dioxin and related chlorinated hydrocarbons (CHCs). Most information currently available concerning CHC toxic action and biological effects focuses on the responses of individual organisms, as opposed to the potential impacts of CHCs on populations, communities, or ecosystems. In support of EPA's reevaluation, survivorship and reproduction data from two previous studies involving the estuarine fish, Fundulus heteroclitus (Linnaeus), exposed as adults to either dioxin or polychlorinated biphenyls (PCBs), were interpreted at the population level using a stage-classified model of F. heteroclitus population dynamics. The studies differed with respect to the route of exposure of the parental stock: dietary exposure to dioxin in the laboratory and natural exposure to PCBs at the New Bedford Harbor, Massachusetts marine Superfund site. The CHC effects documented in these studies were used to modify fertility and survivorship in the population model. The finite population multiplication (growth) rate, estimated using the model, was used as the measure of population-level effect. In both cases, a negative relationship was observed between CHC dose (quantified as dioxin whole-body burden and liver burden of non-ortho-and mono-ortho-substituted PCB congeners) and population growth rate. The dose-response relationships developed in this study provide useful information for assessing the ecological risks of CHCs to estuarine fish populations.

3835 f 1. Since it is much larger than the experimental value (log A = 6.4) for the high-temperature reaction, we conclude that it is unlikely that this reaction occurs by the unimolecular mechanism A (Scheme HI). many more assumptions than the first (e.g., rotational barriers in the starting species and frequencies of the vibrational modes in 3), is rather long and involved, and is not reported in detail here.This second calculation is in general agreement with the first and gives log A = IO.Thus, according to both calculations, the preexponential for the reaction outlined in Scheme VI is approximately log A = 11 Registry No. Ag( 1 lo), 7440-22-4; fert-butyl alcohol, 75-65-0; isobutylene oxide, 558-30-5; deuterium, 7782-39-0. Abstract:We have measured the equilibrium gas-phase acidities for a series of alkyl-and aryl-substituted organosilanes, and the cross sections for electron photodetachment of the corresponding conjugate bases, using ion cyclotron resonance spectrometry. Gas-phase acidities for the conjugate acids of these compounds are AHoa,id(SiH4) = 372.8 f 2 kcal/mol, AHoaCid(C6H5SiH,) = 370.7 f 2 kcal/mol, AHoacid(C6H5(CHJ)SiH2) = 374.2 f 2 kcal/mol, AH0,,,(CH3SiH3) = 378.3 f 2 kcal/mol, and AHoa,id((CH3)3SiH) L 382.8 f 2 kcal/mol. The electron affinities are EA(SiH3') = 32.4 f 0.6 kcal/mol, EA(C6H5SiH2:) = 33.1 f 0.1 kcal/mol, EA(C6H5(CH3)SiH') = 30.7 f 0.9 kcal/mol, EA(CH3SiH2') = 27.5 f 0.8 kcal/mol, and EA((CH,),Si ) = 22.4 f 0.6 kcal/mol. These values were used to determine the Si-H bond dissociation energies in the organosilicon hydrides: Do[SiH3-H] = 91.6 f 2 kcal/mol, Do[C6H5SiH2-H] = 90.2 f 2 kcal/mol, Do[C6H5(CH3)SiH-H] = 91.3 f 3 kcal/mol, Do[CH3SiH2-H] = 92.2 f 3 kcal/mol, and D0[(CH3),Si-H] Z 91 .O f 2 kcal/mol. The electronic structure of the organosilicon compounds studied is discussed and compared with that in the corresponding carbon compounds.Thermochemical properties are indispensable tools for understanding chemical reactivity and making predictions about intermediates, products, equilibria, and reaction mechanisms. It is surprising, therefore, that despite the prolific investigation and utilization of organosilicon chemistry in the past 20 years, thermochemical data for organosilicon compounds remain scarce. For example, the data base of ionic, gas-phase thermochemical measurements' which has grown impressively in recent years has only minimal coverage in other classes such as organometallics. Especially important, yet experimentally challenging, are thermochemical measurements involving reactive silicon intermediates* such as divalent silicon compounds, silyl radicals, cations, and anions. In this paper, we report our measurements of the gas-phase acidities of selected organosilanes and the electron affinities of the corresponding organosilyl radicals.In addition to contributing thermochemical data, gas-phase investigations involving equilibrium basicities and electron photodetachment spectroscopy of silyl and substituted silyl anions can provide information about substituent effec...

A new tool for analyzing compound libraries by NMR has been developed. Aliquots of solution-state samples (between 120 and 350 microL) are directly injected, using a standard liquids handler, into an NMR (LC-NMR) flow probe. Automated NMR software tracks--and suppresses--intense signals arising from the nondeuterated solvents used (if any) and acquires high-sensitivity one-dimensional 1H NMR spectra. An 88-member combinatorial library, dissolved in DMSO and stored in a 96-well microtiter plate, has been analyzed a number of ways using this technique. This nondestructive technique, which we call direct-injection NMR (DI-NMR) and which is embodied in our versatile automated sample changer (VAST) hardware, has proven to be both routine and robust. Our success in automatically acquiring the NMR data for entire plates of library compounds (within 4-8 h) has caused us to develop new ways to display and analyze the resulting NMR data, as will be shown here.