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Laser flash photolysis (LFP) studies, atoms in molecules (AIM) studies, and density functional theory (DFT) calculations have been performed in order to study the mechanism of the hydrogen abstraction by alpha-diketones in the presence of phenols. Laser irradiation of a degassed solution of 1,2-diketopyracene in acetonitrile resulted in the formation of a readily detectable transient with absorption at 610 nm, but with very low absorptivity. This transient decays with a lifetime of around 2 micros. The quenching rate constant for substituted phenols, kq, ranged from 1.10x10(8) L mol-1 s-1 (4-cyanophenol) to 3.87x10(9) L mol-1 s-1 (4-hydroxyphenol). The Hammett plot for the reaction of the triplet of 1,2-diketopyracene with phenols gave a reaction constant rho=-0.9. DFT calculations (UB3LYP/6-311++G**//UB3LYP/6-31G*) of the triplet complex ketone-phenol revealed that hydrogen transfer has predominantly occurred and that the reaction with alpha-diketones are generally 7 kcal/mol less endothermic than the respective reactions of the monoketones. These results together with the geometries obtained from the DFT calculations, natural bond order (NBO) analysis, and AIM results indicate that hydrogen abstraction for alpha-diketones is facilitated by the electrophilicity of the ketone, instead of neighboring group participation by the second carbonyl group.

The hydrogen-hydrogen (H-H) bond or hydrogen-hydrogen bonding is formed by the interaction between a pair of identical or similar hydrogen atoms that are close to electrical neutrality and it yields a stabilizing contribution to the overall molecular energy. This work provides new, important information regarding hydrogen-hydrogen bonds. We report that stability of alkane complexes and boiling point of alkanes are directly related to H-H bond, which means that intermolecular interactions between alkane chains are directional H-H bond, not nondirectional induced dipole-induced dipole. Moreover, we show the existence of intramolecular H-H bonds in highly branched alkanes playing a secondary role in their increased stabilities in comparison with linear or less branched isomers. These results were accomplished by different approaches: density functional theory (DFT), ab initio, quantum theory of atoms in molecules (QTAIM), and electron localization function (ELF).

A aromaticidade tem sido exaustivamente discutida e continua sendo um tema misterioso. Nesse trabalho é proposto um novo índice de aromaticidade chamado índice baseado na densidade-degenerescência-deslocalização ou, simplesmente, D 3 BIA, numa tentativa de lançar nova introspecção sobre esse tema. Esse índice é baseado na teoria de átomos em moléculas (AIM) e, de certa forma, é suportado pela teoria dos spins acoplandos (SC). A aromaticidade diminui com o número de heteroátomos na molécula aromática, pois a degenerescência diminui, e diminui com o aumento do tamanho do anel do composto aromático porque desfavorece a sobreposição dos estados monoeletrônicos. A relação entre planaridade do anel, sua densidade eletrônica e aromaticidade é também observada. A interação atrativa da ressonância de 6 elétrons π no diânion ciclobutadieno compensa sua interação repulsiva carbono-carbono enquanto no seu parente dicatiônico a ressonância de 2 elétrons π é insuficiente para contrabalancear sua interação repulsiva e adota uma estrutura não-plana.Aromaticity has been exhaustedly discussed for several years and it remains as a misterious issue. In this work it is proposed a new index of aromaticity named density, degeneracy and delocalization-based index of aromaticity or simply D 3 BIA in an attempt to cast new insight and perspective over this theme. This index is based on AIM (atoms in molecules) theory and it is somewhat supported by SC (spin-coupled) theory. Aromaticity decreases as the number of heteroatoms in the aromatic molecule increases since degeneracy decreases and it decreases as the ring size of an aromatic compound increases because it disfavors overlap of single-electron states. The relation between planar structures, electron density and aromaticity is also observed. The attractive interaction of 6π-electron resonance in cyclobutadiene dianion compensate its carbon-to-carbon repulsive interaction while in its dicationic parent the 2π-electron resonance is insufficient to counterbalance its carbon-to-carbon repulsive interaction and it adopts a puckered structure.Keywords: delocalization index; degeneracy; ring density; aromaticity IntroductionIn the nineteenth century benzene was the pivot of the aromatic empiricism. However, for cyclobutadiene both theories gave completely different results. 8,11Hückel's MO study predicted the stability of other aromatic compounds and zero stability for conjugated cyclic systems such as cyclobutadiene. Many different magnetic criteria have appeared so far, based on magnetic shielding or ring current. 20Within MO theory, the aromatic character of benzene is explained through delocalized orbitals. Nevertheless by using the spin coupled valence bond theory (SCVB), Gerratt et al. 21 established that all the six π electrons of benzene are localized and symmetrically distorted towards neighboring carbon atoms on each side and possess the same energy and shape. Other studied aromatic molecules have similar features. 22Within the description SCVB, the stability of aromatic syst...

The atoms in molecule theory shows that the spiropentadiene dication has a planar tetracoordinate carbon (ptC) atom stabilized mainly through the sigma bonds and this atom has a negative charge. The bonds to the ptC atom have less covalent character than the central carbon from neutral spiropentadiene. The total positive charge is spread along the structure skeleton. The analysis of the potential energy surface shows that the dication spiropentadiene has a 2.3 kcal/mol activation barrier for ring opening.

