Showing papers by "Paul von Ragué Schleyer published in 2011"

Cleavage of carbene-stabilized disilicon.

Abstract: Reaction of carbene-stabilized disilicon L:Si═Si:L (L: = :C{N(2,6-Pri2C6H3)CH}2, 1) with BH3·THF results in facile cleavage of the silicon–silicon double bond and the formation of two quite different “push–pull” stabilized products with borane- and carbene-coordinated silylene moieties: 2, containing a parent silylene (:SiH2); and 3, containing a unique three-membered cyclosilylene.

A perfectly square-planar tetracoordinated oxygen in a tetracopper cluster-based coordination polymer.

Abstract: A perfectly square-planar, D4h-symmetric, tetracoordinated oxygen has been observed in a tetracopper cluster-based coordination polymer that shows an unusual magnetic moment and ESR signal The stabilization factor of the square-planar tetracoordinated oxygen has been revealed using DFT calculations

Aromaticity in Group 14 Homologues of the Cyclopropenylium Cation

Abstract: The nature of the bonding and the aromaticity of the heavy Group 14 homologues of cyclopropenylium cations E3H3+ and E2H2E'H+ (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 Huckel 4n+2 π 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 π 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)πzz index.

Electrophilic Aromatic Sulfonation with SO3: Concerted or Classic SEAr Mechanism?

Abstract: The electrophilic sulfonation of several arenes with SO3 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 SO3 is concerted and does not involve the conventionally depicted 1:1 σ 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 SO3 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 CCl4 or CFCl3). However, in polar, higher dielectric SO3-complexing media, M06-2X favors an SEAr mechanism for the 2:1 reaction in...

Starlike Aluminum–Carbon Aromatic Species

Abstract: 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.

Designing metal-free catalysts by mimicking transition-metal pincer templates.

Abstract: Whereas nature often promotes reactions by utilizing cheap and abundant light transition metals (TMs; e.g., Fe, Mn, Ni, Cu, and Zn) in enzyme active sites, the majority of man-made catalysts are based on precious heavy TMs (e.g., Ru, Rh, Ir, Pd, and Pt), despite high costs, limited availability, and contamination problems. Development of cheap, green, and effective catalysts (without precious TMs or even TMs at all) is at the forefront of chemical research. The recent discovery of metal-free reversible hydrogen activation by Stephan and co-workers facilitates direct catalytic hydrogenation. On the basis of the frustrated Lewis pair (FLP) principle, 3] we designed metal-free hydrogenation catalysts computationally. The metal-free catalyst 1 c] and the well-known metal–ligand bifunctional hydrogenation catalyst 2 have related electronic and geometric structural features (Scheme 1). Indeed, increasing experimental evidence demonstrates the resemblance between the reactivity of TM-free compounds and TM complexes. Herein, we computationally explore the design of metal-free catalysts by directly mimicking TM catalysts. The M05-2X DFT functional was specifically developed to target nonbonding interactions. We previously calibrated the good performance of the functional in describing weak bonding interactions and applied it to understand the catalytic role of N-heterocyclic carbenes in the metal-free transformation of carbon dioxide into methanol. In our design of FLP-based hydrogen-activation molecules and hydrogenation catalysts, we found that the energetic results given by M05-2X were in good agreement with those provided by CCSD(T) or MP2 calculations. M05-2X (as implemented in the Gaussian 03 program) was thus selected for use in all of the DFT calculations. All of the geometries were optimized and characterized as minima or transition states (TSs) at the M05-2X/6-31GACHTUNGTRENNUNG(d,p) level. The energies were then refined by using M05-2X/6-311++GACHTUNGTRENNUNG(2d,p)// M05-2X/6-31GACHTUNGTRENNUNG(d,p) single-point computations. The M05-2X/ 6-31G ACHTUNGTRENNUNG(d,p) wave functions of the TSs were confirmed to be stable, which indicates the reliability of our single-referencebased calculations. The harmonic frequencies at the same level were employed for zero-point energy corrections and thermal and entropy corrections at 298.15 K and 1 atm. Bulky solvation effects were simulated by using the IEFPCM model with benzene as a representative solvent. The corresponding free energies are discussed below, unless otherwise specified. For clarity, we use Lewis structure drawings to depict the electronic structures of molecules in the main text. The optimized geometries and their Cartesian coordinates are provided in the Supporting Information. We selected a new type of pincer catalyst (3 in Scheme 2) recently developed by Milstein and co-workers as a template to derive TM-free analogues. Such TM pincer catalysts have intriguing electronic structures (see below) and exhibit high s-bond activation reactivity (e.g., involving H H, b] C H, N H, and O H 14b] bonds). They promote the direct synthesis of amides and imines from alco[a] G. Lu, H. Li, L. Zhao, F. Huang, Prof. Dr. Z.-X. Wang College of Chemistry and Chemical Engineering Graduate University of Chinese Academy of Sciences Beijing,100049 (P.R. China) Fax: (+86) 10-88256674 E-mail : zxwang@gucas.ac.cn [b] Prof. P. v. R. Schleyer Center for Computational Chemistry, University of Georgia Athens, Georgia 30602 (USA) [] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201002631. Scheme 1. Comparison of the metal-free (1) and TM (2) hydrogenation catalysts.

