Asymptotic isomer enumeration in chemistry

In the theory of chemical isomerism, considerable attention has been paid in the past to the enumeration of all isomers (of a chosen type) of all molecules with a particular molecular formula of interest, having an appropriate number of atoms of some special interest (e.g., carbon atoms). Some corresponding methods and results are here developed for enumerating ensembles or mixtures of molecules. The main emphasis is on asymptotic enumeration results. This revised version was published online in July 2006 with corrections to the Cover Date.     

Benzenoid isomer enumeration and a new topological paradigm

Benzenoid isomer series possessing a constant number of isomers are reviewed. The benzenoid series having the same isomer number possess a one-to-one matching among their benzenoid membership in regard to molecular symmetry. These results are supplemented and clarified. For a given number of carbons, information regarding all the more stable benzenoids of potential interest to experimentalists is contained in these results.     

Asymptotic isomer enumeration in chemistry. II

This is a sequel to a recent paper by John Knopfmacher (1998) on the problem of asymptotic enumeration of different kinds of isomers of ensembles or mixtures of molecules, involving together with other atoms a large number n of carbon atoms, in total. The new kinds of mixtures to be chosen for application below require and are treated with the aid of both the previous mathematical tools, and new theorems based on different techniques. This revised version was published online in July 2006 with corrections to the Cover Date.     

Single isomer trisubstituted olefins bearing alkyl groups

E-β-Chloro-α-iodo-α,β-unsaturated esters were converted to single isomer trisubstituted olefins bearing alkyl substituents by using and alkyl-Suzuki cross coupling. The process was highly selective, and the products in all cases were isolated as single isomers. Mechanistic investigations indicated that this process transfers a hydrogen from water to the α-position of the substrate, and then an alkyl group is introduced to the β-position of the intermediate template while replacing a chloride.     

Pseudo-point groups and chronality in enumeration of reaction pairs

Summary The pseudo-point group is constructed to characterize the symmetry of a basic pair of hexagonal reaction graphs having no par-bonds on its edges. Any pairs of reaction graphs (reaction pairs) are considered to be obtained by adding par-bonds to the edges of the basic pair; they are counted by the USCI (unit-subduced-cycle-index) approach. Thus, the six edges of the basic pair are assigned to the coset representation (/C _2v ). After the subduction of the is calculated, the partial-cycle-index method of the USCI approach is applied to the combinatorial enumeration of reactions pairs.Reaction pairs are classified to two categories, i.e. isoenergetic and anisoenergetic. An isoenergetic pair is concluded to be a self-reaction pair, while an anisoenergetic pair corresponds to a non-self-reaction pair. The concept of chronality is also discussed to clarify the symmetrical nature of the resulting orbits.     

Kinetic behavior in free-radical polymerization of isomer methacrylic monomers with active functional groups as side substituents

The kinetic behavior of the free-radical polymerization of 2-hydroxy-4-N-methacrylamidobenzoic acid (4-HMA) and 2-hydroxy-5-N-methacrylamidobenzoic acid (5-HMA) in a solution of N,N-dimethylformamide is described. The methacrylic monomers 4-HMA and 5-HMA were isomers in which the phenolic and carboxylic functional groups were in different positions on the side aromatic ring with respect to the methacrylamide group. Semiempirical (AM1 and PM3 treatments) and ab initio (6-31G**) quantum mechanical calculations indicated the existence of intramolecular H-bonding between the phenolic and carboxylic groups. These calculations also indicated a slightly higher reactivity of 4-HMA with respect to 5-HMA under the same experimental conditions as obtained from the frontier orbital interactions between the highest molecular orbital of the monomers and the singly occupied molecular orbital of the radical obtained by the reaction of a methyl radical with the corresponding monomer. Gravimetric study of the free-radical polymerization of 4-HMA and 5-HMA at several temperatures ranging from 50 to 150 °C demonstrated this behavior. The kinetic results obtained and the average molecular weights of the polymers prepared at different temperatures indicated that the monomer 4-HMA had a slightly higher reactivity at low temperatures (50–90 °C), whereas at higher temperatures (120–150 °C), the reactivity of both monomers became similar as a consequence of the “dead-end” radical polymerization. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4528–4535, 1999     

