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The 9-homocubyl cation rearrangement revisited.
Chem. Commun. (Camb.)
PUBLISHED: 11-06-2014
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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.
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Reciprocal hydrogen bonding-aromaticity relationships.
J. Am. Chem. Soc.
PUBLISHED: 09-18-2014
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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 ?-electron delocalization boost aromaticity. Conversely, such interactions are weakened when aromaticity is decreased as a result of more localized quinoidal ? character. Representative examples of the tautomeric equilibria of ?-conjugated heterocyclic compounds in protic solvents and other H-bonding environments also illustrate such H-bonding/aromaticity interplay.
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Al?C monolayer: the planar tetracoordinate carbon global minimum.
Nanoscale
PUBLISHED: 08-07-2014
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Inspired by our theoretical finding that C?Al?(2-) has a planar D?h minimum with two planar tetracoordinate carbons (ptCs), we computationally designed a new two-dimensional (2D) inorganic material, an Al?C monolayer. All carbons in this monolayer are ptC's, stabilized inductively by binding to four electropositive Al atoms in the same plane. The Al?C monolayer is semiconducting with an indirect minimum band gap and a slightly larger direct band gap. Good persistence of the Al?C monolayer is indicated by its moderate cohesive energy, the absence of imaginary modes in its phonon spectrum, and the high melting point predicted by molecular dynamics (MD) simulations. Moreover, a particle-swarm optimization (PSO) global minimum search found the Al?C monolayer to be the lowest-energy 2D structure compared to other Al?C alternatives. Dividing the Al?C monolayer results in one-dimensional (1D) Al?C nanoribbons, which are computed to have quite rich characteristics such as direct or indirect band gaps with various values, depending on the direction of the division and the resulting edge configuration.
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Arenium ions are not obligatory intermediates in electrophilic aromatic substitution.
Proc. Natl. Acad. Sci. U.S.A.
PUBLISHED: 06-27-2014
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Our computational and experimental investigation of the reaction of anisole with Cl2 in nonpolar CCl4 solution challenges two fundamental tenets of the traditional SEAr (arenium ion) mechanism of aromatic electrophilic substitution. Instead of this direct substitution process, the alternative addition-elimination (AE) pathway is favored energetically. This AE mechanism rationalizes the preferred ortho and para substitution orientation of anisole easily. Moreover, neither the SEAr nor the AE mechanisms involve the formation of a ?-complex (Wheland-type) intermediate in the rate-controlling stage. Contrary to the conventional interpretations, the substitution (SEAr) mechanism proceeds concertedly via a single transition state. Experimental NMR investigations of the anisole chlorination reaction course at various temperatures reveal the formation of tetrachloro addition by-products and thus support the computed addition-elimination mechanism of anisole chlorination in nonpolar media. The important autocatalytic effect of the HCl reaction product was confirmed by spectroscopic (UV-visible) investigations and by HCl-augmented computational modeling.
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Dynamic complexation of copper(I) chloride by carbene-stabilized disilicon.
Chemistry
PUBLISHED: 04-15-2014
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Reaction of N-heterocyclic-carbene (NHC)-stabilized disilicon (1) with CuCl gave a carbene-stabilized disilicon-copper(I) chloride complex (2). The nature of the structure and bonding in 2 has been investigated by crystallographic, spectroscopic, and computational methods. The dynamic complexation behavior of 2 was experimentally explored by variable-temperature NMR analysis.
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Aromaticity in transition structures.
Chem Soc Rev
PUBLISHED: 03-19-2014
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Aromaticity is an essential concept in chemistry, employed to account for the unusual stability, reactivity, molecular structures, and other properties of many unsaturated organic compounds. This concept was later extended to inorganic molecules and to saturated systems with mobile electrons, as well as to transition structures, the focus of the present review. Although transition structures are inherently delocalized, not all exhibit aromaticity. We contrast here examples of pericyclic reaction transition structures (where aromaticity is significant) with those for illustrative pseudo-pericyclic reactions (where aromaticity is less or not important). Non-pericyclic reactions may also have aromatic transition structures. State-of-the-art computational methods to evaluate the aromaticity of transition structures are described briefly.
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Covalent hypercoordination: can carbon bind five methyl ligands?
Angew. Chem. Int. Ed. Engl.
PUBLISHED: 03-14-2014
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C(CH3)5(+) is the first reported example of a five-coordinate carbon atom bound only to separate (that is, monodentate) carbon ligands. This species illustrate the limits of carbon bonding, exhibiting Lewis-violating "electron-deficient bonds" between the hypercoordinate carbon and its methyl groups. Though not kinetically persistent under standard laboratory conditions, its dissociation activation barriers may permit C(CH3)5(+) fleeting existence near 0?K.
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Bay-type H···H "bonding" in cis-2-butene and related species: QTAIM versus NBO description.
