NMR spectra of androstane analogs of brassinosteroids

We have used two-dimensional NMR spectroscopy to make a complete assignment of signals from the nuclei of hydrogen and carbon atoms in the spectra of brassinosteroids in the androstane series. We have confirmed the stereochemistry of the chiral centers and the structure of the molecules. We have studied the effect of the configuration of the 2,3-diol groups in the A ring of the steroids on the chemical shift of adjacent atoms in the ¹³C and ¹H NMR spectra. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 583–590, September–October, 2007.     

取代基效应对脂肪醇13C NMR化学位移的影响

用原子电负性、静电作用、极化度作为基本参数,并结合表征原子空间连接方式的立体效应参数,对醇分子中不同环境碳原子的化学位移进行关联, 将120个模型化合物(91个脂肪一元醇, 29个二元醇)中747个碳原子相关参数值和化学位移值带入模型中得到如下估算方程:  δ_C=42.947 9 + 63.064 0Q_i-3.628 6F+5.121 3Σα_x-6.584 8Q_iΣα_x -4.842 7N^α_H-0.585 5N^γ_H-4.104 6N^γ_OH (R=0.998 1 R²=0.996 1 S=1.14 F=27 125.2 n=747)  方程中各参数物理意义比较明确. 通过用“留一法”(LOO)检验(R_cv=0.998 0,R²_cv = 0.996 0,S_cv=1.16)及对样本外5个化合物69个碳原子化学位移的预测值和实验值比较, 结果表明模型方程具有很好的稳定性和预测精度, 该模型的提出为以后计算更加复杂化合物的¹³C NMR化学位移奠定了良好的基础.     

A Refinement of the Hg2F2 Structure by the Use of NMR Data

Abstract

The crystal structure of Hg₂F₂ was studied in works.^1,2 The Hg₂F₂elementary cell of the tetragonal syngony with a=3.66 Å and c=10.89 Å contains two formula units, the space-group being 14/mmm. Hg₂F₂ belongs to those types of structures which possess one non-fixed coordinate (z_F and zHg); hence, the position of the fluorine nuclei may be determined from the value of the second moment of the NMR spectrum of ¹⁹F for powder sample.³ In this case it is ostensibly important, inasmuch as determining the positions of fluorine nuclei near the heavy mercury atoms presents a difficulty for X-ray analysis.     

The Use of Protonation Induced Changes in Carbon-13 NMR Chemical Shifts to Investigate the Solution Microscopic Structure of Partially Protonated Polybasic Molecules

Abstract

Changes in the NMR chemical shift of carbon-13 nuclei upon protonation of a nearby basic center are shown to be a useful method of probing the microscopic site of protonation of molecules containing multiple basic centers, such as diethylenetriaminepenta-acetic acid (DTPA). This approach is also shown to support the predominance of the pyridine amine protonated tautomer in the 4-pyridone ? 4-hydroxypyridine equilibrium.     

Chemical ionization of hydrogen halide molecules in collision with metastable helium atoms

An anomaly of the temperature dependence of the cross section for chemical ionization of HBr molecules by metastable helium atoms He*(2³ S ₁) is discovered. It is shown that the interaction anisotropy induced by rotation of the HBr molecule has a significant influence on the formation of the anomaly. Zh. Tekh. Fiz. 68, 13–15 (February 1998)     

脂肪族醚类化合物核磁共振碳谱模拟

利用原子电性作用矢量(atomic electronegativity interaction vector, AEIV)对脂肪族醚类化合物中碳原子局部化学微环境进行表征, 并结合γ-效应参数与其核磁共振碳谱(¹³C NMR)建立定量构谱相关(QSSR)模型,建模计算值和留一法(leave-one-out, LOO)交互校验(cross-validation, CV)预测值的复相关系数(R)分别为0.995 7和0.994 2. 进一步使用外部样本对所得模型稳定性能进行检验,其外部校验相关系数(Q_ext)为0.996 3,结果表明： AEIV、γ-效应参数与¹³C NMR谱化学位移显著相关.     

沸石17O固体核磁共振研究进展

沸石作为一类最重要的固体氧化物材料,是¹⁷O固体核磁共振最早研究对象之一. 近年来,随着核磁共振谱仪磁体场强的不断提高,以及新脉冲序列的发展,¹⁷O固体核磁共振被越来越多地应用于沸石的结构表征,在研究骨架氧结构以及测定Brønsted酸位的O-H键长等方面都提供了非常丰富的信息. 本文将介绍¹⁷O固体核磁共振的特点,回顾20年来它在沸石研究方面的发展并着重介绍近期这一方面的研究突破.      

