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991.
992.
The N-heterocyclic carbene, imidazole-2-ylidene, and its main group (13-15) analogues contain cyclically conjugated 6π electrons. Experimental 1H nuclear magnetic resonance (NMR) spectra suggest an increase in aromaticity along a period from left to right. Whereas the order along a group is as follows: period 2 > period 5 > period 4 > period 3 due to change in structure. To understand the order of aromaticity, the magnetically induced ring currents of the molecules are calculated using aromatic ring current shielding, gauge-including magnetically induced currents (GIMIC) method and Stanger's σ-model applying the gauge-including atomic orbitals NMR technique. It is found that GIMIC best describes the order of aromaticity especially along a group where current-profile changes on the bivalent atom down a group due to change in electron density. Moreover, the GIMIC provides the visualization of current by sign modulus and the anisotropy of the induced current density plots. 相似文献
993.
A new functional group, the hydroxy group, was inserted into a Betti base by reaction with salicylaldehyde, and the naphthoxazine derivatives thus obtained were converted by ring-closure reactions with formaldehyde, acetaldehyde, propionaldehyde or phosgene to the corresponding naphth[1′2′:5,6][1,3]oxazino[3,2-c][1,3]benzoxazine derivatives. Further, the conformational analysis of these polycyclic compounds by NMR spectroscopy and an accompanying molecular modelling are reported; especially, both quantitative anisotropic ring current effects of the aromatic moieties in these compounds and steric substituent effects were employed to determine the stereochemistry of the naphthoxazinobenzoxazine derivatives. 相似文献
994.
995.
996.
通过分析数字集成芯片多输出中电源电压瞬间下降的原因,对常规驱动电路结构进行改进。改进后单输出电路的峰值电流降为原来的2/3,再通过优化版图的电源布线,能有效防止电源电压下降,使芯片正常稳定地工作。 相似文献
997.
Room temperature ionic liquids (RTIL) are molten salts that are liquids at room temperature. Their liquid state makes them possible candidates as solvents in countercurrent chromatography (CCC), which uses solvents as both the mobile and stationary phases. The study focuses on 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM PF6), an easy to synthesize and purify RTIL whose melting point is –8°C. It is shown that BMIM PF6 behaves like a solvent of significant polarity (comparable with that of ethanol). The ternary phase diagram water–acetonitrile–BMIM PF6 is given, because it was necessary to add acetonitrile to reduce the ionic liquid viscosity. The 40:20:40% w/w water–acetonitrile–BMIM PF6 biphasic liquid system was found to be appropriate as a biphasic liquid system for CCC. Different aromatic solutes, including bases, acids, and neutral compounds, were injected into the CCC column to estimate their distribution constants between the ionic liquid-rich phase and the aqueous phase. The resulting Kil/w constants were compared with the corresponding literature octanol–water partition coefficients, Ko/w. The important drawbacks in the use of RTIL in CCC are clearly pointed out: high viscosity producing pressure build-up, UV absorbance limiting the use of the convenient UV detector, and non-volatility precluding the use of the evaporative light-scattering detector for continuous detection. 相似文献
998.
999.
1000.
提出了一种基于多输出端口电流控制电流传输器(MOCCCII,multiple outputs current controlled conveyor)的双模式二阶通用滤波器,该滤波器结构简单,仅由5个有源器件、2个接地电容构成, 无须外接电阻.该电路在不改变内部电路结构的情况下能实现电压模式和电流模式滤波器,且每一种工作模式都能实现高通、低通、带通等多种滤波功能,因此该电路具有通用性.面向实际电路完成了PSPICE仿真,结果表明提出的电路正确有效. 相似文献