Rasch模型在“化学键”概念教学评价中的应用研究

运用Rasch模型探查高二学生对"化学键"概念的认知情况，运用Conquest软件和Winsteps软件对测试结果进行分析。结果表明：基于Rasch模型编制的测试卷能客观有效地检测学生对"化学键"概念的理解，为一线教师的有效教学提供学情基础。     

LaCO几何、电子结构和化学键的研究

长期以来 ,人们对希土羰基化合物的存在一直持怀疑态度[1 ] ,尽管 Slater[2 ] 等用基质隔离法合成部分希土羰基化合物的成功消除了这种疑问 ,但由于此类化合物高度的不稳定性 ,其几何结构至今尚未确定 ,希土羰基化合物的电子结构和化学键更是困扰人们的重要课题。希土羰基化合物是具有 L n- C键的最简单希土有机金属化合物 ,其结构和性能具有一定的代表性 ,探讨其化学键合本质及其不稳定的原因对深入了解希土金属有机化合物结构和性能的关系具有十分重要的意义。一 .研究方法本文采用洪功义、黎乐民等[3 ] 改进的高精度求解密度泛函理论方…     

Anions from dihydro substituted ethyl benzoates and quinoline. New hydrogen donors for tin-free radical chemistry

The anions of substituted dihydro ethyl benzoates and quinoline are very good hydrogen donors to radicals in liquid ammonia and DMSO. With 4-substituted dihydro ethyl benzoates the rate of hydrogen transfer decreases and excellent yields of products are obtained by 6-exo ring closure reaction followed by reduction.     

High pressure FTIR study of intramolecular hydrogen bonding and conformation of a small peptide in solution

Effect of pressure on conformational equilibria of a small peptide N-acetyl-l-Pro-l-Leu-Gly-NH₂ in a chloroform solution has been studied by FTIR spectroscopy. Absorption in the NH stretching region was measured at pressures in the 1–1080 bar range and at 296 K, and decomposed into component bands by least-squares fitting. Intensity ratios of bands which were assigned to a hydrogen-bond-free conformer and intramolecularly hydrogen-bonded conformers with 10- and 13-membered hydrogen bonded rings, respectively, were examined as a function of pressure. It was found that the conformer with the 13-membered ring has definitely smaller partial molar volume than the other two conformers which have nearly the same volume with each other. On the basis of a simple dielectric model, we have shown that the conformer with the 13-membered hydrogen-bonded ring has a considerably large dipole moment which is consistent with the α-helical structure suggested by the previous variable-temperature FTIR study.     

Synergetic degradation of chitosan with gamma radiation and hydrogen peroxide

Chitosan samples were irradiated by ⁶⁰Co γ-rays in the presence of hydrogen peroxide with radiation dose from 10 kGy to 100 kGy. The degradation was monitored by gel permeation chromatography (GPC), revealing the existence of a synergetic effect on the degradation. Structures of the degraded products were characterized with Fourier-transform infrared spectra (FT-IR), ultraviolet-visible spectral (UV-vis) analysis, and X-ray diffraction (XRD). Results showed that the crystallinity of chitosan decreases with degradation, and the crystalline state of water-soluble chitosan is entirely different from that of water-insoluble chitosan. An elemental analysis method was employed to investigate changes in the element content of chitosan after degradation. Mechanism of chitosan radiation degradation with and without hydrogen peroxide was also discussed.     

The influence of the mercury and thallium atoms in organomercury and organothallium nitroxyl radicals on their reactivity

An increase is found in the reactivity of organomercury and organothallium nitroxyl mono- and biradicals of the imidazoline type in comparison with the analogous compounds without organometallic fragments. This is explained by the formation of coordination bonds NHg, NTl, and NOHg.Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 224–226, January, 1993.     

