EXCIPLEX FORMATION INFLUENCED BY ISOMERS——EXCIPLEX FORMED FROM DIMETHYL TEREPHTHALATE AND PHTHALATE WITH 9-ETHYL CARBAZOLE

Both dimethyl terephthalate (DMTP) and dimethyl phthalate are able to form exciplex with 9-ethyl carbazole (EtCz) but the fluorescence quenching constant of DMTP to EtCz is larger than that of DMP to EtCz. The peak positions of the fluorescence bands of the exciplex formed from DMTP and DMP with EtCz are also different. The former gives a peak at shorter wavelength region. Either the exciplex formation rate constant k_3 or the exciplex dissociation rate constant k_4 is not alike. Both of these constants of DMTP are larger than those of DMP. All these results are difficult to explain with the established theory, so a new model which could be used to explain the results satisfactorily is supposed.     

Kinetic resolution of sec-alcohols by a new class of pyridine catalysts having a conformation switch system

The catalysts having a conformation switch system induced by acylation and deacylation serve as asymmetric acylating catalysts of sec-alcohols. The kinetic resolution of various sec-alcohols resulted in good to excellent selectivities in the presence of 0.5 to 0.05 mol % of catalyst 1a. The conformation switch system plays a key role to attain both good selectivity and high catalytic activity.     

高聚物玻璃化转变温度和分子参数的关系 Ⅲ·聚甲基丙烯酸甲酯立构规整性和T_g的关系

本文研究了聚甲基丙烯酸甲酯(PMAA)的立构规整性与T_8的关系,结果表明立构单元分布和含量均对T_8有显著影响。应用前文导出的理论关系,进一步处理了T_8和构象参数实验数据,用这种简单方法计算得的无扰尺寸温度系数和实验值一致。由此推论,不同立构PMMA链的T_8之差反映了它们局部择优链构象的不同。     

Isomer effects in the excess enthalpies of mixtures of alkanes and cycloalkanes

Using the Picker flow calorimeter, excess molar enthalpies H ^E have been obtained at 25°C for mixtures of 1,2-, 1,3- and 1,4-cis- and trans-dimethylcyclohexane and cis- and trans-decalin with n-hexadecane and the highly branched C₁₆ isomer, 2,2,4,4,6,8,8-heptamethylnonane. Values of H ^E are also obtained for cis- and trans-decalin mixed with C₆, C₇, and C₉ isomers. Anomalously low values of H ^E occur for normal alkanes mixed with cycloalkanes in the di-equatorial configuration, suggesting the presence of a negative contribution in H ^E possibly due to a restriction of n-alkane molecular motion by the flat, plate-like cycloalkane.     

Modeling of the conformational behavior of polyvinylchloride

The conformational behavior of polyvinylchloride was modeled by calculations of different conformers, rotational barriers, and rotation-energy profile diagrams of some model compounds, such as 1,3-dichloropropane, 1,3,5-trichloropentane, 1,3,5,7-tetrachloroheptane, 2,4-dichloropentane and 2,4,6-trichloropentane using the PCILO-and MM2-methods. These calculations allow some assertions about the conformational behavior of PVC. It was found that already a simple increment system derived from the model molecules is able to describe the conformational behavior of the larger molecules in question. There is a qualitative agreement of PCILO- and MM2-results in respect of the energetic order of the calculated conformers.     

Structural characterization and isomer differentiation of chalcones by electrospray ionization tandem mass spectrometry

A series of chalcones were characterized by electrospray ionization tandem mass spectrometry (MS(n)). Several ionization modes were evaluated, including protonation, deprotonation and metal complexation, with metal complexation being the most efficient. Collision-activated dissociation (CAD) was used to characterize the structures, and losses commonly observed include H(2), H(2)O, CO and CO(2), in addition to methyl radicals for the methoxy-containing chalcones. CAD of the metal complexes, especially [Co(II) (chalcone-H) 2,2'-bipyridine](+), allowed the most effective differentiation of the isomeric chalcones with several diagnostic fragment ions appearing upon activation of the metal complexes. MS(n) experiments were performed to support identification of some fragment ions and to verify the proposed fragmentation pathways. In several cases, MS(n) indicated that specific neutral losses occurred by stepwise pathways, such as the neutral loss of 44 u as CH3* and HCO*, or CH(4) and CO, in addition to CO(2).     

Synthesis and Stereochemistry of Some New 1,3-Dioxane Derivatives of 1,4-Diacetylbenzene

Summary.  The synthesis and stereochemistry of new 1,3-dioxane derivatives of 1,4-diacetylbenzene are reported. The anancomeric structure of these compounds, the axial orientation of the aryl group for both 1,3-dioxane rings, and the cis and trans isomerism of some of these compounds is discussed considering data of conformational analysis, NMR investigations, and single crystal X-ray diffractometry. Received September 12, 2001. Accepted (revised) November 5, 2001     

Reversed-phase high-performance liquid chromatographic behavior of peptide isomers including a D-amino acid

Summary The chromatographic behavior of ten peptide isomers was investigated using isocratic eluting with reversedphase high-performance liquid chromatography. These peptides are growth hormone releasing factor, consisting of 29 amino acid residues and its isomers where on amino acid residue in the sequence was replaced with the corresponding D-amino acid. Linear relationship was observed between the logarithm of the capacity factors of the isomers and the acetonitrile concentration in the mobile phase with correlation coefficients of 0.995–0.998. The regression coefficients were almost the same for all the isomers with a relative standard deviation of 5.3%. These results suggest that the chromatographic behavior of peptide isomers is also predictable from the solvophobic theory. The regression coefficient reflects the molecular weight and the Y-intercept represents the hydrophobicity of the isomer. The peptide isomers can be separated from one another, based on the differences in the hydrophobicity of each isomer.     

About some properties of new poly(phenylene vinylene) derivatives: PPV-ether and C1-4PPV-ether

Although, the presence of the substituted groups induce a difference in physical and chemical properties of the co-polymer, it does not affect its symmetry group. We deduce from IR and RRS analyses of C_1-4PPV-ether, compared to those of PPV-ether, that the geometrical structural modifications with a decrease in the planarity of the chain and the vibrational features are possibly ascribed to conformational effects. The molecular packing effect is also outlined. Moreover, the decrease of the optical gap and the higher energy shifted transition could be assigned to the different lattice relaxations and vibrational behaviors shown by this co-polymer. Added to IR, RRS and optical density results, the PL measurements prove a great delocalization of π-electron system with a red shifted emission.     

Synthesis and conformational studies of amide-linked cyclic homooligomers of a thymidine-based nucleoside amino acid

Cyclic homooligomers of a thymidine-based nucleoside amino acid were synthesized from the linear dimer using BOP reagent in the presence of DIPEA under dilute conditions. Conformational analysis by NMR and constrained MD studies revealed that all the cyclic products had symmetrical structures. The NH and CO groups in these molecules point in opposite directions with near perpendicular orientation with respect to the plane of the macrocyclic ring having CO on the same side as the base.