AlPW12O40 was found to be an effective catalyst for the selective acetylation of alcohols, thiols, and amines in the absence of solvent at room temperature. 相似文献
In this paper, density functional theory (DFT) was used to calculate 14N nuclear quadrupole coupling constants (NQCC), χ, and asymmetry parameters, η, for a series of imidazole derivatives: imidazole, 5-methylimidazole and histidine. These calculations were carried out with the PW91P86 method via the Gaussian 98 package. A systematic theoretical investigation of the different environmental effects on (χ, η) values of amino 14N1 and imino 14N2 of imidazole ring of these compounds, reveals that the local surrounding of nitrogen atoms play an important role in determining their χ and η values. Our calculations in solution show that adding explicit solvent molecules to the polarizable continuum model (PCM) has a strong effect on (χ, η) values, thereby indicating that for long-range effects, PCM, is not sufficient to describe the whole solvent effects. We also evaluate the influence of [Fe3+ (S = 1/2)] on the (χ, η) values of proximal and remote nitrogens of an axial ligand and compare with those of free ligands. The results show that Fe3+ has a strong effect on the (χ, η) values of proximal nitrogen unlike remote nitrogen. Finally, our results predict (χ = 1.56 MHz, η = 0.690) for proximal nitrogen and (χ = 2.75 MHz, η = 0.169) for remote nitrogen in PfHRP2–Fe3+-PPIX complex. 相似文献
Quinazoline-2,4(1H,3H)-diones 4 were silylated and condensed with methyl 5-azido-2,5-dideoxy-3-O-(4-methylbenzoyl)-α,β-D-erythro-pentofuranoside (3) using trimethylsilyl trifluoromethanesulfonate (TMS triflate) as the catalyst to afford the corresponding 5′-azidonucleosides 5 . 1-(5-Azido-2,5-dideoxy-α-D-erythro-pentofuranosyl)quinazoline-2,4(1H,3H)-diones 6 and the corresponding β anomers were obtained by treating 5 with sodium methoxide in methanol at room temperature. 6-Methyl-1-(5-amino-2,5-dideoxy-β-D-erythro-pentofuranosyl)quinazoline-2,4(1H,3H)-dione (8) was obtained by treatment of the corresponding azido derivative 7 with triphenylphosphine in pyridine, followed by hydrolysis with ammonium hydroxide. 相似文献
By measuring the GaBr emission enhanced by iodide at 350 nm in a carbon cavity heated by a hydrogen/argon flame, 0–60 ng of bromide is determined with a detection limit of 0.5 ng μl?1. In a similar way, gallium (0–20 ng) can be determined with a detection limit of 0.15 ng μl?1. Interferences are reported. Chloride (0–1200 ng) is determined by means of GaCl emission; the detection limit is 50 ng μl?1. 相似文献
A combination of triphenylphosphine (PPh3) and 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) provides a safe and easily available mixed reagent system for the conversion of 1° and 2° alcohols, thiols, trimethylsilyl-, and tetrahydropyranyl ethers to their corresponding thiocyanates and the 3° ones to isothiocyanates in good to high yields. 相似文献
The mixture of triphenylphosphine (PPh3) and 2,3-dichloro-5,6-dicyanobenzquinone (DDQ) as a neutral system has been used for the preparation of various types of diethyl α-bromo, α-iodo and α-azidophosphonates from their corresponding diethyl α-hydroxyphosphonates in the presence of n-Bu4NBr, n-Bu4NI and NaN3 in good to high yields. 相似文献
The current trend for ultra-high-field magnetic resonance imaging (MRI) technologies opens up new routes in clinical diagnostic imaging as well as in material imaging applications. MRI selectivity is further improved by using contrast agents (CAs), which enhance the image contrast and improve specificity by the paramagnetic relaxation enhancement (PRE) mechanism. Generally, the efficacy of a CA at a given magnetic field is measured by its longitudinal and transverse relaxivities r1 and r2, i.e., the longitudinal and transverse relaxation rates T1−1 and T2−1 normalized to CA concentration. However, even though basic NMR sensitivity and resolution become better in stronger fields, r1 of classic CA generally decreases, which often causes a reduction of the image contrast. In this regard, there is a growing interest in the development of new contrast agents that would be suitable to work at higher magnetic fields. One of the strategies to increase imaging contrast at high magnetic field is to inspect other paramagnetic ions