CO2 laser photoacoustic spectra and vibrational modes of heroin, morphine and narcotine

Heroin, morphine and narcotine are very large molecules having 50, 40 and 53 atoms respectively. Moderately high resolution photoacoustic (PA) spectra have been recorded in 9.6 μm and 10.6 μm regions of CO₂ laser. It is very difficult to assign the modes of vibrations for PA bands by comparison with conventional low resolution IR spectra. The ab initio quantum chemical calculations were used for determining the molecular geometries and normal mode frequencies of vibrations of these molecules for assignments of PA spectra.     

NMR chemical shifts of129Xe dissolved in liquid haloalkanes and their mixtures

Xenon-129 NMR spectra have been measured for solutions containing Xe dissolved in a variety of linear 1-haloalkanes, α, ω-dihaloalkanes, and their mixtures. The data are found to be linearly related to solvent composition expressed as volume fraction of the constituent methyl groups, methylene groups, and halogen atoms. This behavior is consistent with a model in which the large dispersion force contributions to the chemical shift of¹²⁹Xe result from pair interactions between dissolved Xe atoms and the methyl, and methylene groups, and the halogen atom, from which the solvent molecules are composed.     

New methods for determining the polarization state of vacuum ultraviolet radiation

Two new methods are proposed for determining the polarization of vacuum ultraviolet radiation which permit the determination of an arbitrary polarization mode for photons with energies of 10–100 eV. The essence of these methods is to create and detect a nonequilibrium population of the magnetic sublevels of atoms and molecules excited by the original VUV radiation and then determine the polarization of this radiation based on these measurements in accordance with known formulas. Zh. Tekh. Fiz. 69, 115–122 (September 1999)     

Formation of cold Cs ground state molecules through photoassociation in the pure long-range state

We report on the formation of translationally cold Cs₂ ground state molecules through photoassociation in the 1_u attractive molecular state below the 6 s _1/2 +6 p _3/2 dissociation limit. The cold molecules are obtained after spontaneous decay of photoassociated molecules in a MOT and in a dark SPOT. We also used polarized atoms, in the f =3, m f =+3Zeeman ground state. Purely asymptotic and adiabatic calculations including hyperfine interaction and rotation are in excellent agreement with observed structures. As expected, the 1_u state is actually a pure long-range state, consisting of paired atoms, uniquely linked by the first terms of the multipole expansion of the electrostatic interaction. A temperature of 20 K has been measured for the molecular cloud. Received 19 July 1999     

Anion effect on the conformational equilibrium of sulfamide and its N,N′‐diindolyl derivative: Insights on anion transportation

The relatively high acidity of the sulfamide hydrogens suggests a potential for the development of sulfamide derivatives as novel anion receptors. The interactions of sulfamide with F^?, Cl^?, CH₃COO^?, and H₂PO₄^? anions were spectroscopically (¹H and ¹⁹F NMR) and theoretically (density functional theory) analyzed, and the complexation through hydrogen bonds was confirmed by changes in the NMR signals and theoretical calculations. The replacement of 2 sulfamide hydrogens with indolyl groups yields the N,N′‐diindolylsulfamide ( DIS , N‐1H‐indol‐4‐yl‐N′‐1H‐indol‐7‐ylsulfuric diamide), whose bond rotations allow the interaction of 4 H(N) atoms with anions. The conformational preferences of DIS change upon the presence of anions, but they are practically insensitive to the anion type. According to the quantum theory of atoms in molecules, natural bond orbital analysis, and NMR chemical shifts, as well as to a thermodynamic cycle, the complex with fluoride is the most stable, followed by the oxoanion‐derived models.     

NMR chemical shifts of129Xe dissolved in liquid ethers, secondary amines, and alkyl sulfides

Xenon-129 NMR spectra have been measured for solutions containing Xe dissolved in a variety of linear aliphatic ethers, secondary amines, and alkyl sulfides. The chemical shifts obtained from these data are found to be linearly related to the composition of the solvent molecules expressed as volume fraction of the constituent methyl groups, methylene groups, and the respective oxygen, nitrogen, and sulfur atoms. These results support a model used previously to interpret chemical shifts measured for¹²⁹Xe dissolved in a variety ofn-alkanes and their terminally substituted derivatives. This model treats dispersion force contributions to the chemical shift as resulting from pairwise additive interactions between each dissolved Xe atom and the individual constituent groups of each solvent molecule.     

