Buffer-gas pressure broadening for the (3 0(0) 1)III <-- (0 0 0) band of CO2 measured with continuous-wave cavity ring-down spectroscopy

Buffer-gas pressure broadening for the (3 0(0) 1)(III)<--(0 0 0) band of CO(2) in the 1600 nm region was investigated with continuous wave cavity ring-down spectroscopy within the temperature range 263-326 K. The measured absorption profiles were analyzed with Voigt functions. Pressure broadening coefficient, gamma(gas), and the temperature dependent parameter (broadening exponent), n, were determined for a variety of buffer gases: N(2), O(2), He, Ne, Ar, Kr and Xe. gamma(air) values estimated subsequently are 0.096(2) for R(0), 0.085(5) for P(8), 0.075(2) for P(16), 0.070(4) for P(26), and 0.069(2) for P(38) in units of cm(-1) atm(-1), where numbers in parentheses are one standard deviation in units of the last digits quoted. n(air) values are 0.77(4) for R(0), and 0.73(11) for P(8).     

Proton inventory studies of alpha-thrombin-catalyzed reactions of substrates with selected P and P' sites

Deuterium kinetic solvent isotope effects for the human alpha-thrombin-catalyzed hydrolysis of (1) substrates with selected P(1)-P(3) sites, Z-Pro-Arg-7-amido-4-methylcoumarin (7-AMC), N-t-Boc-Val-Pro-Arg-7-AMC, Bz-Phe-Val-Arg-4-nitroanilide (pNA), and H-D-Phe-L-Pip-Arg-pNA, are (DOD)k(cat) = (2.8-3.3) +/- 0.1 and (DOD)(k(cat)/K(m)) = (0.8-2.1) +/- 0.1 and (2) internally fluorescence-quenched substrates (a) (AB)Val-Phe-Pro-Arg-Ser-Phe-Arg-Leu-Lys(DNP)-Asp-OH, an optimal sequence, and (b) (AB)Val-Ser-Pro-Arg-Ser-Phe-Gln-Lys(DNP)-Asp-OH, recognition sequence for factor VIII, are (DOD)k(cat) = 2.2 +/- 0.2 and (DOD)(k(cat)/K(m)) = (0.8-0.9) +/- 0.1, at the pL (L = H, D) maximum, 8.4-9.0, and (25.0-26.0) +/- 0.1 degrees C. The most plausible models fitting the partial isotope effect (proton inventory) data have been selected on the basis of lowest values of the reduced chi squared and consistency of fractionation factors at all substrate concentrations, assuming rate-determining acylation. The data for Z-Pro-Arg-7-AMC are consistent with a single-proton bridge at the transition state phi(TS) = 0.39 +/- 0.05 and components for solvent reorganization phi(S) = 0.8 +/- 0.1 and phi(S) = 1.22 for k(cat) and k(cat)/K(m), respectively. The data for tripeptide amides fit bowl-shaped curves; an example is N-t-Boc-Val-Pro-Arg-7-AMC: phi(TS)(1) = phi(TS)(2) = 0.57 +/- 0.01 and phi(S) = 1 for k(cat) and 1.6 +/- 0.1 for k(cat)/K(m). Proton inventories for the nonapeptide (2b) are linear. The data for k(cat) for H-D-Phe-L-Pip-Arg-pNA and the decapeptide (2a) are most consistent with two identical fractionation factors for catalytic proton bridging, phi(TS)(1) = phi(TS)(2) = 0.68 +/- 0.02 and a large inverse component (phi(S) = 3.1 +/- 0.5) for the latter, indicative of substantial solvent reorganization upon leaving group departure. Proton inventory curves for k(cat)/K(m) for nearly all substrates are dome-shaped with an inverse isotope effect component (phi(S) = 1.2-2.4) originating from solvent reorganization during association of thrombin with substrate. These large contributions from medium effects are in full accord with the conformational adjustments required for the fulfillment of the dual, hemostatic and thrombolytic, functions of thrombin.     

