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51.
The electronic absorption, fluorescence and phosphorescence spectra of s-tetrazine at low temperatures (4.2-1.5 K) are reported and analyzed in the neat crystal and in several mixed crystals. The 3B3u-1Ag (nπ*) origin is at 18414 ± 5 cm?1 for neat tetrazine. In the mixed crystal several sites identified. The lowest energy origin is at 17453 cm?1 for tetrazine in pyrazine; 17 701 cm?1 in pyrimidine; and 17 676 cm?1 in pyridazine. The eB3u-1Ag (nπ*) origin is at 14 096 ± 2 cm?1 for the neat crystal. The phosphorescence lifetime of neat tetrazine is measured to be 96.8 ± 2.1 μs at 4.2 and 1.8 K. All the spectra are predominately composed of members of progressions in a single totally symmetric mode (ν6a) built upon site origins and vibrational fundamentals. The ν6a interval is: 743 (1Ag), 715 (3B3u), and 709 cm?1 (1B3u) in the neat tetrazine crystal; 732 (1Ag) and 705 cm?1 (1B3u in pyrazine host, 737 (1Ag) and 701 cm?1 (1B3u) in pyrimidine host, and 732 (1Ag) and 703 cm?1 (1B3u) in pyridazine host mixed crystals. All emission spectra may be analyzed by Oi → (ν″6a)on (i), i indicating the observed s 相似文献
52.
Following an earlier proposal to evaluate electron repulsion integrals over Gaussian basis functions by a numerical quadrature based on a set of orthogonal polynomials (Rys polynomials), a computational procedure is outlined for efficient evaluation of the two-dimensional integrals Ix, Iy, and Iz. Compact recurrence formulas for the integrals make the method particularly fitted to handle high-angular-momentum basis functions. The technique has been implemented in the HONDO molecular orbital program. 相似文献
53.
Detailed molecular orbital calculations were directed to the cyclopropylcarbinyl radical (1), the cyclopropoxy radical (2), and the cyclopropylaminium radical cation (3) as well as their ring-opened products. Since a considerable amount of data are published about cyclopropylcarbinyl radicals, calculations were made for this species and related ring-opened products as a reference for 2 and 3 and their reactions. Radicals 1-3 have practical utility as "radical clocks" that can be used to time other radical reactions. Radical 3 is of further interest in photoelectron-transfer processes where the back-electron-transfer process may be suppressed by rapid ring opening. Calculations have been carried out at the UHF/6-31G*, MP4//MP2/6-31G*, DFT B3LYP/6-31G*, and CCSD(T)/cc-pVTZ//QCISD/cc-pVDZ levels. Energies are corrected to 298 K, and the barriers between species are reported in terms of Arrhenius E(a) and log A values along with differences in enthalpies, free energies, and entropies. The CCSD(T)-calculated energy barrier for ring opening of 1 is E(a) = 9.70, DeltaG* = 8.49 kcal/mol, which compares favorably to the previously calculated value of E(a) = 9.53 kcal/mol by the G2 method, but is higher than an experimental value of 7.05 kcal/mol. Our CCSD(T)-calculated E(a) value is also higher by 1.8 kcal/mol than a previously reported CBS-RAD//B3LYP/6-31G* calculation. The cyclopropoxy radical has a very small barrier to ring opening (CCSD(T), E(a) = 0.64 kcal/mol) and should be a very sensitive time clock. Of the three series studied, the cyclopropylaminium radical cation is most complex. In agreement with experimental data, bisected cyclopropylaminium radical cation is not found, but instead a ring-opened species is found. A perpendicular cyclopropylaminium radical cation (4) was found as a transition-state structure. Rotation of the 2p orbital in 4 to the bisected array results in ring opening. The minimum onset energy of photoionization of cyclopropylamine was calculated to be 201.5 kcal/mol (CCSD(T)) compared to experimental values of between about 201 and 204 kcal/mol. Calculations were made on the closely related cyclopropylcarbinyl and bicyclobutonium cations. Stabilization of the bisected cyclopropylcarbinyl conformer relative to the perpendicular species is much greater for the cations (29.1 kcal/ mol, QCISD) compared to the radicals (3.10 kcal/mol, QCISD). A search was made for analogues to the bicyclobutonium cation in the radical series 1 and 2 and the radical cation series 3. No comparable species were found. A rationale was made for some conflicting calculations involving the cyclopropylcarbinyl and bicyclobutonium cations. The order of stability of the cyclopropyl-X radicals was calculated to be X = CH2 > X = O > X = NH2+, where the latter species has no barrier for ring opening. The relative rate of ring opening for cyclopropyl-X radicals X = CH2 to X = O was calculated to be 3.1 x 10(6) s(-1) at 298 K (QCISD). 相似文献
54.