than the commonly used Gd(III)-based CAs. For lanthanides, the magnetic moment can be higher than that of the isotropic Gd(III) ion. In addition, the symmetry of electronic ground state influences the PRE properties of a compound apart from diverse correlation times. In this work, PRE of water 1H has been investigated over a wide range of magnetic fields for aqueous solutions of the lanthanide containing polyoxometalates [DyIII(H2O)4GeW11O39]5– (Dy-W11), [ErIII(H2O)3GeW11O39]5– (Er-W11) and [{ErIII(H2O)(CH3COO)(P2W17O61)}2]16− (Er2-W34) over a wide range of frequencies from 20 MHz to 1.4 GHz. Their relaxivities r1 and r2 increase with increasing applied fields. These results indicate that the three chosen POM systems are potential candidates for contrast agents, especially at high magnetic fields. 相似文献
Summary. A facile and efficient synthesis of a series of novel 1,2,4-triazolo[5,1-b]quinazolines is described. 2,3-Diaryl-2,3-dihydro-1H-1,2,4-triazolo[5,1-b]quinazolin-9-ones were obtained by reaction of 3-amino-2-arylamino-3H-quinazolin-4-ones with aromatic aldehydes as well as by ring closure of the corresponding anils. Treatment of 3-amino-2-arylamino-3H-quinazolin-4-ones with aromatic carboxylic acids afforded 2,3-diaryl-3H-1,2,4-triazolo[5,1-b]quinazolin-9-ones which could also be synthesized by dehydrogenation of the corresponding dihydro derivatives. Reaction of
3-amino-2-arylamino-3H-quinazolin-4-ones with diethyl malonate and acetylacetone gave 3-aryl-3,9-dihydro-9-oxo-1,2,4-triazolo[5,1-b]quinazolin-2-yl-acetic acid ethyl ester and 3-aryl-2-methyl-3H-1,2,4-triazolo[5,1-b]quinazolin-9-ones, respectively. The latter compounds were also prepared via reaction with acetic anhydride, whereas acetylation with acetic anhydride in the presence of pyridine afforded the acetyl
derivatives.
Received March 22, 2001. Accepted (revised) May 11, 2001 相似文献
A simple, rapid, and sensitive method was developed for detection and quantitation of lysine (Lys) in various biological samples by isocratic liquid chromatography (LC). Samples containing Lys and other amino acids were derivatized with 9-fluorenylmethyl chloroformate (FMOC-CI). The mobile phase used for isocratic elution was 50 mmol/L sodium acetate buffer (pH 4.20)-acetonitrile (43 + 57, v/v). Lys was detected with a UV detector at 265 nm. The derivatized Lys eluted from a LiChrospher 100 RP-18 (150 x 4.0 mm id) column at a retention time of 5.6 min. The limit of detection was 0.73 mumol/L (signal-to-noise [S/N] ratio, 3:1), and the limit of quantitation was 2.37 mumol/L (S/N ratio, 10:1). Lys recoveries from fortified biological samples were > 97.5%. Average Lys contents found in rumen fluid samples collected before the morning feeding and at 2.0, 4.0, and 6.0 h after feeding were 4.26, 3.34, 3.58, and 3.82 mumol/L, respectively. The hydrolysate of a sample of mixed rumen microorganisms collected before the morning feeding was determined to contain 1.372 mumol/mg microbial nitrogen in the form of Lys. The Lys concentrations of human plasma, goat plasma, human urine, and goat urine were 140.0, 102.0, 58.0, and 32.0 mumol/L, respectively. 相似文献
Roughness of surface has as an important influence on identifying the mechanical behavior and performance of crystalline metals. In this study, nanoindentation simulations are conducted by the two dimensional quasicontinuum method to determine the load–penetration response and the critical load associated with the onset of plasticity in rough surfaces of a face-centered cubic single crystal copper. The arithmetic roughness index, ranging between 2 and 13 Å, is used to specify the roughness of surface. Results of indentation with different roughnesses are in good agreement with previous studies for the indenter size of 10–140 Å. The resultant load–penetration scattering, which stems from the roughness, indicates different dislocation nucleation steps, different subsequent dislocations intervals and varying stiffness values of samples. It can be concluded that the surface roughness has a significant effect on the first dislocation emission because of the indenter position and surface interactions beneath it. Moreover, the critical penetration depth for the first dislocation emission increases by the increase of the contact area between the indenter and surface.