Synthesis and liquid crystalline properties of novel compounds containing a 3-fluoro-4-cyanophenoxy group

A series of novel compounds containing a 3-fluoro-4-cyanophenoxy group were synthesized and fully characterized by IR and ¹H NMR, and their mesomorphic properties were studied. Seven compounds exhibited enantiotropic nematic phases and three compounds exhibited monotropic nematic phases, as confirmed by differential scanning calorimetry and polarizing optical microscopy. Selected properties of the liquid crystalline compounds synthesized were calculated by ab initio methods at a HF/6-31G level. The bond lengths, bond angles and dihedral angles of the fragments with the same structure change little between the compounds. All the compounds with a terminal alkoxy chain approached a planar structure.     

Synthesis and liquid crystalline properties of novel compounds containing a 3‐fluoro‐4‐cyanophenoxy group

A series of novel compounds containing a 3‐fluoro‐4‐cyanophenoxy group were synthesized and fully characterized by IR and ¹H NMR, and their mesomorphic properties were studied. Seven compounds exhibited enantiotropic nematic phases and three compounds exhibited monotropic nematic phases, as confirmed by differential scanning calorimetry and polarizing optical microscopy. Selected properties of the liquid crystalline compounds synthesized were calculated by ab initio methods at a HF/6‐31G level. The bond lengths, bond angles and dihedral angles of the fragments with the same structure change little between the compounds. All the compounds with a terminal alkoxy chain approached a planar structure.     

抗癌药物冬凌草甲素的分子构型研究

本文用同核化学位移相关谱和二维J分解谱归属了冬凌草甲素的^1H谱, 测定了所有的质子标量偶合常数, 根据一系列的Karplus公式计算了二面角, 用自编WUPH计算程序, 计算出冬凌草甲素的分子构型。     

Electric dipole moment studies of carboxylic ester conformations: alkyl acetates,benzoates, 2,4,6-trimethyl-benzoates and 2,3,5,6-tetramethylbenzoates

Electric dipole moments μ in benzene at 30 °C have been determined (Table 3) on methyl, ethyl, isopropyl and t-butyl esters of the title compounds to determine steric effects on conformation in solution. Experimental moments were compared with those calculated for various possible conformations by a 3-dimensional vectorial addition method using bond moments and bond angles. The experimental moments for the alkyl acetates were best interpreted in terms of an out-of-plane deviation of the alkyl group from an s-trans conformation caused by steric interference between the alkyl group and the carbonyl oxygen and increasing in the series from methyl to t-butyl. The dihedral angles 0 (deviations) were calculated using a vector addition method. An increase in the moments of the benzoate series over the acetates was interpreted in terms of conjugative interaction between phenyl and carbonyl groups. Angles of twist φ for the benzoates and trimethylbenzoates were calculated using the Braude-Sondheimer equation. A decrease in the moments of the methyl, ethyl, and isopropyl trimethyl-benzoates as compared with the benzoates was interpreted in terms of steric interference between ortho methyls and both oxygens. The decrease in the angles of twist from methyl to t-butyl for the trimethylbenzoates was tentatively explained by greater steric interaction of the alkyl group with both carbonyl oxygen and ortho methyls, which forces adoption of a more coplanar arrangement between the ring and the carbonyl group than for the other alkyl derivatives, this interaction increasing with the size of the alkyl group. Dipole moments for 2,3,5,6-tetramethylbenzoates were nearly the same as for corresponding trimethyl-benzoates, thus showing no conclusive evidence for operation of a “buttressing” effect.     

取代基效应定量研究新进展

综述了最近20多年有机化学取代基效应定量研究所取得的诸多新进展.主要内容包括：（1）电负性均衡原理得到普遍认同并用于计算分子中电荷分布和基团的诱导效应,分子电负性计算方法在几何平均法、调和平均法、加权平均法的基础上又提出了价电子均衡方法,进一步扩展了电负性均衡原理的应用范围.（2）试剂亲电、亲核能力的实验测定,提出了以二苯甲基正离子和醌甲基化合物等作为参考化合物的标定方法,其研究范围被扩展到气相条件、有机金属反应和自由基体系,得到一系列试剂的亲核参数N和亲电参数E.理论上提出亲电性指数和亲核性指数的概念及其定量计算公式,并对实验测定参数和理论计算指数之间的关系进行了深入研究.（3）极化效应参数从最初的经验方法,进而采用量子化学方法计算,再用统计方法提出了烷基极化效应指数PEI和基团极化效应参数PEIX,被广泛用于解释和估算有机化合物的气相酸碱性、电离能、生成焓、键能、反应速率、水溶解性和色谱保留值.（4）直接由取代苯的紫外光谱吸收能量计算得到的取代基激发态参数exCC,不同于取代基在分子处于基态的极性参数和自由基状态下的自旋离域效应参数,该参数用于对位二取代苯、取代二苯乙烯、二取代氮苄叉苯胺等多类有机化合物紫外光谱的定量相关,结果良好.（5）立体屏蔽效应的提出区分了传统的三种立体效应.在羰基与亲核试剂加成反应立体选择性定量表示,提出了立体选择性指数Ci;在表示基团对反应中心表面积屏蔽的定量提出了屏蔽参数SR,进一步基于反应中心体积被屏蔽的角度提出了拓扑立体效应指数TSEI,这些参数在分子内二面角、反应的立体选择性、烯烃和烷基苯的生成焓、咪唑离子液体的酸性以及烷烃与羟基自由基的反应速率等方面得到良好的应用.此外,论文还对有机化学取代基效应定量研究中值得进一步深入探索的领域和问题提出了建议和展望.     

