In dieser Zuschrift stellten die Autoren eine theoretische Studie zum Mechanismus der Benzolnitrierung mit einer Säuremischung vor. Sie lieferten außerdem qualitative experimentelle UV-spektroskopische Daten zum Reaktionsverlauf. Zusätzliche NMR-Experimente ergaben nun, dass, anders als in der früheren Studie angenommen, bei den verwendeten Reaktantkonzentrationen kein stabiler π-Komplex entsteht. Namentlich wurde die UV-Absorption bei 320 nm (in den fast sofort nach dem Mischen der Reaktanten aufgenommene Spektren; Abbildung 3 der Originalzuschrift) fälschlich einem frühen Reaktionskomplex zugeordnet. Die Autoren danken Professor Svetlana Simova, Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, für die Aufnahme und Analyse der NMR-Spektren.

The classic SEAr mechanism of electrophilic aromatic substitution (EAS) reactions described in textbooks, monographs, and reviews comprises the obligatory formation of arenium ion intermediates (σ complexes) in a two-stage process. Our findings from several studies of EAS reactions challenge the generality of this mechanistic paradigm. This Account focuses on recent computational and experimental results for three types of EAS reactions: halogenation with molecular chlorine and bromine, nitration by mixed acid (mixture of nitric and sulfuric acids), and sulfonation with SO3. Our combined computational and experimental investigation of the chlorination of anisole with molecular chlorine in CCl4 found that addition-elimination pathways compete with the direct substitution processes. Detailed NMR investigation of the course of experimental anisole chlorination at varying temperatures revealed the formation of addition byproducts. Moreover, in the absence of Lewis acid catalysis, the direct halogenation processes do not involve arenium ion intermediates but instead proceed via concerted single transition states. We also obtained analogous results for the chlorination and bromination of several arenes in nonpolar solvents. We explored by theoretical computations and experimental spectroscopic studies the classic reaction of benzene nitration by mixed acid. The structure of the first intermediate in this process has been a subject of contradicting views. We have reported clear experimental UV/vis spectroscopic evidence for the formation of the first intermediate in this reaction. Our broader theoretical modeling of the process considers the effects of the medium as a bulk solvent but also the specific interactions of a H2SO4 solvent molecule with intermediates and transition states along the reaction path. In harmony with the obtained spectroscopic data, our computational results reveal that the structure of the initial π complex precludes the possibility of electronic charge transfer from the benzene π system to the nitronium unit. In contrast to usual interpretations, our computational results provide compelling evidence that in nonpolar, noncomplexing media and in the absence of catalysts, the mechanism of aromatic sulfonation with sulfur trioxide is concerted and does not involve the conventional σ-complex (Wheland) intermediates. Stable under such conditions, (SO3)2 dimers react with benzene much more readily than monomeric sulfur trioxide. In polar (complexing) media, the reaction follows the classic two-stage SEAr mechanism. Still, the rate-controlling transition state involves two SO3 molecules. The reactivity and regioselectivity in EAS reactions that follow the classic mechanistic scheme are quantified using a theoretically evaluated quantity, the electrophile affinity (Eα), which measures the stabilization energy associated with the formation of arenium ions. Examples of applications are provided.

Strategies to construct zwitterionic anions from the parent anions are proposed. Two principles are employed; the cationic counterpart is (a) attached as a substituent or (b) inserted as an integral part at a remote location in the assembly. The optimized geometries reveal that a striking similarity exists between the zwitterions and the respective precursor parent anion. The computed vibrational frequencies emphasize that these novel entities are minima on their respective potential energy surfaces. A substantial HOMO-LUMO gap indicates that the proposed structures do not show instability in their respective electronic states and that the higher energy configuration states do not contribute to the ground state viability. The separation of charge between the monopoles in these zwitterions is demonstrated by moderately large nonzero dipole moments. Significant large energy barriers for rearrangement to the closely related positional isomers, demonstrated in a few cases, advocate the thermal stability (associated with spectroscopic viability) of the novel molecules. The donor capacity (basicity) of the anionic subunit in these zwitterions is comparable to that of the respective parent anions. Since the qualitative and quantitative features in the designed charged compensated complexes are conserved as anions, these molecules may perhaps be employed in synthetic organic or organometallic chemistry.

From a materials perspective, [18]annulene and its precursor reveal fascinating packing motifs in the solid state.

