抗癌药物冬凌草甲素的分子构型研究

本文用同核化学位移相关谱和二维J分解谱归属了冬凌草甲素的^1H谱, 测定了所有的质子标量偶合常数, 根据一系列的Karplus公式计算了二面角, 用自编WUPH计算程序, 计算出冬凌草甲素的分子构型。     

Determination of absolute configuration by 31P NMR

The determination of absolute configuration is crucial for all chiral molecules; therefore many techniques have been developed to serve this purpose. It is especially important for the invention of new medicines, as in some cases only one enantiomer has a positive medical effect, while the other is inactive or even harmful. Herein we present the usefulness of the ³¹P NMR technique in the determination of the absolute configuration of chiral molecules, as an alternative to other methods. The models presented herein, often developed on the basis of ¹H NMR, describe conformations and configurations and following shielding effects in chiral molecules, that can be applied to determine the stereochemistry of a molecule by ³¹P NMR spectroscopy. Numerous examples of the use of some of the described models are presented as well.     

Determination of nanocellulose fibril length by shear viscosity measurement

The lengths of ten types of cellulose nanofibrils were evaluated by shear viscosity measurement of their dilute dispersions. Aqueous dispersions of surface-carboxylated cellulose nanofibrils with a uniform width of ~3 nm were prepared from wood cellulose by 2,2,6,6-tetramethylpiperidine-1-oxyl-mediated oxidation and successive mechanical treatment. Cellulose nanofibril samples with different average lengths were prepared by controlling the conditions of the oxidation or mechanical treatment. The viscosity-average lengths, L _visc, of the nanofibrils were calculated by applying the shear viscosities of the dilute dispersions to an equation for the dilute region flow behavior of rod-like polymer molecules. The obtained L _visc values ranged from 1,100 to 2,500 nm and showed a linear relationship to the length-weighted average length, L _w, measured by microscopic observation; the relation was described as L _visc = 1.764 × L _w + 764. The influences of the electric double-layer of the nanofibrils and surface-carboxylate content on the value of L _visc were also investigated.     

二维NMR测定1,3-双烯膦酸酯的构型

本文报道用1H NMR二维J谱、二维同核相关谱及NOE技术测定了三个共轭双烯膦酸酯的双键构型,并求得各δ和J值。     

Controlled modulation of conductance in silicon devices by molecular monolayers

We have controllably modulated the drain current (I(D)) and threshold voltage (V(T)) in pseudo metal-oxide-semiconductor field-effect transistors (MOSFETs) by grafting a monolayer of molecules atop oxide-free H-passivated silicon surfaces. An electronically controlled series of molecules, from strong pi-electron donors to strong pi-electron acceptors, was covalently attached onto the channel region of the transistors. The device conductance was thus systematically tuned in accordance with the electron-donating ability of the grafted molecules, which is attributed to the charge transfer between the device channel and the molecules. This surface grafting protocol might serve as a useful method for controlling electronic characteristics in small silicon devices at future technology nodes.     

Vibrational spectra and molecular configuration of hexafluoroazomethane

Twenty-one of the 24 fundamental frequencies of CF₃N=NCF₃ have been identified from the infrared and Raman spectra of the vapor and condensed states. The spectra reveal a total lack of infrared-Raman coincidence, i.e., the rule of mutual exclusion is obeyed. This evidence strongly supports the existence of hexafluoroazomethane in the trans configuration and negates the results of a recent electron diffraction study which favor the cis form.     

Energy-level density of a debye solid by the method of steepest descent

It is demonstrated that the method of Lin and Eyring for estimating the energy level density of a collection of harmonic oscillators can be used to calculate the vibrational energy level density of a Debye solid (a continuum of oscillators), and hence to obtain the Debye specific heat law. Haarhoff's formula is also considered.     

Size and persistence length of molecular bottle-brushes by Monte Carlo simulations

Single-chain simulations of densely branched comb polymers, or "molecular bottle-brushes" with side-chains attached to every (or every second) backbone monomer, were carried out by off-lattice Monte Carlo technique. A coarse-grained model, described by hard spheres connected by harmonic springs, was employed. Backbone lengths of up to 100 units were considered, and compared with the corresponding linear chains. The backbone molecular size was investigated as a function of its length at fixed arm size, and as a function of the arm size at fixed backbone length. The apparent swelling exponents obtained by a power-law fit were found to be larger than those for the corresponding linear polymers, indicative of stiffening of the comb backbone. The probability distribution function for the backbone end-to-end distance was also investigated for different backbone lengths and arm sizes. Analysis of this function yielded the critical exponents, which revealed an increase in the swelling exponent consistent with values found from the molecular size. The apparent persistence length of the backbone was also determined, and was found to increase with increasing branching density. Finally, the static structure factors of the whole bottle-brushes and of their backbones are discussed, which provides another consistent estimate of the swelling exponents.     

Stable molecular configuration in crystalline carboxylic acids

The stable (lower enthalpy) molecular configurations of propionic, butyric, Jeric and lauric acids in the crystalline state have been examined via their atom-atom potentials. It was found that the cis configuration is more stable than the trans configuration for propionic, butyric and valeric acids, and that the trans configuration is more stable than the cis configuration for lauric acid, in accord with a previous IR spectral analysis. The potential energy of benzoic acid was recalculated using the positions of atoms given by Speakman, and indicates that the A form is more stable than the B form, in agreement with the results of previous work.     

