The electronic structure of finite and infinite linear, cyclic and M?bius strip polyacenes has been investigated by adopting
Hückel and semiempirical schemes. Using the Hückel approach, it turns out that the M?bius belting process modifies the highest
occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap in such a way its evolution with chain length
is similar to the linear polyacenes rather than their cyclic analogs. These results are corroborated at the Austin model 1
(AM1) level, where the geometry relaxation effects are taken into account. The optimized AM1 structures show that the M?bius
defect is localized and extends over a third of the ring. With respect to the Hückel approach, accounting for geometry distortion
at the AM1 levels results in an increase in the HOMO–LUMO gap of the M?bius strip relative to the linear and cyclic finite-size
structures. On the other hand, when including electron-hole correlation at the configuration interaction singles/Zerner's
intermediate neglect of differential overlap level the behavior with system size of the first excitation energy of cyclic
and M?bius polyacenes differs from their linear analogs and leads to smaller singlet excitation energies.
Received: 20 September 2000 / Accepted: 21 September 2000 / Published online: 28 February 2001 相似文献
The effectiveness of therapeutically used iron compounds is related to their physical and chemical properties. Four different iron compounds used in oral, intravenous, and intramuscular therapy have been examined by X-ray powder diffraction, iron-57 Mössbauer spectroscopy, transmission electron microscopy, BET surface area measurement, potentiometric titration and studied through dissolution kinetics determinations using acid, reducing and chelating agents. All compounds are nanosized with particle diameters, as determined by X-ray diffraction, ranging from 1 to 4.1 nm. The superparamagnetic blocking temperatures, as determined by Mössbauer spectroscopy, indicate that the relative diameters of the aggregates range from 2.5 to 4.1 nm. Three of the iron compounds have an akaganeite-like structure, whereas one has a ferrihydrite-like structure. As powders the particles form large and dense aggregates which have a very low surface area on the order of 1 m2?g?1. There is evidence, however, that in a colloidal solution the surface area is increased by two to three orders of magnitude, presumably as a result of the break up of the aggregates. Iron release kinetics by acid, chelating and reducing agents reflect the high surface area, the size and crystallinity of the particles, and the presence of the protective carbohydrate layer coating the iron compound. Within a physiologically relevant time period, the iron release produced by acid or large chelating ligands is small. In contrast, iron is rapidly mobilized by small organic chelating agents, such as oxalate, or by chelate-forming reductants, such as thioglycolate.
Hyperfine Interactions - High-energy ball milling of Fe–Mn elemental powder mixtures has been carried out for different Mn concentrations, ranging from 0 to 90%. Mössbauer spectroscopy... 相似文献
RE2Fe14B (RE=rare earth) materials are capable of absorbing hydrogen to form a stable solid solution at room temperature. Hydrogenation produces a number of significant changes in the hyperfine interactions. In this work, 57Fe Mössbauer effect spectroscopy and X-ray diffraction measurements were performed on Nd14.01Hf0.08Fe78.91B7.00 alloys submitted to thermal treatment in hydrogen atmosphere. A non-linear increase of the hyperfine fields and isomer shifts with hydrogen concentration was observed. The hyperfine parameters of the 8j1 site exhibit a rather different evolution than those experienced by the other major sites (8j2, 16k1, 16k2). The origin of the hyperfine field enhancement is analyzed in terms of volume expansion and H nearest neighbors to the Fe sites. A linear expression on these two effects to give account of isomer shift evolution for 8j1 site is given. 相似文献
The AOAC 2007.1 quick, easy, cheap, effective, rugged, and safe official method, together with gas chromatography coupled to a triple quadrupole mass spectrometer was applied for the analysis of 38 multiclass pesticides from dried fruits typically cultivated and exported from Colombia: uchuva (Physalis peruviana), lulo (Solanum quitoense), guanabana (Anona muricata), and pitahaya (Hylocereus undatus). The whole method was validated in terms of matrix‐matched calibration, matrix effect, and recovery using atrazine‐d5 as internal standard, triphenylphosphate for quality control of the injection, and a proper mixture of analytes protectants. Matrix‐matched calibration data were found satisfactory for all pesticides and dried fruits, reporting R2 values above 0.99. Matrix effect values evidenced the existence of such effect in most cases. The applied procedure gave satisfactory recovery percentages (70–120%) and relative standard deviation values (< 20%) for 92% of the 456 combinations pesticide/fruit studied (spiked levels of 25, 200, and 400 µg/kg). Finally, 20 real dried fruit samples were analyzed and residues of tebuconazole were found in two samples of uchuva at a concentration below the lowest calibration level of the method for one of them and at 10.8 ± 1.6 µg/kg for the other, being below or similar to the general maximum residue level established for monitoring purposes in food applications. 相似文献