Discrimination in racemates of small chiral molecules

A comparison of similar chiral molecules provides information about the impact of molecular characteristics on selectivity. In this article, the intermolecular structure in racemic fluids is the basis for comparing the molecules: the radial distribution between atoms on identical molecules is compared with the corresponding distribution for atoms from a mirror-image pair. A difference in these distributions signals an enantiomeric imbalance in the local distribution of molecules. The structure in the racemic fluids is explored using Monte Carlo (MC) simulations and the integral equation theory of Chandler, Silbey and Ladanyi (CSL) [1982, Molec. Phys., 46, 1335].

Racemic fluids are examined for several categories of chiral molecules. First, symmetrically shaped molecules have been considered in order to isolate local excesses attributable to energetic contributions. Second, racemates of hard chiral molecules have been examined. Here, enantiomeric imbalances can only originate from asymmetry in the molecular shape. Finally, both the molecular shape and the interaction strengths have been varied in order to explore the competition between steric and energetic effects. Within each category, the number of chiral molecules is large and a selection mechanism is required to identify those molecules which are expected to show large local excesses. An appropriate selection criterion (chirality index) has been defined and evaluated for 400 000 chiral molecules. Based on the results of this assessment, 24 racemates have been chosen for detailed examination by MC simulations and integral equation theories.     

A computational NICS and 13C NMR characterization of the substitution patterns of C70−2x(BN)x fullerenes (x=1–25)

A density functional study has been performed to investigate the electronic and magnetic properties of BN substituted fullerenes C_70?2x(BN)_x (x=1, 2, 3, 6, 9, 12, 15, 17, 19, 21, 23 and 25) based on the NMR parameters and NICS index. The calculated ¹³C chemical shielding (CS) tensors are found to be perturbed at the first and second neighbors of the doped atoms while the other distant carbon atoms not to be influenced significantly. ¹³C Chemical shifts (δ_iso) of the second neighbors of nitrogen and boron are significantly shifted to upfield and downfield (the second neighboring effects), respectively. Besides, chemical shifts are also affected by the curvature of the corresponding sites; for example, the perturbed sites at the caps yield smaller absolute values of chemical shifts than those located at the equator. Nucleus independent chemical shifts (NICS) at the cage centers of heterofullerenes (from ?25.29 to ?8.80) demonstrate that all the substituted species are aromatic, but less than C₇₀ (?27.29). The predicted NICS values may be useful for identification of the heterofullerenes through their endohedral ³He NMR chemical shifts.     

Dynamical and structural properties of adsorbed water molecules at the TiO2 rutile-(110) surface: interfacial hydrogen bonding probed by ab-initio molecular dynamics

ABSTRACT

Ab-initio molecular dynamics (AIMD) simulations have been carried out to study a range of different and energetically-accessible adsorbed-water configurations and motifs for their vibrational and structural characteristics, in contact with rutile-(110) interfaces at 100?K. The radial pair distribution function between the titanium atoms at the interface and the hydrogen and oxygen atoms in the water monolayer show an orientation of the water molecules parallel to the surface of titania, and with hydrogen atoms pointed in the opposite direction to the surface. In some cases, a distinctive vibrational frequency region between 2500 and 3000?cm^?1 has also been observed, due to a strong dispersion interaction between water molecules. This behaviour is also seen in experimental studies of thin-film water coverage on TiO₂ surfaces.     

The quenching of Pb(6p 2 1 D 2) atoms in collisions with molecules

The collisions of metastable Pb(6p ^{2 1} D ₂) atoms with various molecules were studied by the diagnostics of radiation from a hollow cathode lamp and a laser on lead vapor. Experiments were performed for a gas flow of lead atoms with argon. The Pb(6p ^{2 1} D ₂) states were excited in a gas discharge in the presence of reagent gas molecules. The absolute rate constants for the quenching and chemical reactions of lead atoms in the ground and excited states were determined. The quantum efficiency of chemical reactions was close to one for the N₂O, CH₂Cl₂, SF₆, and CuBr molecules. Long-lived chemical compounds were formed in these reactions.     