Molecular SiO 2 and other simple silicon oxides have remained elusive despite the indispensable use of silicon dioxide materials in advanced electronic devices. Owing to the great reactivity of silicon–oxygen double bonds, as well as the low oxidation state of silicon atoms, the chemistry of simple silicon oxides is essentially unknown. We now report that the soluble disilicon compound, L:Si=Si:L (where L: = :C{N(2,6- i Pr 2 C 6 H 3 )CH} 2 ), can be directly oxidized by N 2 O and O 2 to give the carbene-stabilized Si 2 O 3 and Si 2 O 4 moieties, respectively. The nature of the silicon oxide units in these compounds is probed by spectroscopic methods, complementary computations and single-crystal X-ray diffraction.

Experimental evidence is reported for the first intermediate in the classic SEAr reaction of benzene nitration with mixed acid. The UV/Vis spectroscopic investigation of the reaction showed an intense absorption at 320 nm (appearing as a band shoulder) arising from a reaction intermediate. Our theoretical modeling shows that the interaction between the two principal reactants with solvent (H2SO4) molecules significantly affects the structure of the initial complex. In this complex, a larger distance between the aromatic ring and nitronium ion precludes the possibility for electronic charge transfer from the benzene π-system to the electrophile. The computational modeling of the potential energy surface reveals that the reaction favors a stepwise mechanism with intermediate formation of π- and σ-(arenium ion) complexes.

Complexity of the potential energy surface of the 9-homocubyl cation is revealed by Born-Oppenheimer molecular dynamics simulations and high ab initio levels. The stereospecific automerizations observed experimentally involve bridged ions, which have either an aromatic or an anti-aromatic character. New pathways leading to more stable isomers are unveiled.

Natural bond orbital (NBO) analyses and dissected nucleus-independent chemical shifts (NICS π z z ) were computed to evaluate the bonding (bond type, electron occupation, hybridization) and aromatic character of the three lowest-lying Si2CH2 (1-Si, 2-Si, 3-Si) and Ge2CH2 (1-Ge, 2-Ge, 3-Ge) isomers. While their carbon C3H2 analogs favor classical alkene, allene, and alkyne type bonding, these Si and Ge derivatives are more polarizable and can favor “highly electron delocalized”? and “non-classical”? structures. The lowest energy Si 2CH2 and Ge 2CH2 isomers, 1-Si and 1-Ge, exhibit two sets of 3–center 2–electron (3c-2e) bonding; a π-3c-2e bond involving the heavy atoms (C–Si–Si and C–Ge–Ge), and a σ-3c-2e bond (Si–H–Si, Ge–H–Ge). Both 3-Si and 3-Ge exhibit π and σ-3c-2e bonding involving a planar tetracoordinated carbon (ptC) center. Despite their highly electron delocalized nature, all of the Si2CH2 and Ge2CH2 isomers considered display only modest two π electron aromatic character (NICS(0) π z z =--6.2 to –8.9 ppm, computed at the heavy atom ring center) compared to the cyclic-C 3H2 (–13.3 ppm).

AbstractPlanar vierfach koordinierter Kohlenstoff (ptC) wurde seit 1874 für über ein Jahrhundert als unplausibel angesehen. Danach konnten jedoch Beispiele rechnerisch vorhergesagt und experimentell realisiert werden. Sowohl elektronische als auch mechanische Effekte (z. B. in kleinen Ringen und Käfigen) stabilisieren diese ungewöhnlichen Bindungsanordnungen. Konzepte basierend auf den Bindungsmotiven des planaren Methans/planaren Methandikations können erweitert werden, um planar hyperkoordinierte Strukturen anderer chemischer Elemente zu entwerfen. Zahlreiche Konfigurationen von verschiedenen Zentralatomen (Haupt‐ und Übergangsmetallelementen) in einer Ebene mit Koordinationszahlen von bis zu zehn werden diskutiert. Die Evolution von solchen planaren Konfigurationen aus kleinen Molekülen zu Clustern, nanoskopischen Spezies bis hin zu Festkörpern wird aufgezeigt. Für einige experimentell hergestellte planare Materialien wurden außergewöhnliche elektrische und magnetische Eigenschaften nachgewiesen.

