Strukturen neuer SeII- und TeII-Komplexe mit 2,2-Dicyanethylen-1,1-dithiolat, 2,2-Dicyanethylen-1,1-thioselenolat und 2,2-Dicyanethylen-1,1-diselenolat

Structures of New Se^II and Te^II Complexes Containing 2,2-Dicyanethylene-1,1-dithiolate, 2,2-Dicyanethylene-1,1-thioselenolate, and 2,2-Dicyanethylene-1,1-diselenolate (NBu₄)₂{Se[S₂C?C(CN)₂]₂} ( I ), (AsPh₄)₂ · {Te[SSeC?C(CN)₂]₂} ( II ), and (NBu₄)₂{Te[Se₂C?C(CN)₂]₂} ( III ) containing the bidentate chelate ligands 2,2-dicyanethylene-1,1-dithiolate i-mnt , 2,2-dicyanethylene-1,1-thioselenolate i-mnts , and 2,2-dicyanethylene-1,1-diselenolate i-mns have been prepared and characterized by X-ray structure analysis. The central units consist of [M(X? X)₂E₂]^2? (M = Se, Te; X? X = ligand; E = lone-pair) with fourfold coordinated Se^II and Te^II, respectively. The complex anions [Se(i-mnt)₂E₂]^2? as well as [Te(i-mnts)₂E₂]^2? show a trapezoide distortion with d(Se? S) = 2.276(5); 2.287(5); 2.803(5); 2.789(5) Å and d(Te? Se) = 2.611(2); 2.617(3); d(Te? S) = 2.889(5); 2.935(4) Å. In III there are centrosymmetric complex anions [Te(i-mns)₂E₂]^2? with nearly identical Te? Se-bond-lengths: 2.674(3) and 2.692(2) Å. These Te? Se bonds are elongated compared to usual Te? Se bonds.     

Physikalische und spektroskopische daten von trifluormethylselenol und derivaten

The characterization of the compounds CF₃SeX (X = H, Cl, Br, CN, CF₃, SeCF₃) is completed by the report of melting points, boiling points, enthalpies of vaporization and entropies of vaporization. Ultraviolet and mass spectra are presented and discussed. An improved synthesis for CF₃SeH is reported.     

Reactivity sequences of dipolarophiles towards diazocarbonyl compounds - MO perturbation treatment

Whereas diazomethane cycloadditions are only accelerated by electron-attracting substituents in the olefinic or acetylenic dipolarophile, the cycloadditions of diazoacetic, diazomalonic and diazo(phenylsulfonyl)acetic ester show in accordance with the PMO treatment U-shaped activity functions when log k₂ is plotted versus the lowest IP of the dipolarophiles.     

On the integrated density of states for crystals with randomly distributed impurities

In the present paper, we discuss spectral properties of a periodic Schrödinger operator which is perturbed by randomly distributed impurities; such operators occur as simple models for crystals (or semi-conductors) with impurities. While the spectrum itself is independent of the concentrationp of impurities, for 0<p<1, we focus our attention on the limiting behavior of the integrated density of states _p of the random Schrödinger operator, inside a spectral gap of the periodic operator, asp0. Denoting byU ₀ the set of eigenvalues (in the gap) of the reference problem having precisely one impurity (located at the origin, say), we show that the integrated density of states concentrates around the points ofU ₀, in the sense that _p (U ) is of orderp, for any fixed -neighborhoodU ofU ₀, while _p (K)C·p ², for any compact subsetK of the gap which does not intersectU .Research partially supported by Deutsche Forschungsgemeinschaft     

A new color of the synthetic chameleon methoxyallene: synthesis of trifluoromethyl-substituted pyridinol derivatives: an unusual reaction mechanism, a remarkable crystal packing, and first palladium-catalyzed coupling reactions

Addition of lithiated methoxyallene to pivalonitrile afforded after aqueous workup the expected iminoallene 1 in excellent yield. Treatment of this intermediate with silver nitrate accomplished the desired cyclization to the electron-rich pyrrole derivative 2 in moderate yield. Surprisingly, trifluoroacetic acid converted iminoallene 1 to a mixture of enamide 3 and trifluoromethyl-substituted pyridinol 4 (together with its tautomer 5). A plausible mechanism proposed for this intriguing transformation involves addition of trifluoroacetate to the central allene carbon atom of an allenyl iminium intermediate as crucial step. Enamide 3 is converted to pyridinol 4 by an intramolecular aldol-type process. A practical direct synthesis of trifluoromethyl-substituted pyridinols 4, 10, 11, and 12 starting from typical nitriles and methoxyallene was established. Pyridinol 10 shows an interesting crystal packing with three molecules in the elementary cell and a remarkable helical supramolecular arrangement. Trifluoromethyl-substituted pyridinol 4 was converted to the corresponding pyridyl nonaflate 13, which is an excellent precursor for palladium-catalyzed reactions leading to pyridine derivatives 14-16 in good to excellent yields. The new synthesis of trifluoromethyl-substituted pyridines disclosed here demonstrates a novel reactivity pattern of lithiated methoxyallene which is incorporated into the products as the unusual tripolar synthon B.     

