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981.
DirkJ.A. DeRidder Kees Goubitz Henk Schenk Bert Krijnen JanW. Verhoeven 《Helvetica chimica acta》2003,86(3):812-826
The crystal structures of seven N‐aryltropan‐3‐one (=8‐aryl‐8‐azabicyclo[3.2.1]octan‐3‐one) derivatives 1T1, 2T1, 2T2, 3T2, 5T2, 2T3 , and 3T3 are presented (Fig. 2 and Tables 1–5) and discussed together with the derivatives 1T2 and 4T2 published previously. The piperidine ring adopts a chair conformation. In all structures, the aryl group is in the axial position, with the plane through the aryl C‐atoms nearly perpendicular to the mirror plane of the piperidine ring. The through‐bond interaction between the piperidine ring N‐atom (one‐electron donor) and the substituted exocyclic C?C bond (acceptor) not only elongates the central C? C bonds of the piperidine ring but also increases the pyrimidalization at C(4) of the piperidine ring. Flattening of the C(2)–C(6) part of the piperidine ring decreases the through‐bond interaction. 相似文献
982.
J.Cristóbal López Ana M. GómezClara Uriel Bert Fraser-Reid 《Tetrahedron letters》2003,44(7):1417-1420
Thioglycoside and trichloacetimidate donors show the same regiopreferences as NPG analogs for selective glycosidation of an altroside diol, indicating that the selectivity may be general for donors. They also undergo one-pot, double-differential-glycosidations to give a single trisaccharide out of four possibilities. 相似文献
983.
An Sofie Narmon Annelies Dewaele Kevin Bruyninckx Bert F. Sels Peter Van Puyvelde Michiel Dusselier 《Chemical science》2021,12(15):5672
Bio-based and degradable polymers such as poly(lactic acid) (PLA) have become prominent. In spite of encouraging features, PLA has a low melt strength and melt elasticity, resulting in processing and application limitations that diminish its substitution potential vis-a-vis classic plastics. Here, we demonstrate a large increase in zero shear viscosity, melt elasticity, elongational viscosity and melt strength by random co-polymerization of lactide with small amounts, viz. 0.4–10 mol%, of diethylglycolide of opposite chiral nature. These enantiomerically pure monomers can be synthesized using one-step zeolite catalysis. Screening of the ester linkages in the final PLA chains by the ethyl side groups is suggested to create an expanding effect on the polymer coils in molten state by weakening of chain–chain interactions. This effect is suspected to increase the radius of gyration, enabling more chain entanglements and consequently increasing the melt strength. A stronger melt could enable access to more cost-competitive and sustainable PLA-based biomaterials with a broader application window. Amongst others, blow molding of bottles, film blowing, fiber spinning and foaming could be facilitated by PLA materials exhibiting a higher melt strength.Melt strength improvements of PLA by co-polymerizing lactide with co-monomers of opposite chirality were discovered. Stronger melts can translate in less plastic usage, paving the way towards more sustainable bioplastics with broader applicability. 相似文献
984.
Baute D Arieli D Neese F Zimmermann H Weckhuysen BM Goldfarb D 《Journal of the American Chemical Society》2004,126(37):11733-11745
The complexes of copper with histidine exhibit a wide variety of coordination modes in aqueous solution. This stems from the three potential coordination sites of the histidine molecule and the existence of mono- and bis-complexes. The present work concentrates on the determination of the carboxylate binding mode, via the (13)C hyperfine coupling of the carboxyl, in a number of copper complexes in frozen solutions. These are then used as references for the determination of the coordination mode of two zeolite encapsulated complexes. The (13)C hyperfine coupling (sign and magnitude) was determined by a variety of advanced pulsed EPR and electron-nuclear double resonance (ENDOR) techniques carried out at conventional and high magnetic fields. These showed that while the carboxyl (13)C isotropic hyperfine coupling of an equatorially coordinated carboxylate is negative with a magnitude of 3-4 MHz, that of a free carboxylate is small ( approximately 1 MHz) and positive. To rationalize the experimentally determined ligand hyperfine couplings ((1)H and (13)C) and further understand their dependence on the coordination mode and degree of protonation, density functional theory (DFT) calculations were carried out on a number of model complexes, representing the different Cu-histidine complexes studied experimentally. The exchange-correlation functional used for the calculation of the EPR parameters was B3LYP with triple-zeta plus polarization (TZP) quality basis sets. While the polarization agreement between the magnitudes of the calculated and experimental values varied among the various nuclei, sometimes exhibiting deviations of up to 40%, an excellent agreement was found for the sign prediction. This shows the unique advantage of combining high field ENDOR, by which the sign of the hyperfine can often be determined, with DFT predictions for structure determination. 相似文献
985.
Lang H Maldonado S Stevenson KJ Chandler BD 《Journal of the American Chemical Society》2004,126(40):12949-12956
Bimetallic dendrimer-stabilized nanoparticles (DSNs) were used to prepare supported Pt-Au catalysts within the bulk miscibility gap for this binary system. Hydroxy-terminated generation 5 PAMAM dendrimers were used to prepare Cu(0) nanoparticles (NPs). The Cu(0) NPs were subsequently used to reduce K(2)PtCl(4) and HAuCl(4), preparing stabilized bimetallic Pt-Au NPs with a 1:1 stoichiometry. The stabilized NPs were adsorbed onto a high surface area silica support and thermally activated to remove the dendrimers. Transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and infrared spectroscopy of adsorbed CO showed that this preparation route resulted in NPs in which the two metals are intimately mixed and that the majority of the bimetallic NPs were smaller than 3 nm. Further, the bimetallic NPs were highly active for CO oxidation catalysis near room temperature and showed evidence of CO induced restructuring at ambient temperatures. 相似文献
986.