The idea of planar tetracoordinate carbon (ptC) was considered implausible for a hundred years after 1874. Examples of ptC were then predicted computationally and realized experimentally. Both electronic and mechanical (e.g., small rings and cages) effects stabilize these unusual bonding arrangements. Concepts based on the bonding motifs of planar methane and the planar methane dication can be extended to give planar hypercoordinate structures of other chemical elements. Numerous planar configurations of various central atoms (main-group and transition-metal elements) with coordination numbers up to ten are discussed herein. The evolution of such planar configurations from small molecules to clusters, to nanospecies and to bulk solids is delineated. Some experimentally fabricated planar materials have been shown to possess unusual electrical and magnetic properties. A fundamental understanding of planar hypercoordinate chemistry and its potential will help guide its future development.

AbstractReview: [computational tools to describe and quantify the different manifestations of aromaticity in transition structures; 51 refs.

Aromaticity, a highly stabilizing feature of molecules with delocalized electrons in closed circuits, is generally restricted to 'Hückel' systems with 4n+2 mobile electrons. Although the Möbius concept extends the principle of aromaticity to 4n mobile electron species, the rare known examples have complex, twisted topologies whose extension is unlikely. Here we report the realization of osmapentalenes, the first planar Möbius aromatic complexes with 16 and 18 valence electron transition metals. The Möbius aromaticity of these osmapentalenes, documented by X-ray structural, magnetic and theoretical analyses, demonstrates the basis of the aromaticity of the parent osmapentalynes. All these osmapentalenes are formed by both electrophilic and nucleophilic reactions of the in-plane π component of the same carbyne carbon, illustrating ambiphilic carbyne reactivity, which is seldom observed in transition metal chemistry. Our results widen the scope of Möbius aromaticity dramatically and open prospects for the generalization of planar Möbius aromatic chemistry.

Computed association energies and dissected nucleus-independent chemical shifts (NICS) document the mutual enhancement (or reduction) of intermolecular interactions and the aromaticity of H-bonded substrates. H-bonding interactions that increase cyclic 4n + 2 π-electron delocalization boost aromaticity. Conversely, such interactions are weakened when aromaticity is decreased as a result of more localized quinoidal π character. Representative examples of the tautomeric equilibria of π-conjugated heterocyclic compounds in protic solvents and other H-bonding environments also illustrate such H-bonding/aromaticity interplay.

The meta-stability of the hexacoordinate CH62+ dication in the gas phase is confirmed by a detailed computational exploration of its potential energy surface, using a modified "Kick" heuristic methodology and by Born-Oppenheimer Molecular-Dynamics simulations to assess its kinetic persistence. The transition states for deprotonation, decomposition into CH3+ and H-3(+), hydrogen scrambling, and H-H rotation are found. In addition, a nearly perfect correlation between the protonation affinities and their coordination number is obtained.

The conventional view that the σCC and σCH bonds in alkanes and unsaturated hydrocarbons are so highly localized that their non-steric interactions are negligible is scrutinized by the block-localized wavefunction (BLW) method. Even molecules considered conventionally to be “strain free” and “unperturbed” have surprisingly large and quite significant total σ-BLW-delocalization energies (DEs) due to their geminal and vicinal hyperconjugative interactions. Thus, the computed BLW-DEs (in kcal mol−1) for the antiperiplanar conformations of the n-alkanes (CNH2N+2, N = 1-10) range from 11.6 for ethane to 82.2 for n-decane and are 50.9 for cyclohexane and 91.0 for adamantane. Although σ-electron delocalization in unsaturated hydrocarbons usually is ignored, the σ-BLW-DEs (in kcal mol−1) are substantial, as exemplified by D 2h ethylene (9.0), triplet D 2d ethylene (16.4), allene (19.3), butadiene (19.0), hexatriene (28.3), benzene (28.1), and cyclobutadiene (21.1). While each individual geminal and vicinal hyperconjugative interaction between hydrocarbon σ-bonding and σ-antibonding orbitals tends to be smaller than an individual π conjugative interaction (e.g., 10.2 kcal mol−1 in anti-1,3-butadiene, the presence of many σ-hyperconjugative interactions (e.g., a total of 12 in anti-1,3-butadiene, see text), result in substantial total σ-stabilization energies (e.g., 19.0 kcal mol−1 for butadiene), which may surpass those from the π interactions. Although large in magnitude, σ-electron delocalization energies often are obscured by cancellation when two hydrocarbons are compared. Rather than being strain-free, cyclohexane, adamantane, and diamantane suffer from their increasing number of intramolecular 1,4-C…C repulsions resulting in elongated C–C bond lengths and reduced σ-hyperconjugation, compared to the (skew-free) antiperiplanar n-alkane conformers. Instead of being inconsequential, σ-bond interactions are important and merit consideration.