17O NMR spectra of alpha-diesters

17O NMR spectra of title compounds were measured at natural abundance in acetonitrile solutions. Intercarbonyl dihedral angles have been estimated by molecular mechanics, which show invariance except in one case. Because of this invariance, contrary to other alpha-dicarbonyl compounds, a correlation between chemical shifts and dihedral intercarbonyl angles could not be developed. Spectroscopic and computational results allowed us to evaluate other conformational features.     

Conformational Analysis of 1,2:3,4-Diepoxides: Ab Initio and semiempirical molecular-orbital calculations

Using semiempirical and ab initio procedures, the most stable conformations of meso- and rac-bioxirane and of some substituted 1,2:3,4-diepoxides were calculated. For threo-diepoxides (having the same relative configurations as rac-bioxirane, 3 ), two stable conformations with CCCC dihedral angles of ca. 90 and ca. 270° were found. For erythro-diepoxides (derivatives of meso-bioxirane, 4 ) the calculations suggest three preferred conformations with corresponding dihedral CCCC angles of ca. 90°, ca. 180°, and ca. 270°. The calculations are in fair agreement with the experimental data available for the unsubstituted compounds 3 and 4 .     

Half-chair conformations of unsaturated heterocyclic compounds

The vicinal proton coupling constants were obtained for some 6-membered ring unsaturated heterocyclic compounds. The R values and ring dihedral angles were determined and found to be consistent with half-chair conformations. Relative to saturated heterocyclic compounds, the effect of the sulfur atom on ring puckering was attenuated. However, an increase in ring puckering resulted from the sulfone group and was attributed to torsional energy.     

Etude de la conformation des n-aryl azoles par la methode e.h.t. (extended hückel theory)

E.H.T. calculations of total energy as a function of dihedral angle are presented for 8 N-aryl azoles, six with dihedral angle α and two with dihedral angles α and β. The rotational barriers to the planar and to the orthogonal geometries are discussed. The dihedral angles α or β corresponding to the minimum of the calculated potential curve or surface are important; a uniform correction of these values is necessary to find the experimental values.     

A new bisabolene from Stevia tomentosa

The new sesquiterpene (1R,2R,3R,6R,7S)-1-acetoxy-2,3-dihydroxy-2,3-dihydrobisabolene (3) together with ten known terpenes and three known flavonoids were isolated from the aerial parts and from the roots of Stevia tomentosa. The structure of 3 follows from spectral studies, the relative chirality at C-3 follows from 1H NMR coupling constants comparison with the corresponding calculated values obtained by applying a generalized Karplus-type relationship to the dihedral angles of model compounds, and the absolute configuration is assumed in analogy to known (2R,3R,6R,7S)-2,3-epoxy-2,3-dihydrobisabolen-1-one (2).     

J-coupling constants for a trialanine peptide as a function of dihedral angles calculated by density functional theory over the full Ramachandran space

We present 13 (3)J, seven (2)J and four (1)J coupling constants (24 in all) calculated using B3LYP/D95** as a function of the φ and ψ Ramachandran dihedral angles of the acetyl(Ala)(3)NH(2) capped trialanine peptide over the entire Ramachandran space. With the exception of three of these J couplings, all show significant dependence upon both dihedral angles. For each J coupling considered, a two dimensional grid with respect to φ and ψ angles can be used to interpolate the values for any pair of φ and ψ values. Such simple interpolation is shown to be very accurate. Most of these calculated J couplings should prove useful for improving the accuracy of the determination of peptide and protein structures from NMR measurements in solution over that provided by the common procedure of treating the J couplings as functions of a single dihedral angle by means of Karplus-type fittings.     

