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981.
Paul C. Kline Bruce C. Noll Hongqiu Zhao Anthony S. Serianni 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(2):o137-o140
1‐(β‐d ‐Erythrofuranosyl)cytidine, C8H11N3O4, (I), a derivative of β‐cytidine, (II), lacks an exocyclic hydroxymethyl (–CH2OH) substituent at C4′ and crystallizes in a global conformation different from that observed for (II). In (I), the β‐d ‐erythrofuranosyl ring assumes an E3 conformation (C3′‐exo; S, i.e. south), and the N‐glycoside bond conformation is syn. In contrast, (II) contains a β‐d ‐ribofuranosyl ring in a 3T2 conformation (N, i.e. north) and an anti‐N‐glycoside linkage. These crystallographic properties mimic those found in aqueous solution by NMR with respect to furanose conformation. Removal of the –CH2OH group thus affects the global conformation of the aldofuranosyl ring. These results provide further support for S/syn–anti and N/anti correlations in pyrimidine nucleosides. The crystal structure of (I) was determined at 200 K. 相似文献
982.
Liping Zhao Shengfu Ji Fengxiang Yin Zexiang Lu Hui Liu Chengyue Li 《天然气化学杂志》2006,15(4):287-296
A series of CoxMgxO/Al2O3/FeCrAl catalysts (x=0-1) were prepared. The structures of the catalysts were characterized using XRD, SEM, and TPR analyses. The catalytic activity of the catalysts for methane combustion was evaluated in a continuous flow microreactor. The results indicated that the active washcoats adhered well on the FeCrAl foils. The phases in the catalysts were Co--xMgxO solid solutions, α-Al2O3, and γ-Al2O3. The surface particle size of the catalysts varied with variations in the molar ratios of Co to Mg. The Co component of the Co1_xMgxO/Al2O3/FeCrAl catalysts played an important role in the catalytic activity for methane combustion. In the Co1-xMgxO/AluO3/FeCrAl series catalyst (x=0.2-0.8), the catalytic activity in terms of x was in the order of 0.5〉0.2〉0.8 under the experimental conditions. The presence of Mg in these catalysts could promote the thermal stability to a large extent. There were strong interactions between the Co1-xMgxO oxides and the AluO3/FeCrAl supports. 相似文献
983.
X. Li Y. Zhao X. Jing F. Liu F. Hao 《International journal of quantum chemistry》2006,106(5):1086-1092
The equilibrium geometries, vibrational frequencies, and dissociation energies of rare gas iodine clusters Rg2I?(Rg = Ar, Kr, Xe) were calculated at the Hartree–Fock (HF), second‐order Møller–Plesset (MP2), the coupled cluster method with single and double excitation and a noniterative correction for triple excitations method [CCSD(T)] levels. The title species have bent C2v structure of about 60° angle. The electron correlation effects and relativistic effects on the geometry and stability were investigated at CCSD(T) level. Both effects stabilize title species. The calculated electron affinities are in good agreement with the experimental results available. The effect of high angular momentum functions (g and h) was studied. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 相似文献
984.
A structure and kinetic stability study on some complexes with the general formula MN5, where M are the alkali-metal atoms, Li, Na, K, and Rb, has been carried by using hybrid density functional methods. Complex B (C2v) with two points of attachment to the N5 ring is the most energetically favored for all metals considered here. Pyramidal structures A (C5v) are kinetically unstable and they rapidly rearrange to the most stable planar structures B. At the QCISD(T)/6-311 + G*//B3LYP/6-311 + G* + ZPE (B3LYP/6-311 + G*) level, the decomposition barrier heights of LiN5-B, NaN5–B, KN5-B, and RbN5-B are predicted to be 19.9, 22.0, 22.5, and 23.0 kcal/mol, respectively. In addition, the rate constants of the decomposition reaction MN5-B MN3 + N2 (M + Li, Na, K, and Rb) are also predicted using conventional transition state theory and canonical variational transition state theory, respectively. 相似文献
985.
A novel microwave dielectric powder with composition of Ca2Zn4Ti16O38 was synthesized through a citrate sol-gel process. The development of crystalline phases with heat-treating temperature for the gel derived powders was evaluated by using thermo-gravimetric analysis and X-ray powder diffraction analysis techniques. The pure phase of Ca2Zn4Ti16O38 with crichtonite crystal structure was obtained at relatively low temperature of 1000 °C. The synthesized powder has high reactivity and the dense ceramics with single crystalline phase were obtained at low sintering temperature of 1100 °C. Impedance spectroscopy and microwave dielectric measurements on sintered samples showed the present compound to be a modest dielectric insulator with excellent dielectric properties of εr∼47-49, Qf value ∼27,800-31,600 GHz and τf∼+45 to +50 ppm/°C. It shows comparable microwave dielectric properties to other moderate-permittivity microwave dielectrics, but much lower sintering temperature of 1100 °C. 相似文献
986.
