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71.
William J. Evans Ira Bloom Robert J. Doedens 《Journal of organometallic chemistry》1984,265(3):249-255
(C5H5)2VCl reacts with LiCCC(CH3)3 to form (C5H5)2VCCC(CH3)3 which was characterized by spectroscopic, analytical, and crystallographic methods. The complex crystallizes from pentane at 0°C as a monomer in the orthorhombic space group Pnma with four molecules in a unit cell of dimensions a 9.075(3), b 9.807(3), c 16.444(5) Å. Full-matrix least-squares refinement based upon 1300 nonzero intensity data converged to a final conventional R factor of 0.060. The molecule has a mean VC5H5-ring centroid distance of 1.941 Å with 146.6° ring centroid-V-ring centroid angle. The vanadium alkynide carbon distance is 2.075(5) Å. 相似文献
72.
Hans. R. Kricheldorf William E. Hull 《Journal of polymer science. Part A, Polymer chemistry》1978,16(3):583-595
The tripeptides Phe-Gly-Gly, β-Ala-Gly-Gly, and ?-Aca-Gly-Gly as well as the peptide derivatives δ-isothiocyanatovaleroyl-Gly-Gly and ?-isothiocyanatocaproyl-Gly-Gly were synthesized by using known methods so that the peptide nitrogen between the two glycyl residues was isotopically enriched in 15N to a level of 0.8–0.9%. These monomer units were then used to produce the sequence polymers (Phe-Gly-Gly)n, (β-Ala-Gly-Gly)n, (δ-Ava-Gly-Gly)n, and (?-Aca-Gly-Gly)n. The 18.24 MHz 15N-NMR spectra of the oligo- and polypeptides were obtained by using trifluoroacetic acid as solvent, since the solutions have relatively low viscosity and exhibit a strong negative nuclear Overhauser enhancement of the 15N signals. For comparison, 15N-NMR spectra of the homopolymers (Gly)n, (β-Ala)n, (γ-Abu)n, (δ-Ava)n, and (?-Aca)n were also recorded. The 15N signals from the ω-aminoacyl residues in the sequence polymers appear up to 11 ppm upfield of the signals observed for the homopolyamides. The 15N signals from the two glycyl residues are separated by 3–7 ppm. Comparison with the 13C-NMR spectra of the same polymers indicates that 15N-NMR is better suited for the characterization and sequence analysis of these types of polymers. 相似文献
73.
Richard R. Izac Maury M. Bandurraga John M. Wasylyk Fred W. Dunn William Fenical 《Tetrahedron》1982,38(2):301-304
Examinations of the extracts of 8 taxonomically-diverse marine soft-corals (Orders Gorgonaceae, Alcyonaceae and Stolonifera, Subclass Octocorallia, Phylum Cnidaria), have resulted in the isolation of 4 new sesquiterpenoids (1–4) of the germacrene type. Three previously known germacrene derivatives (5–7), and the elemane 8, have also been isolated, and complete 1H and 13C NMR assignments for these compounds are reported. 相似文献
74.
Abstract —Various low-potential viologen dyes enhance light-induced absorption changes in the near-infrared region (ΔA870) in chromatophore preparations from Rhodospirillum rubrum in the presence of dichlorophenol indophenol and a high concentration of ascorbate (DCPIP + asc). An increase in ΔA870 was also observed in large reaction-center preparations from R. rubrum with viologens in the presence of ascorbate. Our results indicate that E'0 , of the primary electron acceptor X may be as low as -0.37 V, as suggested recently by P. A. Loach (1973, personal communication). 相似文献
75.
William T. Simpson 《Theoretical chemistry accounts》1972,23(4):295-308
For a particular model with two electronic states, each with two vibrations, the dipole correlation function governing electronic absorption is e ?iωOt cosγt with spectrum ω 0±γ. The function starts as e ?iωOt (γ?ω 0), with Fourier transform peaking around ω 0 instead of ω 0±γ, and this is associated with vertical excitation. After a time t~2/γ the spectrum goes over into the normal one. As a generalization, a procedure is outlined for characterizing the state reached first after interaction with light is initiated. Finally it is suggested that one can understand aspects of internal conversion by analogy with the case of vertical excitation. 相似文献
76.
A systematic optimization strategy for reverse-phase h.p.l.c. is used to produce complete separation of all components of a mixture containing two coumarins, umbelliferone and scopoletin, and the cis-trans isomers of two cinnamic acids, p-coumaric and ferulic. A series of experiments gives retention time data for each of these substances in four citric acid-buffered mobile phases. A mathematical model fitted to this data gives model parameters that provide the basis for a window diagram of relative retentions vs. mobile phase pH. The window diagram gives an estimate of the mobile phase pH required for optimum separation. 相似文献
77.
Marshall M. Siegel Rushung Tsao William Bouley Theodore Kornas Glen Brubaker 《Journal of the American Society for Mass Spectrometry》1993,4(6):504-506
An inexpensive multisample fast-atom bombardment (FAB) probe assembly was designed for high-throughput analysis of samples on a VG ZAB-SE mass spectrometer. The system consists of a vacuum lock system and a FAB probe whose tip contains five or more sample wells. The probe enters the mass spectrometer source region perpendicular to the secondary ion beam axis, The probe is maintained at high voltage on contact with a spring clip attached to the screen plate of the source block. Sample throughput with the multisample probe is twice that of a coaxial probe, with about twice the sensitivity and no sample cross contamination. 相似文献
78.
2,5-Dihydro-5-oxo-1,2,4-triazine and some of its alkylated derivatives have been prepared. Nmr spectroscopic analysis has established that the 2,5-dihydro-5-oxo tautomers are preferred over the 4,5-dihydro-5-oxo ones. This preference, and the behavior of 1,2,4-triazines in some other chemical reactions has been interpreted in terms of electron-electron repulsions between the lone pairs of electrons of N1 and N2 in this ring system. 相似文献
79.
William R. Dolbier Jr. Jian-Xin Duan Xiao X. Rong 《Journal of fluorine chemistry》2007,128(10):1091-1093
Selective, high yield partial fluorination of p-bis-(trichloromethyl)benzene to p-bis-(chlorodifluoromethyl)benzene has been accomplished by warming a slurry of the p-bis-(trichloromethyl)benzene in anhydrous HF which also contains a small quantity of inert solvent, such as 1,2-dichloroethane. 相似文献
80.
A method of analysing steady-state fluorescence quenching in compartmentalized systems is proposed which allows the evaluation of partition and binding constants where both association processes occur simultaneosly. The number of binding sites is shown to be dependent on the nature of the quencher. 相似文献