The quasi-symmetric OHN and ODN bridges in the adducts of 3,5-dimethylpyridine with 3,5-dinitrobenzoic acid

The crystal structure of the adduct of 3,5-dimethylpyridine and 3,5-dinitrobenzoic acid (DMP-DNB) has been determined at room temperature and 80 K for both undeuterated and deuterated compounds. The monoclinic crystals are isomorphous, space group P2₁/c and Z = 4. Very strong OHN hydrogen bonds are almost linear with fully disordered (1:1) bridge hydrogen atoms between oxygen and nitrogen atoms. This is well reflected in the difference in electron density maps the contours of which depend both on cooling and deuteration. The intramolecular hydrogen bond lengths are 2.550(2) Å for the (OHN) and 2.563(2) Å for (ODN) at room temperature and 2.529(2) Å for (OHN) and 2.531(2) Å for (ODN) at 80 K. Therefore, there is a small but meaningful isotope effect upon the O…N hydrogen bridge length at room temperature and no Ubbelohde isotope effect is observed at 80 K. The infra-red spectra show very broad stretching protonic bands in the 200–1600 cm⁻¹ range. The isotopic ratio v(H)/v(D) at room temperature is about 1.1.     

One—dimensional Anion—chain Containing [Mo2O6（pic）2]^2—Unit Connected by Unclassical Hydrogen Bond

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EFFECT OF THE COMPOSITION OF m-CRESOL/H2O SOLVENT ON HYDRODYNAMIC PROPERTIES OF NYLON 6

The intrinsic viscosity [ η], Huggins constant (KH), [ η]0, α3 and flow activation energy values of nylon 6 have been measured in water/m-cresol (0/100-20/80) systems at different temperatures (20-60℃). It has been found that the intrinsic viscosity, [η]0 and α3 increase with the increase in water contents in m-cresol up to 15% and then decrease. They increase with the increase in temperature irrespective of solvent composition. It has been noted that the percent increase of α3 is the highest at 60℃ and the lowest at 20℃ for a particular solvent system. The intrinsic viscosity data obey Arrhenius equation over the considered conditions. The activation energy and the KH values decrease very sharply with the addition of water,giving a minimum value at 15% of water and then increase slowly. The variation of all the parameters has been explained in terms of variation in thermodynamic quality of solvent with the addition of water to m-cresol and change in temperature,resulting in the change of conformational and orientational properties of polymer molecules. This change of solvent quality also results in variation of selective sorption of solvent over the polymer, such as hydrogen bonding, etc.     

Spatial partitioning of the molecular wave function: Reexamination of the bond‐charge model of covalent binding

Owing to the complexity of many‐body quantum interactions, theoretical problems in chemistry almost always necessitate numerical methods of solution. In turn, and somewhat paradoxically, this reliance on computation often precipitates a need for approximate models that can provide a conceptual framework for the interpretation of the accurate numerical solutions. In this report, we reexamine and improve upon the simple bond‐charge (SBC) model of Parr and Borkman with the express purpose of examining its role as a suitable intuitive model for bonding energetics. We first discuss the role of spatial partitioning in the SBC model and the molecular‐structure approximations that are implicit within the model. We then develop an improved version of the model by including electrostatic repulsion between shared electron pairs; we refer to this enhanced version as a semiempirical charge‐cloud (SCC) model. Applying the SCC model to a broad range of diatomics, we find that it is able to predict dissociation energies on a par with simple variational calculations. As an illustrative application of the SCC model, we use it to estimate unknown dissociation energies and rotational constants for a number of alkali dimers. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Infrared spectra of cyclic and non-cyclic ureas in solution: structures and interactions

Infrared spectra of 1,3-dimethyl-2-imidazolidinone, 2-imidazolidinone, 1,1,3,3-tetramethylurea, 1,3-dimethylurea, and 1,1-dimethylurea have been collected in a variety of solvents. When the C=O stretching frequencies for the molecules in a common solvent were recorded, higher frequencies were found for the cyclic ureas as compared to the non-cyclic ureas. When data were analyzed within each class of urea derivatives, it was found that an increase in the number of methyl groups present on the nitrogen atoms contributed to a decrease in the C=O stretching frequency values. The frequencies of the C=O stretching modes have been correlated with the electron accepting ability of each solvent (represented by Gutmann electron acceptor numbers). The urea derivatives in the studied series exhibit similar behavior with respect to changing solvent environment despite the differences that exist among the ureas. These observations have been interpreted in terms of the structural characteristics of the ureas.     