We use comparative natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) methods to analyze the proximal bay‐type H···H interactions in cis‐2‐butene and related species, which lead to controversial interpretation as attractive “HH bonding” in the QTAIM framework. We address the challenging questions concerning well established structural, conformational, and vibrational properties of such species that appear to be sharply at odds with the QTAIM interpretation. In contrast to the purported “HH bonding” of QTAIM theory, NBO‐based evaluation of steric (donor–donor) and hyperconjugative (donor–acceptor) interactions unambiguously portrays such H···H contacts as dominated by steric clashes that are only partially softened by weak secondary hyperconjugative interactions, contributing negligibly (bHH < 0.01) to H···H bond order. Additional details of NBO‐based versus QTAIM‐based description are provided by natural bond critical point analysis of topological bond critical point properties, which further emphasizes the contrast between the problematic bay‐type H···H contacts and remaining noncontroversial (consensus) chemical bonds. NBO analysis is thereby shown to be fully consistent with the traditional physical organic concept of repulsive bay‐type H···H contacts, including the corollary array of structural, conformational, and vibrational properties. © 2014 Wiley Periodicals, Inc.

The proposal by Winstein and Trifan that the 2-exo-norbornyl cation has a bridged, non-classical structure initiated a number of investigations into this system. In 1962, H. C. Brown entered into the fray by proposing a pair of rapidly-equilibrating cations and steric effects as alternative explanations. The 2-norbonyl cation was investigated via rate studies, nmr experiments, theoretical calculations, and other methods over a number of years by a number of workers. While the X-ray determination of the 2-exo-norbomyl structure by Scholz et al. (Science, 2013, 341, 62) brought long overdue closure to the vituperative structure controversy, this chapter summarizes other remarkable issues currently engaging research groups worldwide. Unexpectedly, Duncan et al.'s gas phase protonation of norbornene gives the 1,3-dimethylcyclopentyl cation, the C7H11+ global minimum. Merino et al.'s molecular dynamics simulations reveal many acyclic as well as monocyclic intermediates along the reaction pathway. Finally, the 2-endo-norbornyl cation also has a bridged minimum, but "leakage" to the 2-exo cation has a very low barrier. This bridging of the 2-endo cation explains products found from the reaction of alpha-pinene with acetic acid. Thus, even at this late date, norbornyl cation isomers still fascinate.

In an attempt to produce the 2-norbornyl cation (2NB(+)) in the gas phase, protonation of norbornene was accomplished in a pulsed discharge ion source coupled with a supersonic molecular beam. The C7H11(+) cation was size-selected in a time-of-flight mass spectrometer and investigated with infrared laser photodissociation spectroscopy using the method of "tagging" with argon. The resulting vibrational spectrum, containing sharp bands in the C-H stretching and fingerprint regions, was compared to that predicted by computational chemistry. However, the measured spectrum did not match that of 2NB(+), prompting a detailed computational study of other possible isomers of C7H11(+). This study finds five isomers more stable than 2NB(+). The spectrum obtained corresponds to the 1,3-dimethylcyclopentenyl cation, the global minimum-energy structure for C7H11(+), which is produced through an unanticipated ring-opening rearrangement path.

Rigorous quantum chemical investigations of the S(N)2 identity exchange reactions of methyl, ethyl, propyl, allyl, benzyl, propargyl, and acetonitrile halides (X = F-, CI-) refute the traditional view that the acceleration of S(N)2 reactions for substrates with a multiple bond at C-beta (carbon adjacent to the reacting C-alpha center) is primarily due to pi-conjugation in the S(N)2 transition state (TS). Instead, substrate-nucleophile electrostatic interactions dictate S(N)2 reaction rate trends. Regardless of the presence or absence of a C-beta multiple bond in the S(N)2 reactant in a series of analogues, attractive C-beta(delta(+)) ... X(delta(-)) interactions in the S(N)2 TS lower net activation barriers (E-b) and enhance reaction rates, whereas repulsive C-beta(delta(-)) ... X(delta(-)) interactions increase Eb barriers and retard S(N)2 rates. Block-localized wave function (BLW) computations confirm that pi-conjugation lowers the net activation barriers of S(N)2 allyl (1t, coplanar), benzyl, propargyl, and acetonitrile halide identity exchange reactions, but does so to nearly the same extent. Therefore, such orbital interactions cannot account for the large range of E-b values in these systems.