Determination of low-level agricultural residues in soft drinks and sports drinks by liquid chromatography/tandem mass spectrometry: single-laboratory validation

In this study, sponsored by PepsiCo Inc., a method was validated for measurement of 11 pesticide residues in soft drinks and sports drinks. The pesticide residues determined in this validation were alachlor, atrazine, butachlor, isoproturon, malaoxon, monocrotophos, paraoxon-methyl, phorate, phorate sulfone, phorate sulfoxide, and 2,4-dichlorophenoxyacetic acid (2,4-D) when spiked at 0.100 microg/L (1.00 microg/L for phorate). Samples were filtered (if particulate matter was present), degassed (if carbonated), and analyzed using liquid chromatography with tandem mass spectrometry. Quantitation was performed with matrix-matched external standard calibration solutions. The standard curve range for this assay was 0.0750 to 10.0 microg/L. The calibration curves for all agricultural residues had coefficient of determination (r2) values greater than or equal to 0.9900 with the exception of 2 values that were 0.9285 and 0.8514. Fortification spikes at 0.100 microg/L (1.00 microg/L for phorate) over the course of 2 days (n=8 each day) for 3 matrixes (7UP, Gatorade, and Diet Pepsi) yielded average percent recoveries (and percent relative standard deviations) as follows (n=48): 94.4 (15.2) for alachlor, 98.2 (13.5) for atrazine, 83.1 (41.6) for butachlor, 89.6 (24.5) for isoproturon, 87.9 (24.4) for malaoxon, 96.1 (9.26) for monocrotophos, 101 (25.7) for paraoxon-methyl, 86.6 (20.4) for phorate, 101 (16.5) for phorate sulfone, 93.6 (25.5) for phorate sulfoxide, and 98.2 (6.02) for 2,4-D.     

Synthesis and coordination properties of new macrocyclic ligands: equilibrium studies and crystal structures

The synthesis and characterization of two new macrocyclic ligands, the bis-macrocyclic compound 2,6-bis(1,4,13-triaza-7,10-dioxacyclopentadec-1-ylmethyl)phenol (L) and 38-methoxy-1,4,13,16,19,28-hexaaza-7,10,22,25-tetraoxatricyclo[14.14.7.1(32,36)]octatriconta-32,34,Delta(36,38)-triene (L1) are reported. Equilibrium studies of basicity and coordination properties toward metal ions such as Cu(II), Zn(II), Cd(II) and Pb(II) were performed for ligand by potentiometric measurements in aqueous solution (298.1 +/- 0.1 K, I= 0.15 mol dm(-3)). L behaves as a hexaprotic base (logK(1)= 10.93, logK(2)= 9.70, logK(3)= 8.79, logK(4)= 8.05, logK(5)= 6.83, logK(6)= 2.55). All metal ions form stable mono- and dinuclear complexes: logK(MLH(-1))= 25.61 for Cu(II), 15.37 for Zn(II), 12.58 for Cd(II) and 13.79 for Pb(II); logK(M(2)LH(-1))= 31.61 for Cu(II), 23.38 for Zn(II), 24.49 for Cd(II) and 23.68 for Pb(II). All these dinuclear species show a great tendency to add the OH(-) group: the equilibrium constant for the addition reaction was found to be logK(M(2)LH(-1)OH)= 4.77 for Cu(II), 5.66 for Zn(II), 2.8 for Cd(II) and 3.18 for Pb(II). In the case of Ni(II), kinetic inertness prevents the possibility of solution studies. The dinuclear solid adducts [Ni(2)H(-1)L(N(3))(3)].EtOH and [Cu(2)H(-1)L(N(3))](ClO(4))(2) were characterized by X-ray analysis.     

Liquid-liquid distribution of ion-associates of dichlorocuprate(I) with quaternary ammonium counter-ions

The dichlorocuprate(I) anion CuCl(-)(2) can be extracted as its ion-associates Q(+).CuCl(-)(2) with quaternary ammonium cations (Q(+)) into chloroform. The extraction constants K(ex) have been determined, and the log K(ex) values found for the various counter-ions used are 1.93 for (C(3)H(7))(4)N(+), 4.10 for (C(4)H(9))(4)N(+), 6.57 for (C(5)H(11))(4)N(+), 1.57 for C(8)H(17)N(+) (CH(3))(3), 2.83 for C(10)H(21)N(+) (CH(3))(3) 4.12 for C(12)H(25)N(+) (CH(3))(3) and 5.21 for C(14)H(29)N(+)(CH(3))(3), respectively. A linear relationship was found between log K(ex) and the total number of carbon atoms in Q(+); from the slope of the line, the contribution of a methylene group to log K(ex) was calculated to be 0.59. The extractability with alkyltrimethylammonium cations was larger than that with symmetrical tetra-alkylammonium cations and the difference in log K(ex) for two cations (one of each type) with the same number of carbon atoms was about 0.4. From the extraction constants obtained, the extractability of CuCl(-)(2) was found to lie between that of ReO(-)(4) and ClO(-)(4).     