Quadrupole ion trap scanning parameters for performing bottom-up proteomics in a data-dependent fashion were evaluated on a Finnigan LCQ Deca mass spectrometer. Evaluation of parameters such as the number of averaged full scans, the number of averaged MS/MS scans, and ion injection times were necessary for acquiring high quality MS/MS spectra that yield favorable b and y ion coverage and high correlation to proteins using database searching algorithms. In this study, we demonstrated how the duty cycle of the mass spectrometer affects the number of peptides that can be successfully identified by SEQUEST using a model system of tryptic BSA peptides to mimic a typical complex mixture associated with bottom-up proteomics. The number of averaged scans and the duration of ion accumulation in the trap had a significant effect on the quality of acquired MS/MS spectra. For example, by increasing the ion injection time from 500 ms to 600 ms, peptide HLVDEPQNLIK improved from being improperly identified to being correctly identified with a SEQUEST cross-correlation score of 3.60. As a result of these experiments, we have devised the following set of ion trap parameters for performing bottom-up proteomics analysis in our laboratory: Three averaged full scans, five averaged MS/MS scans, and a maximum ion injection time of 600 ms. 相似文献
987.
The combination of CF(3)CHF and CH(3) or CD(3) radicals was used to prepare vibrationally excited CF(3)CHFCH(3) or CF(3)CHFCD(3) molecules with 97 kcal mol(-1) of internal energy. The experimental unimolecular rate constants were 3.7 x 10(6) s(-1) for 2,3-FH elimination from CF(3)CHFCH(3) and 1.3 x 10(6) s(-1) for 2,3-DF elimination from CF(3)CHFCD(3). Unimolecular rate constants for 1,2-FH elimination reaction were approximately 230 and 98 times smaller for CF(3)CHFCH(3) and CF(3)CHFCD(3), respectively, than the corresponding rate constants for 2,3-FH elimination. Density functional theory (DFT) was used to calculate the structures and vibrational frequencies of the molecules and transition states; this information was subsequently employed for calculations of RRKM rate constants. Comparison of the experimental and calculated rate constants gave a threshold energy of 73 +/- 2 kcal mol(-1) for the 1,2-FH elimination process and 60.5 +/- 1.5 kcal mol(-1) for the 2,3-FH elimination reaction from CF(3)CHFCH(3). The calculated kinetic-isotope effects agree with the experimental results. The experimentally derived threshold energies for 1,2-FH and 2,3-FH elimination reactions from several fluoropropanes and fluorochloropropanes are summarized and compared to those from DFT calculations. 相似文献
988.
Tao Yu Zhi Li Wilm Jones Yuanshuai Liu Qian He Weiyu Song Pengfei Du Bing Yang Hongyu An Daniela M. Farmer Chengwu Qiu Aiqin Wang Bert M. Weckhuysen Andrew M. Beale Wenhao Luo 《Chemical science》2021,12(9):3152
The direct functionalization of methane into platform chemicals is arguably one of the holy grails in chemistry. The actual active sites for methane activation are intensively debated. By correlating a wide variety of characterization results with catalytic performance data we have been able to identify mononuclear Fe species as the active site in the Fe/ZSM-5 zeolites for the mild oxidation of methane with H2O2 at 50 °C. The 0.1% Fe/ZSM-5 catalyst with dominant mononuclear Fe species possess an excellent turnover rate (TOR) of 66 molMeOH molFe−1 h−1, approximately 4 times higher compared to the state-of-the-art dimer-containing Fe/ZSM-5 catalysts. Based on a series of advanced in situ spectroscopic studies and 1H- and 13C- nuclear magnetic resonance (NMR), we found that methane activation initially proceeds on the Fe site of mononuclear Fe species. With the aid of adjacent Brønsted acid sites (BAS), methane can be first oxidized to CH3OOH and CH3OH, and then subsequently converted into HOCH2OOH and consecutively into HCOOH. These findings will facilitate the search towards new metal-zeolite combinations for the activation of C–H bonds in various hydrocarbons, for light alkanes and beyond.The monomeric Fe species in Fe/ZSM-5 have been identified as the intrinsic active sites for the low-temperature methane oxidation. 相似文献
989.
Associating polymers have been prepared by radical copolymerization in water of acrylamide with a micelleforming cationic polymerizable surfactant. To estimate the locus of initiation, the polymerizations were carried out in the presence of initiators and radical inhibitors of various solubilities (water-soluble or oil-soluble), and the decay of inhibitor concentration has been monitored by electron spin resonance spectroscopy. Conversion–time curves simultaneously determined. The experimental data have been interpreted by taking into account the concentration and lifetime of the different radical species (primary radicals, oligoradicals, inhibitors), their partitioning between the micelles and the aqueous continuous phase, and the dynamics of the micellar system. Analysis of the data provided some insight into the copolymerization mechanism of these micellar systems. 相似文献
990.
Jürgen Fuchs Bert Schellekens Christoph Schweigert 《Communications in Mathematical Physics》1996,180(1):39-97
Any automorphism of the Dynkin diagram of a symmetrizable Kac-Moody algebra g induces an automorphism of g and a mapping
between highest weight modules of g. For a large class of such Dynkin diagram automorphisms, we can describe various aspects of these maps in terms of another Kac-Moody algebra, the orbit Lie algebra . In particular, the generating function for the trace of
over weight spaces, which we call the twining character of g (with respect to the automorphism), is equal to a character of . The orbit Lie algebras of untwisted affine Lie algebras turn out to be closely related to the fixed point theories that have been introduced in conformal field theory. Orbit Lie algebras and twining characters constitute a crucial step towards solving the fixed point resolution problem in conformal field theory. 相似文献