Direct Measurement of Perchlorate Exposure Biomarkers in a Highly Exposed Population: A Pilot Study

Exposure to perchlorate is ubiquitous in the United States and has been found to be widespread in food and drinking water. People living in the lower Colorado River region may have perchlorate exposure because of perchlorate in ground water and locally-grown produce. Relatively high doses of perchlorate can inhibit iodine uptake and impair thyroid function, and thus could impair neurological development in utero. We examined human exposures to perchlorate in the Imperial Valley among individuals consuming locally grown produce and compared perchlorate exposure doses to state and federal reference doses. We collected 24-hour urine specimen from a convenience sample of 31 individuals and measured urinary excretion rates of perchlorate, thiocyanate, nitrate, and iodide. In addition, drinking water and local produce were also sampled for perchlorate. All but two of the water samples tested negative for perchlorate. Perchlorate levels in 79 produce samples ranged from non-detect to 1816 ppb. Estimated perchlorate doses ranged from 0.02 to 0.51 µg/kg of body weight/day. Perchlorate dose increased with the number of servings of dairy products consumed and with estimated perchlorate levels in produce consumed. The geometric mean perchlorate dose was 70% higher than for the NHANES reference population. Our sample of 31 Imperial Valley residents had higher perchlorate dose levels compared with national reference ranges. Although none of our exposure estimates exceeded the U. S. EPA reference dose, three participants exceeded the acceptable daily dose as defined by bench mark dose methods used by the California Office of Environmental Health Hazard Assessment

Investigating the Effects of Basis Set on Metal–Metal and Metal–Ligand Bond Distances in Stable Transition Metal Carbonyls: Performance of Correlation Consistent Basis Sets with 35 Density Functionals

Density functional theory (DFT) is a widely used method for predicting equilibrium geometries of organometallic compounds involving transition metals, with a wide choice of functional and basis set combinations. A study of the role of basis set size in predicting the structural parameters can be insightful with respect to the effectiveness of using small basis sets to optimize larger molecular systems. For many organometallic systems, the metal–metal and metal–carbon distances are the most important structural features. In this study, we compare the equilibrium metal–ligand and metal–metal distances of six transition metal carbonyl compounds predicted by the Hood-Pitzer double-ζ polarization (DZP) basis set, against those predicted employing the standard correlation consistent cc-pVXZ (X = D,T,Q) basis sets, for 35 different DFT methods. The effects of systematically increasing the basis set size on the structural parameters are carefully investigated. The Mn–Mn bond distance in Mn2(CO)10 shows a greater dependence on basis set size compared to the other M–M bonds. However, the DZP predictions for re(Mn–Mn) are closer to experiment than those obtained with the much larger cc-pVQZ basis set. Our results show that, in general, DZP basis sets predict structural parameters with an accuracy comparable to the triple and quadruple-ζ basis sets. This finding is very significant, because the quadruple-ζ basis set for Mn2(CO)10 includes 1308 basis functions, while the equally effective double-ζ set (DZP) includes only 366 basis functions. Overall, the DZP M06-L method predicts structures that are very consistent with experiment

Investigating the Effects of Basis Set on Metal–Metal and Metal–Ligand Bond Distances in Stable Transition Metal Carbonyls: Performance of Correlation Consistent Basis Sets with 35 Density Functionals

Density functional theory (DFT) is a widely used method for predicting equilibrium geometries of organometallic compounds involving transition metals, with a wide choice of functional and basis set combinations. A study of the role of basis set size in predicting the structural parameters can be insightful with respect to the effectiveness of using small basis sets to optimize larger molecular systems. For many organometallic systems, the metal–metal and metal–carbon distances are the most important structural features. In this study, we compare the equilibrium metal–ligand and metal–metal distances of six transition metal carbonyl compounds predicted by the Hood-Pitzer double-ζ polarization (DZP) basis set, against those predicted employing the standard correlation consistent cc-pVXZ (X = D,T,Q) basis sets, for 35 different DFT methods. The effects of systematically increasing the basis set size on the structural parameters are carefully investigated. The Mn–Mn bond distance in Mn₂(CO)₁₀ shows a greater dependence on basis set size compared to the other M–M bonds. However, the DZP predictions for r_<i>e</i>(Mn–Mn) are closer to experiment than those obtained with the much larger cc-pVQZ basis set. Our results show that, in general, DZP basis sets predict structural parameters with an accuracy comparable to the triple and quadruple-ζ basis sets. This finding is very significant, because the quadruple-ζ basis set for Mn₂(CO)₁₀ includes 1308 basis functions, while the equally effective double-ζ set (DZP) includes only 366 basis functions. Overall, the DZP M06-L method predicts structures that are very consistent with experiment