Structures of the hexafluorocyclopropane, octafluorocyclobutane, and decafluorocyclopentane radical anions probed by experimental and computational studies of anisotropic electron spin resonance (ESR) spectra

Anisotropic electron spin resonance (ESR) spectra are reported for the radical anions of hexafluorocyclopropane (c-C(3)F(6)(-)), octafluorocyclobutane (c-C(4)F(8)(-)), and decafluorocyclopentane (c-C(5)F(10)(-)) generated via gamma-irradiation in plastically crystalline tetramethylsilane (TMS) and rigid 2-methyltetrahydrofuran (MTHF) matrices. By combining the analysis of these experimental ESR spectra involving anisotropic hyperfine (hf) couplings with a series of quantum chemical computations, the geometrical and electronic structure of these unusual perfluorocycloalkane radical anions have been characterized more fully than in previous studies that considered only the isotropic couplings. Unrestricted Hartree-Fock (UHF) computations with the 6-311+G(d,p) basis set predict planar ring structures for all three radical anions, the ground electronic states being (2)A(2)(") for c-C(3)F(6)(-) (D(3h) symmetry), (2)A(2u) for c-C(4)F(8)(-) (D(4h)), and (2)A(2)(") for c-C(5)F(10)(-) (D(5h)), in which the respective six, eight, and ten 19F-atoms are equivalent by symmetry. A successful test of the theoretical computation is indicated by the fact that the isotropic 19F hf couplings computed by the B3LYP method with the 6-311+G(2df,p) basis set for the optimized geometries are in almost perfect agreement with the experimental values: viz., 19.8 mT (exp) vs 19.78 mT (calc) for c-C(3)F(6)(-); 14.85 mT (exp) vs 14.84 mT (calc) for c-C(4)F(8)(-); 11.6 mT (exp) vs 11.65 mT (calc) for c-C(5)F(10)(-). Consequently, the same computation method has been applied to calculate the almost axially symmetric anisotropic 19F hf couplings for the magnetically equivalent 19F atoms: (-4.90 mT, -4.84 mT, 9.75 mT) for c-C(3)F(6), (-3.54 mT, -3.48 mT, 7.02 mT) for c-C(4)F(8)(-), and (-2.62 mT, -2.56 mT, 5.18 mT) for c-C(5)F(10)(-). ESR spectral simulations performed using the computed principal values of the hf couplings and the spatial orientations of the 19F nuclei as input parameters reveal an excellent fit to the experimental anisotropic ESR spectra of c-C(3)F(6)(-), c-C(4)F(8)(-), and c-C(5)F(10)(-), thereby providing a convincing proof of the highly symmetric D(nh) structures that are predicted for these negative ions. Furthermore, using the computed 19F principal values and their orientations, the effective 19F anisotropic hf couplings along the molecular symmetry axes were evaluated for c-C(3)F(6)(-) and c-C(4)F(8)(-) and successfully correlated with the positions of the characteristic outermost features in both the experimental and calculated anisotropic spectra. In addition, the electronic excitation energies and oscillator strengths for the c-C(3)F(6)(-) , c-C(4)F(8)(-), and c-C(5)F(10)(-) radical anions were computed for the first time using time-dependent density functional theory (TD-DFT) methods.     

Interaction between biimidazole complexes of ruthenium and acetate: hydrogen bonding and proton transfer

The hydrogen bonding and deprotonation processes between four ruthenium biimidazole complexes, namely [Ru(bpy)(2)(BiimH(2))](PF(6))(2) (1, bpy is bipyridine, BiimH(2) is 2,2'-biimidazole), [Ru(bpy)(2)-(BbimH(2))](PF(6))(2) (2, BbimH(2) is 2,2'-bibenzimidazole), and [Ru(bpy)(2)(DMBbimH(2))](PF(6))(2) (3, DMBbimH(2) is 7,7'-dimethyl-2,2'-bibenzimidazole) and [Ru(bpy)(2)(TMBbimH(2))](2+) (4, TMBbimH(2) is 5,6,5',6'-tetramethyl-2,2'-bibenzimidazole), and acetate are investigated. Their hydrogen bonded adducts are indeed trapped and observed by absorption spectra and electrochemical experiments in acetonitrile solution in the presence of an excess of acetic acid for the first time. The binding constants log K(B) for these adducts are 6.74 for 1·OAc, 7.11 for 2·OAc, 7.26 for 3·OAc, and 6.99 for 4·OAc. A new approach to calculate the deprotonation constant is also developed by establishing a set of circular equilibria. The equilibrium constants for the first deprotonation step of the complexes log K(A) are 2.74 for 1, 5.19 for 2, 4.54 for 3, and 3.78 for 4. The pK(a1) values of the complexes in acetonitrile solution are calculated by subtracting log K(A) from pK(a) (HOAc in acetonitrile), giving 19.6 for 1, 17.1 for 2, 17.8 for 3, and 18.5 for 4. The degree of proton transfer (D(PT)) can be quantified by the calculation of absorption spectral and redox data, which is 0.41 for 1·OAc, 0.53 for 2·OAc, 0.57 for 3·OAc, and 0.47 for 4·OAc. Interestingly, the binding constant log K(B) (7.26) and D(PT) value (0.57) both reach their maxima at a critical point, where pK(a1) for the complex is 17.8 and ΔpK(a) for the adduct is 4.5 (ΔpK(a) = pK(a)(HOAc) - pK(a1), in acetonitrile solution). Moreover, the binding constant log K(B) shows linear correlation with the degree of proton transfer D(PT).     

