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21.
The results of Monte Carlo simulations are reported for a model nematogen in the presence of an intense magnetic field. The predictions of a molecular field theory for the system are found to be in reasonable accord with the simulations. It would appear therefore that the critical field for a real nematogen may be unobservably high. 相似文献
22.
Gutiérrez MC Sleegers A Simpson HD Alphand V Furstoss R 《Organic & biomolecular chemistry》2003,1(20):3500-3506
The first fluorogenic assay allowing for detection of microbial enzymes able to perform Baeyer-Villiger oxidation is described. This is based on the use of 4-oxopentyl umbelliferyl ether 1 as a fluorogenic substrate. When Baeyer-Villigerases active against this test ketone are present in the selected whole cells, 1 is transformed into 3-hydroxypropyl umbelliferyl ether 3, which, in a subsequent step, releases the fluorescent product umbelliferone. Different microorganisms, known to be endowed with Baeyer-Villigerase activity, were assayed. 相似文献
23.
The reactions of NaCo(CO)4 with MenSiCl4?n (n = 0–3) in diethylether (Et2O) and in tetrahydrofuran (THF) have been studied. Three distinct reaction pathways were recognised which depend on the acidity of the chlorosilane and basicity of solvent. Attack at the silicon centre via the Co atom of Co(CO)4? leads to formation of a SiCo bond; reaction involving a CO ligand of Co(CO)4? gives clusters R3SiOCCo3(CO)9; and chlorosilane induced attack of Co(CO)4? on the solvent gives products derived from THF molecules. 相似文献
24.
25.
The collisionless infrared excitation by short CO2 laser pulses of the molecules SO2, OCS, NO2, NH3 and DN3 is compared with that of larger molecules. The average number of photons absorbed per molecule and the fraction of molecules dissociated depends predominantly on the laser intensity, while for larger molecules with higher densities of vibrational states the excitation is primarily determined by the laser fluence. 相似文献
26.
Hersh WH Xu P Simpson CK Grob J Bickford B Hamdani MS Wood T Rheingold AL 《The Journal of organic chemistry》2004,69(6):2153-2163
Reaction of the N-toluenesulfonyl derivatives of (S)-alanine, phenylalanine, and valine (4-6) with PhPCl(2) gave in high yield the 4-methyl, benzyl, and isopropyl derivatives (7-9) of 2-phenyl-1-p-toluenesulfonyl-1,3,2-oxazaphospholidin-5-one. The ratios of the (2S,4S)/(2R,4S) diastereomers (cis/trans isomers) were 1:1, 2:1, and 10:1 for the methyl, benzyl, and isopropyl derivatives 7a,b, 8a,b, and 9a,b, respectively. For 7a,b, both isomers could be crystallized, but for the others only the major isomers were isolable. The X-ray crystal structure of 9a shows that the isopropyl and phenyl groups are mutually cis and that the tolyl moiety is oriented s-trans to both the isopropyl and phenyl groups. Reaction of 6 with Cl(2)PCH(2)CH(2)PCl(2) (10) gave a 56:38:7 mixture of the cis/cis, cis/trans, and trans/trans diphosphorus heterocycles 11a-c. The major isomer could be crystallized and isolated free of the other diastereomers. Reaction of 6 with EtPCl(2) gave a 6:1 mixture of cis/trans isomers of the ethyl-substituted heterocycles 12a,b as an inseparable oil but allowed confirmation of the structure of 11a. Slow epimerization at phosphorus may occur by inversion but more likely by ring opening/closure, since 7b, 9a, and 11a give rise upon standing in solution to mixtures containing starting material and 7a, 9b, and 11b, respectively, along with the free amino acid derivatives 4 and 6. The NMR spectra, and in particular the coupling constants between the alpha-hydrogen atom of the amino acid moiety and phosphorus, were used to establish the identities of the cis and trans isomers. Reaction of 9a with (THF)W(CO)(5) gave the phosphorus-ligated adduct (9a)W(CO)(5) (13), and the IR spectrum of this complex shows that 9a is a strongly electron-withdrawing ligand. The geometry of the sulfonamide moiety is discussed in detail, as are the (1)H NMR coupling constants. The data are consistent with the presence of little steric interaction between the cis isopropyl and phosphorus substituent in 9a, 11a, and 12a and orientation of the tolyl moiety s-cis to the isopropyl group in 9b, 12b, and 13. 相似文献
27.
Supercritical fluid extraction (SFE) exploits the solvation power of fluids at temperatures and pressures close to their critical point. Use of SFE with supercritical CO2 is reported for the extraction of caffeine and quinine from various plant materials and of morphine from serum. Results are compared with those obtained by extractions with subcritical methanol and tetrahydrofuran, normal organic Soxhlet extractions and solid-phase extraction. 相似文献
28.
Garth J Simpson 《Chemphyschem》2004,5(9):1301-1310
Recent observations of remarkably large chiroptical effects in second-harmonic generation (SHG) and sum-frequency generation (SFG) measurements suggest exciting possibilities for the development of new chiral-specific spectroscopies and novel chiral materials for nonlinear optics. Several fundamental studies designed to elucidate the molecular and macromolecular origins of the chiral responses are reviewed to provide a framework for development of this emerging field. In general, the chiral activity in SHG and SFG has the potential to arise from complex interactions between hosts of different competing effects. Fortunately, relatively simple electric dipole-allowed mechanisms routinely dominate the nonlinear optical chiral activities of most practical systemsexpressions can often be generated to link the. This substantial reduction in complexity allows for the development of simple models connecting the macroscopic nonlinear optical response to intuitive molecular and supramolecular properties. 相似文献
29.
P. Jagam J. -X. Wang J. J. Simpson 《Journal of Radioanalytical and Nuclear Chemistry》1993,171(2):277-286
Improvements in the detection limits of thorium and uranium in acrylics by neutron activation analysis are described. The average of results from 10 g samples was found to be higher than the results from 450 g samples taken from the same acrylic sheet. Using eighteen large (450 g) samples from a single sheet of acrylic gave an average value of 3.2±0.3 pg/g thorium and 1.1±0.7 pg/g uranium. Interpretation of the results is discussed. Epithermal neutron activation with Cd shielded irradiations did not improve the detection sensitivity for thorium. 相似文献
30.
The interaction of optically active ethyl(1-naphthyl)phenylgermyllithium, R′3-Ge*Li, with alkyl halides, RX, to give optically active R′3Ge*R compounds, occurs with predominant retention of configuration at germanium in the case of MeBr, i-PrCl, i-PrBr, n-BuCl, n-BuBr, t-BuCl, t-BuBr, CH2CHCH2Cl, CH2CHCH2Br and PhCH2Cl, but with predominant inversion in the case of MeI, i-PrI, CH2CHCH2I, PhCH2I, and PhCH2Br. It is suggested that the retention reactions involve direct coupling between R′3Ge*Li and RX, in a four-centre process, while the inversion reactions involve halogen—lithium exchange, to give R′3GeX and RLi, also in a four-centre retentive process, followed by coupling between R′3GeX and RLi in an invertive process. 相似文献