J Comput Chem
PUBLISHED: 02-26-2014
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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 ?
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Do ?-conjugative effects facilitate SN2 reactions?
J. Am. Chem. Soc.
PUBLISHED: 02-13-2014
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Rigorous quantum chemical investigations of the SN2 identity exchange reactions of methyl, ethyl, propyl, allyl, benzyl, propargyl, and acetonitrile halides (X = F(-), Cl(-)) refute the traditional view that the acceleration of SN2 reactions for substrates with a multiple bond at C? (carbon adjacent to the reacting C? center) is primarily due to ?-conjugation in the SN2 transition state (TS). Instead, substrate-nucleophile electrostatic interactions dictate SN2 reaction rate trends. Regardless of the presence or absence of a C? multiple bond in the SN2 reactant in a series of analogues, attractive C?(?(+))···X(?(-)) interactions in the SN2 TS lower net activation barriers (E(b)) and enhance reaction rates, whereas repulsive C?(?(-))···X(?(-)) interactions increase E(b) barriers and retard SN2 rates. Block-localized wave function (BLW) computations confirm that ?-conjugation lowers the net activation barriers of SN2 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.
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On the large ?-hyperconjugation in alkanes and alkenes.
J Mol Model
PUBLISHED: 01-22-2014
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The conventional view that the ?CC and ?CH 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 ?-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 (C(N)H(2N+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 ?-electron delocalization in unsaturated hydrocarbons usually is ignored, the ?-BLW-DEs (in kcal mol(-1)) are substantial, as exemplified by D2h ethylene (9.0), triplet D2d 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 ?-bonding and ?-antibonding orbitals tends to be smaller than an individual ? conjugative interaction (e.g., 10.2 kcal mol(-1) in anti-1,3-butadiene, the presence of many ?-hyperconjugative interactions (e.g., a total of 12 in anti-1,3-butadiene, see text), result in substantial total ?-stabilization energies (e.g., 19.0 kcal mol(-1) for butadiene), which may surpass those from the ? interactions. Although large in magnitude, ?-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 ?-hyperconjugation, compared to the (skew-free) antiperiplanar n-alkane conformers. Instead of being inconsequential, ?-bond interactions are important and merit consideration.
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Splitting molecular oxygen en route to a stable molecule containing diphosphorus tetroxide.
J. Am. Chem. Soc.
PUBLISHED: 12-11-2013
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In contrast to stable phosphorus oxides such as P4O6 and P4O10 that possess iconic adamantane-like cage structures, highly reactive phosphorus oxides such as PO, PO2, and P2Ox (x = 1-5) only have been studied in the gas phase or by matrix isolation techniques. Elusive diphosphorus tetroxide, the long sought phosphorus analogue of N2O4, is particularly noteworthy. Computations predict that the oxo-bridged O2POPO form of P2O4 is energetically more favored than the P-P bonded O2P-PO2 isomer. Herein, we report the experimental realization of diphosphorus tetroxide-in its energetically disfavored O2P-PO2 form-via carbene-stabilization. The synthesis of the title compound involves the splitting of molecular oxygen by carbene-stabilized diphosphorus.
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Correlation effects on the relative stabilities of alkanes.
J. Am. Chem. Soc.
PUBLISHED: 08-22-2013
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The "alkane branching effect" denotes the fact that simple alkanes with more highly branched carbon skeletons, for example, isobutane and neopentane, are more stable than their normal isomers, for example, n-butane and n-pentane. Although n-alkanes have no branches, the "kinks" (or "protobranches") in their chains (defined as the composite of 1,3-alkyl-alkyl interactions-including methine, methylene, and methyl groups as alkyl entities-present in most linear, cyclic, and branched alkanes, but not methane or ethane) also are associated with lower energies. Branching and protobranching stabilization energies are evaluated by isodesmic comparisons of protobranched alkanes with ethane. Accurate ab initio characterization of branching and protobranching stability requires post-self-consistent field (SCF) treatments, which account for medium range (?1.5-3.0 Å) electron correlation. Localized molecular orbital second-order Møller-Plesset (LMO-MP2) partitioning of the correlation energies of simple alkanes into localized contributions indicates that correlation effects between electrons in 1,3-alkyl groups are largely responsible for the enhanced correlation energies and general stabilities of branched and protobranched alkanes.
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A Hückel theory perspective on Möbius aromaticity.
Org. Lett.
PUBLISHED: 06-25-2013
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Heilbronners Hückel molecular orbital treatment of Möbius 4n-? annulenes is revisited. When uneven twisting in ?-systems of small Möbius rings is accounted for, their resonance energies become comparable to iso-?-electronic linear alkenes with the same number of carbon atoms. Larger Möbius rings distribute ?-twisting more evenly but exhibit only modest aromatic stabilization. Dissected nucleus independent chemical shifts (NICS), based on the LMO (localized molecular orbital)-NICS(0)? index confirm the magnetic aromaticity of the Möbius annulenes considered.
