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Neopentylallylsodium (NpANa) has been prepared by the reaction of neopentylallyllithium with an equivalent amount of sodium 2,2-dimethyl-1-butoxide in hydrocarbon solvent. NpANa is stable in diethyl ether and THF solvents for extended periods of time, and proton NMR and UV data are reported for NpANa in THF at various temperatures. A more substantial degree of ionic delocalization is indicated for NpANa as compared to NpALi and apparently is greater for the trans isomer of NpANa. UV absorption maxima for NpANa and NpALi are explained in terms of cis/trans ratios rather than in terms of ion pairing. 相似文献
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Ni(+)(CO(2))(n), Ni(+)(CO(2))(n)Ar, Ni(+)(CO(2))(n)Ne, and Ni(+)(O(2))(CO(2))(n) complexes are generated by laser vaporization in a pulsed supersonic expansion. The complexes are mass-selected in a reflectron time-of-flight mass spectrometer and studied by infrared resonance-enhanced photodissociation (IR-REPD) spectroscopy. Photofragmentation proceeds exclusively through the loss of intact CO(2) molecules from Ni(+)(CO(2))(n) and Ni(+)(O(2))(CO(2))(n) complexes, and by elimination of the noble gas atom from Ni(+)(CO(2))(n)Ar and Ni(+)(CO(2))(n)Ne. Vibrational resonances are identified and assigned in the region of the asymmetric stretch of CO(2). Small complexes have resonances that are blueshifted from the asymmetric stretch of free CO(2), consistent with structures having linear Ni(+)-O=C=O configurations. Fragmentation of larger Ni(+)(CO(2))(n) clusters terminates at the size of n=4, and new vibrational bands assigned to external ligands are observed for n> or =5. These combined observations indicate that the coordination number for CO(2) molecules around Ni(+) is exactly four. Trends in the loss channels and spectra of Ni(+)(O(2))(CO(2))(n) clusters suggest that each oxygen atom occupies a different coordination site around a four-coordinate metal ion in these complexes. The spectra of larger Ni(+)(CO(2))(n) clusters provide evidence for an intracluster insertion reaction assisted by solvation, producing a metal oxide-carbonyl species as the reaction product. 相似文献
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Fischer CJ Gafni A Steel DG Schauerte JA 《Journal of the American Chemical Society》2002,124(35):10359-10366
The interpretation of room temperature phosphorescence studies of proteins requires an understanding of the mechanisms governing the tryptophan triplet-state lifetimes of residues fully exposed to solvent and those deeply buried in the hydrophobic core of proteins. Since solvents exposed tryptophans are expected to behave similarly to indole free in solution, it is important to have an accurate measure of the triplet state lifetime of indole in aqueous solution. Using photon counting techniques and low optical fluence (J/cm(2)), we observed the triplet-state lifetime of aqueous, deoxygenated indole and several indole derivatives to be approximately 40 micros, closely matching the previous reports by Bent and Hayon based on flash photolysis (12 micros; Bent, D. V.; Hayon, E. J. Am. Chem. Soc. 1975, 97, 2612-2619) but much shorter than the 1.2 ms lifetime observed more recently (Strambini, G. B.; Gonnelli, M. J. Am. Chem. Soc. 1995, 117, 7646-7651). However, we have now been able to reproduce the long lifetime reported by the latter workers for aqueous indole solutions and show that it likely arises from geminate recombination of the indole radical cation and solvated electron, a conclusion based on studies of the indole radical cation in water (Bent and Hayon, 1975). The evidence for this comes from a fast rise in the phosphorescence emission and measurements of a corresponding enhanced quantum yield in unbuffered solutions. This species can be readily quenched, and the corresponding fast rise disappears, leaving a monoexponential 40 micros decay, which we argue is the true indole triplet lifetime. The work is put in the context of room temperature phosphorescence studies of proteins. 相似文献
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McRee D 《Chemistry & biology》2003,10(4):295-297
The first X-ray structure of human carboxylesterase 1 (hCE1) and the structures of hCE1 with drug analogs bound reveal important molecular details of how the drugs cocaine, heroin, and tacrine are metabolized and cleared. 相似文献