Secondary kinetics of methanol decomposition: theoretical rate coefficients for 3CH2 + OH, 3CH2 + 3CH2, and 3CH2 + CH3

Direct variable reaction coordinate transition state theory (VRC-TST) rate coefficients are reported for the (3)CH(2) + OH, (3)CH(2) + (3)CH(2), and (3)CH(2) + CH(3) barrierless association reactions. The predicted rate coefficient for the (3)CH(2) + OH reaction (approximately 1.2 x 10(-10) cm(3) molecule(-1) s(-1) for 300-2500 K) is 4-5 times larger than previous estimates, indicating that this reaction may be an important sink for OH in many combustion systems. The predicted rate coefficients for the (3)CH(2) + CH(3) and (3)CH(2) + (3)CH(2) reactions are found to be in good agreement with the range of available experimental measurements. Product branching in the self-reaction of methylene is discussed, and the C(2)H(2) + 2H and C(2)H(2) + H2 products are predicted in a ratio of 4:1. The effect of the present set of rate coefficients on modeling the secondary kinetics of methanol decomposition is briefly considered. Finally, the present set of rate coefficients, along with previous VRC-TST determinations of the rate coefficients for the self-reactions of CH(3) and OH and for the CH(3) + OH reaction, are used to test the geometric mean rule for the CH(3), (3)CH(2), and OH fragments. The geometric mean rule is found to predict the cross-combination rate coefficients for the (3)CH(2) + OH and (3)CH(2) + CH(3) reactions to better than 20%, with a larger (up to 50%) error for the CH(3) + OH reaction.     

Potentiometric determination of stepwise stability constants of vanadium, molybdenum and tungsten chelates formed with asparagine and glutamine

The metal chelates of V(IV), Mo(VI) and W(VI) formed with asparagine and glutamine have been studied potentiometrically. Stepwise stability constants in O.1M sodium perchlorate at 25 degrees C are reported as follows. Asparagine chelates--log K(1)K(2) 14.50 and log K(3) 4.04 for V, log K(1) 8.06, log K(2) 7.29 and log K(3) 3.45 for Mo, and log K(1) 5.84, log K(2) 5.11 and log K(3) 3.30 for W. Glutamine chelates--log K(1)K(2) 14.45, and log K(3) 4.07 for V, log K(1) 7.90, log K(2) 6.93, and log K(3) 3.35 for Mo, and log K(1) 5.76, log K(2) 5.09 and log K(3) 3.20 for W.     

Reactivity of metallic nitride endohedral metallofullerene anions: electrochemical synthesis of a Lu3N@Ih-C80 derivative

Electrochemically generated Lu(3)N@I(h)-C(80) dianions react with the electrophile, PhCHBr(2), to produce a methano derivative of Lu(3)N@I(h)-C(80)(CHC(6)H(5)) (1) with high regioselectivity. The compound was characterized by MALDI-TOF, NMR, and UV-vis-NIR absorption spectroscopy. Electrochemical characterization of this Lu(3)N@I(h)-C(80)(CHC(6)H(5)) derivative showed the typical irreversible reductive behavior of the pristine Lu(3)N@I(h)-C(80), similar to those observed for Bingel adducts of Lu(3)N@I(h)-C(80). Using the same conditions, the reaction between Sc(3)N@I(h)-C(80) dianions and PhCHBr(2) was conducted for comparison. Unexpectedly, no nucleophilic reaction was observed, indicating that Sc(3)N@I(h)-C(80) dianions are not reactive toward the electrophile. Theoretical studies for both dianionic Lu(3)N@I(h)-C(80) and Sc(3)N@I(h)-C(80) showed that the HOMO is more highly localized on the fullerene cage for [Lu(3)N@I(h)-C(80)](2-) and more localized on the inside cluster for [Sc(3)N@I(h)-C(80)](2-), providing an explanation for the drastically different reactivities observed.     