Electronic structure,geometry, and energetics of carbanion and carbocation of [1.1]ferrocenophane: An INDO study

INDO-SCF calculations with constrained geometry optimization have been performed to determine the bridge geometries in [1.1]ferrocenophane and its carbocation and carbanion to address the question of possible C-H-C hydrogen bonding in the carbanion derivative. In the equilibrium geometry of the carbanion, the endo-hydrogen is bonded to one of the bridge carbon atoms and the calculated distance between the two bridge carbons seems too large to accommodate a stable C-H-C hydrogen bond. The results indicate that the observed proton NMR spectrum of carbanion should be interpreted in terms of rapid proton exchange between two bridge carbon atoms rather than a symmetric hydrogen bond. The ground state charge distributions show that the ionic bridges in both carbanion and carbocation are highly conjugated and most of the ionic charge in both molecules is distributed over the ferrocene ring system. The charge on the iron varies only slightly among the three molecules and the formal oxidation state of iron remains +2. The role of the iron seems to be that of a conduit for charge transfer between ferrocene rings upon conjugation.     

新颖无限三股螺旋钴配位聚合物{[Co(4,4′-bipyridine)(4,4′-azobispyridine)2(NCS)2]·H2O}n的合成、结构和性质的研究

The novel complex {[Co(bipy)(azpy)₂(NCS)₂]·H₂O}_n (where bipy=4,4′-bipyridine, azpy=4,4′-azobisp- yridine) has been synthesized and characterized by elemental analyses, IR, UV, thermal analyses, and vari-able-temperature magnetic susceptibility. The crystal (C₃₂H₂₆CoN₁₂OS₂, M_r=717.70) belongs to the orthorhombic, space group Pnna, a=2.21312(16)nm, b=1.40403(10)nm, c=1.14237(8)nm, V=3.5497(4)nm³, Z=4, D_c=1.343g·cm^-3, μ=0.645mm^-1, F(000)=1476, and final R₁=0.0691, wR₂=0.1129 for 231 parameters and 1674 observed reflections [I >2.00σ(I)]. The Co(Ⅱ) atom is, in a distorted octahedral geometry, coordi-nated by six nitrogen atoms from two bridging bipy, two monodentate azpy, and two thiocyanate groups. The bridging ligand bipy links Co(Ⅱ) atoms to form the infinite “rod＂ with terminal coordination azpy ligand acting as sidearms. Unprecedented three parallel interpenetrating two-dimensional (4,4) networks and novel infinite triple helices are formed via hydrogen-bonding interactions. CCDC: 155588.     

_f-A_aD_d氢键体系的统计特征

基于氢键的形成和缩聚反应机理在统计意义下的相似性,利用高分子反应统计理论和反应动力学理论对氢键溶液的一个模型体系进行了相关讨论.给出了体系的溶胶分数和发生溶胶-凝胶相变的条件,指出质子受体基团间的竞争作用对溶胶凝胶相变点的影响,进而讨论了体系的数量分布函数和相关问题.     

Hydrogen bonds in solid hydroxides, a bond valence approach

The bond valences s_OH due to the O-H bonds of OH⁻ ions in solids have been calculated indirectly from intermolecular H?O distances, viz. those within the Wigner Seitz cell around the respective hydrogen atom, by using the equation s_OH=1−∑s_H?O. The bond valences thus derived are an excellent measure of the strength of O-H bonds [J. Mol. Struct. 351 (1995) 205]. This is shown by their almost linear correlation with the wave numbers of the stretching modes of matrix isolated OD⁻ ions observed with IR or Raman experiments. In the case of very weak or lacking hydrogen bonds, this correlation fails because then other interionic bonding phenomena than hydrogen bonds as metal-oxygen interactions and hydrogen-hydrogen repulsion etc. gain in importance or dominate finally and, hence, partly or fully determine the wave numbers of the OD stretching modes, which, however, still remain a measure of the respective bond strengths. The relation of the distances r_OH, the bond valences s_OH, and the stretching modes ν_OD of both free, gaseous OH⁻ ions and H₂O molecules and those embedded in crystalline matrices is discussed.     

Intramolecular hydrogen bonding allows simple enaminones to structurally mimic the i, i + 4, and i + 7 residues of an α-helix

An intramolecularly hydrogen bonded enaminone scaffold was designed and synthesized in order to mimic the i, i + 4, and i + 7 residues of an α-helix. The resonance stabilized vinylogous amide group serves as an aromatic ring isostere and allows the positioning and angular projection of the R-groups in a manner similar to an α-helix.