Computational study of the Stone-Wales transformation in C36

The transition of the D6h neutral and charged isomers to D2d isomers of C36 via Stone-Wales transformation has been studied by means of the hybrid density functional method (B3LYP). The results show that the transition state (TS) and reaction pathway could be identified for the rearrangement from C36-D6h to C36-D2d on the potential energy surface. We found that the neutral and charged transition states all have C2 molecular point group symmetry with the two migrating carbon atoms remaining close to the fullerene surface. The other kind of possible TS with a carbene-like structure along the stepwise reaction path does not exist as a stationary point with the density functionals utilized here. The classical barriers are 6.23 eV through the neutral TS, 6.37 eV through the anionic TS, and 6.29 eV through the cationic TS at the B3LYP/6-31G level of theory.     

富勒烯和富勒烯衍生物中的Stone-Wales旋转

Stone-Wales旋转是富勒烯异构化的基本方式,了解其特征和规律对于理解富勒烯和富勒烯衍生物的形成至关重要.本文采用密度泛函理论方法系统研究了富勒烯和富勒烯衍生物的Stone-Wales旋转.结果显示,富勒烯异构体趋向于从高B55键(两个五元环共用的边)结构向低B55结构转化,满足独立五元环原则的结构或具有低B55键数的异构体在热力学上更为有利.相反,对于富勒烯衍生物,具有更多B55的异构体不仅在热力学上更有利,而且从动力学角度讲,从满足独立五元环原则的结构向不满足的结构的转变比相反过程更容易.这些结果可以解释目前的相关实验事实,暗示了富勒烯衍生物可能是先衍生化后异构化而形成.     

Ozone adsorption on carbon nanotubes: the role of Stone-Wales defects

First-principles calculations within the density functional theory have been performed in order to investigate ozone adsorption on carbon nanotubes. Particular emphasis is placed on the effects of Stone-Wales-like defects on the structural and electronic properties of (i) ideal tubes and (ii) tubes in the presence of ozone. Our results show that structural deformations induced on the pure carbon nanotubes by Stone-Wales defects are similar, as expected, to those induced on graphite; for the (10,0) tube, the semiconducting character is kept, though with a small reduction of the band gap. As for the ozone adsorption, the process on ideal nanotubes is most likely physisorption, though slightly stronger if compared to other previously studied molecules and consistent with the strong oxydizing nature of O(3). However, when ozone adsorbs on Stone-Wales defects, a strong chemisorption occurs, leading to relevant structural relaxations and to the formation of a CO covalent bond; this is consistent with experimental observations of CO functional groups, as well as of the liberation of CO gas phase and of the formation of C vacancies, thus explaining the consumption of the nanotube film upon ozone exposure.     

Ni adsorption on Stone-Wales defect sites in single-wall carbon nanotubes

Ni adsorption on Stone-Wales defect sites in (10,0) zigzag and (5,5) armchair single-wall carbon nanotubes was studied using the density functional theory. The stable adsorption sites and their binding energies on different Stone-Wales defect types were analyzed and compared to those on perfect side walls. It was determined that the sites formed via fusions of 7-7 and 6-7 rings are the most exothermic in the cases of (10,0) and (5,5) defective tubes. In addition C-C bonds associated with Stone-Wales defects are more reactive than the case for a perfect hexagon, thus enhancing the stability of the Ni adsorption. Moreover, the Ni adsorption was found to show a noticeable relationship to the orientation of the Stone-Wales defects with respect to the tube axis. The nature of the Ni adsorption on Stone-Wales defects that have the similar orientation is identical, in spite of the different chiralities.     

Water phase transition induced by a Stone-Wales defect in a boron nitride nanotube

Boron nitride nanotubes (BNNTs) have been reported to possess superior water permeation properties. In this work, using molecular dynamics simulations with partial charges, capturing BNNT polarization effects obtained from quantum calculations, we found that Stone-Wales (SW) defects in a (5,5) BNNT result in phase transition of water, i.e., a transition between liquid-like phase and vapor-like phase was observed. The 90 degree rotation of the B-N bond, SW transformation, in an SW-defective (5,5) BNNT results in breaking of hydrogen bonding with neighboring water molecules and leads to the existence of a vapor-like phase near the SW defect. Water transport rate was evaluated by measuring translocation time. Water in an SW-defective (5,5) BNNT has fewer translocation events, longer translocation time, and a higher axial diffusion coefficient compared to water in a nondefective (5,5) BNNT.     

Intramolecular rearrangements

It has been shown that S--aminoalkyl derivatives of 2-mercaptopyrimidine and 2-mercapto-4, 6-diaminopyrimidine, containing primary and secondary amino groups, undergo S N rearrangement in neutral or weakly alkaline solution, with the formation of the corresponding aminothiols. In the case of derivatives of 2-mercapto-4, 6-diaminopyrimidine, however, the S N rearrangement proceeds more slowly, apparently as a result of the difficulty of formation of the intermediate cyclic compound.For part II, see [9].     