On the Advantages of Hydrocarbon Radical Stabilization Energies Based on R−H Bond Dissociation Energies

Abstract: Hydrocarbon radical stabilization energy (RSE) estimates based on the differences in R−H vs CH3−H bond dissociation energies have inherent advantages over RSEs based on R−CH3 vs CH3−CH3, as well as R−R vs CH3−CH3 comparisons, since the R−CH3 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−CH3 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.

A study of aromatic three membered rings

Abstract: The resonance energies (REs) of neutral three membered ring analogs of the cyclopropenyl cation, computed using block localized wave function (BLW) methods, reveal considerable variations. The RE's of cyclopropenes substituted with exocyclic double bonded groups CX, (X = O, NH, CH2, S, PH, SiH2) increase with the electronegativity of X in the same row (SiH2 < PH < S and CH2 < NH < O). The extra cyclic resonance energies (ECREs) (an energetic measure of aromaticity based on comparisons with the RE's of acyclic models) of these derivatives range from +10.5 kcal/mol for cyclopropenone (X = O) (somewhat aromatic; the benzene ECRE is 29.3 kcal/mol) to −2.4 kcal/mol (slightly antiaromatic) for X = SiH2. Additional disubstitution of the CC double bond by X′ groups (X′ = CH3, NH2, OH, SiH3, PH2, SH) increases the REs considerably, but has only small effects on the ECREs. Even the ECRE of deltic acid (X = O, X′ = OH) is only increased to +13.3 kcal/mol. The conclusion based on ECRE's, that all 12 of the three membered rings are only marginally aromatic/antiaromatic, is supported by the satisfactorily plot (R2 = 0.92) of ECRE against values of NICS(0)πzz (a superior nucleus chemical independent shift magnetic index of aromaticity), which range only from −6.1 ppm (diatropic) for deltic acid (cf., −35.5 ppm for benzene and −14.2 ppm for the parent cyclopropenium ion) to +8.9 ppm (paratropic) for the silicon derivative, X = SiH2, X′ = SiH3. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

Why Benchmark-Quality Computations Are Needed To Reproduce 1-Adamantyl Cation NMR Chemical Shifts Accurately

Abstract: While the experimental 1H 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 Moller-Plesset perturbation theory, fail to reproduce its experimental 13C 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-ζ 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.

Starlike Aluminum—Carbon Aromatic Species.

Abstract: Born-Oppenheimer molecular dynamics simulations and high-level quantum chemical computations (B3LYP, MP2, CCSD(T)) reveal the starlike D5h C5Al5- global minimum with five planar tetracoordinate carbon atoms to be a promising candidate for detection by photoelectron detachment spectroscopy.