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Substituent Effects on "Hyperconjugative" Aromaticity and Antiaromaticity in Planar Cyclopolyenes.
Org. Lett.
PUBLISHED: 05-31-2013
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Computed aromatic stabilization energies (ASEs) and dissected nucleus independent chemical shifts (NICS?zz) quantify the effect of hyperconjugation on the (anti)aromaticities of the planar conformations of three, five, seven, and nine membered (CnHn)CR2 (R = H, SiH3, F) rings. CH2 and especially C(SiH3)2 groups supply two "pseudo" ? electrons hyperconjugatively along with the olefinic ? electrons in the ring, whereas a CF2 group acts like a partially vacant p orbital. Following the Hückel rule, compounds with 4n+2 (or 4n) pseudo ? electrons are "hyperconjugatively" aromatic (or antiaromatic).
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Evaluation of Triplet Aromaticity by the Isomerization Stabilization Energy.
Org. Lett.
PUBLISHED: 05-02-2013
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The many manifestations of aromaticity have long fascinated both experimentalists and theoreticians. Due to their degenerate half-filled MOs, triplet [n]annulenes with 4n ?-electrons are also aromatic, but the degree of their stabilization has been difficult to quantify. The isomerization stabilization energy (ISE) method has been applied to evaluate the triplet aromaticity. The reliability of this approach is indicated by the strong correlation of the ISE results with NICS(1)zz, a magnetic indicator of triplet state aromaticity.
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Nonamethylcyclopentyl cation rearrangement mysteries solved.
Org. Lett.
PUBLISHED: 03-27-2013
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The C1 nonamethylcyclopentyl cation minimum undergoes complete methyl scrambling in SbF5 with a 7 kcal/mol barrier. This corresponds to the rate-limiting conformational interconversion of enantiomeric hyperconjomers via a C(s) transition structure (above right). A remarkable, more rapid, second process only exchanges methyls within sets of four and five (blue and red, see above), as has been observed experimentally at low temperatures. The computed ?2 kcal/mol barrier involves a C(s) [1s,2s] sigmatropic methyl shift transition structure (above left).
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Electrophilic aromatic sulfonation with SO3: concerted or classic S(E)Ar mechanism?
J. Am. Chem. Soc.
PUBLISHED: 11-07-2011
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The electrophilic sulfonation of several arenes with SO(3) was examined by electronic structure computations at the M06-2X/6-311+G(2d,2p) and SCS-MP2/6-311+G(2d,2p) levels of theory. In contrast to the usual interpretations, the results provide clear evidence that in nonpolar media and in the absence of catalysts the mechanism of aromatic sulfonation with a single SO(3) is concerted and does not involve the conventionally depicted 1:1 ? complex (Wheland) intermediate. Moreover, the computed activation energy for the 1:1 process is unrealistically high; barriers for alternative 2:1 mechanisms involving attack by two SO(3) molecules are 12-20 kcal/mol lower! A direct 2:1 sulfonation mechanism, involving a single essential transition state, but no Wheland type intermediate, is preferred generally at MP2 as well as at M06-2X in isolation (gas phase) or in noncomplexing solvents (such as CCl(4) or CFCl(3)). However, in polar, higher dielectric SO(3)-complexing media, M06-2X favors an S(E)Ar mechanism for the 2:1 reaction involving a Wheland-type arene-(SO(3))(2) dimer intermediate. The reaction is slower in complexing solvents, since the association energy, e.g., with nitromethane, must be overcome. But, in accord with the experimental kinetics (second-order in SO(3)), attack by two sulfur trioxide molecules is still favored energetically over reaction with a single SO(3) in CH(3)NO(2). The theoretical results also reproduce the experimental reactivity and regioselectivity trends for benzene, toluene, and naphthalene sulfonation accurately.
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[n]Imperilenes: stacked [n]trannulenes separated by planar cycloalkane rings.
Org. Lett.
PUBLISHED: 06-13-2011
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Two trannulene moieties fused to each other by means of perfectly planar cycloalkane rings comprise an interesting class of molecules (above) named "imperilenes". Based on computed geometries and NICS(zz) values, only the [5], [7], and [9]imperilene singlet states as well as the 4+ charged [4], [6], and [8]imperilenes and their higher energy neutral quintet states are aromatic. The ? electron systems of the individual trannulene rings, rather than the overall electron count, determine the behavior.
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On the advantages of hydrocarbon radical stabilization energies based on R-H bond dissociation energies.