Co-Ln mixed-metal phosphonate grids and cages as molecular magnetic refrigerants

The synthesis, structures, and magnetic properties of six families of cobalt-lanthanide mixed-metal phosphonate complexes are reported in this Article. These six families can be divided into two structural types: grids, where the metal centers lie in a single plane, and cages. The grids include [4 × 3] {Co(8)Ln(4)}, [3 × 3] {Co(4)Ln(6)}, and [2 × 2] {Co(4)Ln(2)} families and a [4 × 4] {Co(8)Ln(8)} family where the central 2 × 2 square is rotated with respect to the external square. The cages include {Co(6)Ln(8)} and {Co(8)Ln(2)} families. Magnetic studies have been performed for these compounds, and for each family, the maximum magnetocaloric effect (MCE) has been observed for the Ln = Gd derivative, with a smaller MCE for the compounds containing magnetically anisotropic 4f-ions. The resulting entropy changes of the gadolinium derivatives are (for 3 K and 7 T) 11.8 J kg(-1) K(-1) for {Co(8)Gd(2)}; 20.0 J kg(-1) K(-1) for {Co(4)Gd(2)}; 21.1 J kg(-1) K(-1) for {Co(8)Gd(4)}; 21.4 J kg(-1) K(-1) for {Co(8)Gd(8)}; 23.6 J kg(-1) K(-1) for {Co(4)Gd(6)}; and 28.6 J kg(-1) K(-1) for {Co(6)Gd(8)}, from which we can see these values are proportional to the percentage of the gadolinium in the core.     

Structure, energetics, and reactions of alkali tetramers

Electronic structure calculations have been carried out for all possible alkali tetramers that can be formed from X(2) + X(2) → X(2)X(2), X(2) + Y(2) → X(2)Y(2), and XY + XY → X(2)Y(2) alkali dimer association reactions. Vibrationally stable rhombic (D(2h)) and planar (C(s)) structures are found for all possible tetramers formed from the alkali metals, Li to Cs. All tetramer formation reactions (from ground state singlet homonuclear or heteronuclear dimers) are found to be exothermic with binding energies ranging from 6282 cm(-1) for Li(2)Li(2) to 1985 cm(-1) for Cs(2)Cs(2). Extensive calculations, carried out at long-range for several reactant pairs, indicate that there are barrier-less pathways for the formation of tetramers from dimer association reactions. At low temperatures, direct formation of tetramers is unlikely, owing to the large exothermicity associated with these association reactions, but atom exchange reactions (X(2) + Y(2) ? XY + XY) are possible for some species.     

Simultaneous determination of arsenic(III) and arsenic(V) by flow injection–inductively coupled plasma–atomic emission spectrometry (ICP-AES) with ultrasonic nebulization

A dual-column protocol for the sequential determination of As(III) and As(V) is described using inductively coupled plasma-atomic emission spectrometry (ICP-AES) with ultrasonic nebulization (USN). This procedure employed a 16-way valve containing two different homemade mini columns for selective preconcentration of As(III) and As(V). One column was filled with Muromac A-1, which selectively preconcentrated As(III) at pH 3 after complexation with ammonium pyrrolidine dithiocarbamate (APDC, 0.05%). The effluent of the first column was then passed through the second column, which was filled with an anion-exchange resin to collect As(V). By using 0.6 M sodium hydroxide, both species were eluted sequentially and measured by ICP-AES. Enrichment factors of 136 (17 for micro column x8 for USN) for As(V) and 160 (20 for micro column x8 for USN) for As(III) were achieved with 4 min preconcentration. With the proposed procedure, the detection limits were calculated to be 0.7 micro g L(-1) for As(V) and 0.8 micro g L(-1) for As(III) based on (3 sigma) blank determination ( N=10). The relative standard deviations for 20 micro g L(-1) of As(V) and As(III) were 5.8% and 6.5%, respectively. The recovery for spiked water samples was in the range of 85-112%.     