Ab initio through‐space/bond interaction analysis of the long C–C bonds in Bi(anthracene‐9,10‐dimethylene) photoisomers

The bi(anthracene‐9,10‐dimethylene) photoisomer has remarkably long C–C single bonds. To examine the lengthening of the C–C bond, we propose a novel procedure for quantitatively analyzing orbital interactions in a molecule at the level of the ab initio molecular orbital method. In this procedure, we can cut off the specific through‐space/bond interactions in a molecule by artificially increasing the absolute magnitude of the exponents in a Gaussian function. Then, the spatial orbital interactions were perfectly cut off, and, each term that makes up the total energy, that is, the nuclear–electron attractions, the electron–electron repulsions, and the nuclear–nuclear repulsions cancel each other. Several model molecules of the photoisomer were analyzed by this procedure. It was found that the orbital interaction between the p orbital on the benzene ring and the σ* orbital on the C–C bond in question, σ→σ* electron transfer through π orbital, weakens the C–C bond efficiently when these orbitals were located in the “periplanar” conformation. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

X-Ray Investigation of Chiral β-Hydroxyketone Derived From (+)-Isomenthone

X-ray analysis was carried out to study the molecular structure of β-hydroxyketone, which is one of the minor products of selective aldol condensation of (−)-menthone with 4-carbomethoxybenzaldehyde. The cyclohexanone ring of the compound has a practically undistorted chair conformation with an axial orientation of the 1-methyl group and the bulkiest 2-[1′-hydroxy-1′ -(4-carbomethoxyphenyl)]methyl substituent and the equatorial orientation of the isopropyl group in the 4 position. The stereochemical configuration was found to be (1R,2R,4 R, 1′ S), supporting that the compound belongs to the group of (+)-isomenthones; i.e., the starting (-)-menthone undergoes (to a certain extent) epimerization under the reaction conditions used. In crystal, there is intermolecular hydrogen bonding >C=O...HO- (the -O...H- bond length is 2.15 Å). The molecular conformation in hydroxyketone crystals differs from the one which prevails in solutions (according to previous NMR data) and which is characterized by an inverted cyclohexanone ring and intramolecular hydrogen bonding >C=O...HO.Original Russian Text Copyright © 2004 by S. V. Shishkina, T. G. Drushlyak, L. A. Kutulya, N. S. Pivnenko, and O. V. Shishkin__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 933–940, September–October, 2004.     

Phenol O–H bond dissociation energy in water clusters

We are reporting ab initio and density functional theory (DFT) calculations for the phenol O–H bond dissociation energy in the gas phase and in phenol–water clusters. We have tested a series of recently proposed functionals and verified that DFT systematically underestimates the O–H bond dissociation energy of phenol. However, O–H bond dissociation energies in water clusters are in reasonable agreement with experimental data for phenol in solution. We have evaluated electronic difference densities in phenol–water, phenoxy–water, and water, and we are suggesting that the representation of this quantity gives an interesting picture of the electronic density rearrangement induced by hydrogen bond interactions in phenol–water clusters. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

A sequence and structural study of transmembrane helices

A comparison is made between the distribution of residue preferences, three dimensional nearest neighbour contacts, preferred rotamers, helix-helix crossover angles and peptide bond angles in three sets of proteins: a non-redundant set of accurately determined globular protein structures, a set of four-helix bundle structures and a set of membrane protein structures. Residue preferences for the latter two sets may reflect overall helix stabilising propensities but may also highlight differences arising out of the contrasting nature of the solvent environments in these two cases. The results bear out the expectation that there may be differences between residue type preferences in membrane proteins and in water soluble globular proteins. For example, the -branched residue types valine and isoleucine are considerably more frequently encountered in membrane helices. Likewise, glycine and proline, residue types normally associated with `helix-breaking' propensity are found to be relatively more common in membrane helices. Three dimensional nearest neighbour contacts along the helix, preferred rotamers, and peptide bond angles are very similar in the three sets of proteins as far as can be ascertained within the limits of the relatively low resolution of the membrane proteins dataset. Crossing angles for helices in the membrane protein set resemble the four helix bundle set more than the general non-redundant set, but in contrast to both sets they have smaller crossing angles consistent with the dual requirements for the helices to form a compact structure while having to span the membrane. In addition to the pairwise packing of helices we investigate their global packing and consider the question of helix supercoiling in helix bundle proteins.