J. Org. Chem.
PUBLISHED: 03-18-2011
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Hydrocarbon radical stabilization energy (RSE) estimates based on the differences in R-H vs CH(3)-H bond dissociation energies have inherent advantages over RSEs based on R-CH(3) vs CH(3)-CH(3), as well as R-R vs CH(3)-CH(3) comparisons, since the R-CH(3) and R-R reference systems are prone to unbalanced contaminating intramolecular interactions involving the R groups. When the effects of steric crowding, branching, protobranching, conjugation, and hyperconjugation are taken into account, R-CH(3) and R-R based RSE values are nearly identical to R-H RSEs. Corrections for electronegativity differences between H and R are not needed to achieve agreement.
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Why benchmark-quality computations are needed to reproduce 1-adamantyl cation NMR chemical shifts accurately.
J Phys Chem A
PUBLISHED: 03-01-2011
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While the experimental (1)H NMR chemical shiftsof the 1-adamantyl cation can be computed within reasonably small error bounds, the usual Hartree-Fock and density functional quantum-chemical computations, as well as those based on rather elaborate second-order Møller-Plesset perturbation theory, fail to reproduce its experimental (13)C NMR chemical shifts satisfactorily. This also is true even if the NMR shielding calculations treat electron correlation adequately by the coupled-cluster singles and doubles model augmented by a perturbative correction for triple excitations (i.e., at the CCSD(T) level) with quadruple-? basis sets. We demonstrate that good agreement can be achieved if highly accurate 1-adamantyl cation equilibrium geometries based on parallel computations of CCSD(T) gradients are employed for the NMR shielding computations.
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Aromaticity in Group 14 homologues of the cyclopropenylium cation.
Chemistry
PUBLISHED: 01-19-2011
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The nature of the bonding and the aromaticity of the heavy Group 14 homologues of cyclopropenylium cations E3H3+ and E2H2EH+ (E, E = C-Pb) have been investigated systematically at the BP86/TZ2P DFT level by using several methods. Aromatic stabilization energies (ASE) were evaluated from the values obtained from energy decomposition analysis (EDA) of charged acyclic reference molecules. The EDA-ASE results compare well with the extra cyclic resonance energy (ECRE) values given by the block localized wavefunction (BLW) method. Although all compounds investigated are Hückel 4n+2 ? electron species, their ASEs indicate that the inclusion of Group 14 elements heavier than carbon reduces the aromaticity; the parent C3H3+ ion and Si2H2CH+ are the most aromatic, and Pb3H3+ is the least so. The higher energies for the cyclopropenium analogues reported in 1995 employed an isodesmic scheme, and are reinterpreted by using the BLW method. The decrease in the strength of both the ? cyclic conjugation and the aromaticity in the order C ? Si>Ge>Sn>Pb agrees reasonably well with the trends given by the refined nucleus-independent chemical shift NICS(0)?zz index.
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Consistent aromaticity evaluations of methylenecyclopropene analogues.
J. Org. Chem.
PUBLISHED: 11-03-2010
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Quantitative evaluations of the aromaticity (antiaromaticity) of neutral exocyclic substituted cyclopropenes (HC)(2)C=X (X = BH to InH (group 13), CH(2) to SnH(2) (group 14), NH to SbH (group 15), O to Te (group 16)) by their computed extra cyclic resonance energies (ECRE, via the block-localized wave function method) and by their aromatic stabilization energies (ASEs, via energy decomposition analyses) correlate satisfactorily (R(2) = 0.974). Electronegative X-based substituents increase the aromaticity of the cyclopropene rings, whereas electropositive substituents have the opposite effect. For example, (HC)(2)C=O is the most aromatic (ECRE = 10.3 kcal/mol), and (HC)(2)C=InH is the most antiaromatic (ECRE = -15.0 kcal/mol). The most refined dissected nucleus-independent chemical shift magnetic aromaticity index, NICS(0)(?zz), also agrees with both energetic indexes (R(2) = 0.968, for ECRE; R(2) = 0.974, for ASE), as do anisotropy of the induced current density plots.
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Aromaticity and relative stabilities of azines.
Org. Lett.
PUBLISHED: 10-12-2010
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The most refined nucleus-independent chemical shift index (NICS(0)(?zz)) and the extra cyclic resonance energies (ECREs), based on the block localized wave function (BLW) method, show that the aromaticity of all azines is like that of benzene. The same is true for aza-naphthalenes relative to naphthalene. The lower relative energies of isomers with vicinal Ns are due to the weakness of NN bonds rather than to reduced aromaticity.
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Starlike aluminum-carbon aromatic species.