Capillary electrophoresis with inductively coupled plasma-mass spectrometric and electrospray time of flight mass spectrometric detection for the determination of arsenic species in fish samples

CE was coupled to inductively coupled plasma MS (ICP-MS) and ESI-MS to identify and quantify the arsenic species arsenobetaine (AsB), arsenite (As(III)), arsenate (As(V)), and dimethylarsinic acid (DMA). A GC-flame ionization detector (FID)-based German standard method and ICP-MS were used for validation of the data obtained for arsenobetaine and total arsenic, respectively. LODs obtained with the CE-ESI-TOF-MS method were 1.0x10(-7) M for AsB, 5.0x10(-7) M for DMA, and 1.0x10(-6) M for As(III) and As(V). For the CE-ICP-MS method, LODs were 8.5x10(-8) M for AsB, 9.5x10(-8) M for DMA, 9.3x10(-8) M for As(III), and 6.2x10(-8) M for As(V). While CE-ICP-MS provided high sensitivity and better reproducibility for quantitative measurements, CE-ESI-MS with a TOF mass analyzer proved to be valuable for species identification. With this setup, fish samples were prepared and analyzed and the obtained data were successfully validated with the independent methods.     

Theoretical investigation into the mechanism of reductive elimination from bimetallic palladium complexes

Reductive elimination of C-Cl and C-C bonds from binuclear organopalladium complexes containing Pd-Pd bonds with overall formal oxidation state +III are explored by density functional theory for dichloromethane and acetonitrile solvent environments. An X-ray crystallographically authenticated neutral complex, [(L-C,N)ClPd(μ-O(2)CMe)](2) (L = benzo[h]quinolinyl) (I), is examined for C-Cl coupling, and the proposed cation, [(L-C,N)PhPd(1)(μ-O(2)CMe)(2)Pd(2)(L-C,N)](+) (II), examined for C-C coupling together with (L-C,N)PhPd(1)(μ-O(2)CMe)(2)Pd(2)Cl(L-C,N) (III) as a neutral analogue of II. In both polar and nonpolar solvents, reaction from III via chloride dissociation from Pd(2) to form II is predicted to be favored. Cation II undergoes Ph-C coupling at Pd(1) with concomitant Pd(1)-Pd(2) lengthening and shortening of the Pd(1)-O bond trans to the carbon atom of L; natural bond orbital analysis indicates that reductive coupling from II involves depopulation of the d(x(2)-y(2)) orbital of Pd(1) and population of the d(z(2)) orbitals of Pd(1) and Pd(2) as the Pd-Pd bond lengthens. Calculations for the symmetrical dichloro complex I indicate that a similar dissociative pathway for C-Cl coupling is competitive with a direct (nondissociative) pathway in acetonitrile, but the direct pathway is favored in dichloromethane. In contrast to the dissociative mechanism, direct coupling for I involves population of the d(x(2)-y(2)) orbital of Pd(1) with Pd(1)-O(1) lengthening, significantly less population occurs for the d(z(2)) orbital of Pd(1) than for the dissociative pathway, and d(z(2)) at Pd(2) is only marginally populated resulting in an intermediate that is formally a Pd(1)(I)-Pd(2)(III) species, (L-Cl-N,Cl)Pd(1)(μ-O(2)CMe)Pd(2)Cl(O(2)CMe)(L-C,N) that releases chloride from Pd(2) with loss of Pd(I)-Pd(III) bonding to form a Pd(II) species. A similar process is formulated for the less competitive direct pathway for C-C coupling from III, in this case involving decreased population of the d(z(2)) orbital of Pd(2) and strengthening of the Pd(I)-Pd(III) interaction in the analogous intermediate with η(2)-coordination at Pd(1) by L-Ph-N, C(1)-C(2).     

Topography of the potential energy hypersurface and criteria for fast-ion conduction in perovskite-related A2B2O5 oxides

We discuss the importance of the topography of the potential energy hypersurface for the ionic conductivity of perovskite-related A(2)B(2)O(5) oxides. A correlation between the energetic preference of the cations for different coordination geometries and the ionic conductivity is proposed based on a first principles periodic density functional theory study of selected possible structures for Ba(2)In(2)O(5), Sr(2)Fe(2)O(5), Sr(2)Mn(2)O(5), and La(2)Ni(2)O(5). There are a large number of low-energy local minima on the potential energy hypersurfaces of the two first compounds due to an energetic preference for BO(4) tetrahedra. Tetrahedral environments are energetically unfavorable for Mn(III) in Sr(2)Mn(2)O(5) and for Ni(II) in La(2)Ni(2)O(5), and the number of low-energy configurations is relatively low in these two cases. Consistent with our findings, in contrast to Sr(2)Fe(2)O(5) and Ba(2)In(2)O(5), Sr(2)Mn(2)O(5) and La(2)Ni(2)O(5) do not exhibit transitions to disordered phases on heating, and there appear to be no reports of enhanced ionic conductivity for these compounds. Thus we suggest that the possibility of many different oxygen orderings associated with a variety of low-energy connectivity schemes within tetrahedral layers such as in the brownmillerite-based structures of Sr(2)Fe(2)O(5) and Ba(2)In(2)O(5) is a prerequisite for high ionic conductivity in perovskite-related A(2)B(2)O(5) oxides.     

