Energetics,electronic structures and geometries of didehydroazines

When calibrated against the available experimental data for didehydrobenzenes, RB3LYP/cc-pVTZ, QCISD/cc-pVTZ, CCSD(T)/cc-pVTZ, and G3 electronic structure calculations provide reliable predictions of standard enthalpies and singlet—triplet splittings in all possible isomers of didehydroazines that (with a possible exception of 2,6-didehydropyridine) possess singlet ground states. Singlet didehydroazines with larger numbers of nitrogen atoms turn out to be more prone to ring opening, as indicated by the fact that out of the 6, 11, 6 and 3 possible didehydropyridines, didehydrodiazines, didehydrotriazines, and didehydrotetrazines, respectively, 5, 7, 2 and none are actually found. Immediate proximity of the nitrogen atom to the formally triple carbon—carbon bond confers decreased thermodynamic stabilities and smaller singlet—triplet splittings on the species of the 1,2-didehydro type. Some of the aza analogues of singlet 1,3-didehydrobenzene are as stable as their 1,2-didehydro counterparts. The only existing aza analogue of singlet 1,4-didehydrobenzene is 2,5-didehydropyrazine, which is particularly stable and possesses a large singlet—triplet splitting, making it a feasible synthetic target. The present calculations indicate that the experimental standard enthalpies of formation of pyrimidine and pyrazine are in error.     

Conformations and thermodynamic properties of sulphur homocycles. II. The fluxional S+ 8 radical cation

According to both UB3LYP/6-311G* and UMP2/6-311G* calculations, the lowest energy conformer of the S⁺ ₈ radical cation possesses C_s symmetry. However, there are three other structures with low relative energies that do not exceed 6.5 kcal mol^?1. These conformers, which are found to be very prone to pseudorotation, are predicted to interconvert readily. The fluxional nature of S⁺ ₈ is expected to facilitate its reactions with organic species where a specific conformation is demanded by steric constraints, which may explain its high reactivity towards PAHs with crowded hydrogen atoms.     

Energetics,electronic structures and geometries of naphthalene,quinoline and isoquinoline analogues of 1,2-didehydrobenzene

B3LYP/cc-pVTZ electronic structure calculations employed in conjunction with additive corrections derived from experimental data for 1,2-didehydrobenzene predict the standard enthalpies of formation of 1,2- and 2,3-didehydronaphthalenes to be equal to 121.0 and 123.7 kcal mol^?1, respectively. The corresponding singlet-triplet splittings amount to 40.1 and 35.4 kcal mol^?1. The positional dependence of both of these quantities is preserved in those didehydroquinolines and didehydroisoquinolines in which the didehydrogenation sites are separated by at least one carbon from the heteroatoms. The effect of the adjacent heteroatoms on the singlet-triplet splittings is significantly more pronounced than that on the standard enthalpies of formation. Test G3 calculations on 2,3-didehydronaphthalene confirm the reliability of the additive correction scheme in the prediction of properties of annelated analogues of 1,2-didehvdrobenzene. Such a scheme opens an avenue to facile electronic structure calculations on didehydrogenation reactions of polycondensed heterocyclic compounds with six-membered rings.     

Theoretical thermochemistry of the C60F18, C60F36, and C60F48 fluorofullerenes

Relative energies of C₆₀F_N fluorofullerenes are reproduced reasonably well at the B3LYP/6- 311G** level of theory employed in conjunction with isodesmic transfluorination reactions, although overestimation of steric repulsions among non-bonded atoms is evident for species with larger values of N. On the other hand, the MNDO method is found to be less suitable for studies of fluorofullerene thermochemistry. The gas-phase standard enthalpy of formation of the C₆₀F₁₈ species is predicted to lie between ?1500 kJ mol^?1 and ?1400 kJ mol^?1.     

On Some Properties of the Hoede-Bakker Index

In this article, we analyze the decisional power index, the so called Hoede-Bakker index. This index takes the preferences of the players into account, as well as the social structure in which players may influence each other. We investigate the properties of the Hoede-Bakker index. We adapt some postulates for power indices and voting power paradoxes for the Hoede-Bakker set up. We investigate whether this decisional power index displays the adapted voting power paradoxes and whether it satisfies the adapted postulates for power indices.