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van Heijnsbergen D von Helden G Meijer G Maitre P Duncan MA 《Journal of the American Chemical Society》2002,124(8):1562-1563
The organometallic ions V+-(benzene) and V+-(benzene)2 are produced by laser vaporization in a pulsed nozzle source. They are trapped and mass selected in an ion-trap/time-of-flight mass spectrometer, and their infrared spectra are measured with resonance-enhanced multiphoton photodissociation (IR-REMPD) spectroscopy with a tunable free-electron laser. Vibrational bands in the 600-1800 cm-1 region are characteristic of the benzene molecular moiety perturbed by the metal cation bonding. Experimental data are compared to the IR spectra derived from density functional calculations. Vibrational patterns in V+-(C6H6) indicate that the metal is bound in an eta6 pi-bonding configuration, while V+-(C6H6)2 is a sandwich. Trapped-ion IR-REMPD is a general method to access the vibrational spectroscopy of organometallic ions and their clusters. 相似文献
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David Littlejohn Iain S. Duncan James B.M. Hendry John Marshall John M. Ottaway 《Spectrochimica Acta Part B: Atomic Spectroscopy》1985,40(10-12)
The analytical performance of three uncoated electrographite tubes, three pyro-coated electrographite tubes, one tantalum carbide (TaC) coated electrographite tube, and three totally pyrolytic graphite (TPG) tubes has been evaluated and compared. A test programme was devised to determine the useful operational lifetime of each tube, and assess the influence of tube age on the sensitivity of lead, manganese and vanadium measurements by atomic absorption spectrometry. The TPG tubes were found to be more durable than the other types studied, but the lifetime advantage depended on the thickness of the pyrolytic graphite. The best TPG tube, of 720 μm wall-thickness, lasted 2.5 times longer than the pyro-coated tubes, and 5 times longer than the uncoated electrographite tubes.The TPG tubes gave slightly poorer AAS sensitivity for lead, equivalent sensitivity for manganese, and 4 times better sensitivity for vanadium than the pyro-coated electrographite tubes. Also, with TPG, signal magnitude was more consistent throughout the lifetime of a tube. For each of the test elements studied, poorest sensitivity was encountered with the TaC-coated electrographite tube. 相似文献
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Christopher J. H. Smalley Harriet E. Hoskyns Colan E. Hughes Duncan N. Johnstone Tom Willhammar Mark T. Young Christopher J. Pickard Andrew J. Logsdail Paul A. Midgley Kenneth D. M. Harris 《Chemical science》2022,13(18):5277
We report the crystal structure of a new polymorph of l-tyrosine (denoted the β polymorph), prepared by crystallization from the gas phase following vacuum sublimation. Structure determination was carried out by combined analysis of three-dimensional electron diffraction (3D-ED) data and powder X-ray diffraction (XRD) data. Specifically, 3D-ED data were required for reliable unit cell determination and space group assignment, with structure solution carried out independently from both 3D-ED data and powder XRD data, using the direct-space strategy for structure solution implemented using a genetic algorithm. Structure refinement was carried out both from powder XRD data, using the Rietveld profile refinement technique, and from 3D-ED data. The final refined structure was validated both by periodic DFT-D calculations, which confirm that the structure corresponds to an energy minimum on the energy landscape, and by the fact that the values of isotropic 13C NMR chemical shifts calculated for the crystal structure using DFT-D methodology are in good agreement with the experimental high-resolution solid-state 13C NMR spectrum. Based on DFT-D calculations using the PBE0-MBD method, the β polymorph is meta-stable with respect to the previously reported crystal structure of l-tyrosine (now denoted the α polymorph). Crystal structure prediction calculations using the AIRSS approach suggest that there are three other plausible crystalline polymorphs of l-tyrosine, with higher energy than the α and β polymorphs.A new polymorph of l-tyrosine is reported, with the crystal structure determined by combined analysis of 3D-ED data and powder XRD data, augmented by information from periodic DFT-D calculations and solid-state 13C NMR data. 相似文献