Chemistry
PUBLISHED: 05-11-2010
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Is it possible to achieve molecules with starlike structures by replacing the H atoms in (CH)(n)(q) aromatic hydrocarbons with aluminum atoms in bridging positions? Although D(4h) C(4)Al(4)(2-) and D(2) C(6)Al(6) are not good prospects for experimental realization, a very extensive computational survey of fifty C(5)Al(5)(-) isomers identified the starlike D(5h) global minimum with five planar tetracoordinate carbon atoms to be a promising candidate for detection by photoelectron detachment spectroscopy. BOMD (Born-Oppenheimer molecular dynamics) simulations and high-level theoretical computations verified this conclusion. The combination of favorable electronic and geometric structural features (including aromaticity and optimum C-Al-C bridge bonding) stabilizes the C(5)Al(5)(-) star preferentially.
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What is the preferred structure of the Meisenheimer-Wheland complex between sym-triaminobenzene and 4,6-dinitrobenzofuroxan?
J. Org. Chem.
PUBLISHED: 05-11-2010
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The geometries, energies, and electronic properties of possible configurations of Meisenheimer-Wheland (M-W) complexes of sym-triaminobenzenes and 4,6-dinitrobenzofuroxan (DNBF) were investigated theoretically by MP2 and a variety of DFT methods. The pi-pi complex is preferred thermodynamically by more than 15 kcal/mol over the sigma-complexes for the unsubstituted species. However, the N-substituents of the 1,3,5-triaminobenzenes influence the relative stabilities of the alternative configurations significantly. The sigma-syn configuration of the M-W complex of 1,3,5-tris(N-piperidyl)benzene and DNBF has the lowest energy, followed closely by the sigma-anti and pi-pi forms. The small energy differences between different configurations are consistent with the dynamic interconversion of three homomeric structures observed experimentally by NMR. The ca. 1.63 A C-C interring bond exchanges among three equivalent sites. Quantum theory of atoms in molecules (QTAIM) analysis provided insights into the nature of the intermonomer interactions. Charge transfer and sigma bonding account for the stability and remarkably large binding energies of the M-W complexes.
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On the electronic structure and stability of icosahedral r-X2Z10H12 and Z12H(12)(2-) clusters; r = {ortho, meta, para}, X = {C, Si}, Z = {B, Al}.
Phys Chem Chem Phys
PUBLISHED: 03-26-2010
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We report on the electronic structure of the 12-vertex icosahedral clusters r-X(2)Z(10)H(12) and Z(12)H(12)(2-), where X = {C, Si} and Z = {B, Al}. The least stable cluster--with the lowest HOMO-LUMO gap (E(g))--corresponds to the ortho-X(2)Z(10)H(12) isomers for all values of X = {C, Si} and Z = {B, Al}. The well-known energetic order E(para) < E(meta) < E(ortho) for r-carboranes is also valid for all compounds except r-C(2)Al(10)H(12). Substitution of two atoms of carbon or silicon into the icosahedral cage B(12)H(12)(2-) enhances considerably the stability of the system as analyzed from E(g) gaps, as opposite to Al(12)H(12)(2-), where similar gaps are found upon double carbon or silicon substitution regardless of the positions in the cage. In order to highlight similarities and differences in the title clusters, topological analysis of the electron density was performed, together with analysis of the deviation from polyhedron icosahedral form with (i) volumes, skewness and kurtosis calculations; and (ii) continuous shape measures.
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Why are some (CH)4X6 and (CH2)6X4 polyheteroadamantanes so stable?
Org. Lett.
PUBLISHED: 02-26-2010
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The representative isodesmic reactions shown in the Abstract graphic for (CH)(4)X(6) hexaheteroadamantane derivatives reveal energetic nonadditivity to remarkably different extents: while the electropositive element stabilizations are exceptionally large and the pnictide and hexaoxaadamantane stabilizations are more modest, the sulfur and selenium analogues are destabilized. Similar behavior is exhibited by (CH(2))(6)X(4) tetraheteroadamantanes. Analysis shows that aromaticity is not involved; the sign and magnitude of the nonadditivity depends on the interplay of hyperconjugation, electrostatic, and steric (lone pair repulsion) effects.
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Why are perfluorocyclobutadiene and some other (CF)(n)(q) rings non-planar?
Org. Lett.
PUBLISHED: 01-26-2010
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Although surprising, the nonplanarity of C(2h) C(4)F(4) is not unique. While C(6)F(6) is planar, other members of the (CF)(n) family, for example, C(5)F(5)(-), C(6)F(6)(-), C(7)F(7)(-), and triplet C(7)F(7)(-) are not. C(2h) C(4)F(4) is not aromatic, as claimed (see above), but its antiaromaticity is reduced relative to the planar D(2h) form due to decreased pi antibonding and enhanced cross-ring pi overlap. The nonplanar C(2h) geometry also benefits from the relief of repulsive FC-CF bond eclipsing interactions.
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Electrophile affinity: a reactivity measure for aromatic substitution.
