The geometries of four different series of D(6h)-symmetric polybenzenoid hydrocarbons (PBH) up to and including C(222)H(42) have been optimized at the B3LYP/6-31G(d) level of theory. Excluding C(48)H(24) and C(138)H(42), which have D(3d) minima due to 1,5 H...H repulsions between adjacent perimeter rings, optimized geometries are planar D(6h) minima. Nucleus Independent Chemical Shifts (NICS), at the same level, indicate the presence of individual aromatic rings, which correspond to Clar's qualitative sextets rule (Clar, E. TheAromatic Sextet; Wiley: London, 1972). NICS and the Clar valence electron topologies agree perfectly in the molecule plane; however, the NICS values computed in parallel planes further away from the molecular surface converge, indicating the presence of a uniform magnetic shielding field. For each series, PBH total NICS values (i.e., the sum of NICS values for all rings in the PBH) correlate linearly with the number of carbon atoms, indicating constant magnetic field development within a series. The C-C lengths depend on their proximity to the more olefinic rich molecular perimeters. However, the large PBH (> or =C(48)H(24)) internal C-C distances converge to approximately 1.426 A. In agreement with Clar's rule, HF/6-31G(d)//B3LYP/6-31G(d) vertical ionization potentials and B3LYP/6-31G(d) HOMO-LUMO gaps are largest within the "fully benzenoid" series, where all carbon atoms are members of a single sextet. The largest members of the four series studied are predicted to exhibit semiconducting properties.

Donor-acceptor complexes MX 3 -D (M ) Al, Ga, In; X ) F, Cl, Br, I; D ) YH 3 , PX 3 , X -; Y ) N, P, As) and their components have been studied using self-consistent field and hybrid Hartree-Fock/density functional (B3LYP) methods with effective core potentials. The theoretical dissociation energies of the MX 3 -D complexes decrease in the orders F > Cl > Br > I, Al > Ga < In, and N . P g As for all investigated complexes. The calculated (B3LYP/LANL2DZP) dissociation energies for ammonia adducts are on average 7 kJ mol -1 higher than those from experiment. There is no correlation between the dissociation energy and the degree of charge transfer. Complexes of ammonia and metal fluorides have mostly ionic metal-donor bonds, while the other donor-acceptor adducts are mostly covalently bonded. In addition, a significant charge redistribution between the terminal atoms leads to further electrostatic stabilization of ammonia adducts. Coulomb interactions destabilize MX 3 -PX 3 complexes, and despite some experimental indications, the existence of these particular complexes in the gas phase is improbable. Distortion of MX 3 from planarity under complex formation leads to decreasing X-M-X angles. These decreasing angles correlate well with increasing M-X bond lengths. For all investigated MX 3 -Xsystems a strong correlation of the MX 3 -Xdissociation energy with the M-X bond length increase is found. Correlations between the pyramidal angle X-M-Y and the length of the adjacent M-Y bond have been found for each donor atom Y. All observed trends in structural and thermodynamic properties are qualitatively explained on the basis of a simple electrostatic model.

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.