The voltammetric behavior of the LMF-Mg(II) complex with DNA at a mercury electrode is reported for the first time. In NH3–NH4Cl buffer (pH=9.10), the adsorption phenomena of the LMF–Mg(II) complex were observed by linear sweep voltammetry. The mechanism of the electrode reaction was found to be a reduction of LMF in the complex, and the composition of the LMF–Mg(II) complex is 2:1. In the presence of calf thymus DNA (ctDNA), the peak current of LMF–Mg(II) complex decreased considerably, and a new well-defined adsorptive reduction peak appeared at −1.63 V (vs. SCE). The electrochemical kinetic parameters and the binding number of LMF–Mg(II) with ctDNA were also obtained. Moreover, the new peak currents of LMF–Mg(II)–DNA system increased linearly correlated to the concentration of DNA in the 4.00×10−7–2.60×10−6 g ml−1 range when the concentrations of LMF–Mg(II) complex was fixed at 5.00×10−6 mol l−1, with the detection limits of 2.33×10−7 g ml−1. An electrostatic interaction was suggested by electrochemical method. 相似文献
987.
Hanhua Zhao Curtis P. Berlinguette John Bacsa Shane E. Tichy Kim R. Dunbar 《Journal of Cluster Science》2003,14(3):235-252
The preparation and characterization of two new mixed-valence, trinuclear species, [Mn3O(O2CCF3)6(H2O)3]CF3COOH4/3H2O (1) and [Mn3O(O2CCF3)6(CH3COOH)3] (2), is reported. Compound 1 crystallizes in the triclinic space group, P¯1 (No. 2), with the parameters, a=12.3131(9) Å, b=12.4427(9) Å, c=12.965(1) Å, =72.593(4)°, =73.453(5)°, =68.345(4)°, V=1727.2(2) Å3, and Z=2. A total of 14060 reflections were collected in the range 1.6827.52°. The final weighted and non-weighted agreement indices, R1=0.0589 and wR2=0.1445 were based on a total of 6953 unique reflections with an R
int value of 0.0542. Compound 2 crystallizes in the monoclinic space group, P21/n (No. 14), with the parameters, a=12.876(3) Å, b=12.212(4) Å, c=17.732(4) Å, =100.40(3)°, V=3640.4(1) Å3, and Z=4. A total of 32197 reflections were collected in the range 1.7227.13°. The final weighted and non-weighted agreement factors, R1=0.0647 and wR2=0.1609 were based on a total of 8018 unique reflections with an R
int
value of 0.0462. An investigation of the physical properties revealed that both compounds display an intermediate ground state of S=3/2 as a consequence of intramolecular antiferromagnetic coupling. The magnetic data for compound 1 was best fit to the parameters g=2.09, J=–5.5 cm–1, J=–3.4 cm–1, and D
Mn(III)=–4.5 cm–1; the data for compound 2 was best fit to the parameters g=2.10, J=–2.9 cm–1, J=–5.5 cm–1, and D
Mn(III)=–4.5 cm–1. 相似文献
988.
The photocatalytic degradation for some kinds of dye-constituent aromatics with TiO2 in the presence of phosphate anions in aqueous dispersion was investigated under both visible light (λ>480 nm) and UV irradiation. The influences of phosphate anion upon the degradation of organics under these different conditions was revealed by the measurement of point of zero ξ-potential (P ZC) of TiO2, UV-VIS spectra, HPLC and LC-MS. The adsorption and photodegradation of some organics, which adsorb on the surface of TiO2 by a dominating group bearing a positive charge, was enhanced, while that of others, which adsorb on the surface of TiO2 by a dominating group bearing negative charge, was depressed by the presence of phosphate anions under UV irradiation at the experimental conditions (pH 4.3). It was confirmed that better adsorption of organics on the surface of TiO2 had an advantage in their photocatalytic degradation under UV irradiation. On the other hand, although the adsorption of rhodamine B (RhB) and methylene Blue (MB) markedly increased, their degradation under visible light irradiation was depressed in the presence of phosphate anions. It is suggested that phosphate anion greatly blocked the electron transfer from excited RhB and MB molecules as RhB and MB molecules predominantly adsorbed on the surface of TiO2 through the electrostatic interaction with surface adsorbed phosphate anions. 相似文献
989.
Junbiao Chang Rongfeng Chen Ruiyun Guo Chunhong Dong Kang Zhao 《Helvetica chimica acta》2003,86(6):2239-2246
Two biphenyl lignans, α‐ and β‐DDB ( 1 and 2 , respectively) were efficiently synthesized without contamination by other regio‐isomers. The different yields of the Ullmann coupling reactions for the synthesis of 1 and 2 were rationalized by calculating steric hindrance, stability, entropy change, and heat‐of‐formation values. The enantiomers of 1 and 2 were readily separated by HPLC on a chiral stationary phase. Their configurations were assigned based on the Cotton effect of the authentic natural products. 相似文献
990.
Rong-Qiao He Chuan-You Li Jung Xu Xiu-An Zhao 《Applied biochemistry and biotechnology》1996,60(3):229-244
Estimation of the optimal concentrations of residual sugar in medium for a fed-batch culture of Baker’s yeast has been studied and practiced. The concentrations, however, depended on different species and targets of the biomass, which was expected to be made. Kinetic changes of the residual phosphate salt in the medium conformed to a logarithmic process until the fourth hour during an 11-h culture. The parabolic method (see ref. 9 later in article) might be qualified to maintain the concentrations of residual sugar around 0.15 g/L. It was demonstrated that cell growth followed a sigmoid process during a fed-batch culture, because the cells consumed the nutrient with two metabolic pathways, one was for cell conversion and another was for non-cell conversion. With the parabolic method, we can estimate kinetics of cell growth and cell growth rate during the culture. 相似文献