Effect Of Positional Isomerism Research Articles

An investigation of the positional isomeric effect of terpyridine derivatives: Self-assembly of novel cadmium coordination architectures driven by N-donor covalence and π⋯π non-covalent interactions

Four cadmium compounds based on two pyridyl substituted terpyridine isomers, namely {[Cd 3(L1) 2(SCN) 6](THF) 2} n ( 1), [Cd 2(L1)(CH 3OH)Cl 4] n ( 2), [Cd 2(L2) 2(N 3) 4] ( 3) and [Cd 2(L2) 2(SCN) 4] ( 4) (L1 = 4′-(3-pyridyl)-2,2′:6′,2″-terpyridine, L2 = 4′-(2-pyridyl)-2,2′:6′,2″-terpyridine) have been synthesized and characterized by IR, elemental analyses, and luminescent spectroscopy with the purpose of investigating the influence of isomeric effect on network assembly. In compounds 1 and 2, L1 as a mono-tridentate bridging ligand links Cd II atoms into 1D grid-like and 2D wave-like polymers. In contrast, in compounds 3 and 4, L2 as a chelating ligand forms Cd II dimeric structures. The different coordination behaviors and the positional isomer effect were discussed under the same reaction conditions. In addition, three kind of π⋯π interactions in the four compounds were summarized, two of which could not be observed on tpy-based complexes.

Isomerism in Coordination Complexes and Polymers Derived from Bispyridylurea Ligands: Effect of Solvents, Conformational Flexibility, and Positional Isomerism of the Ligands

Reactions between Cd(NO3)2·6H2O and N,N‘-bis(3-pyridyl)urea (3bpu)/N,N‘-bis(4-pyridyl)urea (4bpu) with a 1:2 metal:ligand ratio under various solvent systems (polar/nonpolar) afforded four discrete coordination complexes, namely, [{(H2O)2Cd(3bpu)4}·(NO3)2·2H2O·X] 1, [{(H2O)2Cd(3bpu)4}·(NO3)2] 3, [{(H2O)2Cd(3bpu)4}·(NO3)2·2H2O·p-xylene] 4, [{(H2O)2 (NO3)2Cd(μ-4bpu)2}] 8, and four coordination polymers, namely, [{(H2O)3(NO3)Cd(μ-3bpu)}·(NO3)·H2O·1/2o-xylene]n 2, [{(H2O)3(NO3)Cd(μ-3bpu)}·(NO3)·X]n 5, [{(NO3)2Cd(μ-3bpu)2}]n 6, [{(H2O)(NO3)Cd(μ-4bpu)2}·(NO3)·EtOH·nitrobenzene]n 7, which were mainly characterized by single-crystal X-ray diffractometry. The effect of solvents, conformational flexibility, and positional isomerism of the ligands on the various isomerisms (supramolecular isomerism and polymorphism) is discussed.

Molecular Tectonics of Mixed-Ligand Metal−Organic Frameworks: Positional Isomeric Effect, Metal-Directed Assembly, and Structural Diversification

A series of nine mixed-ligand metal-organic frameworks (MOFs) have been prepared by the combination of a bent dipyridyl linker 4-amino-3,5-bis(4-pyridyl)-1,2,4-triazole (bpt) and three benzenedicarboxylate isomers (pa = phthalate, ip = isophthalate, and tp = terephthalate), respectively, with different metal ions such as CoII, NiII, CuII, ZnII, and CdII. The framework structures of these neutral polymeric complexes have been determined by the X-ray single-crystal diffraction technique. Structural analysis reveals that the benzenedicarboxylate isomers display versatile coordination modes to manage the metal ions to form 1-D chain or ribbon arrays, which are further extended via the exo-bidentate bpt connectors to give rise to a variety of coordination networks, such as a simple (4,4) layer, 2-D double layer with decorated (4,4) topology, 2-D layer with decorated (3,6) topology, 2-D bilayer with 82.10 topology (2-fold interpenetration), 3-D polythreaded architecture (1-D + 2-D), and 2-fold interpenetrating porous lattice of (4,4) layers. The accessorial secondary interactions such as hydrogen bonding and/or aromatic stacking are also helpful for the extension and stabilization of the final supramolecular aggregates. This work evidently indicates that the isomeric effect of the anionic benzenedicarboxylate is significant in the construction of these network structures, which are also well regulated by the metal centers. The ZnII and CdII MOFs exhibit strong solid-state luminescence emissions at room temperature, which originate differently from intraligand transition or ligand-to-metal charge transfer. Thermal stability of these crystalline materials has been explored by thermogravimetric analysis of mass loss. The 3-D host frameworks of MOFs 8 and 9 show similar porous cavities, and their desorption/adsorption behaviors of guest solvents have also been investigated.