J. Am. Chem. Soc.
PUBLISHED: 09-26-2009
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The reactivity and regioselectivity of the electrophilic chlorination, nitration, and alkylation of benzene derivatives were rationalized by comparing literature data for the partial rate factors (ln f) for these S(E)Ar processes with theoretical reactivity parameters. The Electrophile Affinity (Ealpha), a new quantity, is introduced to characterize reactivity and positional selectivity. Ealpha is evaluated theoretically by the energy change associated with formation of an arenium ion by attachment of a model electrophile to the aromatic ring. The dependence between Ealpha and ln f values for chlorination for 11 substitutions of benzene and methyl benzenes had a high correlation coefficient (r = 0.992). Quite satisfactory correlations between Ealpha values and partial rate factors also were obtained for the nitration of substituted benzenes (r = 0.971 for 12 processes) and benzylation of benzene and halobenzenes (r = 0.973 for 13 processes). These results provide clear evidence for the usefulness of the electrophile affinity in quantifying reactivity and regiochemistry. Satisfactory relationships (r >0.97) also were found between EPN (electrostatic potential at nuclei) values, which reflect the variations of electron density at the different arene ring positions, and the experimental partial rate factors (ln f) for the chlorination and nitration reactions, but not for the benzylation. This disaccord is attributed to strong steric influences on the reaction rates for substitutions involving the bulky benzyl moiety.
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2-Norbornyl ion-pair leakage in electrophilic addition of HCl to nortricyclene and norbornene.
Org. Lett.
PUBLISHED: 09-25-2009
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Ion pair "leakage" pathways, located computationally by means of multidimensional potential energy surface scans, rationalize the unsymmetrical D-label scrambling observed experimentally in the DCl addition products of nortricyclene and norbornene. "Classical" addition transition structures can interconvert ("leak") to symmetrical nonclassical 2-norbornyl ion pair species, either TSs or a minimum, before products form.
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Homoconjugation/homoaromaticity in main group inorganic molecules.
J. Am. Chem. Soc.
PUBLISHED: 07-16-2009
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Quantum chemical computations show that three groups of inorganic ions and neutral molecules, whose structures have long been known and characterized, are aromatic due to through-space homoconjugation: (i) I(4)(2+), S(6)N(4)(2+), and S(2)I(4)(2+) dications and the (O(2))(4) cluster with pericyclic transition-state-like (PTS-like) homoaromaticity; (ii) the bishomoaromatic Te(6)(2+) and 1,5-diphosphadithiatetrazocines; and (iii) the spherically homoaromatic Te(6)(4+). The S(2)I(4)(2+) dication has an unusually high S-S bond order (approximately 2.3) and dual PTS-like aromaticity arising from two separate sets of four-center, six-electron (4c-6e) in-plane through-space conjugation. The diamagnetic (O(2))(4) structural unit recently observed in epsilon-phase oxygen solid has quadruple PTS-like aromaticity, each arising from 4c-6e in-plane through-space conjugation within an O(2)-O(2) plane. Finally, we note that the lighter S(6)(4+) and Se(6)(4+) homologues of Te(6)(4+) also are spherically homoaromatic and might be observable in complexes.
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Is cyclopropane really the sigma-aromatic paradigm?
Chemistry
PUBLISHED: 06-30-2009
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Dewar proposed the sigma-aromaticity concept to explain the seemingly anomalous energetic and magnetic behavior of cyclopropane in 1979. While a detailed, but indirect energetic evaluation in 1986 raised doubts-"There is no need to involve sigma-aromaticity,"-other analyses, also indirect, resulted in wide-ranging estimates of the sigma-aromatic stabilization energy. Moreover, the aromatic character of "in-plane", "double", and cyclically delocalized sigma-electron systems now seems well established in many types of molecules. Nevertheless, the most recent analysis of the magnetic properties of cyclopropane (S. Pelloni, P. Lazzeretti, R. Zanasi, J. Phys. Chem. A 2007, 111, 8163-8169) challenged the existence of an induced sigma-ring current, and provided alternative explanations for the abnormal magnetic behavior. Likewise, the present study, which evaluates the sigma-aromatic stabilization of cyclopropane directly for the first time, fails to find evidence for a significant energetic effect. According to ab initio valence bond (VB) computations at the VBSCF/cc-PVTZ level, the sigma-aromatic stabilization energy of cyclopropane is, at most, 3.5 kcal mol(-1) relative to propane, and is close to zero when n-butane is used as reference. Trisilacyclopropane also has very little sigma-aromatic stabilization, compared to Si(3)H(8) (6.3 kcal mol(-1)) and Si(4)H(10) (4.2 kcal mol(-1)). Alternative interpretations of the energetic behavior of cyclopropane (and of cyclobutane, as well as their silicon counterparts) are supported.
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The effect of perfluorination on the aromaticity of benzene and heterocyclic six-membered rings.