The electronic structures and the stability of tetrahedrane, substituted tetrahedranes and silicon and germanium parents have been studied at oB97XD/6-311++G(d,p) level of theory. The quantum theory of atoms in molecules (QTAIM) was used to evaluate the substituent effect on the carbon cage in the tetrahedrane derivatives. The results indicate that electron withdrawing groups (EWGs) have two different behaviors, i.e., a stronger EWG makes the tetrahedrane cage slightly unstable while a weak EWG causes greater instability in the tetrahedrane cage. On the other hand, the sigma electron donating group, s-EDG, stabilizes the tetrahedrane cage and the p-EDG leads to tetrahedrane disruption. NICS and D3BIA indices were used to evaluate the sigma aromaticity of the studied molecules, where EWGs and EDGs result in the decrease and increase, respectively, of both aromaticity indices, showing that sigma aromaticity plays an important role in the stability of tetrahedrane derivatives. Moreover, for tetra-tert-butyltetrahedrane there is another stability factor: hydrogen-hydrogen bonds which impart a high stabilization in this cage. Generalized valence bond (GVB) theory was also used to explain the stability effect of the substituents directly bonded to the carbon of the tetrahedrane cage. Moreover, the ADMP simulations are in accordance with our thermodynamic results indicating the unstable and stable cages under dynamic simulation.

There are four types of aromaticity criteria: energetic, electronic, magnetic and geometric. The delocalization, density and degeneracy-based index of aromaticity, D3BIA, is an electronic aromaticity index from QTAIM that is not reference dependent and can be used for aromatic, homoaromatic, sigma aromatic and other aromatic systems with varying ring size containing hetereoatoms or not. We used B3LYP, MP2 and MP3 methods to search for linear relations between well-known aromaticity indices and D3BIA for a series of acenes. We found that the D3BIA versus FLU correlation exceeded 91 % and reasonably good correlations exist between D3BIA and HOMA and between D3BIA and PDI. Previous works have shown that D3BIA can be used for homoaromatic systems and tetrahedrane derivatives (sigma aromaticity), but no previous work has validated D3BIA for benzenoid systems. This is the first time we have shown that D3BIA can be used successfully for benzenoid systems, for example, acenes. This work supports and validates the use of D3BIA in classical aromatic systems.

A estabilidade relativa do dicátion 1,3-desidro-5,7-adamantanediila é atribuída a sua aromaticidade tridimensional. Contudo, sua natureza eletrônica não é bem conhecida. A fim de entendê-la melhor, os di-e monocátions do adamantanodiil e alguns de seus análogos foram estudados utilizando a teoria de átomos e moléculas (AIM). Eles foram comparados com análogos de adamantano não-aromáticos. Os resultados de AIM indicam que a densidade eletrônica no centro da estrutura em gaiola e a média de todos os índices de deslocalização, envolvendo seus átomos cabeças-de-ponte são maiores em compostos aromáticos do que em não-aromáticos. A degenerescência energética dos átomos cabeça-de-ponte, a uniformidade e magnitude da carga compartilhada entre estes, distingue os dicátions 1,3-adamantila e 1,3-desidro-5,7-adamantanediila. Contudo, ambos são aromáticos, assim como o 1,3-desidro-5,7-diboroadamantano. O cátion 1,3-desidro-7-adamantila tem uma homoaromaticidade planar característica.The relative stability of the 1,3-dehydro-5,7-adamantanediyl dication is ascribed to its tridimensional aromaticity. However, its electronic nature is not well known. In order to improve its understanding, dicationic and monocationic adamantanedyil species and some key analogues were studied by atoms in molecules (AIM) theory. They were compared to non-aromatic adamantane analogues. AIM results indicate that the density in center of the cage structure and the average of all delocalization indexes involving its bridged atoms are higher in aromatic than in non-aromatic compounds. Degeneracy in energy of the bridged atoms, uniformity and magnitude of their shared charge distinguish the dications 1,3-adamantyl and the 1,3-dehydro-5,7-adamantanediyl. However, both are aromatic as well as the 1,3-dehydro-5,7-diboroadamantane. The 1,3-dehydro-7-adamantyl cation has a characteristic planar homoaromaticity.Keywords: adamantyl dication, adamantyl cation, degeneracy, delocalization index, ring density, aromaticity, tridimensional aromaticity, 1,3-dehydro-5,7-adamantanediyl dication IntroductionSome cationic adamantanediyl species are relatively stable intermediates, e.g., Schleyer's 1,3-dehydro-5,7-adamantanediyl dication (Scheme 1). The adamantanediyl monocation is a stable species and can be synthesized from the 1-adamantanol and fluorosulfonic acid-antimony pentafluoride.1 The adamantanediyl dication has not been observed as persistent long-lived species so far.2 Ionization of 1,3-difluoroadamantane in superacids afforded only the monocation complex 3 C 10 H 14 F-SbF 5 (Scheme 1). Attempts of obtaining other adamantane-1,3-diyl dications were not successful. 4 However, the 1,3-dehydro-5,7-adamantanediyl dication (or 1,3,5,7-bisdehydroadamantane dication) is a stable species.5 It exhibits shielded bridgehead carbons at 6.6 ppm with methylene carbon resonances appearing at 35.6 ppm which is characteristic of hypercoordinate carbocations. 5 Despite that 1,3,5,7-tetrasilaadamantane (Scheme 1) is easily obtained 6 from the 1,3,5-hexamethyl-1,3,5-trisilacycl...