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71.
Electron pulse radiolysis at ?298°K of 2 atm H2 containing 5 torr O2 produces HO2 free radical whose disappearance by reaction (1), HO2 + HO2 →H2O2 + O2, is monitored by kinetic spectrophotometry at 230.5 nm. Using a literature value for the HO2 absorption cross section, the values k1 = 2.5×10?12 cm3/molec·sec, which is in reasonable agreement with two earlier studies, and G(H) G(HO2) ?13 are obtained. In the presence of small amounts of added H2O or NH3, the observed second-order decay rate of the HO2 signal is found to increase by up to a factor of ?2.5. A proposed kinetic model quantitatively explains these data in terms of the formation of previously unpostulated 1:1 complexes, HO2 + H2O ? HO2·H2O (4a) and HO2 + NH3? HO2·NH3 (4b), which are more reactive than uncomplexed HO2 toward a second uncomplexed HO2 radical. The following equilibrium constants, which agree with independent theoretical calculations on these complexes, are derived from the data: 2×10?20?K4a?6.3 × 10?19 cm3/molec at 295°K and K4b = 3.4 × 10?18 cm3/molec at 298°K. Several deuterium isotope effects are also reported, including kH/kD = 2.8 for reaction (1). The atmospheric significance of these results is pointed out. 相似文献
72.
We report the first example of pentavalent-uranium silicate synthesized by a high-temperature, high-pressure hydrothermal method. The 3-D framework consists of four-membered single rings of corner-sharing SiO4 tetrahedra and 1-D UO4/1O2/2 chains. Magnetic susceptibility and XPS were measured to identify the valence state of uranium. 相似文献
73.
Jenn-Huei Lii Steven Gallion Charles Bender Hkan Wikstrm Norman L. Allinger Kenneth M. Flurchick M. M. Teeter 《Journal of computational chemistry》1989,10(4):503-513
The MM2 potential functions for amides and peptides have been further extended by examining the experimental crystal structures for cyclo-(-Ala-Ala-Gly-Gly-Ala-Gly-), I, and cyclo-(-Ala-Ala-Gly-Ala-Gly-Gly-), II. The force field obtained was then applied to a study of the structure of the hydrophobic protein Crambin, for which a high resolution crystal structure is available. The energy minimization was carried out using a version of MM2 adapted to the CYBER 205. 相似文献
74.
75.
A new uranium(V) silicate, K3(U3O6)(Si2O7), and the germanate analogue, Rb3(U3O6)(Ge2O7), have been synthesized under high-temperature, high-pressure hydrothermal conditions and characterized by single-crystal X-ray diffraction. Their structures contain uranate columns formed of triple octahedral chains of the alpha-UF5 type linked by disilicate (or digermanate) units to form a 3-D framework structure. The valence state of uranium is confirmed by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and magnetic susceptibility. 相似文献
76.
A new niobium(V) silicate, Rb(2)(Nb(2)O(4))(Si(2)O(6)).H(2)O, has been synthesized by a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction, thermogravimetric analysis, and solid-state NMR spectroscopy. It crystallizes in the tetragonal space group P4(3)22 (No. 95) with a = 7.3431(2) A, c = 38.911(3) A, and Z = 8. Its structure contains tetrameric units of the composition Nb(4)O(18), which share corners to form a layer of niobium oxide. The Nb-O layer is a slice of the pyrochlore structure. Neighboring Nb-O layers are linked by vierer single-ring silicates generating eight-ring and six-ring channels running parallel to <100> directions, in which the Rb(+) cations and water molecules reside. The tantalum analogue was prepared and characterized by powder X-ray diffraction. Upon heating to 500 degrees C, Rb(2)(Nb(2)O(4))(Si(2)O(6)).H(2)O loses lattice water molecules, while the framework structure is retained to give the anhydrous compound Rb(2)(Nb(2)O(4))(Si(2)O(6)), whose structure was also characterized by single-crystal X-ray diffraction. The dehydrated sample absorbs water reversibly, as indicated by powder X-ray diffraction. Rb(2)(Nb(2)O(4))(Si(2)O(6)) crystallizes in the tetragonal space group I4(1) (No. 80) with a = 10.2395(6) A, c = 38.832(3) A, and Z = 16. 相似文献
77.
MM3(96) parameterization for camptothecin analogs: An ab initio and molecular mechanics study 总被引:1,自引:0,他引:1
Sean W. Carrigan Jenn-Huei Lii J. Phillip Bowen 《Journal of computer-aided molecular design》1997,11(1):61-70
Torsional parameters for MM3(96) were derived for the missing atom types present in thenatural product camptothecin (CPT). Potential energy curves were calculated via ab initiocalculations on representative compounds for dihedral angles containing these missingparameters. Gaussian 92 at the restricted Hartree–Fock level of theory using thestandard 6-31G** and 4-31G** basis sets, was used for all the quantum-mechanicscalculations. Missing MM3 torsional terms were obtained by optimizing the V1, V2 and V3parameters such that MM3 could reproduce the ab initio torsional profile. MM3 calculatedmolecular structures that compare well with the ab initio results. Using the newly developedparameters, conformational analyses and QSAR studies of camptothecin analogs wereundertaken. MM3 predicts two distinct boatlike conformations for the -hydroxy lactonemoiety. The low-energy lactone conformation predicted by MM3 is in general agreement withreported X-ray crystal structures of CPT iodoacetate and 7-ethyl-10-(4-piperidino)piperidinylcarbonyloxy CPT HCl as well as the ab initio structure of a CPT-like-hydroxy lactone. 相似文献
78.
Gelatin (G) and xanthan gum (X) formed complexes either on bringing their blends to pH 2.3 or carrying out the electrode process in aqueous blends of X and G at pHs from 9 to 11. X carboxylic groups and G peptide moieties were involved in the interactions between the partners. Also non-coulombic interactions with involvement of NH and OH groups, as well as hydrophobic interactions were involved, as proved by FTIR and thermal analyses, respectively. The reaction yield declined with increase of concentration of G in the blends. Simultaneously, the thermal stability of the complexes slightly increased with increase in G content in the blend. The latter increased with its concentration in the reaction mixture. Slow formation by electrosynthesis provided more an organised matrix. 相似文献
79.
A theory of excitation of ultrasonic waves in the stimulated Brillouin
scattering (SBS) process is presented in this paper. By using several
reasonable approximations, a numerical calculation of the transient
longitudinal SBS shows that large amplitude of acoustic waves can be built
up by the nanosecond pulse of high-power laser, which may result in the
damage of optical glasses. The maximal density change and the maximal
acoustic wave intensity in optical glasses of 5\,cm in thickness are
calculated by using different parameters of the high-energy laser, such as
the intensity, the pulse width, and the wave length. The damage threshold of
the optical glasses is about 80 GW/cm^{2} when using a 1064 nm laser. The
dynamic mechanism of SBS is the electrostriction effect of the components
coupling with the high-power laser. 相似文献
80.
Time-dependent degradation of threshold voltage in AIGaN/GaN high electron mobility transistorsTime-dependent degradation of threshold voltage in AIGaN/GaN high electron mobility transistors 下载免费PDF全文
This paper gives a detailed analysis of the time-dependent degradation of the threshold voltage in A1GaN/GaN high electron mobility transistors (HEMTs) submitted to off-state stress. The threshold voltage shows a positive shift in the early stress, then turns to a negative shift. The negative shift of the threshold voltage seems to have a long recovery time. A model related with the balance of electron trapping and detrapping induced by shallow donors and deep acceptors is proposed to explain this degradation mode. 相似文献