Supercritical fluid extraction (SFE) is combined with supercritical fluid chromatography (SFC) in an analytical mode to develop a system for fractionating and enriching high value ferulate-phytosterol esters (FPE) contained in corn bran oil. Corn bran is initially extracted with neat supercritical carbon dioxide (SC-CO2) at various pressures (13.8, 34.5, and 69 MPa) and temperatures (40, 60, and 80 degrees C) to see if the FPE can be enriched in the extracts. These initial studies show the greatest percentage of FPE could be extracted under two sets of conditions: 69 MPa at 80 degrees C and 34.5 MPa at 40 degrees C. Both sets of parameters yield an extract containing approximately 1.25% FPE. A stock supply of corn bran oil is then produced by scaled-up SFE at 34.5 MPa and 40 degrees C for subsequent chromatographic fractionation. The SFE-obtained corn bran oil is then applied to the head of a minichromatographic column containing an amino-propyl sorbent. SFC is than commenced using neat SC-CO2 at 69 MPa and 80 degrees C to remove the majority of the triglyceride-based oil. Pressure and temperature are then lowered to 34.5 MPa and 40 degrees C, respectively, and ethanol is added as a modifier. The modifier is added in an increasing stepwise gradient program, and fractions are collected at equal volume intervals. The resultant fractions are analyzed by analytical high-performance liquid chromatography with evaporative light-scattering detection and show that FPE could be enriched to a 14.5% (w) level. 相似文献
55.
Lindsey?Gillian?MackayEmail author C.?Peter?Taylor Richard?Bruce?Myors Ruth?Hearn Bernard?King 《Accreditation and quality assurance》2003,8(5):191-194
The selection of an appropriate isotope dilution mass spectrometry technique for the practical application of this potentially primary method of analysis is highly important. The NARL approach for the application of the exact matching double isotope dilution mass spectrometry technique developed by Henrion is presented. NARL's approach utilises exact matching to minimise the effect of measurement biases within the method but also includes the thorough examination of all other biasing factors. The approach has been successfully tested in international CCQM intercomparisons with other national metrology institutes. 相似文献
56.
Griffiths PC Fallis IA Willock DJ Paul A Barrie CL Griffiths PM Williams GM King SM Heenan RK Görgl R 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(8):2022-2028
The morphology of micelles formed by two novel metallosurfactants has been studied by small-angle neutron scattering (SANS) and small-angle-X-ray scattering (SAXS). The two surfactants both contain a dodecyl chain as the hydrophobic moiety, but differ in the structure of the head group. The surfactants are Cu(II) complexes of monopendant alcohol derivatives of a) the face-capping macrocycle 1,4,7-triazacyclanonane (tacn), and b) an analogue based upon the tetraazamacrocycle 1,4,7,10-tetraazacyclododecane. Here, neutron scattering has been used to study the overall size and shape of the surfactant micelles, in conjunction with X-ray scattering to locate the metal ions. For the 1,4,7,10-tetraazacyclododecane-based surfactant, oblate micelles are observed, which are smaller to the prolate micelles formed by the 1,4,7-triazacyclononane analogue. The X-ray scattering analysis shows that the metal ions are distributed throughout the polar head-group region, rather than at a well-defined radius; this is in good agreement with the SANS-derived dimensions of the micelle. Indeed, the same model for micelle morphology can be used to fit both the SANS and SAXS data. 相似文献
57.
An analytical method was developed for measuring adenosine, inosine and hypoxanthine in freshly delivered human term placentas. Representative freeze-clamped samples were taken from the sub-maternal surface of each placenta. Acid-soluble extracts of the samples were analyzed by reversed-phase high-performance liquid chromatography on columns packed with 10-micron porous octadecylsilica, using gradient elution with a linear increase in methanol concentration in ammonium phosphate buffer. Resolution of hypoxanthine from xanthine and adenosine from adenine, and quantitation of hypoxanthine and adenosine were achieved using 0.05 M ammonium dihydrogen phosphate, pH 6.5, as the low-strength eluent. Resolution of inosine from a prominent peak of beta-NAD was optimized using 0.02 M ammonium dihydrogen phosphate, pH 5.6, as low-strength eluent. Recovery of standards was greater than 90%. Mean contents (+/- S.D.) of the analytes in placentas from seven normal deliveries were, adenosine 30.6 +/- 11.5 nmol/g, inosine 68.0 +/- 25.8 nmol/g and hypoxanthine 217 +/- 127.5 nmol/g. 相似文献
58.
