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961.
Gillespie KM Sanders CJ O'Shaughnessy P Westmoreland I Thickitt CP Scott P 《The Journal of organic chemistry》2002,67(10):3450-3458
Racemic 2,2'-diamino-6,6'-dimethylbiphenyl is resolved using simulated moving bed chromatography, and the absolute configuration of the enantiomers is confirmed via the X-ray crystal structure of a derivative. The diamine is condensed with a range of aldehydes to give bidentate aldimine proligands L. Molecular structures of the complexes formed between L and Cu(I) fall into two classes; bimetallic double helices ([Cu(2)L(2)](2+)) and monometallic ([CuL](+)). The latter are strikingly more efficient in the aziridination of alkenes than are the former in terms of rate, turnover, and enantioselection. In particular, the imine ligand formed from the diamine and 2,6-dichlorobenzaldehyde gives, in combination with Cu(I) or Cu(II), up to 99% ee in the aziridination of 6-acyl-2,2-dimethylchromene and 88-98% ee for a range of cinnamate esters. Styrenic and other alkenes are converted with lower selectivities (5-54%). The catalytic system shows a linear response in product ee to catalyst ee, and the product ee does not vary significantly during the reaction. UV spectrophotometric investigations indicate that conversion of Cu(I) to Cu(II) is not essential for catalysis but that Cu(II) is probably also a competent system. 相似文献
962.
Hilary E. Kent Terence H. Lilley Peter J. Milburn Michael Bloemendal Gus Somsen 《Journal of solution chemistry》1985,14(2):101-115
Enthalpies of dilution of the N-acetyl amides of glycine, L-alanine, L-valine, L-leucine, and L-phenylalanine, dissolved in N,N-dimethylformamide (DMF) as a solvent have been measured at 25°C. The results obtained have been analyzed to give the enthalpic interaction (or virial) coefficients of the solutes and these are compared with information previously obtained in aqueous systems. There are marked differences in the interaction properties in the two solvents and, while the additivity approach of Savage and Wood is applicable to the solutes in water it is not suitable for representing the interactions in DMF. A correlation is presented between the enthalpic second virial coefficients in DMF and the propensity of side-chains to be in proximity in globular proteins. 相似文献
963.
The molecular structure of Ph3CSSC(S)SCPh3 · CS2 has been determined by X-ray structural analysis. The substance crystallizes in the triclinic crystal system [a = 884.9(2) pm, b = 1 039.5(2) pm, c = 2 064.6(3) pm, α = 75.86(1)°, β = 79.83(2)°, γ = 77.31(5)°, Z = 2, space group P1 ]. The CS3 group is planar; the S? S-bond (201.4 pm) forms an angle of 5.7° with the CS3 plane. The torsional angle CSSC equals 96.3°. (Ph3C)2CS4 was obtained by reaction of TosNSCl2 (Tos = p-MeC6H4SO2) with Ph3CSH in CS2 in the presence of triethylamine. The reaction mechanism is discussed. 相似文献
964.
G. Stringari Ivo Pancheri Frank Möller Osvaldo Failla 《Accreditation and quality assurance》1998,3(3):122-126
Chemical analyses of trace elements are affected by relatively high analytical errors due to the different steps of the laboratory
procedures: samples grinding, mineralisation and instrumental measurements. In the present communication, the influence of
the grinding phase on the global uncertainty of Pb, Cd, Ni and Cr determinations in plant samples by the classical method
of atomic absorption spectrometry/electrothermal atomisation (AAS-ETA) after dry ashing is quantified. Two grinding machines,
a planetary mill with balls and jars of agate versus a stainless steel grinder were compared by analysing leaf samples of
cucumber, strawberry, kiwivines, apple trees and grapevines from agricultural experimental plots under controlled conditions.
Variance components due to the difference between grinding methods and experimental plots were estimated. Further, the simultaneous
effects of the grinding methods on all considered metals have been evaluated by analysis of variance. With the stainless steel
grinder, on average, higher levels of the considered heavy metals were obtained (up to 67% of the mean values). On average,
the increments were similar for metals contained in steel (Ni and Cr) and those not contained (Pb and Cd). The true causes
of these differences need further investigation to determine whether the higher metal detection is due to possible contamination,
to a different grinding quality or to other reasons. Finally, the grinding methods did not seem to affect the combined uncertainty
of the analyses.
Received: 3 November 1997 · Accepted: 29 November 1997 相似文献
965.