Probe the accumulation modes of the Au–C22H14 dimer on the structure and NLO properties

In 2006, the Au–C₂₂H₁₄ with a covalent bond between an individual pentacene (C₂₂H₁₄) and a gold (Au) atom was synthesised and characterised, and its nonlinear optical (NLO) properties were explored. To further investigate the NLO properties from molecules to materials, three kinds of different dimers (Au–C₂₂H₁₄)₂ (2, 3 and 4) were designed to probe the monomer accumulation modes on the structures and NLO properties. The results indicate that Au atoms doping breaks the conjugate structures of the two pentacenes to different extent. On the other hand, their NLO properties investigated by three density functional theory methods Becke-Half-and-Half-LYP (BHandHLYP), Coulomb-attenuating method Becke-3-Lee–Yang–Parr (CAM-B3LYP) and Minnesota 2005 double the amount of nonlocal exchange (M05-2X) show the same order, and 2 has the largest first hyperpolarisability (β_tot) than the other molecules. At the same time, natural bond orbital analysis shows the Au atoms play a crucial role in pushing electron density. Meanwhile, the frontier molecular orbital analysis shows that charge transfer has occurred between the two pentacene molecules and Au atoms. As a result, the order of transition energy is opposite to the order of β_tot values. Because the pentacene is taken as a simplified fragment of the graphene, our present work may be beneficial to the development of high-performance NLO materials.     

Application of 15N NMR Spectroscopy to Studies of the Intermolecular Interaction of Biomolecules

Abstract

Nitrogen nuclei are frequently located at the interaction sites of biomolecules; for example, amide nitrogens in the peptide are the key to maintaining the peptide backbone conformation by hydrogen bonding. Histidine, tryptophan, and arginine side chains contain nitrogen atoms which are often located at the active sites of enzymes. Heterocyclic compounds like purine and pyrimidine bases are substances essential to information transfer by base pair formation of nucleic acids. Also some other co-factors and dyes, such as flavins, porphyrins and chlorophylls, are nitrogen-containing substances which regulate energy tranduction in biological systems. Lecithin and phosphatidylethanolamine are the principal components of the phospholipids from biomembranes. To detect interaction sites and to study the interaction mechanism of these biomolecules, the use of nitrogen NMR seems promising. Although more than 99% of naturally occurring nitrogen element has the ¹⁴N nuclei, a disadvantage of the use of ¹⁴N magnetic resonance has been recognized. It has a spin quantum number I = 1 and the associated quadrupole moment provides a very broad resonance signal of about 100～1000 Hz. Thus, for detection of small changes of the chemical environment, use of ¹⁴N magnetic resonance is not adequate. The natural abundance of ¹⁵N nuclei with a spin quantum number I = 1/2 (which give a sharp resonance signal) is only 0.3% (Table 1). But recent developments in the instrumentation of NMR spectroscopy have made it possible to observe the resonance of the nuclei with low natural abundance. Fourier transform (FT) NMR can save thousands of times the accumulation time to improve the signal to noise ratio of ¹⁵N spectra [1-3]. Also superconducting magnets with wide bores have made possible the use of thick sample tubes of 25 mmΦ and observation of the ¹⁵N resonance of substances of low solubility [4]. In spite of such instrumental development, the observation of the 15N resonance is still not easy because of its low sensitivity; about of proton magnetic resonance. In the application of ¹⁵N NMR in biological systems, we often encounter quite low solubility of biomacromolecules and also sometimes need to measure the concentration dependency of ¹⁴N chemical shifts. For such experiments, enrichment of ¹⁵N nuclei in the molecules is required. Chemical syntheses starting from the simple ¹⁵N containing compounds as an ¹⁵N source and also biological syntheses by bacterial fermentation using the ¹⁵N source in culture media are employed for ¹⁵N enrichment. Enrichment at specific positions of biomolecules is useful for spectral assignments and also for analyses of the pathways of biosyntheses [5].     

The differential coefficient (∂P/∂n)ne for properties of diatomic molecules and atoms

The coefficient (?P/?n)_ne, where n is the stage of ionization and n_e is the total number of electrons, has been investigated for atoms and for diatomic molecules by the use of tabulated data. The P are (a) orbital configurations, (b) state symbols, (c) ionization potentials, (d) internuclear separations of ground state molecules, (e) crystal radii of ionized atoms, (f) relativistic HF radii of neutral atoms, (g) dissociation potentials of molecules, and (h) the logarithms of the transition moments of electron transitions from the first excited to the ground states of atoms and molecules. The differentials for atoms in cases (a), 0870 0891 V (c) and (h) add to form those for the diatomic molecules, agreeing with a prediction based on the matrix formulation of the periodic system of diatomic molecules diatomic molecules.