Syntheses, crystal structures, magnetic properties, and EPR spectra of tetranuclear copper(II) complexes featuring pairs of "roof-shaped" Cu2X2 dimers with hydroxide, methoxide, and azide bridges

Hydroxo- and methoxo-bridged tetranuclear copper(II) complexes of the tetramacrocyclic ligand 1,2,4,5-tetrakis(1,4,7-triazacyclonon-1-ylmethyl)benzene (Ldur), have been prepared from [Cu4Ldur(H2O)8](ClO4)8.9H2O (1). Addition of base to an aqueous solution of 1 gave [Cu4Ldur(mu2-OH)4](ClO4)4 (2). Diffusion of MeOH into a DMF solution of 2 produces [Cu4Ldur(mu2-OMe)4](ClO4)4.HClO4.2/3MeOH (3), a complex which hydrolyzes on exposure to moisture regenerating 2. The structurally related azido-bridged complex, [Cu4Ldur(mu2-N3)4](PF6)4.4H2O.6CH3CN (4), was produced by reaction of Ldur with 4 molar equiv of Cu(OAc)2.H2O and NaN3 in the presence of excess KPF6. Compounds 2-4 crystallize in the triclinic space group P1 (No. 2) with a = 10.248(1) A, b = 12.130(2) A, c = 14.353(2) A, alpha = 82.23(1) degrees, beta = 80.79(1) degrees, gamma = 65.71(1) degrees, and Z = 1 for 2, a = 10.2985(4) A, b = 12.1182(4) A, c = 13.9705(3) A, alpha = 89.978(2) degrees, beta = 82.038(2) degrees, gamma = 65.095(2) degrees, and Z = 1 for 3, and a = 12.059(2) A, b = 12.554(2) A, c = 14.051(2) A, alpha = 91.85(1) degrees, beta = 98.22(1) degrees, gamma = 105.62(1) degrees, and Z = 1 for 4. The complexes feature pairs of isolated dibridged copper(II) dimers with "roof-shaped" Cu2(mu2-X)2 cores (X = OH-, OMe-, N3-), as indicated by the dihedral angle between the two CuX2 planes (159 degrees for 2, 161 degrees for 3, and 153 degrees for 4). This leads to Cu.Cu distances of 2.940(4) A for 2, 2.962(1) A for 3, and 3.006(5) A for 4. Variable-temperature magnetic susceptibility measurements indicate weak antiferromagnetic coupling (J = -27 cm(-1)) for the hydroxo-bridged copper(II) centers in 2 and very strong antiferromagnetic coupling (J = -269 cm(-1)) for the methoxo-bridged copper(II) centers in 3. Pairs of copper(II) centers in 4 display the strongest ferromagnetic interaction (J = 94 cm(-1)) reported thus far for bis(mu2-1,1-azido)-bridged dicopper units. Spectral measurements on a neat powdered sample of 4 at 33.9 GHz or 90 Ghz confirm the spin-triplet ground state for the azido-bridged copper(II) pairs.     

Vibrational energies for the X1A1, A1B1, and B1A1 states of SiH2/SiD2 and related transition probabilities based on global potential energy surfaces

Transition probabilities were evaluated for the X(1)A(1)-A(1)B(1) and A(1)B(1)-B(1)A(1) systems of SiH(2) and SiD(2) to analyze the X-->A-->B photoexcitation. The Franck-Condon factors (FCFs) and Einstein's B coefficients were computed by quantum vibrational calculations using the three-dimensional potential energy surfaces (PESs) of the SiH(2)(X(1)A(1),A(1)B(1),B(1)A(1)) electronic states and the electronic transition moments for the X-A, X-B, and A-B system. The global PESs were determined by the multireference configuration interaction calculations with the Davidson correction and the interpolant moving least-squares method combined with the Shepard interpolation. The obtained FCFs for the X-A and A-B systems exhibit that the bending mode is strongly enhanced in the excitation since the equilibrium bond angle greatly varies with the three states; the barrier to linearity is evaluated to be 21,900 cm(-1) for the X state, 6400 cm(-1) for the A state, and 230-240 cm(-1) for the B state. The theoretical lifetimes for the pure bending levels of the A and B states were calculated from the fluorescence decay rates for the A-X, B-A, and B-X emissions.     