R. B. King 《Journal of Cluster Science》1995,6(1):5-20
Many trinuclear metal clusters have structures based on isolated metal triangles with either single bonds (e.g.,M
3(CO)12 whereM = Fe, Ru, Os) or double bonds (e.g., Re3 Cl
12
3–
) along each edge of the triangle. Individual metal triangles can be joined in the following ways to form more complicated triangulated networks: (1) Bridging an edge of a triangle with a new vertex to give rafts in which adjacent triangles share edges; (2) Bridging a vertex of a triangle with a new edge to give bowties in which adjacent triangles share vertices; (3) Capping a triangular face with a new vertex to give a chain of tetrahedra in which adjacent tetrahedra share faces. Such triangulated metal networks are particularly common in osmium carbonyl chemistry and in mixed osmium/platinum carbonyl derivatives. Platinum triangles of the type Pt3L6 are analogous to cyclopropenyl rings and can form sandwiches with one or two mercury atoms in the center such as the mercuric derivative Hg[Pt.3(µ2-2,6-Me2C6H3NC)3] (2,6-Me2C6H3NC)3]2 and the mercurous derivative Hg2[Pt3(µ2-CO)3L3]2. Platinum triangles can also be stacked in the absence of filling to give [Pt3(µ2-CO)3(CO)3]
n
2–
(n=2, 3, 4, 5, 6, 10). Metal triangles also form the faces of metal deltahera of which the octahedron, bicapped square antiprism, and icosahedron are found in globally delocalized transition metal clusters.This article is dedicated to Prof. L. F. Dahl in recognition of his many seminal contributions to metal cluster chemistry. 相似文献
59.
Chin JW Santoro SW Martin AB King DS Wang L Schultz PG 《Journal of the American Chemical Society》2002,124(31):9026-9027
We report the selection of a new orthogonal aminoacyl tRNA synthetase/tRNA pair for the in vivo incorporation of a photocrosslinker, p-azido-l-phenylalanine, into proteins in response to the amber codon, TAG. The amino acid is incorporated in good yield with high fidelity and can be used to crosslink interacting proteins. 相似文献
60.
Griffiths PC Hirst N Paul A King SM Heenan RK Farley R 《Langmuir : the ACS journal of surfaces and colloids》2004,20(16):6904-6913
The effect of ethanol on the interaction between the anionic surfactant sodium dodecyl sulfate (SDS) and the nonionic polymer poly(vinylpyrrolidone) (PVP) has been investigated using a range of techniques including surface tension, fluorescence, electron paramagnetic resonance (EPR), small-angle neutron scattering (SANS), and viscosity. Surface tension and fluorescence studies show that the critical micelle concentration (cmc) of the surfactant decreases to a minimum value around 15 wt % ethanol; that is, it follows the cosurfactant effect. However, in the presence of PVP, the onset of the interaction, denoted cmc(1), between the surfactant and the polymer is considerably less dependent on ethanol concentration. The saturation point, cmc(2), however, reflects the behavior of the cmc in that it decreases upon addition of ethanol. This results in a decrease in the amount of surfactant bound to the polymer [C(bound) = cmc(2) - cmc] at saturation. The viscosity of simple PVP solutions depends on ethanol concentration, but since SANS studies show that ethanol has no effect on the polymer conformation, the changes observed in the viscosity reflect the viscosity of the background solvent. There are significant increases in bulk viscosity when the surfactant is added, and these have been correlated with the polymer conformation extracted from an analysis of the SANS data and with the amount of polymer adsorbed at the micelle surface. Competition between ethanol and PVP to occupy the surfactant headgroup region exists; at low ethanol concentration, the PVP displaces the ethanol and the PVP/SDS complex resembles that formed in the absence of the ethanol. At higher ethanol contents, the polymer does not bind to the ethanol-rich micelle surface. 相似文献