Improving description of hydrogen bonds at the semiempirical level: water–water interactions as test case

Hydrogen bonding is not well described by available semiempirical theories. This is an important restriction because hydrogen bonds represent a key feature in many chemical and biochemical processes, besides being responsible for the singular properties of water. In this study, we describe a possible solution to this problem. The basic idea is to replace the nonphysical gaussian correction functions (GCF) appearing in the core–core repulsion terms of most MNDO‐based semiempirical methods by a simple function exhibiting the correct physical behavior in the whole range of intermolecular separation distances. The parameterized interaction function (PIF) is the sum of atom‐pair contributions, each one having five adjustable parameters. In this work, the approach is used to study water–water interactions. The parameters are optimized to reproduce a reference ab initio intermolecular energy surface for the water–water dimer obtained at the MP2/aug‐cc‐pVQZ level. OO, OH, and HH parameters are reported for the PM3 method. The results of PM3‐PIF calculations remarkably improve qualitatively and quantitatively those obtained at the standard PM3 level, both for water–dimer properties and for water clusters up to the hexamer. For example, the root‐mean‐square deviation of the PM3‐PIF interaction energies, with respect to ab initio values obtained using 700 points of the water dimer surface, is only 0.47 kcal/mol. This value is much smaller than that obtained using the standard PM3 method (4.2 kcal/mol). The PM3‐PIF water dimer energy minimum (−5.0 kcal/mol) is also much closer to ab initio data (−5.0±0.01 kcal/mol) than PM3 (−3.50 kcal/mol). The method is therefore promising for the development of new semiempirical approaches as well as for application of combined quantum mechanics and molecular mechanics techniques to investigate chemical processes in water. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 572–581, 2000     

Thermorheological analysis of PVC blends

The effect of temperature on dynamic viscoelastic measurements of miscible poly (vinyl chloride) (PVC)/ethylene‐vinyl acetate–carbon monoxide terpolymer (EVA‐CO) and immiscible PVC/high‐density polyethylene (HDPE) and PVC/chlorinated polyethylene (CPE) molten blends is discussed. PVC plasticized with di(2 ethyl hexyl) phthalate (PVC/DOP) and CaCO₃ filled HDPE (HDPE/CaCO₃) are also considered for comparison purposes. Thermorheological complexity is analyzed using two time–temperature superposition methods: double logarithmic plots of storage modulus, G′, vs. loss modulus, G″, and loss tangent, tan δ, vs. complex modulus, G*, plots. Both methods reveal that miscible PVC/EVA‐CO and PVC/DOP systems are thermorheologically complex, which is explained by the capacity of PVC to form microdomains or crystallites during mixing and following cooling of the blends. For immiscible PVC/HDPE and PVC/CPE blends the results of log G′ vs. log G″ show temperature independence. However, when tan δ vs. log G* plots are used, the immiscible blends are shown to be thermorheologically complex, indicating that the morphology observed by microscopy and constitued by a PVC phase dispersed in a HDPE or CPE matrix, is reflected by this rheological technique. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 469–477, 2000     

Chiral C2-Symmetric CuII Complexes as Catalysts for Enantioselective Hetero-Diels–Alder Reactions

Air-stable and recyclable , the Cu^II–(bis)oxazoline complex 1 efficiently catalyzes diastereo- and enantioselective hetero-Diels–Alder reactions between β,γ-unsaturated α-keto acid derivatives 2 and vinyl ethers for the synthesis of substituted dihydropyrans 3 [Eq. (1)]. Results of the crystal structure analysis of 1 in combination with calculations on model compounds indicate the formation of a reactive intermediate through replacement of the two H₂O ligands with a chelating substrate. X=OEt, N(OMe)Me; R=alkyl, aryl, alkoxy, thiobenzyl.     

聚苯乙烯型阴离子交换剂的合成方法

对聚苯乙烯型阴离子交换剂的各种合成方法作了较为全面的评述。指出使用氯甲醚为原料的氯甲基化法由于毒性问题已经受到限制，分析了其它方法在实现工业化生产并最终取代氯甲基化法方面的可能性。     

细胞凋亡分析测试方法的研究进展*

本文对细胞凋亡的测试方法进行了较全面的综述, 介绍了细胞凋亡的形态学特征和生化特征, 重点评述了细胞凋亡的各种分析测试方法, 包括形态学观察法、琼脂糖凝胶电泳法、流式细胞计数法、原位DNA 标记检测法、anti-ssDNA MAbs 分析和PCR 技术。对细胞凋亡测试方法的发展趋势作了讨论。     

Comparison of localization and delocalization indices obtained with Hartree-Fock and conventional correlated methods: effect of Coulomb correlation

Atomic populations and localization [lambda(A)] and delocalization [delta(A,B)] indices (LIs and DIs) are calculated for a large set of molecules at the Hartree-Fock (HF), MP2, MP4(SDQ), CISD, and QCISD levels with the 6-311++G(2d,2p) basis set. The HF method and the conventional correlation methods [MP2, MP4(SDQ), CISD, and QCISD] yield distinct sets of LIs and DIs. Yet, within the four conventional correlation methods the differences in atomic populations and LIs and DIs are small. Relative to HF, the conventional correlation methods [MP2, MP4(SDQ), CISD, QCISD] yield virtually the same LIs and DIs for molecules with large charge separations while LIs and DIs that differ significantly from the HF values--the LIs are increased and DIs decreased--are obtained for bonds with no or small charge separations. Such is the case in the archetypal homopolar molecules HC(triple bond)CH, H2C=CH2, CH3-CH3, and "protonated cyclopropane" C(3)H(7) (+), in which case the bonding may be atypical. Relative to HF, the typical effect of the conventional correlation methods is to decrease the DI between atoms.     

Low-temperature IR and NMR studies of the interaction of group 8 metal dihydrides with alcohols

The reactions of the octahedral dihydrido complexes [MH(2)(PP(3))] [M=Fe, Ru, Os; PP(3)=P(CH(2)CH(2)PPh(2))(3)] with a variety of weak ROH acids have been studied by IR and NMR methods in either CH(2)Cl(2) or THF in the temperature range from 190 to 290 K. This study has allowed the determination of the spectral and thermodynamic properties associated with the formation of dihydrogen bonds (DHB) between the terminal hydrides and the OH group. Both the DHB enthalpy values and the hydride basicity factors (E(j)) have been found to increase in the order Fe < Ru < Os. The proton transfer process, leading to the DHB complexes, and eventually to eta(2)-H(2) products, has been found to depend on the acidic strength of the alcohol as well as the nature of the solvent. Low temperature IR and NMR techniques have been used to trace the complete energy profile of the proton transfer process involving the osmium complex [OsH(2)(PP(3))] with trifluoroethanol.     

后处理对TiO_2纳米薄膜的表面结构和光催化活性的影响

半导体TiO2光催化材料对环境中各种污染物的明显去除效果已引起人们的广泛关注犤1～6犦。在普通钠钙玻璃表面涂制高光催化活性的TiO2纳米薄膜,制成环保建筑材料,不仅可以自洁玻璃表面,而且可以用于净化空气、处理废水等。但由于在热处理过程中玻璃基体内的Na+离子扩散到薄膜表面,形成光生电子和空穴的复合中心,使TiO2纳米薄膜的光催化活性大大降低犤7～10犦。为了减少玻璃中Na+离子对TiO2薄膜光催化活性的有害影响,Paz等犤8犦研究了将钠钙玻璃浸入酸溶液中,使玻璃表面的钠离子被酸中的氢离子置换出来;在玻璃表面形成SiO2凝胶层,以便阻…