Effect of positional isomerism on the abiotic degradation of pesticides: Case of m- and p-imazamethabenz-methyl

The effect of positional isomerism on chemical and photochemical degradations of the Imazamethabenz-methyl (IMBM), a pesticide of the imidazolinone family, has been studied. IMBM is proposed in the form of a mixture of the two positional isomers: meta and para. The development of an effective HPLC method (resolution factor R=1.3) allows us to follow either the abiotic disappearance of the meta and para IMBM and the formation of their breakdown products. The abiotic degradation studies include the chemical hydrolysis, as well as the direct and the indirect photodecomposition. We used TiO(2), a well-known initiator of hydroxyl radicals, to highlight the role of *OH in the indirect photodegradation. This work confirms the different behaviours of positional isomers in the environment. Indeed the chemical or direct photochemical degradation is faster for the meta isomer than for the para. Whereas, concerning IMBM, there is not any prevalent influence of this type of isomerism on the indirect photochemical degradation. The degradation products were tentatively identified by LC-MS, NMR and IR and a degradation pathway was proposed.

Nonionic Urea Surfactants: Influence of Hydrocarbon Chain Length and Positional Isomerism on the Thermotropic and Lyotropic Phase Behavior

The thermotropic and lyotropic phase behavior of 1- and 5-decyl urea, and 1-, 2-, 4-, and 6-dodecyl urea have been studied. This allowed the effect of positional isomerism to be examined. Intermolecular hydrogen bonding by the urea moiety is the dominant factor in determining the solid-state thermal behavior and crystal solubility boundary of these linear nonionic surfactants. The positional isomers where the urea moiety was not situated at the terminus of the hydrocarbon chain exhibited higher melting points than the 1-alkyl ureas. This has been rationalized by postulating interdigitated chains in the solid state. In the urea surfactant-water systems, three phases are observed, viz. crystalline solid, a dilute aqueous solution of the alkyl urea, and an isotropic liquid. The last two phases coexist in the low-surfactant, high-temperature region of the binary phase diagram. An overview of structure-property correlations for linear nonionic urea surfactants is presented in light of the new physicochemical data obtained for the decyl urea and dodecyl urea positional isomers.

A Noninterpenetrated 1D Molecular Ladder and 2D Butterfly Network: Effect of Positional Isomerism of Semirigid Bis(pyridylmethyl)pyromellitic Diimide Ligands on the Architecture of Their Metal(II) Complexes

Three new complexes [Cd(4-pmpmd)1.5 (NO3)2] x CHCl3 (1), [Cd(3-pmpmd)1.5 (NO3)2] x EtOH (2), and [Zn(3-pmpmd)1.5 (NO3)2] x MeOH (3) (3- or 4-pmpmd = N,N'-bis(3- or 4-pyridylmethyl)pyromellitic diimide) containing T-shaped building blocks have been obtained from reactions between the long semirigid ligands 3- or 4-pmpmd and either cadmium or zinc nitrate. 1 forms noninterpenetrated 1D molecular ladders that are linked via multiple, complementary intermolecular C-H...O hydrogen bonds that effect the 3D alignment. 2 and 3 are isostructural and feature a noninterpenetrated 2D butterfly-shaped network with (6,3) topology. Multiple intermolecular C-H...O hydrogen bonds exist between the 2D layers and generate the 3D framework. The structural differences between 1 and 2 or 3 are attributed to the different conformations adopted by the ligands, which illustrate the influence of positional isomerism on the resultant supramolecular architectures of metal complexes.

In Vitro Metabolism of Nitric Oxide-Donating Aspirin: The Effect of Positional Isomerism

