Paul von Ragué Schleyer

A Stable Silicon(0) Compound with a Si=Si Double Bond

Dative, or nonoxidative, ligand coordination is common in transition metal complexes; however, this bonding motif is rare in compounds of main group elements in the formal oxidation state of zero. Here, we report that the potassium graphite reduction of the neutral hypervalent silicon-carbene complex L:SiCl4 {where L: is:C[N(2,6-Pri2-C6H3)CH]2 and Pri is isopropyl} produces L:(Cl)Si–Si(Cl):L, a carbene-stabilized bis-silylene, and L:Si=Si:L, a carbene-stabilized diatomic silicon molecule with the Si atoms in the formal oxidation state of zero. The Si-Si bond distance of 2.2294 ± 0.0011 (standard deviation) angstroms in L:Si=Si:L is consistent with a Si=Si double bond. Complementary computational studies confirm the nature of the bonding in L:(Cl)Si–Si(Cl):L and L:Si=Si:L.

Induced magnetic fields in aromatic [n]-annulenes: interpretation of NICS tensor components

The components of nucleus-independent chemical shift (NICS) tensors for Dnhn-annulenes are discussed as indexes of the aromatic character of electronic π systems. The component corresponding to the principal axis perpendicular to the ring plane, NICSzz, is found to be a good measure for the characterisation of the π system of the ring. Isotropic NICS values at ring centres contain large influences from the σ system and from all three principal components of the NICS tensor. At large distances away from the ring center, NICSzz, which is dominated by contributions from the π system, characterizes NICS well.

Predicting molecules--more realism, please!

The body of computations of molecules for which there is as yet no experimental evidence is growing very rapidly. This is simply wonderful—as a marker of the reliability of theory, and, sociologically, in creating a tense and fruitful balance between theory and synthesis in chemistry. Claims of “stability,” implicit and explicit, are made for the calculated molecules; we have been as guilty of this as others. We would like to suggest that literature reports of these claims be qualified, and that the computations performed be described in a circumspect way.

Facts and artifacts about aromatic stability estimation

Abstract The stability of a set of 105 five-membered π-electron systems (involving aromatic, non-aromatic and anti-aromatic species) was evaluated using six isodesmic reactions of which two belong to the subclass of homodesmotic reactions, which are based on cyclic and acyclic reference systems. We demonstrate that the ‘Resonance Energies’ derived from isodesmotic schemes have obvious flaws and do not correct or cancel other contributions to the energy, such as the changes of hybridization, homoconjugation of heterosubstituted cyclopentadienes, conjugative interactions of CC or CX (X=N or P) with a π or pseudo π orbital at Y (Y=O, S, NH, PH), strain, etc. as effectively as possible. Likewise, ‘aromatic stabilization energies (ASE)’ derived from homodesmotic schemes based on the acyclic reference compounds do not give satisfactory results. We strongly recommend that only cyclic reference compounds should be used for ASE and other aromaticity evaluations. The analysis is based on ab initio optimized geometries at B3LYP/6-311+G∗∗.

CH+5: The never‐ending story or the final word?

The closely related Cs(1), Cs(2), and C2v(3) structures of CH5+ have been reinvestigated with high level ab initio theory through the coupled cluster with single and double substitutions (CCSD), and CCSD with perturbatively included connected triple excitations [CCSD(T)] levels, employing a triple‐ζ plus double polarization functions basis set, with f‐functions on carbon as well as d‐functions on the hydrogens [TZ2P(f,d)]. Vibrational frequencies have been computed up to TZ2P+f CCSD; the inclusion of f‐functions on carbon is critical for the configuration interaction with single and double excitations (CISD) and coupled cluster methods using the triple‐ζ basis sets. The changes in geometries between the CISD and CCSD levels are very small, e.g., the C–H bond lengths vary by at most 0.005 A. Thus, the optimizations are essentially converged within theoretical limits. The differences in energies of 1, 2, and 3 decrease and essentially vanish at the most sophisticated levels when the zero point vibrational e...

Are N,N-Dihydrodiazatetracene Derivatives Antiaromatic?