The reactivity of defects at the sidewalls of single-walled carbon nanotubes: the Stone-Wales defect

The reactivity of 5/7/7/5 (Stone-Wales, SW) defects is compared to that of the pristine sidewalls of (5,5) and (10,0) carbon nanotubes (CNTs) using density functional theory (PBE). Infinite tube models (periodic boundary conditions) are used to investigate the reaction energy for CH(2) addition to the ten [5,6], [5,7], [6,7], and [7,7] C-C junctions resulting from SW rotations of the two unique bonds in (5,5) and (10,0) CNTs. In all cases, at least one of the junctions associated with the SW defects is more highly reactive than the pristine tubes. The orientation of these junctions with respect to the tube axis mainly determines the exothermicity. The [7,7] junctions are not the most reactive sites in SW defects of (5,5) and (10,0) CNTs.     

Iodine-mediated rearrangements of diallylsilanes

Diallylsilanes can be made to rearrange upon treatment with I₂. Of the silanes tested, diallyldiphenylsilane showed the greatest propensity to undergo this intramolecular carbocation allylation process. After etherification of the initially formed iodosilane, the products from this transformation represent useful synthetic intermediates, suitable for alkylation and cross-metathesis/annulation reactions.     

The rearrangements of hydroperoxyindolenines

Examination of a series of substituted 3-)hydroxyperoxy) indolenines favours a mechanism for base catalysed decomposition of the dioxetan rather than Criegee type.     

Pummerer rearrangements using chlorotrimethylsilane

Pummerer rearrangements of 3-carbomethoxythian-4-one $\text{S}$-oxide ($\text{1}$) have been investigated and chlorotrimethylsilane found to be the reagent of choice for preparing the corresponding α,β-unsaturated sulphides ($\text{2}$ and $\text{3}$). Related chlorotrimethylsilane-induced Pummerer rearrangements are also reported.     

Acylthioketene-thioacylketene-thiet-2-one rearrangements

Flash vacuum thermolysis (FVT) of 6-aryl-1,3-dioxine-4-thiones 9 leads to the formation of acylthioketenes 10, which are characterized by Ar matrix IR spectroscopy as well as on-line tandem mass spectrometry. The thioketenes 10 undergo a 1,3-shift of the aryl group to generate thioacylketenes 11. Ketenes 11 cyclize to 3-aryl-thiet-2-ones 12, which are also characterized by matrix IR spectroscopy and tandem mass spectrometry. The thiet-2-ones 12 undergo two kinds of reaction under the FVT conditions: (i) cheletropic CO extrusion with formation of arylthioketenes 13, and (ii) cycloreversion to COS and arylacetylene.     

Microwave accelerated aza-Claisen rearrangements

The microwave-assisted thermal aza-Claisen rearrangement of allylic imidates and thiocyanates to the corresponding amides and isothiocyanates is investigated. Significant accelerations of the rearrangement of allylic imidates to amides and of allylic thiocyanates to isothiocyanates in comparison with standard thermal reactions is observed.     

Polyhedral rearrangements in clusters

Alternative reaction mechanisms for the rearrangements of cluster compounds containing from five to twelve atoms (or units of atoms) are deduced using purely geometrical arguments and the fact that cluster structures are determined by strong attractive forces between component units. This forms the first stage of a two-stage procedure. The results of the first stage apply to all clusters of a given geometry and the second stage involves quantitative consideration of the electronic states of a given system to determine the relative energetics of the geometrically determined mechanisms.     

Hydrogen atom mediated Stone-Wales rearrangement of pyracyclene: a model for annealing in fullerene formation

We have investigated the Stone-Wales (SW) rearrangement of pyracyclene (C(14)H(12)) using quantum mechanical molecular modeling. Of particular interest in this study is the effect of an added hydrogen atom on the barriers to SW rearrangement. Hydrogen atoms are found in high abundance during combustion, and their effect upon isomerization of aromatic compounds to more stable species may play an important role in the combustion synthesis of fullerenes. We have calculated the barriers for the SW rearrangement in pyracyclene using density functional theory B3LYP/6-31G(d) and B3LYP/6-311G(d,p). Two mechanisms have been investigated: (i) a mechanism with two identical transition states of C(1) symmetry and a cyclobutyl intermediate and (ii) a mechanism with one transition state containing an sp(3) carbon (J. Am. Chem. Soc. 2003, 125, 5572-5580; Nature 1993, 366, 665-667). We find that the barriers for these mechanisms are 120.0 kcal mol(-1) for the cyclobutyl mechanism and 130.1 kcal mol(-1) for the sp(3) mechanism. Adding a hydrogen atom to the internal bridge carbon atoms of pyracyclene reduces the barrier of the cyclobutyl mechanisms to 67.0 kcal mol(-1) and the sp(3) mechanism to 73.1 kcal mol(-1). The bonding of carbon atoms in pyracyclene is similar to those found in isomers of C(60), and the barriers are low enough so that these reactions can become significant during fullerene synthesis in flames. Adding hydrogen atoms to the external bridge atoms on pyracyclene produces a smaller reduction in the SW barrier and adding hydrogen atoms to nonbridge external carbon atoms results in no reduction of the barrier.     

Carbocation rearrangements in aspernomine biosynthesis

Quantum chemical calculations (B3LYP/6-31G(d)) on carbocation rearrangements that are proposed to occur in the biosynthesis of aspernomine are described. Based on these calculations, a pathway involving a concerted but asynchronous [4+2] cycloaddition that avoids the formation of a secondary carbocation is proposed for small model systems.