NO-donating aspirin (NO-ASA) is a potentially important chemopreventive agent against cancer. Since positional isomerism affects strongly its potency in inhibiting colon cancer cell growth, we studied the metabolic transformations of its ortho-, meta-, and para-isomers in rat liver and colon cytosolic, microsomal, and mitochondrial fractions as well as in intact HT-29 human colon cancer cells. NO-ASA and metabolites were determined by high-performance liquid chromatography and products identified by mass spectroscopy, as required. For all three isomers, the acetyl group on the ASA moiety was hydrolyzed rapidly. This was followed by hydrolysis of the ester bond linking the salicylate anion to the spacer. The ortho- and para-isomers produced salicylic acid and a putative intermediate consisting of the remainder of the molecule, which via a rapid step generated nitrate, (hydroxymethyl)phenol, and a conjugate of spacer with glutathione. The meta-isomer, in contrast, generated salicylic acid and (nitroxymethyl)phenol, the latter leading to (hydroxymethyl)phenol and the glutathione-spacer conjugate. This metabolic pathway takes place in its entirety only in the cytosolic fraction of the tissues tested and in intact human colon cancer cells, perhaps reflecting exposure to the cytosolic glutathione S-transferase, which catalyzes the formation of the spacer-glutathione conjugate. Thus, the three positional isomers of NO-ASA differ in their metabolism and these differences correlate with their differential effects on cancer cell growth, underscoring the importance of positional isomerism in modulating drug effects.

Synthesis and electrogenerated chemiluminescence of donor-substituted phenylquinolinylethynes and phenylisoquinolinylethynes: effect of positional isomerism.

In furtherance of our research on the design, synthesis and study of electrogenerated chemiluminescence (ECL) of new donor substituted phenylquinolinylethynes, we report here more new series with the aim of studying the effect of positional isomerism on their overall photophysical properties with a special focus on ECL. For this study we have chosen 2-, 3-, and 4-(p-substituted phenyl)ethynylquinolines, and 1- and 4-(p-substituted phenyl)ethynylisoquinolines. These ethynes were synthesized in good yields by modified Sonogashira coupling of the corresponding terminal alkyne with the respective haloquinolines. The photophysical properties and ECL were studied in acetonitrile solvent and the various results are discussed.

Novel piperidinyloxy oxazolidinone antimicrobial agents

Oxazolidinone antibacterial agents, where the N-substituted piperazinyl group of eperezolid was replaced with a N-substituted piperidinyloxy moiety, were synthesized and shown to be active against a variety of resistant and susceptible Gram-positive organisms. The effect of ring size, positional isomerism, and fluorine substitution on antibacterial activity was examined.

Comparative Studies on Isomerization Behavior and Photocontrol of Nematic Liquid Crystals Using Polymethacrylates with 3,3‘- and 4,4‘-Dihexyloxyazobenzenes in Side Chains

Azobenzene moieties with 3,3‘-dihexyloxy-2,2‘-dimethyl and 4,4‘-dihexyloxy substituents were attached to backbones of polymethacrylates at low contents (7 mol %) to investigate the effect of positional isomerism of the substituents on photochemical and thermal isomerization behavior and the ability to perform the surface-assisted alignment control of liquid crystals photochemically. Photoisomerization in thin films in a glassy state through π−π* excitation deviates markedly from those in solutions, but that through n−π* excitation takes the behavior of solution photoisomerization. This can be explained in terms of the difference in sweep volumes (critical volumes needed for isomerization) of the azobenzenes in relation to the mechanisms of the isomerization. Polarized light-induced anisotropy of the polymer thin films gave rise to the photocontrollability of alignment of a nematic liquid crystal, despite low azo-chromophore contents. Photoorientation and photoreorientation of the nematic liquid crystal depend significantly on sweep volumes for the photoisomerization which is controlled by the selection of the excitation wavelength and the molecular conformation of the azo-chromophores.

Mobility of oligopeptides on normal-phase silica: effect of positional isomerism.

AbstractIsomeric oligopeptides composed of five methionyl residues and one glycyl residue or of five γ‐methyl‐L‐glutamyl residues and one glycyl residue all exhibit marked differences in retention on normal‐phase silica. When the glycyl residue is at internal positions of hexa or heptapeptides, the peptide elutes most rapidly form the μPorasil column. Comparison of the effect of positional isomerism on retention in short oligopeptides with the effect on retention of hexamers and heptamers suggests that a change in peptide conformation may be responsible for the change in oligopeptide mobility.

Effect of distal positional isomerism on peptide fragmentation. A comparison of dimethylaminobenzylidene and benzoyl derivatives

AbstractThe electron‐impact‐induced fragmentation patterns of the positional isomers of N,N‐dimethylaminobenzaldehyde and their Schiff base derivatives of valylisoleucylalanine and leucyltryptophylleucine ethyl esters were determined. The electron‐impact‐induced fragmentation patterns of the positional isomers of N‐hydroxysuccinimido N′,N′‐dimethylaminobenzoate and of the corresponding N,N‐dimethylaminobenzoyl ethyl esters of the above peptides were compared. Significant differences among the isomers were noted between the Schiff base set and the substituted benzoyl set. The N‐blocking groups m‐ and p‐dimethylaminobenzylidene and m‐dimethylaminobenzoyl gave the most useful sets of sequence peaks of relatively uniform intensity with intense molecular ions. Positional isomerism was found to influence cleavage of both proximal and distal bonds.

