Clar's sextet rule is a consequence of the sigma-electron framework

A number of condensed PAHs are examined to identify the underlying reasons governing empirical Clar's rule taking benzene as a limiting case. It is found that the so-called Clar's structures are the only minima on the MP2(fc) potential energy hypersurfaces, meaning that other conceivable valence isomers are nonexistent. The influence of the electron correlation energies to the stability of Clar's structures is substantial with predominating influence of the sigma-electrons. However, the contributions arising from the sigma- and pi-electron correlation energies are approximately the same, if Clar's structures are compared with some artificial pi-electron localized or graphite-like delocalized planar systems. Analysis of the Hartree-Fock (HF) energies provides a compelling evidence that the origin of stability of Clar's structures lies in a decrease of the positive T, V(ee) and V(nn) energy terms relative to some characteristic virtual "delocalized" or "localized" model geometries. Partitioning of the mixed and terms in the sigma- and pi-type contributions, by using the stockholder (SHR), equipartitioning (EQP) and standard pi (SPI) schemes, unequivocally shows that the driving force leading to Clar's structures are more favorable sigma-type interactions. All these conclusions hold for the archetypal benzene too, which could be considered as a limiting Clar system. Finally, the boundaries of Clar's hypothesis and some common misconceptions are briefly discussed. Perusal of the geometric parameters and pi-bond orders reveals that there are no benzene rings completely "vacant" or "fully occupied" by the pi-electrons, envisaged by Clar in his picture of condensed benzenoid compounds. Instead, there are six-membered rings with higher and lower total pi-electron density. The bond length anisotropy of the former rings is smaller. It is concluded that Clar's proposition is a useful rule of thumb providing qualitative information on the stability of the PAH systems, which in turn should not be overinterpreted.     

On the Reaction of Glycerol Dehydratase with But‐3‐ene‐1,2‐diol

Intriguing inactivation : Calculations suggest that the ability of relatively high‐energy radical intermediates to inactivate glycerol dehydratase (GDH) may reflect a general and hitherto unidentified inactivation mechanism in the reaction of coenzyme B₁₂‐dependent enzymes and 3‐unsaturated 1,2‐diols (see scheme; AdoCbl: adenosylcobalamin or coenzyme B₁₂).

     

On perfect Lee codes

In this paper we survey recent results on the Golomb-Welch conjecture and its generalizations and variations. We also show that there are no perfect 2-error correcting Lee codes of block length 5 and 6 over Z. This provides additional support for the Golomb Welch conjecture as it settles the two smallest cases open so far.     

Generalized uniformly continuous semigroups and semilinear hyperbolic systems with regularized derivatives

We adopt the theory of uniformly continuous operator semigroups for use in Colombeau generalized function spaces. The main objective is to find a unique solution to a class of semilinear hyperbolic systems with singularities. The idea of regularized derivatives is to transform unbounded differential operators into bounded, integral ones. This idea is used here to permit working with uniformly continuous operators.     

Generalized uniformly continuous semigroups and semilinear hyperbolic systems with regularized derivatives

We adopt the theory of uniformly continuous operator semigroups for use in Colombeau generalized function spaces. The main objective is to find a unique solution to a class of semilinear hyperbolic systems with singularities. The idea of regularized derivatives is to transform unbounded differential operators into bounded, integral ones. This idea is used here to permit working with uniformly continuous operators.     

Investigations on the chromatographic behavior of hybrid reversed-phase materials containing electron donor-acceptor systems. I. Contribution of sulfur-aromatic interactions

The influence of sulfur-aromatic interactions on the chromatographic separation behavior of hybrid RP phases, containing thiol-groups and/or embedded sulfide-groups (S-RP) has been investigated. To allow a precise outline of this new interaction mode, a wide variety of S-RP phases with different alkyl chain length, with and without residual thiol-groups and silanol-endcapping were prepared and tested in comparison to some conventional monomerical as well as polymerical n-alkyl type RP phases. The solute test sets employed in this study comprised the classical chromatographic column tests from Engelhardt and Tanaka as well as test assemblies containing polycyclic aromatic hydrocarbons, stilbene-based cis/trans isomers and functional isomers of benzene. In general, a pronounced strong planar recognition ability as well as a strong increase in the retention for aromatic compounds have been noticed. It was furthermore found that not only the sulfur atom incorporated into the alkyl chain, but also the residual thiol-groups of the 3-propylthiol silica backbone contribute to the overall retention behavior of these novel S-RP type phases.     

Isomorphic factorizations of trees

A tree is even if its edges can be colored in two colors so that the monochromatic subgraphs are isomorphic. All even trees of maximum degree 3 in which no two vertices of degrees 1 or 3 are adjacent are determined. It is also shown that, for every n, there are only finitely many trees of maximum degree 3 and with n vertices of degree 3 that are not even. © 1995 John Wiley & Sons, Inc.     

Large-scale purification of the mycotoxins aflatoxin B1, B2 and G1

The isolation and purification of gram quantities of the important mycotoxins aflatoxin B1, B2 and G1 are described. The method involves final purification on a Waters Prep LC-500 instrument, loaded with silica cartridges, and elution with chloroform.     

Technetium removal from the aqueous solution using zeolites A and Y containing transition metal ions Co2+ and Zn2+

Journal of Radioanalytical and Nuclear Chemistry - The adsorption properties of two zeolite types, faujasite (NaY, ZnY, CoY) and LTA-4A zeolite (NaA, ZnA, CoA), towards technetium were studied in...     

The origin of aromaticity: important role of the sigma framework in benzene.

The physical nature of aromaticity is addressed at a high ab initio level. It is conclusively shown that the extrinsic aromatic stabilization energy of benzene E(ease)B, estimated relative to its linear polyene counterpart(s), is very well-reproduced at the Hartree-Fock (HF) level. This is a consequence of the fact that the contributions arising from the zero-point vibrational energy (ZPVE) and electron correlation are rather small. More specifically, they yield together 2.0 kcalmol(-1) to the destabilization of benzene. A careful scrutiny of the HF energies by virial theorem shows further that the kinetic energies of the sigma and pi electrons E(T)HF(sigma) and E(T)HF(pi) are strictly additive in the gauge linear zig-zag polyenes, which also holds for their sum Et(T)HF This finding has the important corollary that E(ease)B is little dependent on the choice of the homodesmic reactions involving zig-zag polyenes. A detailed physical analysis of the sigma- and pi-electron contributions to extrinsic aromaticity requires explicit introduction of the potential energy terms Vne, Vee, and Vnn, which signify Coulomb interactions between the electrons and the nuclei. The Vee term involves repulsive interaction Vee(sigmapi) between the sigma and pi electrons, which cannot be unequivocally resolved into sigma and pi contributions. The same holds for the Vnn energy, which implicitly depends on the electron density distribution via the Born-Oppenheimer (BO) potential energy surface. Several possibilities for partitioning Vee(sigmapi) and Vnn terms into sigma and pi components are examined. It is argued that the stockholder principle is the most realistic, which strongly indicates that E(ease)B is a result of favorable sigma-framework interactions. In contrast, the pi-electron framework prefers the open-chain linear polyenes.