Nucleus-Independent Chemical Shifts (NICS) as an Aromaticity Criterion

A comprehensive review is presented on nucleus-independent chem. shift as a criterion for aromaticity. [on SciFinder (R)

The Design of “Neutral” Carbanions with Intramolecular Charge Compensation

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

Octahedral and Tetrahedral Coinage Metal Clusters: Is Three-Dimensional d-Orbital Aromaticity Viable?

The first quant. evidence for the viability of three-dimensional arom. clusters involving d-orbitals in pseudo-octahedral coinage metal cages M6Lie2 (M = Cu, Tag, Au) as well as in tetrahedral coinage metal cages M'4Li4 (M' = Cu, Ag) was obtained computationally. These cages exhibit many features similar to those of their square planar M4Li2 analogs. The large neg. nucleus-independent chem. shifts (NICS) at the cage centers indicate three-dimensional delocalization. This diatropic character arises mostly from d-orbital delocalization combined with substantial contributions from the lowest-valence orbitals. The bonding MOs of the pseudo-octahedral clusters M6Li2 (M = Cu, Ag, Au) are analogous to those in similar octahedral clusters involving p-orbital delocalization (e.g., B6H62-). The M'4Li4 clusters exhibit two isomeric forms: metal tetrahedral cages tetracapped by lithium cations on the outside [(M'4).4Li] and lithium tetrahedra on the inside capped by coinage metal atoms on each of the four faces [(Li4).4M]. Whereas the (M'4).4Li type structure is preferred for copper, gold and silver favor the (Li4).4M arrangement. NBO-NICS anal. shows that the large diatropic character in (M'4).4Li structures is due to the favorable contribution from both s- and d-orbitals, whereas the small NICS values in the center of (Li4).4M are due only to the diatropic contributions from the s-orbitals. [on SciFinder (R)

Planar Tetracoordinate Carbon Atoms Centered in Bare Four-membered Rings of Late Transition Metals

Planar tetracoordinate carbons (ptC's) can be stabilized by four-membered ring perimeters composed of four bare transition metal atoms. DFT analyses of the MOs, electronic structures, energies, and magnetic properties of these CM4 species (where M represents isoelectronic combinations of Cu, Ni, Ag, and Pd) reveal striking similarities with main group metal ptC analogs (e.g., CAl2Si2, CAl4Na-, and C5Li2). While the CCu42+, CAg42+, and CNiCu3+ ions have the largest HOMO-LUMO sepns., CCu42+ is the best candidate for detection by gas-phase photoelectron spectroscopy. [on SciFinder (R)

Which NICS Aromaticity Index for Planar pi Rings Is Best?

Five increasingly sophisticated aromaticity indexes, based on nucleus-independent chem. shifts (NICS), were evaluated against a uniform set of arom. stabilization energies (ASE) for 75 mono- and polyheterocyclic five-membered rings. While acceptable statistical correlations were given by all of the NICS methods, the most fundamentally grounded index, NICS(0)pizz (based on the pi contribution to the out-of-plane zz tensor component), performed best statistically (cc = 0.980) and in practice. The easily computable NICS(1)zz index is a useful alternative (cc = 0.968). [on SciFinder (R)

Downfield Proton Chemical Shifts Are Not Reliable Aromaticity Indicators

The downfield chem. shifts of arene hydrogens (delta 1H) are due only in part to the pi ring current contribution [sigma.dblvert.(pi)]; local framework effects are equally important. Neither proton chem. shifts nor even sigma.dblvert.(pi) tensor elements, per se, are reliable aromaticity indicators. Unsatd. polycyclic hydrocarbons with nonarom. quinoid structures have delta 1H and sigma.dblvert.(pi) values in the "arom. range". Conversely, numerous arom. protons, including those in five-membered ring heterocycles, resonate in the "nonarom. range". [on SciFinder (R)

Evidence for d Orbital Aromaticity in Square Planar Coinage Metal Clusters

Quant. evidence for the existence of aromaticity involving the d orbitals of transition metals is provided for the first time. The doubly bridged square planar (D4h) coinage metal clusters (M4Li2, M = Cu (1), Ag (2), and Au (3)) are characterized as arom. by their substantial nucleus independent chem. shifts (NICS) values in the centers (-14.5, -14.1, and -18.6, resp.). Nevertheless, the participation of p orbitals in the bonding (and cyclic electron delocalization) of 1-3 is negligible. Instead, these clusters benefit strongly from the delocalization of d and to some extent s orbitals. The same conclusion applies to Tsipis and Tsipis' H-bridged D4h Cu4H4 ring (4). Canonical MO-NICS anal. of structures 1-3 shows the total diatropic d orbital contributions to the total NICS to be substantial, although the individual contributions of the five sets of filled d orbitals vary. The d orbital aromaticity of Cu4Li2 also is indicated by its atomization energy, 243.2 kcal/mol, which is larger than Boldyrev's doubly (sigma and pi) arom. Al4Li2 (215.9 kcal/mol). [on SciFinder (R)