J Phys Chem A
PUBLISHED: 05-29-2009
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Despite having six highly electronegative Fs, perfluorobenzene C(6)F(6) is as aromatic as benzene. Ab initio block-localized wave function (BLW) computations reveal that both C(6)F(6) and benzene have essentially the same extra cyclic resonance energies (ECREs). Localized molecular orbital (LMO)-nucleus-independent chemical shifts (NICS) grids demonstrates that the Fs induce only local paratropic contributions that are not related to aromaticity. Thus, all of the fluorinated benzenes (C(6)F(n)H((6-n)), n = 1-6) have similar ring-LMO-NICS(pi zz) values. However, 1,3-difluorobenzene 2b and 1,3,5-trifluorobenzene 3c are slightly less aromatic than their isomers due to a greater degree of ring charge alternation. Isoelectronic C(5)H(5)Y heterocycles (Y = BH(-), N, NH(+)) are as aromatic as benzene, based on their ring-LMO-NICS(pi zz) and ECRE values, unless extremely electronegative heteroatoms (e.g., Y = O(+)) are involved.
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4n pi electrons but stable: N,N-dihydrodiazapentacenes.
J. Org. Chem.
PUBLISHED: 05-15-2009
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Despite having 4n pi electrons, dihydrodiazapentacenes are more viable than their 4n+2 pi azapentacene counterparts. Ab inito valence bond block-localized wave function (BLW) computations reveal that despite having 4n pi electrons, dihydrodiazapentacenes are stabilized and benefit substantially from four dihydropyrazine ethenamine (enamine) conjugations. Almost all of these dihydrodiazapentacenes have large negative overall nucleus independent chemical shifts NICS(0)(pizz) values even though their dihydropyrazine rings (e.g., for 6-H(2)) are modestly antiaromatic, as their paratropic contributions are attenuated by delocalization throughout the system.
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Why do two ?-electron four-membered Hückel rings pucker?
Org. Lett.
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Notwithstanding their two (i.e., 4n + 2) ? electrons, four-membered ring systems, 1-4, favor puckered geometries (1a-4a) despite the reduction in vicinal ? overlap and in the ring atom bond angles. This nonplanar preference is due to ? ? ?* hyperconjugative interactions across the ring (A) rather than to partial 1,3-bonding (B). Electronegative substituents (e.g., F in C(4)F(4)(2+)) reduce the ? ? ?* electron delocalization, and planar geometries result. In contrast, electropositive groups (e.g., SiH(3) in C(4)(SiH(3))(4)(2+)) enhance hyperconjugation and increase the ring inversion barriers substantially.
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Many M©B(n) boron wheels are local, but not global minima.
Phys Chem Chem Phys
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Twenty-six planar boron wheels with a central hypercoordinate atom (M©B(n), M is a 2nd or 3rd period element) were designed following the Schleyer-Boldyrev concept of geometric and electronic fit whereby in-plane ?- as well as ?-aromaticity contribute to the chemical bonding. Global minimum searches using an efficient newly implemented method reveal that most of these boron wheels are only local, rather than global minima. However, the Be©B(8) triplet planar wheel global minimum is a new member of the planar hypercoordinate M©B(n) family. Six categories classify the structures of the other global minima: planar wheels, planar non-wheel forms, quasi-two-center-wheels, as well as leaf-like, pyramid-like, and umbrella-like geometries.
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Is C60 buckminsterfullerene aromatic?
Phys Chem Chem Phys
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C(60) does not have "superaromatic" or even aromatic character, but is a spherically ? antiaromatic and enormously strained species. This explains its very large and positive heat of formation (610 ± 30 kcal mol(-1)). The ? electron character of C(60) was analyzed by dissected nucleus independent chemical shifts (NICS). The results were employed to examine the scope and limitations of Hirschs 2(N + 1)(2) spherical aromaticity rule for several globular cages. C(20)(2+) (18 ? electrons) and C(60)(10+) (50 ? electrons) are spherically ? aromatic, but C(20) (20 ? electrons) and C(60) (60 ? electrons) are spherically ? antiaromatic, due to the high paratropicity of their half-filled ? subshells. Limitations for Hirschs rule, for clusters with more than 50 ? electrons, are illustrated by e.g. the ? aromaticity of the 66 ? electron C(60)(6-) and the lack of ? aromaticity of the 72 ? electron C(48)N(12) and C(60)(12-).
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Verification of stereospecific dyotropic racemisation of enantiopure D and L-1,2-dibromo-1,2-diphenylethane in non-polar media.
Chem. Commun. (Camb.)
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The first example of a dyotropic rearrangement of an enantiomerically pure, conformationally unconstrained, vicinal dibromide confirms theoretical predictions: D and L-1,2-dibromo-1,2-diphenylethane racemise stereospecifically in refluxing benzene without crossover to the meso-isomer. An orbital analysis of this six-electron pericyclic process is presented.
