Substructure, subgraph, and walk counts as measures of the complexity of graphs and molecules.

In discussions of unsaturated compounds represented by multigraphs it is necessary to distinguish between the notions of substructure and subgraph. Here the difference is explained and exemplified, and a computer program is introduced which for the first time is able to construct and count all substructures and subgraphs for a colored multigraph (a molecular compound which may contain unsaturation and heteroatoms). Construction of all substructures and subgraphs is computationally demanding; therefore, two alternatives are pointed out for the treatment of large sets of compounds: (i) Often it will suffice to consider counts of substructures/subgraphs up to a certain number of edges only, information which is provided by the program much more rapidly. (ii) It is shown that information equivalent to that gained from substructure or subgraph counts is often far more easily available using walk counts. Some problems and their consequences for substructure/subgraph/walk counts are discussed that arise from the models used in organic chemistry for certain compounds such as aromatics and from the necessity to express qualitative features of molecular structures numerically.     

Halide binding and self-assembling behavior of Triazole-based acyclic and cyclic molecules

The present study demonstrates the use of triazole moiety in designing molecules endowed with the ability for self-assembly and molecular recognition. The receptors 7a and 9a having open structures bind to fluoride ion with good affinity. Various cyclophanes with 19-, 20-, 21-, 38-, and 40-membered rings containing triazole units were designed and synthesized. X-ray crystal structure of macrocycle 16 showed a tubular like architecture. Triazolophane 22 possesses bifurcated CH?N intramolecular hydrogen bonds and it further organizes in the solid state using CH?N interactions. Triazole based compounds are potential store house for exploiting CH?O and CH?N hydrogen bonding interactions for molecular self-assembly.     

Isospectral graphs and molecules

Isospectral molecules are nonidentical species whose molecular graphs have the same set of eigenvalues within HMO calculations. Several new concepts and methods for constructing pairs of isospectral graphs are discussed. Some possible experimental and theoretical applications of isospectral molecular pairs are outlined.     

On acyclic molecular graphs with maximal Hosoya index, energy, and short diameter

The total number of matchings of a graph is defined as its Hosoya index. Conjugated and non-conjugated acyclic graphs that have maximal Hosoya index and short diameter are characterized in this paper, explicit expressions of the Hosoya indices of these extremal graphs are also presented.     

Hydrogen bond formation to cyclic and acyclic polyethers

Equilibrium constants and enthalpies of hydrogen-bond formation of mcresol to various cyclic (crown) and acyclic polyethers have been determined in benzene solvent. Equilibrium constants indicated no evidence for an operative macrocyclic effect; the relationship between the increasing size of the equilibrium constant and the number of ether oxygens was rationalized with a simple statistical thermodynamic model. Enthalpies of interactions ranged between –19 and –23 kJ-mol^–1. In agreement with PCILO calculations, enthalpies of interaction were essentially independent of the number of oxygen atoms in the ether; no significant difference in enthalpies of interaction between cyclic and acyclic ethers was found.     

Reactivity of nitrovinylquinones with cyclic and acyclic enol ethers

The nitrovinyl-substituted quinones 2-(2-nitrovinyl)-1,4-benzoquinone and 2-(2-nitrovinyl)-1,4-naphthoquinone react with a variety of cyclic and acyclic enol ethers via two competing pathways. In one pathway, the nitrovinylquinone acts as an inverse electron-demand [4 + 2] diene. This gives quinoid carbocycles, which readily tautomerize to their hydroquinone form. The other pathway involves conjugate (Michael) addition of the enol ether to the nitrovinylquinone, followed by ring closure. This gave dihydrobenzofurans, which can eliminate an alcohol to give benzofurans. Hindered enol ethers tended to favor the conjugate addition pathway, while less hindered enol ethers favored cycloaddition.     

A simple synthesis of some cyclic and acyclic cyclobutenediones

Cyclobutenediones can be prepared from cyclic and acyclic ketones by a simple four step procedure.     

On minimal energy ordering of acyclic conjugated molecules

The energy of a graph is defined as the sum of the absolute values of all the eigenvalues of the graph. Gutman (Acyclic conjugated molecules, trees and their energies, J. Math. Chem. 1 (1987) 123–143) proposes two conjectures about the minimum of the energy of conjugated trees (trees with a perfect matching), which are verified by Zhang and Li (On acyclic conjugated molecules with minimal energies, Discrete Appl. Math. 92 (1999) 71–84). This paper focuses on the trees of conjugated hydrocarbon /EquationSource> trees in the class in the increasing order of their energies.     

Nonisomorphic graphs with identical atomic counts of self-returning walks: Isocodal graphs

On the basis of endospectral graphs, we present a graphical method for obtaining pairs of nonisomorphic graphs with identical atomic counts of self-returning walks.     

Study of luminescence properties of cyclic and acyclic thiadiazole-containing compounds

Luminescence properties of thiadiazole-containing compounds of cyclic and acyclic structure have been investigated. All compounds are characterized by fluorescence in the region 500–600 nm. The highest quantum yield of luminescence in the studied series is exhibited by acyclic product of the ABA type (where A is thiadiazole cycle, B is isoindole cycle) with N-pentylthiadiazoline fragments.     

