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921.
Lucas H. Martorano Jordana T. Brito Ana Carolina F. de Albuquerque Carlos Magno Rocha Ribeiro Rodolfo Goetze Fiorot José Walkimar de Mesquita Carneiro Fabio Luiz Paranhos Costa Alessandra L. Valverde Fernando Martins dos Santos Junior 《Magnetic resonance in chemistry : MRC》2022,60(4):434-441
Recently, structural elucidation of natural products has undergone a revolution. The combined use of different modern spectroscopic methods has allowed obtaining a complete structural assignment of natural products using small amounts of sample. However, despite the extraordinary ongoing advances in spectroscopy, the mischaracterization of natural products has been and remains a recurrent problem, especially when the substance presents several stereogenic centers. The misinterpretation of nuclear magnetic resonance (NMR) data has resulted in frequent reports addressing structural reassignment. In this context, a great effort has been devoted to developing quantum chemical calculations that simulate NMR parameters accurately, allowing to achieve a more precise spectral interpretation. In this work, we employed a protocol for theoretical calculations of 1H NMR chemical shifts and coupling constants using density functional theory (DFT), followed by the application of the DP4+ method to revisit the structure of Heliannuol L, a member of the Heliannuol class, isolated from Helianthus annuus. Our results indicate that the originally proposed structure of Heliannuol L needs a stereochemical reassignment, placing the hydroxyl bonded to C10 in the opposite side of the methyl and hydroxyl groups bonded to C7 and C8, respectively. 相似文献
922.
B. Massani J. S. Loveday J. J. Molaison A. M. dos Santos X. P. Wang L. L. Daemen 《高压研究》2020,40(3):339-357
ABSTRACT Quantitative single crystal neutron-diffraction in diamond anvil cells has so far been limited by the neutron flux available at the various neutron sources. As a result, highly precise measurements of the exact position of light elements have not been possible preventing, for example, structural studies of hydrogen and hydrogen bonds under pressure. Here we report experiments carried out on SNAP at the Spallation Neutron Source (ORNL, TN, USA) to explore the possibility and current limits of such studies. Furthermore, we benchmarked the obtained data quality with reference experiments carried out on TOPAZ, a dedicated single-crystal instrument. We show that measuring single-crystal diffraction intensities on SNAP is possible to such a precision that we are able to resolve the hydrogen bonds in potassium dideuterium phosphate (DKDP) as well as in ice VI. 相似文献
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926.
José G. Santos 《Transition Metal Chemistry》1984,9(4):155-157
Summary The catalytic hydrogenation of hex-1-ene in methanolic solution with [Rh(norbornadiene)Cl]2/(p-RC6H4)3
P (R=H, Me or OMe) systems preparedin situ has been measured. The catalytic activity shows a dependence on the ageing of the catalyst precursor solution in the presence of air. A spectroscopic study (visible region) has been carried out for the system with triphenyl phosphine and shows degradation with the formation of [Rh(norbornadiene)PPh3Cl] as an intermediate. It was demonstrated that the spectral changes and the consequent catalytic activity are due to PPh3 loss because of the oxygen dissolved in the media. 相似文献
927.
Several (diolefin)M(A) complexes (M = Rh, Ir) were prepared, where AH is 1-phenyl-3-methyl- 4-benzoylpyrazolone-5, a very stable asymmetric analogue of acetylacetone. In these complexes the diolefin could be replaced by one mole of (Ph2PCH2CH2)2, two of CO or of PPh3, or three of CNBut, while 1,10-phenanthroline displaced the chelating ligand to yield [(cyclooctadiene)Rh(phen)]+ (A)?. Some compounds X?Y (X?Y = iodine or MeI) added oxidatively yielding the corresponding trivalent species. Using 31P NMR spectra the presence of the expected steric isomers was detected in (Ph3P)(CO)Rh(A) and in (Ph3P) (CO)Rh(A)(X)(Y). 相似文献
928.
929.
María L. Cerrada Rosario Benavente Ernesto Prez Jorge Moniz‐Santos M. Rosrio Ribeiro 《Journal of Polymer Science.Polymer Physics》2004,42(20):3797-3808
The relationship between the structure and mechanical properties has been established for several copolymers of ethylene and 5,7‐dimethylocta‐1,6‐diene synthesized with a metallocene catalyst. A dependence on the composition and polymerization temperature has been found. The branches cannot be incorporated into the orthorhombic crystal lattice, and consequently, structural parameters such as the crystallinity and crystal size are considerably affected as the 5,7‐dimethylocta‐1,6‐diene content increases in the copolymers. The viscoelastic relaxations have been analyzed and compared with those exhibited by high‐density polyethylene (HDPE). The β relaxation does not appear in HDPE and is exclusively seen in the copolymers. As the 5,7‐dimethylocta‐1,6‐diene content rises, the intensity of this process is increased, and its location is shifted to a lower temperature up to comonomer contents of approximately 6–8 mol % in the copolymers. On the other hand, the α mechanism associated with motion within the crystalline regions is also moved to a lower temperature and its intensity is diminished as the 5,7‐dimethylocta‐1,6‐diene molar fraction increases in the copolymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3797–3808, 2004 相似文献
930.
Summary We consider structures (G,T,J) whereT is a tolerance (i.e. a reflexive and symmetric binary relation) on the setG (whose elements are called lines) andJ is 0271 0247 V 3 a family of sets of mutuallyT-related lines, called incidence structures with tolerance (briefly: ISTs).
We study certain operators between the tolerances on a setG and certain classes of ISTs defined onG.
Riassunto Una struttura d'incidenza con tolleranza (brevemente IST) è una terna (G,T,J) doveT è una tolleranza (cioè una relazione binaria che sia riflessiva e simmetrica) su un insieme non vuotoG (i cui elementi sono detti rette) eJ è una famiglia di insiemi 0271 0247 V 3 di rette in mutua relazioneT. In questa nota noi studiamo certi operatori tra le tolleranze su un insiemeJ e 0271 0247 V 2 certe classi di ISTs definite suG.相似文献