Caio L. Firme

Hydrogen–Hydrogen Bonds in Highly Branched Alkanes and in Alkane Complexes: A DFT, ab initio, QTAIM, and ELF Study

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).

Understanding the planar tetracoordinate carbon atom: spiropentadiene dication.

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.

Density, Degeneracy, Delocalization-Based Index of Aromaticity (D 3 BIA)

A aromaticidade tem sido exaustivamente discutida e continua sendo um tema misterioso. Nesse trabalho e proposto um novo indice de aromaticidade chamado indice baseado na densidade-degenerescencia-deslocalizacao ou, simplesmente, D 3 BIA, numa tentativa de lancar nova introspeccao sobre esse tema. Esse indice e baseado na teoria de atomos em moleculas (AIM) e, de certa forma, e suportado pela teoria dos spins acoplandos (SC). A aromaticidade diminui com o numero de heteroatomos na molecula aromatica, pois a degenerescencia diminui, e diminui com o aumento do tamanho do anel do composto aromatico porque desfavorece a sobreposicao dos estados monoeletronicos. A relacao entre planaridade do anel, sua densidade eletronica e aromaticidade e tambem observada. A interacao atrativa da ressonância de 6 eletrons π no diânion ciclobutadieno compensa sua interacao repulsiva carbono-carbono enquanto no seu parente dicationico a ressonância de 2 eletrons π e insuficiente para contrabalancear sua interacao repulsiva e adota uma estrutura nao-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.

Electronic nature of carbonium ions and their silicon analogues.

Nonclassical ions or carbonium ions have multi-center bonding from delocalized sigma or pi electrons. The 2-norbornyl cation, its derivative 6,6-difluoro-2-norbornyl cation, tris-homocyclopropenyl cation, 7-norbornenyl cation, and 4-cyclopentenyl cation and their corresponding silicon analogues were studied in this work. All carbocations have topologically different 3c-2e systems. The magnitude of all delocalization indexes between each atomic pair of the 3c-2e bond can be used to predict homoaromaticity. The silicon analogues have a topologically different 3c-2e bond from their corresponding carbocation.

Theoretical study of photochemical hydrogen abstraction by triplet aliphatic carbonyls by using density functional theory.

The density functional theory (DFT) and quantum theory of atoms in molecules (QTAIM) have been used to study the lowest lying spin states of the photochemical hydrogen abstraction reaction by formaldehyde, acetaldehyde, and acetone in the presence of different hydrogen donors: propane, 2-propanol, and methylamine. Calculations of all the critical points on the PES of these reactions were performed at uB3LYP/6-311++G(d,p). Methylamine is the best hydrogen donor, in thermodynamic and kinetic terms, followed by 2-propanol and finally propane. Secondary C-H hydrogen abstraction in 2-propanol and C-H abstraction in methylamine is thermodynamically and kinetically favored with respect to hydrogen abstraction from the OH and NH functional groups. Charge transfer takes place before the transition state when methylamine is the hydrogen donor, and for other hydrogen donors, charge transfer begins only in the transition state. The extent of the charge transfer in the transition states corresponds to about 50% of the total change in electron density of the oxygen atom of the T(1) carbonyl compounds during the course of the hydrogen abstraction reactions. The effect of solvent was investigated using the continuum solvation model for the reaction of triplet acetaldehyde in acetonitrile, which resulted in a barrierless transition state for hydrogen abstraction from methylamine.

Electronic nature of the aromatic adamantanediyl ions and its analogues

A estabilidade relativa do dication 1,3-desidro-5,7-adamantanediila e atribuida a sua aromaticidade tridimensional. Contudo, sua natureza eletronica nao e bem conhecida. A fim de entende-la melhor, os di- e monocations do adamantanodiil e alguns de seus analogos foram estudados utilizando a teoria de atomos e moleculas (AIM). Eles foram comparados com analogos de adamantano nao-aromaticos. Os resultados de AIM indicam que a densidade eletronica no centro da estrutura em gaiola e a media de todos os indices de deslocalizacao, envolvendo seus atomos cabecas-de-ponte sao maiores em compostos aromaticos do que em nao-aromaticos. A degenerescencia energetica dos atomos cabeca-de-ponte, a uniformidade e magnitude da carga compartilhada entre estes, distingue os dications 1,3-adamantila e 1,3-desidro-5,7-adamantanediila. Contudo, ambos sao aromaticos, assim como o 1,3-desidro-5,7-diboroadamantano. O cation 1,3-desidro-7-adamantila tem uma homoaromaticidade planar caracteristica. 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.