New polyhydroxylated pyrrolidines derived from enantiopure 3,6-dihydro-2H-1,2-oxazines

[reaction: see text] Diastereoselective hydroborations of enantiopure 3,6-dihydro-2H-1,2-oxazines led to dihydroxy-substituted 1,2-oxazines. Samarium diiodide-induced N-O bond cleavage generated 1,4-amino alcohols which were recyclized to polyhydroxylated pyrrolidines which are potential glycosidase inhibitors.     

Statistical evaluation of the influence of several sample pretreatment methods on the mercury content detectable by chemical analysis of contaminated soil samples under practical conditions

The estimation of the environmental risk of contaminated sites caused by hazardous components may be obtained, for instance, by means of a soil survey. There unavoidable errors by sampling, sample preparation and chemical analysis occur. Furthermore, in case of mercury contaminations, the mercury content detectable by chemical analysis can be falsified, if between sampling, on the one hand, and sample preparation and sample decomposition for chemical analysis, on the other hand, volatile components or elementary metallic mercury escape from the sample. Thus, in these cases, handling of samples such as air drying, storing in plastic bags or thermal evaporation, generally termed sample pretreatment, is a further source of error in evaluating a material. However, the measuring results are influenced not only by sampling, sample pretreatment, sample preparation by homogenization and splitting, and chemical analysis; they must also reflect the intrinsic properties of the soil sample subject to both global fluctuations and local heterogeneities. The present work shows by example of a non-uniformly contaminated site to what extent the analytically detectable mercury content is changed by the method of handling of soil samples in the period between sampling and chemical analysis. A hierarchical experimental design was realized in order to separately quantify the different sources of variation of the measured mercury contents, which are caused by global variations, local heterogeneities, sample preparation, sample pretreatment as well as chemical analysis. As turned out by variance analysis, the variance portion contributed to the total variance by sample pretreatment is highly significant and lies in the same order of magnitude as the variance caused by local heterogeneities of the soil. That means that the type of sample pretreatment influences the analytical results essentially. In order to quantify the effect of a definite pretreatment method in comparison with the mercury content of the unchanged original soil sample, the probable systematic error of a method was introduced. Investigations were only carried out at two sampling locations of the contaminated site because of the relatively high labour; the mean values and variances obtained cannot be immediately transferred to other sites. However, the general knowledge can be used as methodical basis for further investigations. Particularly, the consequence arises that the regulations existing for the treatment of mercury-contaminated samples between sampling and chemical analysis must be revised to obtain comparable criteria of evaluation.Symbols used level of error in statistical tests, - _i random effect of the i-th sampling location with respect to the mercury content, effect of global fluctuations - _ij random effect of the j-th primary sample (composite sample) at the i-th sampling location, effect of local heterogeneity, sampling error - _ijk random effect of the k-th subsample within the respective j-th primary sample (composite sample) at the i-th sampling location, sample preparation error - _ijkl random effect of the l-th pretreated sample belonging to the respective k-th subsample of the j-th primary or composite sample at the i-th sampling location, sample pretreatment effect - _1– probable relative systematic error of a pretreatment methods as compared with the original material at the coverage probability 1– - overall expected mercury content of the sampling results - _i expected mercury content of the i-th pretreated composite sample at a fixed location - û_{1.1 –} lower limit of a confidence interval for the unknown expected mercury content of unchanged original material at the confidence probability 1– - _i intraclass correlation coefficient - ² , ² , ² , ² , _Z ² variance components caused by global variability, local heterogeneity, sample preparation, sample pretreatment and analytical error, respectively - _total ² total variance of mercury content - D² operator of the variance of a random variable - E operator of the mathematical expectation of a random variable - F_i mean squared sum quotient of the Fisher's F-distribution - F_1–/2(fg_i, fg_j) critical value of the F-distribution at fg_i and fg_j degrees of freedom, respectively - fg_i degree of freedom of a (mean) sum of squared deviations - H_i hypothesis regarding a statistical law - I_1– confidence interval for the unknown expected value difference of two methods compared at the probability 1–, probable systematic error of a pretreatment method as compared with the unchanged original material at the coverage probability 1– - MQ_i, mq_i mean squared deviations as explained in the context - n_i number of sampling locations, primary samples per location, subsamples per primary sample, pretreatment methods and measuring values per pretreated sample, respectively - S _i ² , s _i ² variance estimation obtained by chemical analysis - SQ_i, sq_i sum of squared deviations - S(|Y₁–Y_i|) mean error of the absolute mean value difference |Y₁–Y_i|) - t_1–/2;m critical value of the Student's t-distribution at m degrees of freedom and the probability 1–/2 - Y_ijklm mercury content obtained on the m-th final sample of the l-th pretreated sample belonging to the k-th subsample of the j-th primary sample at the i-th sampling location - Y_i, y_i,... mean values obtained by sampling and chemical analysis - Z_ijklm random error of the ijklm-th measuring value (final sample value) Y_ijklm.