The synthesis and X-ray characterization of two new dialkynylated diazatetracenes and the corresponding N, N-dihydrodiazatetracenes are reported. The dialkynylated heteroacenes are packed in a brick-wall motif that enforces significant overlap of their pi-faces. Cyclic voltammetry indicates that the dehydrogenated forms are easily reduced to their radical anions in solution. The planarity of these species validates the discussion of their aromaticity. Nucleus Independent Chemical Shift (NICS) computations demonstrate that both of these 20 pi and 24 pi electron systems are aromatic. Both experimental and computational results suggest that the aromaticity of the dihydroheteroacenes is reduced.

Planar Pentacoordinate Carbon in CAl5+: A Global Minimum

We report evidence for the first global-minimum structure having a planar pentacoordinate carbon. High-level ab initio computations and quantum molecular dynamics simulations at 300 and 400 K reveal that the most stable CAl5(+) isomer has D5h symmetry and is approximately 3.80 kcal/mol lower in energy than the second most stable alternative. The latter has a nonplanar structure based on a tetrahedral CAl4 moiety. The unexpectedly high proclivity for two-dimensional chemical bonding of the carbon in D5h CAl5(+), the robust thermal stability indicated computationally, and its mass spectrometric detection suggest that experimental characterization of this planar pentacoordinate carbon cation at room temperature is a likely prospect.

Four Decades of the Chemistry of Planar Hypercoordinate Compounds.

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.

Crystal Structure Determination of the Nonclassical 2-Norbornyl Cation

A Nonclassical Conclusion The concept of valence, which underlies the Periodic Table, originated in studies of reactivity rather than structure. Nonetheless, when studies in the mid-20th century suggested that the transient norbornyl cation (C7H11+) reacted as though a carbon center had adopted a formally pentacoordinate motif, this nonclassical structural hypothesis engendered tremendous controversy. Scholz et al. (p. 62) have now succeeded in characterizing the norbornyl cation by x-ray crystallography and confirm the symmetrical fivefold motif. A long-standing debate about the structural symmetry of a positively charged hydrocarbon is settled by x-ray diffraction. After decades of vituperative debate over the classical or nonclassical structure of the 2-norbornyl cation, the long-sought x-ray crystallographic proof of the bridged, nonclassical geometry of this prototype carbonium ion in the solvated [C7H11]+[Al2Br7]– • CH2Br2 salt has finally been realized. This achievement required exceptional treatment. Crystals obtained by reacting norbornyl bromide with aluminum tribromide in CH2Br2 undergo a reversible order-disorder phase transition at 86 kelvin due to internal 6,1,2-hydride shifts of the 2-norbornyl cation moiety. Cooling with careful annealing gave a suitably ordered phase. Data collection at 40 kelvin and refinement revealed similar molecular structures of three independent 2-norbornyl cations in the unit cell. All three structures agree very well with quantum chemical calculations at the MP2(FC)/def2-QZVPP level of theory.

THE STRUCTURE AND STABILITY OF BH5. DOES CORRELATION MAKE IT A STABLE MOLECULE ? QUALITATIVE CHANGES AT HIGH LEVELS OF THEORY

Six BH5 structures were examined in detail using the self‐consistent field (SCF), configuration interaction including single and double excitations (CISD), and coupled cluster including single, double, and perturbatively included connected triple excitations [CCSD(T)] methods in conjunction with a double‐ζ plus polarization (DZP), a triple‐ζ plus polarization (TZ2P), and an augmented TZ(3d1f1g,2p1d) basis set. The C4v and the D3h isomers are high in energy [23 and 45 kcal mol−1, respectively, relative to the Cs(I) structure at DZP CCSD]. Although structure Cs(I) is the global minimum, both Cs structures, where BH5 is comprised of nearly planar monoborane (BH3) and a hydrogen molecule, are essentially equal in energy and allow virtually free rotation of the hydrogen moiety. The global minimum was characterized by vibrational frequency analyses at the TZ2P CCSD(T) level. Final energies were obtained with the TZ(3d1f1g,2p1d) basis set and the CCSD(T) method. At room temperature, the borane–hydrogen complex B...