Determination of absolute configuration using X-ray diffraction

Methods for determination of absolute structure using X-ray crystallography are described, with an emphasis on applications for absolute configuration assignment of enantiopure light-atom organic compounds. The ability to distinguish between alternative absolute structures by X-ray crystallography is the result of a physical phenomenon called resonant scattering, which introduces small deviations from the inherent inversion symmetry of single-crystal X-ray diffraction patterns. The magnitude of the effect depends on the elements present in the crystal and the wavelength of the X-rays used to collect the diffraction data, but it is always very weak for crystals of compounds containing no element heavier than oxygen. The precision of absolute structure determination by conventional least squares refinement appears to be unduly pessimistic for light-atom materials. Recent developments based on Bijvoet differences, quotients and Bayesian statistics enable better and more realistic precision to be obtained. The new methods are sensitive to statistical outliers, and techniques for identifying these are summarised.     

Determination of molecular nitrogen by electron-impact induced fluorescence

When a sample of nitrogen gas is bombarded by 100-eV electrons, fluorescent emission from the nitrogen molecular ion, N(+)(2), is observed. The intensity of this fluorescence can be related to the pressure (or molar concentration) of the parent molecule, N(2). The limit of detection for N(2) by means of electron-impact induced fluorescence is 5.0 x 10(-8)mmHg (1.6 x 10(9) molecules/cm(3) or 2.7 x 10(-12) mole/l.). The fluorescence signal is linearly related to N(2) pressure over a range of four orders of magnitude, and relative standard deviations of less than 2% are observed for replicate measurements.     

分子荧光法测定水中的微量铁

基于Fe（Ⅱ）与邻菲口罗啉生成络合物使其荧光猝灭的特性,讨论了在室温下,pH 5.5的乙酸-乙酸铵的缓冲介质中,以λex=270 nm为激发波长,于eλm=365 nm波长处测定该络合物荧光强度。结果表明Fe（Ⅱ）质量浓度在0.8-120.0 ng/mL范围内与络合物的荧光强度具有良好的线性关系,相关系数0.9994,检出限0.50 ng/mL。对60 ng/mL的Fe RSD（n=9）为2.8%。Fe的回收率为98.03%-102.9%。方法已用于水中微量铁的测定。     

Temperature-dependent modulation by biaryl-based monomers of the chain length and morphology of biphenyl-based supramolecular polymers

Supramolecular copolymerizations offer attractive options to introduce structural and functional diversity in supramolecular polymer materials. Yet, general principles and structure–property relationships for rational comonomer design remain lacking. Here, we report on the supramolecular (co)aggregation of a phenylpyridine and bipyridine derivative of a recently reported biphenyl tetracarboxamide-based monomer. We show that both arylpyridines are poor monomers for supramolecular homopolymerizations. However, the two arylpyridines efficiently influence supramolecular polymers of a biphenyl-based polymer. The phenylpyridine derivatives primarily sequestrate biphenyl monomers, while the bipyridine intercalates into the polymers at high temperatures. Thereby, these two poorly homopolymerizing monomers allow for a fine control over the length of the biphenyl-based supramolecular polymers. As such, our results highlight the potential to control the structure and morphology of supramolecular polymers by tailoring the electronic properties of additives.

Supramolecular copolymerizations offer attractive options to introduce structural and functional diversity in supramolecular polymer materials.     

Determination of the chain length in polydiacetylenes by scanning electron microscopy

Partially polymerized TS and HD-Ph crystals were irradiated through a two-dimensional grid by UV light. Surface structure which appears after dissolving unreacted monomer indicates that polymer chains grow into unirradiated crystal areas. From the etch profile the average chain length is ≈5 μm. A quantum yield ≈10^?4 for chain initiation via excitation of the singlet is deduced.     

Determination of the absolute configuration of three as-hydrindacene compounds by vibrational circular dichroism

[Structure: see text]. The absolute configurations of three compounds with a rigid 1,8-disubstituted as-hydrindacene skeleton have been determined using vibrational circular dichroism spectroscopy and quantum chemical calculations. Experimental spectra were compared to B3LYP/6-31G and B3LYP/cc-pVTZ level predicted spectra. Based on the agreement between the predicted and experimental spectra, the stereochemistry could be assigned with high confidence. The results were found to be in agreement with ECD determinations and/or predictions based on the applied asymmetric methods in the synthetic route.     

Determination of absolute configuration of optically active 1-aminoalkanephosphonic acids by chemical correlations

Abstract

The absolute configurations of optically active phosphonic analogues of serine, β-chloroalanine, phenylalanine, tyrosine and 2-aziridinephosphonic acid have been established via chemical correlations with phosphonic analogues of alanine or aspartic acid of known configuration.     

Determination of absolute configuration of axially chiral biaryls

Absolute configurations of axially chiral biaryls with hydroxyl, amino, and carboxyl groups can be easily determined by pmr spectra using MTPA derivatives and shift reagents.