Luminescence spectra and triplet lifetimes of neutral and protonated azaphenanthrenes

Abstract Luminescence spectra of azaphenanthrenes and their conjugate acids were studied in solid solutions at 77°K. The effect of nitrogen positional isomerism and of substitution in neutral and acid medium is reported for both fluorescence and phosphorescence emissions, which appear to start from π, π ∗ levels. From the decay times of the delayed emission, the triplet lifetimes in solid solutions were obtained. By spectrophotometric measurements of the ground state p K 's, the corresponding values in the excited states have been calculated. The basicity increases in both triplet and singlet lowest excited states, particularly in the latter.

The gas-liquid chromatographic behavior of some closely related disubstituted phenylacetic acids

The gas-liquid chromatography of a number of aromatic acids (as their methyl esters) was investigated with two stationary phases in order to study the effect of positional isomerism and substituent groups upon retention behavior. In the case of 3-methoxy-4-hydroxy-and 3-hydroxy-4-methoxy-substituted compounds the isomer separation pattern observed is held to be due to intramolecular hydrogen bonding.

Étude du Comportement D'Isomères Géométriques Sous L'Impact Électronique IV. Cyclopropane, méthylcyclopropane et diméthylcyclopropanes gem, cis et trans

AbstractMass spectra of cyclopropane, methylcyclopropane, 1,1‐dimethylcyclopropane, cis‐1,2‐ and trans‐1,2‐dimethylcyclopropane have been recorded in the electron energy range 50‐10 volts, especially in order to study the effect of isomerism in the three dimethyl compounds.Surprisingly, only a slight effect of position isomerism shows up in 1,1 and 1,2 isomers and no effect of geometrical isomerism can be detected in cis and trans stereoisomers, contrarily to the published data. The identity of behavior of cis and trans isomers under electron impact seems to be related to the pseudoethylenic character of C‐C bonds in C3 ring which, in the present case, overcomes the expected steric effect observed in larger cyclanic isomers.Ionization potential of the three isomers and appearance potentials of main fragment ions are found to be the same within experimental uncertainties. Metastable transitions in the three isomers being identical, corresponding types of fragmentation are also the same. Fragment ions have therefore to be produced by a common molecular ion whose most probable structure is an isopentanic chain magnified image. Fragility of C‐C bonds close to substituting alkyl groups in branched hydrocarbons is confirmed by the present study of methylcyclopropane and gem‐dimethylcyclopropane.

A Note on 3-Thienyl-β-dialkylaminoethyl Ketones and Related Compounds

A group of ketonic bases were synthesized from 3-acetylthiophene and 4-(3-thienyl)-3-buten-2-one, by means of the Mannich reaction. These compounds were prepared in an attempt to study the effect of position isomerism in this series on the physiological activity of the compounds.

The effect of positional and geometrical isomerism on the dilatometric properties of some octadecenoic acids

SummaryFour isomeric octadecenoic acids, oleic, elaidic, petroselinic, and petroselaidic, were prepared and their expansibility determined. The melting dilation of each acid was calculated and found to increase in the order oleic, petroselinic, petroselaidic, and elaidic. In each instance the melting dilation of the trans acid was greater than that of its cis isomer. Of the two cis acids studied, the melting dilation was less for the acid with the double bond farther from the carboxyl end of the carbon chain. The trans acids did not follow this pattern.At temperatures above 52°C., at which all acids were liquid, the absolute specific volumes of the acids were very nearly equal whereas at temperatures below −7°C, at which all were in the solid state, the specific volumes of petroselinic and elaidic acids were at variance with the specific volumes of oleic and petroselaidic acids. This variance must be attributed to differences in crystal packing.Oleic acid clearly showed the two polymorphic forms previously recognized and, in addition, apparently reversibly transformed at about −5°C., indicating the possible existence of a third polymorphic form. These transformations of oleic acid occurred regardless of tempering, and without visual melting. The other three acids did not exhibit polymorphism under the conditions employed in the dilatometric measurements.

The spectral absorption of certain monoazo dyes -1. The effect of position isomerism on the spectral absorption of methyl derivatives of benzeneazophenol

The spectral absorption of certain monoazo dyes -1. The effect of position isomerism on the spectral absorption of methyl derivatives of benzeneazophenol