Molecular size and conformational effects on oligophenylene's electronic and vibrational properties

Electronic and vibrational properties of phenylene‐based oligomers from biphenyl P2P to para‐sexiphenil P6P, and their dependence on torsion angle are calculated using semiempirical quantum mechanic (AM1, ZINDO/S) and ab initio linear combination of atomic orbitals methods. The systematic relations between molecular size and geometry, and numerous molecular properties have been established, providing the basis both for the spectroscopic identification of different structures that could appear during material processing and for tailoring of devices with desired properties. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1783–1794, 2006     

Condensed-phase effects on the conformational equilibrium of ethylene glycol

Density functional calculations for ethylene glycol (CH₂OHCH₂OH) in the gas and in a dielectric medium are reported. The condensed-phase calculations are based on the self-consistent reaction field approach and the environment has the dielectric constant of liquid methanol. NPT Monte Carlo simulations of ethylene glycol (ETG) in liquid methanol are also reported. The simulations were carried out for three conformers of ETG (tGg′, gGg′, and tTt). Comparison between SCRF results for the conformational equilibrium in the gas and in the dielectric suggests that the tGg′ conformer is slightly stabilized relative to the gGg′ conformer in the solvent. However, the energy difference between them is less the 1.0 kJ/mol, which indicates that frequent interconversions between the tGg′ and gGg′ conformers are expected in the condensed phase. The all-trans conformer (tTt) is higher than the most stable conformer in the gas by 14 kJ/mol. Monte Carlo simulations predict that the tGg′ and gGg′ conformers have very similar energies in the solvent. However, the simulations also show, in agreement with experimental data, that the tTt conformer is stabilized in liquid methanol, relative to the gas phase. The microscopic mechanism leading to the stabilization of the tTt conformer in the liquid is related to the differential hydrogen-bonding formation between the ETG conformers and the methanol molecules. © 1996 John Wiley & Sons, Inc.     

Large concentration effects on conformational equilibria

The conformational equilibrium in $\text{cis}$-3-Hydroxythiane S-oxide (Scheme 1) is strongly concentration dependent, being affected by intermolecular hydrogen bonding at high concentration.     

Entropy effects during sorption of alkanes in zeolites

Recent developments in Configurational-Bias Monte Carlo (CBMC) techniques allow the accurate calculation of the sorption isotherms for alkanes, and their mixtures, in various zeolites. The CBMC simulations give new insights into subtle entropy effects affecting mixture adsorption. Three types of entropy effects can be distinguished. (1) Size entropy effects favour the component with the smaller number of C atoms because the smaller molecule finds it easier to fill in the 'gaps' within the zeolite matrix at high molecular loadings. (2) Configurational entropy effects come into play for mixtures of alkanes that differ in the degree of branching. For a mixture of linear and branched alkanes with the same number of C atoms, configurational entropy effects favour the linear isomer because such molecules 'pack' more efficiently within, say, the intersecting channel topology of MFI zeolite. (3) Length entropy effects comes into force for sorption of linear and branched alkanes within the cylindrical channels of say AFI and MOR zeolites; here the double branched alkane has the shortest length and can be packed more efficiently within the channels. We demonstrate that CBMC simulations allow the efficient screening of zeolite structures for a given separation duty and aid the development of novel separation processes exploiting entropy effects.     

Halogenation of phenacylpyrimidines

The action of bromine and chlorine of phenacylpyrimidines yields a series of -mono- and ,-dihalophenacylpyrimidines. IR, UV and PMR spectroscopy showed that, in contrast to the starting compounds, the monohalo derivatives exist in the keto form.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 663–667, May, 1987.     

Quantum-mechanical studies on the conformational and electronic properties of steroids

The conformation of the sterane (perhydrocyclopentanophenanthrene) ring system, the fundamental ring system of steroids, and the conformation of its constituent subunits have been investigated quantum mechanically by the PCILO method, taking into account all the valence electrons of the system. The results indicate that the conformations which correspond to the B/C and C/D trans ring fusion, with the A/B fusion trans or cis, are by far the more stable ones. The result explains one of the fundamental structural properties of all natural steroids.

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Zusammenfassung Die Konformation des Sterans (Perhydrocyclopentanophenanthren), des grundlegenden Ring-Systems der Steroide, und die Konformation der es aufbauenden Untereinheiten wurde quanten-mechanisch mit der PCILO-Methode, bei der alle Bindungselektronen berücksichtigt wurden, untersucht. Die Ergebnisse zeigen, da\ die Konformationen bei weitem die stabilsten sind, bei denen die Ringe B/C und C/D trans stÄndig sind, wÄhrend die Ringe A/B trans oder cis stehen. Dieses Ergebnis erklÄrt eine der fundamentalen Struktur-Eigenschaften aller natürlichen Steroide.