An isomer of coronopilin

Conclusions From the racemes ofC. xanthifolia Fresen. have been isolated a sesquiterpene lactone C₁₆H₂₀O₄, the structural (+)-isomer of (–)-coronopilin, which has not been studied previously; structure I has been proposed for it.Khimiya Prirodnykh Soedinenii, Vol. 5, No. 5, pp. 375–377, 1969     

Alkane isomer family and superfamily classification

Tbilisi University. Translated from Zhurnal Strukturnoi Khimii, Vol. 28, No. 5, pp. 151–153, September–October, 1987.     

Computer enumeration and generation of physical trees

A computer-oriented method for the enumeration and generation of physical trees is presented. Physical trees depict acyclic chemical structures, but the term physical is used to stress the process by which the structures are produced.     

Isomer enumeration of practical benzenoids

Recent development of the leapfrog operation in enumeration and understanding the chemical properties of benzenoid isomer sets having at least one synthesized representative is stressed. Some new enumeration results and correlations of the periodic table for the more stable total resonant sextet (TRS) benzenoid hydrocarbon subset [Table PAH6(sextet)] are presented.     

Regarding enumeration of molecular isomers

In this paper, a simple theoretical approach to counting of substitution isomers is described. It is based on Polya's theorem and on point groups as recently described by us [Baraldi, I.; Fiori, C.; Vanossi, D. J. Math. Chem. 1999, 25, 23-30] and extended in this paper. Several applications are developed that range from molecules without symmetry to molecules with icosahedral symmetry (Ih). The problem of the appearance of stereoisomers is also analyzed.     

Exhaustive enumeration of molecular substructures

This article addresses the systematic and complete enumeration of all the substructures of any size present in a given molecule. The study is not restricted to features which could be defined a priori such as rings or chains. Contrary to prior expectation the exhaustive enumeration is tractable with current computational tools. Results are presented for several families of skeletons which are widespread in chemistry. It is shown that the numbers of constituent substructures of each size are related to the molecular topology, in particular the degree of branching. The number substructures which are distinct depends additionally on the number of different atom and bond types present. The overall shapes of the distribution of substructure counts as a function of substructure size are found to be similar within particular classes of molecules. These distributions are compared and found to be characteristic of certain topologies. For several simple classes of molecule, analytic expressions are provided for the numbers of substructures as a function of fragment and molecule size. These results hold promise for identifying potentially useful scaffolds for use in combinatorial chemistry. © 1997 by John Wiley & Sons, Inc.     

Theory of isomer shift in hemin

Using the self-consistent charge extended Hückel procedure, the charge density difference at Fe⁵⁷ nucleus, between hemin and Fe⁺³ ion is calculated. This is combined with the recent value of the calibration constant, –0.23±0.02a ₀ ³ mm/sec to obtain an isomer shift of –0.374 mm/sec between hemin and Fe⁺³ in good agreement with the value –0.392 mm/sec derived from experimental data and the calculated value of the isomer shift of Fe⁺³ with respect to K₃FeF₆ from first principle covalency investigations in the latter compound. is composed of contributions from core and valence electrons of the same order of magnitude, with the latter being more than one-half of the former. The core contribution is composed of a number of terms of comparable magnitude and differing signs, whose significance is discussed.Supported by grant HL 15196-02 from the Heart and Lung Institute of National Institute of Health.     

Laser-spectrochromatographic identification of isomer in mixture

The combined “Gas Chromatograph — Laser Optoacoustic Spectrometer” (GC-LOAS) has been used to perform selective analysis of a mixture of fatty acid esters stereocisomers and a mixture of methylcyclopentadiene positional isomers. The GCLOAS system allows simultaneous recording of the retention time of the components in the mixture under analysis and their IR absorption spectra in the spectral operating range of the ¹²CO₂, ¹³CO₂ and HeNe lasers. It has been shown that the hydrogenated sample of fatty acids contains a lot of trans-isomer. Its content has been quantitatively estimated. The identification of methylcyclopentadiene isomers (both monomers and dimers) has been carried out, too.