RESUMO

Alloy clusters of NaxLiy (4 ≤ x + y ≤ 10) are studied by exploring the potential energy surface in the ab initio MP2 level with the support of a quantum genetic algorithm (QGA). In some cases, the structures have been also refined with DFT and coupled-cluster methods. The general trends of sodium-lithium structures are in line with previous studies. The ionization potentials and polarizabilities to all structures were calculated with MP2 method and the average error between these two properties compared with experimental data was 6% and 13%, respectively. The topological analysis based on quantum theory of atoms in molecules (QTAIM) showed that by increasing the cluster size of the diatomic system there was a decrease of atomic interaction energies. The degree of degeneracy from D3BIA aromaticity index and the analysis of the atomic charges showed the influence (by charge transfer) of the chemical element in lower quantity in the cluster with respect to the other atoms. Our achievements of comparing our theoretical results with available experimental data have demonstrated that our approach can also predict satisfactorily quantum atomic and alloy clusters properties, at least, for low nuclearities.

Assuntos

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Various kinetic studies of the addition of hydrogen halides to alkenes were carried out in the 1930s, 1940s, and 1970s. Since then, there have been theoretical analyses of several aspects of alkene reactivity and regioselectivity during hydrohalogenation. A few works have studied the influence of the hydrogen halide when it is acting as a catalyst (as well as a reactant) under third-order kinetics. However, there has not been any theoretical investigation of the possibility that there is a carbocation intermediate in the contact ion pair. Therefore, we revisited the mechanistic aspects of this reaction at different levels of theory and using IRC (in a gaseous medium) and ONIOM (with an explicit solvent model). We found that (i) there is only one transition state (one energy barrier) for all possible reaction mechanisms in an apolar medium, (ii) there is no carbocation intermediate when a hydrohalogenation reaction is performed in apolar and polar protic media, (iii) hydrochlorination in apolar and polar protic media occurs through an asynchronous concerted mechanism (not through the stepwise mechanism described in undergraduate/graduate literature), and (iv) there are three possible competing mechanisms (with second-, third-, and fourth-order kinetics) in an apolar medium; the mechanism with fourth-order kinetics has the smallest energy barrier, while that with second-order kinetics (a mechanism in which the hydrogen halide does not also act as a catalyst) has the highest energy barrier. Graphical abstract Hydrochlorination of 2,3-dimethylbutene through an asynchronous concerted mechanism.

RESUMO

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.

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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.

RESUMO

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.

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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.

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The cyclobutadiene dication was not experimentally characterized to the date. However, some of its derivatives were. Most of them have planar geometry, but tetramethylcyclobutadiene dication has a nonplanar geometry according to ab initio calculations. From the atoms in molecules (AIM) theoretical analysis, common electronic features for the planar and puckered cyclobutenyl dication derivatives were observed. The planar cyclobutenyl dication derivatives have bond order of chemical bonds in the ring close to unity and relatively small electronic density in the ring. The puckered cyclobutadiene dication and its puckered derivatives have relatively high electronic density in the ring.

RESUMO

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.

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