Stability and electronic structures of substituted tetrahedranes, silicon and germanium parents – a DFT, ADMP, QTAIM and GVB study

The electronic structures and the stability of tetrahedrane, substituted tetrahedranes and silicon and germanium parents have been studied at ωB97XD/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, σ-EDG, stabilizes the tetrahedrane cage and the π-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.

Electronic structures of bisnoradamantenyl and bisnoradamantanyl dications and related species

AbstractThe highly pyramidalized molecule bisnoradamantene is extremely reactive toward nucleophiles and dienes. In this work, we studied the electronic structure of bisnoaradamantene, as well as those of its cation and dication, which are previously unreported carbonium ions. According to QTAIM and MO analysis, there is a 3c-2e bonding system in the bisnoradamantenyl cation and a 4c-2e bonding system in the bisnoradamantenyl dication. A topological study indicated that, on going from bisnoradamantene to its dication, π-bond interaction with the bridgehead carbon atom increases. Additional study of the bisnoradamantanyl dication also indicated that it has two multicenter bonding systems. Comparison of the D3BIA and NICS aromaticity indices of these molecules and other derivatives indicates that these indices are well correlated, and analysis of these indices shows that the cationic and dicationic bisnoradamantenyl species are homoaromatic. FigureMolecular graphs and HOMO of bisnoradamantene, bisnoradamantenyl cation and bisnoradamantenyl dication

Derivatives of Spiropentadiene Dication : New Species with Planar Tetracoordinate Carbon (ptC) atom

In this work, nine tetrasubstituted derivatives [NH(2), OCH(3), Li, Na, Si(CH(3))(3)/SiH(2)CH(3,) P(CH(3))(2), Cl, F, and CN] of the spiropentadiene dication were analyzed within the framework of QTAIM. In the studied series, the electron-withdrawing substituents destabilize the ptC-containing spiropentadiene dication. On the other hand, stabilization of this dication is possible for electron-donating substituents only through sigma bonds, such as Li and Na. In all studied systems, according to QTAIM, the pi-electron system does not participate in the stabilization of the ptC atom in the spiropentadiene dication. sigma-electron-donating groups stabilize the spiropentadiene dication system by increasing the charge density of C(ext)-ptC bonds, whereas electron-withdrawing groups remove the charge density from C(ext)-ptC bonds.

Experimental and NMR theoretical methodology applied to geometric analysis of the bioactive clerodane trans-dehydrocrotonin

trans-Dehydrocrotonin (t-DCTN) a bioactive 19-nor-diterpenoid clerodane type isolated from Croton cajucara Benth, is one of the most investigated clerodane in the current literature. In this work, a new approach joining X-ray diffraction data, nuclear magnetic resonance (NMR) data and theoretical calculations was applied to the thorough characterization of t-DCTN. For that, the geometry of t-DCTN was reevaluated by X-ray diffraction as well as 1H and 13C NMR data, whose geometrical parameters where compared to those obtained from B3LYP/6-311G++(d,p) level of theory. From the evaluation of both calculated and experimental values of 1H and 13C NMR chemical shifts and spin-spin coupling constants, it was found very good correlations between theoretical and experimental magnetic properties of t-DCTN. Additionally, the delocalization indexes between hydrogen atoms correlated accurately with theoretical and experimental spin-spin coupling constants. An additional topological analysis from quantum theory of atoms in molecules (QTAIM) showed intramolecular interactions for t-DCTN.