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D3h CN3Be3+ and CO3Li3+: viable planar hexacoordinate carbon prototypes.
Phys Chem Chem Phys
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Searches for planar hexacoordinate carbon (phC) species comprised of only seven atoms uncovered good CX(3)M(3) prototypes, D(3h) CN(3)Be(3)(+) and CO(3)Li(3)(+). The latter is the global minimum. It might also be possible to detect the deep-lying kinetically-viable D(3h) CN(3)Be(3)(+) local minimum, based on its robustness toward molecular dynamic simulations and its very high isomerization barrier.
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Is cyclobutadiene really highly destabilized by antiaromaticity?
Chem. Commun. (Camb.)
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The high energy of cyclobutadiene (CBD) is not due primarily to "anti-aromaticity," but rather to angle strain, torsional strain, and Pauli repulsion between the parallel CC bonds. Estimations including block-localized wavefunction (BLW) computations conclude that the enormous ring strain (ca. 60 kcal mol(-1)) far exceeds its antiaromatic destabilization (only 16.5 kcal mol(-1)).
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Why the mechanisms of digermyne and distannyne reactions with H2 differ so greatly.
J. Am. Chem. Soc.
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Despite their formal relationship to alkynes, ArGeGeAr, ArSnSnAr, and Ar*SnSnAr* [Ar = 2,6-(2,6-iPr(2)C(6)H(3))(2)C(6)H(3); Ar* = 2,6-(2,4,6-iPr(3)C(6)H(2))(2)-3,5-iPr(2)C(6)H] exhibit high reactivity toward H(2), quite unlike acetylenes. Remarkably, the products are totally different. ArGeGeAr can react with 1-3 equiv of H(2) to give mixtures of ArHGeGeHAr, ArH(2)GeGeH(2)Ar, and ArGeH(3). In contrast, ArSnSnAr and Ar*SnSnAr* react with only 1 equiv of H(2) but give different types of products, ArSn(?-H)(2)SnAr and Ar*SnSnH(2)Ar*, respectively. In this work, this disparate behavior toward H(2) has been elucidated by TPSSTPSS DFT computations of the detailed reaction mechanisms, which provide insight into the different pathways involved. ArGeGeAr reacts with H(2) via three sequential steps: H(2) addition to ArGeGeAr to give singly H-bridged ArGe(?-H)GeHAr; isomerization of the latter to the more reactive Ge(II) hydride ArGeGeH(2)Ar; and finally, addition of another H(2) to the hydride, either at a single Ge site, giving ArH(2)GeGeH(2)Ar, or at a Ge-Ge joint site, affording ArGeH(3) + ArHGe:. Alternatively, ArGe(?-H)GeHAr also can isomerize into the kinetically stable ArHGeGeHAr, which cannot react with H(2) directly but can be transformed to the reactive ArGeGeH(2)Ar. The activation of H(2) by ArSnSnAr is similar to that by ArGeGeAr. The resulting singly H-bridged ArSn(?-H)SnHAr then isomerizes into ArHSnSnHAr. The subsequent facile dissociation of the latter gives two ArHSn: species, which then reassemble into the experimental product ArSn(?-H)(2)SnAr. The reaction of Ar*SnSnAr* with H(2) forms in the kinetically and thermodynamically more stable Ar*SnSnH(2)Ar* product rather than Ar*Sn(?-H)(2)SnAr*. The computed mechanisms successfully rationalize all of the known experimental differences among these reactions and yield the following insights into the behavior of the Ge and Sn species: (I) The active sites of ArEEAr (E = Ge, Sn) involve both E atoms, and the products with H(2) are the singly H-bridged ArE(?-H)EHAr species rather than ArHEEHAr or ArEEH(2)Ar. (II) The heavier alkene congeners ArHEEHAr (E = Ge, Sn) cannot activate H(2) directly. Instead, ArHGeGeHAr must first isomerize into the more reactive ArGeGeH(2)Ar. Interestingly, the subsequent H(2) activation by ArGeGeH(2)Ar can take place on either a single Ge site or a joint Ge-Ge site, but ArSnSnH(2)Ar is not reactive toward H(2). The higher reactivity of ArGeGeH(2)Ar in comparison with ArSnSnH(2)Ar is due to the tendency of group 14 elements lower in the periodic table to have more stable lone pairs (i.e., the inert pair effect) and is responsible for the differences between the reactions of ArEEAr (E = Ge, Sn) with H(2). Similarly, the carbene-like ArHGe: is more reactive toward H(2) than is ArHSn:. (III) The doubly H-bridged ArE(?-H)(2)EAr (E = Ge, Sn) species are not reactive toward H(2).
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