Stereochemistry of cyclopentane derivatives from (2,3)J(CH) dependence on dihedral angle (theta H--C--C--X)

The (2,3)J(CH) dependence on dihedral angle (theta H--C--C--X) for cyclopentane derivatives was investigated. We observed that the combined use of experimentally obtained (2,3)J(CH) values and the theoretically determined dihedral angles between the corresponding nuclei can be used to infer the relative stereochemistry of the ring substituents in cyclopentane derivatives. There is a good correlation between the magnitude of (3)J(CH) and the dihedral angle between the hydrogen and the coupled carbon (R2 = 0.88).     

Recent progress in quantifying substituent effects

This paper summarizes significant progress in quantifying organic substituent effects in the last 20 years. The main content is as follows: (1) The principle of electronegativity equalization has gained wide acceptance, and has been used to calculate the intramolecular charge distribution and inductive effect of groups. A valence electrons equalization method was proposed to compute the molecular electronegativity on the basis of geometric mean method, harmonic mean method, and weighted mean method. This new calculation method further extended the application of the principle of electronegativity equalization. (2) A scale method was established for experimentally determining the electrophilic and nucleophilic ability of reagents, in which benzhydryliumions and quinone methides were taken as the reference compounds, and the research field was extended to the gas phase conditions, organometallic reaction and radicals system. Moreover, the nucleophilicity parameters N and electrophilicity parameters E for a series of reagents were obtained. The definition and quantitative expression of electrophilicity index and nucleophilicity index were proposed theoretically, and the correlation between the parameters from experimental determination and the indexes from theoretical calculation was also investigated. (3) The polarizability effect parameter was initially calculated by empirical method and further developed by quantum chemistry method. Recently, the polarizability effect index of alkyl (PEI) and groups (PEI X ) were proposed by statistical method, and got wide applications in explaining and estimating gas-phase acidity and basicity, ionization energy, enthalpy of formation, bond energy, reaction rate, water solubility and chromatographic retention for organic compounds. (4) The excited-state substituent constant ex CC obtained directly from the UV absorption energy data of substituted benzenes, is different from the polar constants in molecular ground state and the radical spin-delocalization effects constants in molecular radical state. The proposed constant ex CC correlated well with the UV absorption energy of many kinds of organic compounds, such as 1,4-disubstituted benzenes, substituted stilbenes, and disubstituted N-benzylidenebenzenamine. (5) The establishment of the steric shielding effect distinguished the three traditional steric effects. The stereoselectivity index C i was proposed to quantify the stereoselectivity of the addition reaction of carbonyl with nucleophilic reagent. The shielding parameter S R was defined to quantitatively express the specific surface of the reaction center screened by a group. Further, the Topological Steric Effect Index (TSEI) of a group was proposed on the basis of the relative specific volume of reaction center screened by the atoms of substituents. These parameters can be applied in estimating the intramolecular dihedral angles, stereoselectivity of reaction, enthalpies of formation of alkenes and alkylbenzene, acidity of substitutedimidazolium ionic liquid, and the reaction rate of alkane and hydroxyl radical. In addition, some suggestions and prospects for further studies on quantifying the organic substituent effects were presented in this paper.     

Nuclear magnetic resonance and the conformation of some derivatives of Michler's ketone

¹H NMR spectra are reported for Michler's ketone and twenty of its derivatives, along with ¹³C chemical shifts for five of the compounds. ¹H chemical shifts show only small changes with increasing substitution at the sterically hindered 2-position. Estimates of the dihedral angle between the benzene ring and C? CO? C planes have been obtained from the chemical shifts of C?O and 4-C for ketones 1, 3, 5, 15 and 16 . These angles are both internally consistent and very similar to dihedral angles determined by other methods for corresponding compounds in the benzophenone series.     

THE PHOSPHAZENES–STRUCTURAL PARAMETERS AND THEIR RELATIONSHIPS TO PHYSICAL AND CHEMICAL PROPERTIES

Abstract

A systematic study of crystallographic data has revealed a number of correlations between structural parameters, (e.g., bond angles, bond lengths, dihedral angles) and physical and chemical properties in phosphazenes and related compounds. These include (a) endocyclic bond angles NPN and PNP and their relationships to group electronegativities; (b) endocyclic bond angles NPN and PNP in geminal cyclotriphosphazatrienes, N₃P₃RR′X₄ (X = F, Cl) and their relationships to basicity; (c) deviations from some relationships due to ring puckering and the use of mathematical techniques to “flatten out” these rings; (d) ³⁵Cl nuclear quadrupole resonance frequencies and P–Cl bond lengths and other relationships; (e) exocyclic bond angles viz. OPO, NPN, ClPCl, CPC etc. versus ³¹P NMR chemical shifts; (f) ring compression and deformation and unusual mass spectrometric and bulk polymerisation properties; (g) relationships in phosphazenylcyclophosphazenes between crystal structure, ⁴ J(PP) coupling constants, basicity and conformation; (h) relationships between spin-spin coupling constants, dihedral angles in, and conformations of, spiroderivatives of cyclophosphazenes–trigonal planar versus pyramidal nitrogen atoms.     