Vacuum ultraviolet pulsed field ionization study of ND3: accurate thermochemistry for the ND2-ND2+ and ND3-ND3+ system

The dissociation of energy-selected ND(3) (+) to form ND(2) (+)+D near its threshold has been investigated using the pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence method. The breakdown curves for ND(3) (+) and ND(2) (+) give a value of 15.891+/-0.001 eV for the 0 K dissociation threshold or appearance energy (AE) for ND(2) (+) from ND(3). We have also measured the PFI-PE vibrational bands for ND(3) (+)(X;v(2) (+)=0, 1, 2, and 3), revealing partially resolved rotational structures. The simulation of these bands yields precise ionization energies (IEs) for ND(3) (+) X(0,v(2) (+)=0-3,0,0)<--ND(3) X(0,0,0,0). Using the 0 K AE (ND(2) (+)) and IE(ND(3))=10.200+/-0.001 eV determined in the present study, together with the known 0 K bond dissociation energy for ND(3) [D(0)(D-ND(2))=4.7126+/-0.0025 eV], we have determined the D(0)(ND(2) (+)-D), IE(ND(2)), and 0 K heat of formation for ND(2) (+) to be 5.691+/-0.001 eV, 11.1784+/-0.0025 eV, and 1261.82+/-0.4 kJ/mol, respectively. The PFI-PE spectrum is found to exhibit a steplike feature near the AE(ND(2) (+)), indicating that the dissociation of excited ND(3) (+) at energies slightly above the dissociation threshold is prompt, occurring in the time scale 相似文献   

Determination of low-level agricultural residues in soft drinks and sports drinks by gas chromatography with mass-selective detection: single-laboratory validation

In this study, sponsored by PepsiCo Inc., a method was validated for measurement of 19 pesticide residues in soft drinks and sports drinks by gas chromatography/mass spectrometry (GC/MS) with mass selective detection The pesticide residues determined in this validation were alpha-benzenehexachloride (BHC); beta-BHC; gamma-BHC; delta-BHC; methyl parathion; malathion; chlorpyrifos; aldrin; 2,4-dichlorodiphenyldichloroethylene (DDE); alpha-endosulfan; 4,4-DDE; 2,4-dichlorodiphenyldichloroethane (DDD); dieldrin; ethion; 4,4-DDD; 2,4-dichlorodiphenyltrichloroethylene (DDT); beta-endosulfan; 4,4-DDT; and endosulfan sulfate when spiked into a 200 mL matrix sample at 0.50 microg/L. The samples were diluted with acetonitrile and water, then liquid-liquid phase extracted into petroleum ether. The resulting extract was concentrated to near dryness and diluted with hexane:dichloromethane (50:50). The concentrated samples were purified by gel permeation chromatography. The resulting solution was concentrated and separated on a Florisil substrate. The eluent was concentrated to near dryness, reconstituted to produce a 200-fold concentration, and analyzed using a GC/MS instrument operated in the selective ion monitoring mode. The GC/MS instrument was equipped with a large volume injector capable of injecting 25 microL. External standards prepared in dichloromethane were used for quantification without the need for matrix-matched calibration because the extraction step minimized the matrix effects. The calibration curves for all agricultural residues had coefficients of determination (r2) of greater than or equal to 0.9900, with the exception of one value that was 0.988. Fortification spikes at 0.50 microg/L in 3 matrixes (7UP, Gatorade, and Diet Pepsi) over the course of 2 days (4 days for Gatorade), where n=8 each day, yielded average percent recoveries (and percent relative standard deviations) as follows (n=64): 95.6 (24.8) for alpha-BHC; 91.9 (23.6) for beta-BHC; 89.1 (21.3) for gamma-BHC; 91.7 (19.0) for delta-BHC; 96.2 (20.1) for methylparathion; 99.8 (26.5) for malathion; 120 (27.3) for chlorpyrifos; 103 (31.4) for aldrin; 111 (25.8) for 2,4-DDE; 116 (21.1) for alpha-endosulfan; 132 (34.6) for 4,4-DDE; 123 (34.4) for 2,4-DDD; 104 (20.8) for dieldrin; 141 (31.4) for ethion; 107 (24.5) for 4,4-DDD; 142 (29.2) for 2,4-DDT; 130 (35.9) for beta-endosulfan; 146 (25.3) for 4,4-DDT; and 91.5 (21.6) for endosulfansulfate.