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PubMed1981

Microdiffusion and fluoride-specific electrode determination of fluoride in infant foods: collaborative study.

Twelve laboratories analyzed (1 replicate) 12 samples of infant foods--milk, pears, and peas--containing 0.2-5 ppm F. There was one laboratory outlier. Mean coefficients of variation were 7.06% for intralaboratory determination of 3 sets of blind duplicates and 21.6% for interlaboratory determination of 12 samples. Variance analysis for all samples yielded a reproducibility standard deviation of 0.41 ppm; for 3 sets of blind duplicates, repeatability standard deviation was 0.26 ppm and reproducibility standard deviation was 0.32 ppm.

Diffusion↗
PubMed2002

Interpretation of indirect nuclear spin-spin coupling tensors for polyatomic xenon fluorides and group 17 fluorides: results from relativistic density-functional calculations.

Significant improvements have been made recently in the calculation of NMR indirect nuclear spin-spin coupling tensors (J). In particular, the relativistic zeroth-order regular approximation density-functional theory (ZORA-DFT) approach holds great promise for the calculation of spin-spin coupling constants for a variety of chemical systems containing heavy nuclei. In the present work, the ZORA-DFT method is applied to the calculation of the complete reduced coupling tensors, K, for a range of chlorine-, bromine-, iodine-, and xenon-containing species: K(Cl,F) for ClF(2)(+), ClF(3), ClF(4)(+), ClF(5), ClF(6)(-), and ClF(6)(+); K(Br,F) for BrF(3), BrF(6)(-), and BrF(6)(+); K(I,F) for IF(4)(+) and IF(6)(+); K(Xe,F) for XeF(+), XeF(2), XeF(3)(+), XeF(4), XeF(5)(-), XeF(5)(+), and XeF(7)(+). These species represent a wide variety of geometrical bonding arrangements. Agreement between the calculated coupling constants and available experimental data is excellent, and the absolute sign of the coupling constants is provided. It is shown that (1)K(iso) may be positive or negative even within the same molecule, e.g., K(Cl,F)(iso) may be of either sign, depending on the local environment. Periodic trends in (1)K(iso) for isovalent and isostructural molecules are evident. The spin-spin coupling anisotropies, Delta K, and the orientations of the K tensors are also determined. The success of the calculations is a direct result of employing reliable geometries and considering both scalar and spin-orbit relativistic effects. The dependence of K(Cl,F)(iso) and K(Xe,F)(iso) on the local molecular and electronic structure is discussed in terms of the paramagnetic spin-orbit (PSO) and combined Fermi-contact spin-dipolar (FC+SD) coupling mechanisms. The PSO term depends strongly on the number of valence shell electron lone pairs on the central heavy atom, and the FC+SD contribution increases with the Cl[bond]F or Xe[bond]F bond length for a given series of compounds. This interpretation allows for the successful rationalization of the existing experimental data.

Journal Article↗
PubMed1997

Theoretical Evidence for Two New Intermediate Xenon Species: Xenon Azide Fluoride, FXe(N(3)), and Xenon Isocyanate Fluoride, FXe(NCO).

The reaction behavior of xenon difluoride, XeF(2), toward HN(3), NaN(3), and NaOCN was investigated in H(2)O, aHF (anhydrous HF), and SO(2)ClF solution. The analysis of the final reaction products (XeF(2) + HN(3) (NaN(3)) in H(2)O --> HF, N(2), N(2)O, Xe; XeF(2) + HN(3) in aHF --> N(2), Xe, N(2)F(2); XeF(2) + HOCN (NaOCN) in H(2)O --> HF, N(2), N(2)O, NH(3), CO(2), Xe) indicated the intermediate formation of FXe(N(3)) and FXe(NCO) and revealed different reaction mechanisms for both compounds. Both intermediates, FXe(N(3)) and FXe(NCO), were studied on the basis of ab initio computations at HF and correlated MP2 levels using a quasirelativistic LANL2DZ pseudopotential for Xe. Both were shown to possess stable minima at HF and MP2 levels (no imaginary frequencies) with the following structural parameters (MP2/LANL2DZ). FXe(N(3)): C(s)(); d(F-Xe) = 2.051, d(Xe-N1) = 2.318, d(N1-N2) = 1.241, d(N2-N3) = 1.180 Å; angle(FXeN1) = 178.1, angle(XeN1N2) = 112.2, angle(N1N2N3) = 174.7 degrees. FXe(NCO): C(s)(); d(F-Xe) = 2.024, d(Xe-N) = 2.206, d(N-C) = 1.194, d(C-N) = 1.231 Å; angle(FXeN) = 178.7, angle(XeNC) = 125.4, angle(NCO) = 174.2 degrees. The experimentally unobserved cyanate isomer, FXe(OCN), was calculated to be higher in energy than the isocyanate isomer FXe(NCO): DeltaE = 19.8 (HF), 18.3 (MP2) kcal mol(-)(1).

Journal Article↗