α-氯甲硫醚自由基的构型和推拉效应

推拉效应(captodative effect)指的是,当自由基中心原子上同时连有推电子基及吸电子基时,该自由基具特殊稳定性的现象.近年来,许多化学工作者对此产生了很大的兴趣.研究该效应的方法,主要有理论计算及实验考察.Crans、Leroy等用ab initio 法首先计算了一些简单有机自由基的推拉效应.Pasto 最近探讨了动力学及热力学推拉效应.我们对氨基酸自由基作过考察,发现其有较大的推拉效应.最近,我们对α-氯甲硫醚自由基的稳定性进行了ab initio 计算.结果表明其热力学推拉效应值ΔE_(cd)~T 及动力学推拉效应值ΔE_(cd)~K 都小于零.我们先在STO-3G 水平寻找分子及自由基的     

Force-field parametrization of retro-inverso modified residues: Development of torsional and electrostatic parameters

Summary Torsional and the electrostatic parameters for molecular mechanics studies of retro-inverso modified peptides have been developed using quantum mechanical calculations. The resulting parameters have been compared with those calculated for conventional peptides. Rotational profiles, which were obtained spanning the corresponding dihedral angle, were corrected by removing the energy contributions associated to changes in interactions different from torsion under study. For this purpose, the torsional energy associated to each point of the profiles was estimated as the corresponding quantum mechanical energy minus the bonding and nonbonding energy contributions produced by the perturbations that the variation of the spanned dihedral angle causes in the bond distances, bond angles and the other dihedral angles. These energies were calculated using force-field expressions. The corrected profiles were fitted to a three-term Fourier expansion to derive the torsional parameters. Atomic charges for retro-inverso modified residues were derived from the rigorously calculated quantum mechanical electrostatic potential. Furthermore, the reliability of electrostatic models based on geometry-dependent charges and fixed charges has been examined.     

Relations between edge lengths,dihedral and solid angles in tetrahedra

The tetrahedron, fundamental in organic chemistry, is examined in view of two important kinds of angles: to each tetrahedron edge belongs a dihedral angle (internal intersection angle between two faces having the edge as common side) and to each tetrahedron vertex a solid angle (area of the surface inside the tetrahedron on the unit sphere with the vertex as center). Based on preliminary lemmas, these angles are expressed in terms of edge lengths by an essential use of determinants. The resulting formulae enable to specify angle properties by edge lengths, especially with regard to equality and inequality of single solid angles or certain sums of dihedral angles. A special kind of equal solid angles leads to symmetry aspects. Finally, it is shown that by a particular rearrangement of edges in tetrahedra of a specific class some derived angle properties will be preserved.     

A theoretical study on three conformational structures of 2,6‐distyrylpyridine

Ab initio methods at the levels HF/cc‐pVDZ, HF/6‐31G(d,p), MP2/cc‐pVDZ, and MP2/6‐31G(d,p), as well as methods based on density functional theory (DFT) employing the hybrid functional B3LYP with the basis sets cc‐pVDZ and 6‐31G(d,p), have been applied to study the conformers of 2,6‐distyrylpyridine. Bond distances, bond angles, and dihedral angles have been calculated at the B3LYP level. The calculated values were in good agreement with those measured by X‐ray diffraction analysis of 2,6‐distyrylpyridine. The values calculated using the Hartree‐Fock method and second‐order perturbation theory (MP2) were inconsistent. The optimized lowest‐energy geometries were calculated from the reported X‐ray structural data by the B3LYP/cc‐pVDZ method. Three conformations, A, B, and C, were proposed for 2,6‐distyrylpyridine. Calculations at the three levels of theory indicated that conformation A was the most stable structure, with conformations C and B being higher in energy by 1.10 and 2.57 kcal/mol, respectively, using the same method and basis function. The same trend in the relative energies of the three possible conformations was observed at the two levels of theory and with the different basis sets employed. The reported X‐ray data were utilized to optimize total molecular energy of conformation A at the different calculation levels. The bond lengths, bond angles, and dihedral angles were then obtained from the optimized geometries by ab initio methods and by applying DFT using the two basis functions cc‐pVDZ and 6‐31G(d,p). The values were analyzed and compared. The calculated total energies, the relative energies of the molecular orbitals, the gap between them, and the dipole moment for each conformational structure proposed for 2,6‐distyrylpyridine are also reported. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010