Concise syntheses of trifluoromethylated cyclic and acyclic analogues of cADPR

A novel trifluoromethylated analogue of cADPR, 8-CF3-cIDPDE (5) was designed and synthesized via construction of N1,N9-disubstituted hypoxanthine, trifluoromethylation and intramolecular condensation. A series of acyclic analogues of cADPR were also designed and synthesized. These compounds could be useful molecules for studying the structure-activity relationship of cADPR analogues and exploring the cADPR/RyR Ca2+ signalling system.     

Electrochemical bromination of cyclic and acyclic enes using biphasic electrolysis

A simple method of electrochemical bromination of a series of cyclic and acyclic enes (styrene and substituted styrenes, stilbene, indene, and cyclooctene) in a biphasic water–chloroform mixture mediated by bromide/bromine redox system is reported. Aqueous 25% NaBr/H₂SO₄ is used as the electrolyte. Regio- and stereoselective dibromination of enes is achieved. Moderate to excellent yields of the product (83–98%) is obtained depending on the substrate. Electrolyte reuse has also been demonstrated successfully using HBr in the dibromination of styrene.     

Synthesis of carbazole-linked cyclic and acyclic peptoids with antibacterial activity

This paper describes the synthesis of several novel cyclic and acyclic peptoids, the former structurally comprising a tripeptide moiety linked through a carbazole scaffold. In a key step, a ring-closing metathesis reaction was used giving efficient access to this new class of cyclic peptoids. The target compounds were tested against Staphylococcus aureus (ATCC 6538P) and their minimum inhibitory concentration (MIC) values were determined. These compounds showed moderate to poor activities with MIC values ranging from 15-250 μg/mL.     

Ion transport properties of cyclic and acyclic neutral carrier-containing membranes

The K(+)-valinomycin permselective membrane provides a standard system for describing the normal current-voltage and current-time responses of the "closed circuit shuttle" carrier mechanism. The responses of a selection of cyclic and acyclic carrier-systems have been measured and compared with the standard system responses. Deviations from expected normal behavior suggest that failure of Donnan exclusion is caused by use of high-permittivity plasticizers and too few fixed sites in the membrane supports. The closed circuit shuttle model has been extended to allow for failure of Donnan exclusion, consumption of free carriers, and additional transport by extracted salts. The predicted limiting currents and current-time responses show many features of failure of Donnan exclusion.     

Perfluoro alkyl 1,3-diketonates of cyclic and acyclic secondary amines

Cyclic and acyclic secondary amines were quaternized with perfluoroalkyl 1,3-diketones to form secondary ammonium perfluoroalkyl 1,3-diketonates in good yields. Saturated cyclic secondary amines, including morpholine, N-acetylmorpholine, 3-methyl-N-acetylpiperidine, 2,2,6,6-tetramethyl-4-aminopiperidine, 4,4-dimethylimidazoline, 1,3-bispiperidine propane, and acyclic secondary amines, di(n-propyl) amine, di(isopropyl) amine, di(n-butyl) amine, and di(isobutyl) amine were reacted with various fluorine-containing 1,3-diketones (1-5) to yield the corresponding 1,3-diketonates (6-29). The compounds were characterized by ¹H, ¹⁹F, and ¹³C NMR, electrospray MS, and elemental analyses. Melting points and T_g values were obtained by DSC. Thermal decomposition data (5% weight loss temperature) were recorded by TGA. X-ray single-crystal structures show that 9 and 26 crystallize in monoclinic space group P2(1)/c.     

Interactions of acyclic and cyclic bis-phenanthridinium derivatives with ss- and ds-polynucleotides

The acyclic bis-phenanthridinium ligands 1, 2 and the cyclic analogue 3 bind to ss-RNA by bis-intercalation. Due to it's shorter linker 1 exhibits mono-intercalative binding to ds-polynucleotides, while a mixed mode of binding with 2 is shown to strongly dependent on the base composition and tertiary structure of ds-DNA and RNA. The cyclic analogue 3 binds to ds-polynucleotides by non-intercalative mode. Comparing the ss-/ds-polynucleotide selectivity obtained for 3 and previously reported for 3,8-linked bis-phenanthridinium analogues, it is clear that the more rigid structure and sterically more restricted cleft of the latter could better distinguish ss- from ds-polynucleotide regions.     

Quantum-chemical simulation of acyclic and cyclic forms of glucose in ethanol

Equilibrium between acyclic and cyclic forms of glucose in ethanol has been simulated by quantum chemistry methods. The cyclic form prevails in the ethanolic solution. Solvation of glucose molecules is accompanied by additional structurization, the effect being more prominent in the case of the linear form.     

An efficient, asymmetric organocatalyst-mediated conjugate addition of nitroalkanes to unsaturated cyclic and acyclic ketones

5-Pyrrolidin-2-yltetrazole is a versatile organocatalyst for the asymmetric conjugate addition of nitroalkanes to enones. Using this catalyst, this transformation requires short reaction times, tolerates a broad substrate scope, and possibly proceeds via generation of an iminium species.