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Résumé La conformation du sterane (perhydrocyclopentanophenanthrene), qui représente le squelette fondamental des steroides, ainsi que la conformation des sous-unités qui le constituent, ont été étudiés par la méthode PCILO, une approximation perfectionnée de la méthode des orbitales moléculaires, tenant compte de tous les électrons de valence. Le résultant principal indique que les deux conformations de beaucoup les plus stables sont celles dans lesquelles la jonction des noyaux B et C ainsi que celle des noyaux C et D est trans, alors que la jonction des noyaux A et B peut Être trans ou cis, la première représentant un arrangement légèrement préféré. Ce résultat théorique explique l'une des propriétés structurales fondamentales de tous les composés stéroides naturels.

     

Adjacent gauche stabilization in linear alkanes: implications for polymer models and conformational analysis

High-level ab initio quantum mechanical calculations are used to study various gauche conformational energies of n-pentane to n-decane. The destabilizing "pentane effect" (adjacent gauche states of opposite sign) for alkanes is confirmed, but the energies were found to depend slightly on chain length. In contrast, introducing an adjacent gauche of the same sign requires only 0.22-0.37 kcal/mol, approximately half of the single gauche state energy. This adjacent gauche stabilization should be taken into account when formulating or analyzing rotational isomeric models, carrying out conformational analysis, and developing force fields for alkanes, lipids, and related polymers.     

Influence of intramolecular hydrogen-bonding on the conformational properties of sugar thioureas

Reaction of deoxyisothiocyanato derivatives of 1,2:3,4-di-O-isopropylidene--D-galactopyranose, 1,2:3,5-di-O-isopropylidene--D-glucofuranose, and 2,3:4,5-di-O-isopropylidene-β-D-fructopyranose with ammonia afforded the conesponding sugar thioureas. Both the Z and E stereoisomers around the NH---C(=S) bond were observed in the ¹H and ¹³C NMR spectra of the later in the low temperature range, their relative proportions being a function of the sugar configuration. Experimental evidence for the existence of seven-membered NHO intramolecular hydrogen bonds in the E isomers of thioureas has been obtained from DNMR experiments, rotational barrier height calculations, measurements of temperature coefficients for the ¹H chemical shifts of the NH signals, and study of the influence of the solvent polarity in the rotameric ratio.     

饱和链烃图特征和热力学性质

本文讨论饱和链烃分子异构化和热力学性质之间的关系.首先利用图论方法引进描写分子支化度的α_N指数,该指数能分辨仅有微小结构差异的饱和链烃分子,给出支化结构的合理描述.在此基础上,得到了利用分子图特征参量计算饱和链烃热力学量(如:沸点、密度、生成焓、原子生成热、折射率和色谱保留指数等)的统一表达式.     

用路径数预测链烷的物理化学性质

路径数P~1, P~2, P~3等是表征链烷分子大小、支化度和形状等结构特征的重要参量。据此, 本文将P~1, P~2, P~3等作为独立参量, 用多元线性模型系统地研究了链烷结构-性质的定量关系, 预测了含2-9个碳原子链烷的原子化焓、标准生成焓、汽化焓、摩尔体积和摩尔折射度等多种物理化学性质。计算方法简单, 结构意义明确, 结果令人满意。     

Charge distributions and chemical effects in simple alkanes

A simple parametric procedure for calculating the electronic structure of hydrocarbons has been tested using a set of parametric relations suggested by Fliszár. The calculated inductive charges are satisfactory and are compared with values derived from a least-squares analysis of theoretical charges. Further improvement is possible by applying the least-squares analysis first and then the parametric equations. The procedure is also used to calculate the energy of atomization of the hypothetical vibrationless state, giving results which agreee well with experimental values.     

Influence of conformational rigidity on membrane properties of polyimides

Several polyimides were studied with regard to the influence of their conformational rigidity on the packing in glassy state, and consequently on their physical properties such as glass transition temperature and selectivity of gas separation membranes made from these polymers. The values of their physical properties were taken from literature, while the conformational rigidity parameters such as Kuhn segment, characteristic ratio, and occupied, free, and accessible volumes were calculated here and were correlated with physical properties. It was shown that there are correlations between selectivity of gas separation membranes made from these polyimides on one hand, and characteristic ratio, on another hand.     

蒎酮酸的卤代反应

国外对蒎酮酸的制备、反应及其衍生物的应用报道颇多。为了开发利用我国丰富的松节油资源,我们对顺式蒎酮酸(1)的卤代反应进行了研究。关于蒎酮酸的溴代反应已有研究,而氯代反应还未见报道。我们选用了氯气和氯化硫酰作为氯代反应试剂。在酸性条件下1与氯反应时,得到顺式-2,2-二甲基-3-氯乙酰基环