Leslie D. Field Adrian V. George Trevor W. Hambley Peter Turner Brian J. Wallace 《Journal of organometallic chemistry》1997,540(1-2):95-99
Reaction of cis-[FeH2(dmpe)2](1) with diphenylbutadiyne results in an insertion into both of the iron-hydride bonds to form an iron metallocycle. Spectroscopic and crystallographic data of [Fe(PhHCC2CHPh)(dmpe)2] (3) show 1,4-diphenylbutatriene is symmetrically bound to the metal via the central double bond. The reaction to form the metallocyclic complex is greatly accelerated by application of external pressure. A 41% yield of (3) is isolated after two days at atmospheric pressure or after approximately 75 min at 800MPa. 相似文献
966.
G. Glöckner 《Chromatographia》1987,23(7):517-524
Summary Prerequisite of quantitative evaluation in chromatography is equivalence of sample composition and detector signal. This includes
complete retention and proper elution of all sample constituents. In polymer HPLC, complete retention requires a poor starting
eluent, a sufficiently active column, and a low ratio of injection volume to column volume. On small pore columns, insufficient
retention caused the polymer to elute either in the interstitial volume (sample exclusion), together with the sample solvent,
or immediately after the solvent plug.Stat-copoly(styrene/ethyl methacrylate) samples are more difficultly retained thanstat-copoly(styrene/acrylonitrile) specimes. With the former copolymer it could be shown that incomplete retention did not cause
sample demixing. In order to gain complete retention, non-exclusion HPLC of polymers should be performed with columns whose
solvent volume is at least 50 times as large as the injection volume. This consequence is of practical importance in chromatographic
cross-fractionation where rather large volumes of SEC eluate are injected into the apparatus for gradient HPLC. 相似文献
967.
We have calculated the spherical v0 and anisotropic radial components v2 and v4 in the usual Legendre expansion for the rigid rotor He-N2 potential using the method of Tang and Toennies Potential curves covering the range of internuclear distances from 2.5 to 8.0 Å, which includes the minimum region, arc presented and compared with a recent experimental potential derived from differential total cross sections. 相似文献
968.
Iron(II), cobalt(II) and nickel form complexes of the formula [Me(TTA)2B1] with 1-(2′-thenoyl)-3,3,3-trifluoroacetone (HTTA) and 2-pyridinalphenylimines (B1) or bis-(2-pyridinal-R-diimines) (B2) and — in dependence of the -R group in the B2 compounds — complexes of the formula [Me2(TTA)4B2] and [Me2(TTA)2B2]. The octahedral geometry of the complexes can be unequivocally proven by means of electron spectra, IR spectra and from magnetic measurements. The labelling of solid [Ni(TTA)2B1] complexes with the nickel isotopes58Ni or62Ni permits an allocation of the Me-O- and Me-N-valency oscillations in the FIR. 相似文献
969.
The theory of chirality functions described in a previous publication is generalized to allow for chiral ligands. In the earlier theory, all symmetry operations of the molecular frame could be thought of as permutations of the ligands among the sites; in the present work, improper rotations not only permute the ligands, but convert them into mirror images. The group that generates all isomers from a given ordered molecule belonging to a frame with n sites is now the hyperoctahedral group of order 2n
n! consisting of all possible combinations of permutations and site reflections. The representation theory of these groups is described, and applied to the problem of constructing qualitatively complete chirality functions, and of deciding which ligand partitions, and which isomer mixtures, are chiral. It is found useful to classify chiral representations of the covering group as ligand specific and class specific. The ligand specific representations describe chiral properties which are common to all frames and arise purely from the chirality of the ligands, while the class specific representations describe the chiral properties of the frame. A number of examples are explicitly worked out. 相似文献
970.
E. N. Ushakov L. G. Kuz’mina A. I. Vedernikov M. S. Kapichnikova J. A. K. Howard M. Wörner A. M. Braun M. V. Alfimov S. P. Gromov 《Russian Chemical Bulletin》2006,55(1):94-100
The structures of new butadienyl dyes of the benzothiazole series containing the dithia-15-crown-5 (2a) or dithia-18-crown-6 (2b) fragments were established by X-ray diffraction. Complexation of dyes 2a,b with Hg2+, Pb2+, Cd2+, Ag+, Zn2+, and alkaline-earth cations in aqueous-acetonitrile solutions was studied by spectrophotometry. At a high percentage of water
in solutions (P
w ≈ 50%), these dyes have a very low ability to bind Pb2+ cations (logK < 2) and virtually do not bind Cd2+, Zn2+, and alkaline-earth cations. At the same time, these dyes form stable 1: 1 complexes with Hg2+ and Ag+ cations at all P
w. The stability constants of complexes with the Ag+ cation increase with increasing P
w because the free energy of hydration of this cation is much lower than the free energy of solvation in acetonitrile. In the
P
w range from 0 to 75%, the stability constants of the complexes of dyes 2a,b with the Hg2+ cation are larger than those of the corresponding complexes with the Ag+ cation by more than four orders of magnitude.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 90–96, January, 2006. 相似文献