Synthesis of azulitriphyrin(2.1.1)

A ring-contracted carbaporphyrin, azulitriphyrin(2.1.1), was synthesized via intramolecular McMurry coupling reaction. Absorption, NMR spectra and NICS calculations of protonated azulitriphyrin(2.1.1) revealed its Hückel aromatic character as a 14[Formula: see text] porphyrinoid.

Oxidative nitration reaction of antiaromatic 5,15-dioxaporphyrin

Upon oxidation of 20[Formula: see text]-electron antiaromatic 5,15-dioxaporphyrin (DOP) using nitrosonium ions as oxidants, a tetrakis-[Formula: see text]-nitrated compound was formed instead of the expected 18[Formula: see text]-electron aromatic dication species via an oxidative nitration reaction mechanism. Compared with the original DOP, this tetranitro DOP product exhibited a blue shift of absorption and downfield shifts of the [Formula: see text]-pyrrolic proton signals. The unique antiaromatic electronic structure of the tetranitro DOP was disclosed experimentally by electrochemistry and theoretically by DFT and NICS calculations.

Aplicyanins – brominated natural marine products with superbasic character

By using quantum chemical methods (B3LYP/6-311+G(2df,p)//B3LYP/6-31G(d)), we investigated the structures of aplicyanin A, aplicyanin B, aplicyanin C, aplicyanin D, aplicyanin E, and aplicyanin F along with their protonated structures. The calculated gas phase proton affinities of aplicyanin A, aplicyanin C, and aplicyanin E are around –250 kcal mol−1 and therefore more than 10 kcal mol−1 higher as in typical proton sponges such as 1,8-bis(dimethylamino)naphthalene. The compounds aplicyanin B, aplicyanin D, and aplicyanin F show reduced proton affinities of approximately –240 kcal mol−1 because of the acetyl group being conjugated with the imine N=C moiety. Nucleus-independent chemical shift (NICS) calculations on the same level of theory do not show any peculiarities, and a reasonable correlation between the toxicity of aplicyanins and the gas phase proton affinity is not observed.

HETEROATOM EFFECTS ON THE TRIAFLUAVENE AND HEAVIER ANALOGUES, XC5H4 AND XC5H3 (X = C, Si, Ge, N, P, AND As): DFT CALCULATIONS

DFT calculations were carried out on molecular structure of triafluavenes and their heavier analogues, X C 5 H 4 and X C 5 H 3; 1X and 2X (X = C , Si , Ge , N , P , and As ); by using B3LYP/6-311++G** and MP2/6-311++G** //B3LYP/6-311++G** levels of theory with the Gaussian 03 program. The thermal energies (E), enthalpies (H), and free Gibbs energies (G) of 1X and 2X were calculated. The geometrical parameters, natural bonding orbital charge at atoms HOMO and LUMO, the chemical hardness (η), chemical potential (μ), electrophilicity (ω), and the maximum amount of electronic charge, ΔN max , were obtained. The NICS calculations were preformed for both rings of 1X as well as 2X due to determination of the aromatic character of each rings. Molecular electrostatic potential maps were plotted and the infrared and ultraviolet spectra were calculated for 1X and 2X.

Novel aromatic species containing group 14 atoms

The dianions of tetraphenylsilole and tetraphenylgermole behave as aromatic species, as shown by the equalization of C-C bond lengths in these rings and by nucleus-independent chemical shift (NICS) calculations. Similarly, the dianions of 9-silafluorene and 9-germafluorene are aromatic. In the latter species, the heteroatom five-membered rings are more aromatic than the attached benzene rings. Also to be recognized as aromatically stabilized compounds are the stable unsaturated diiminocarbenes, diiminosilylenes, and diiminogermylenes. Evidence for the aromatic delocalization in these molecules comes from NMR, NICS calculations, Raman spectroscopy, isodesmic molecular orbital (MO) calculations, and calculated isomerization stabilization energies. From the presently available evidence, the aromaticity appears to alternate, with C > Ge > Si, going down group 14.

Tests for aromaticity applied to the pentalenoquinones — A computational study

Criteria for aromaticity and antiaromaticity were applied to the four pentalenoquinones, 1,2-, 1,5-, 1,4-, and 1,6-pentalenoquinone, i.e., bicyclo[3.3.0]octa-4,6,8-triene-2,3-dione (7a), bicyclo[3.3.0]octa-3,5,8-triene-2,7-dione (7b), bicyclo[3.3.0]octa-1(5),3,7-triene-2,6-dione (7c), and bicyclo[3.3.0]octa-1(5),3,6-triene-2,8-dione (7d). Geometry optimizations and frequency calculations were done with the pBP/DN* DFT method as implemented in Spartan, and single-point HF/3-21G calculations to obtain Löwdin bond orders (Spartan), as well as HF/6-31G* NICS calculations (Gaussian 98) were also carried out. Geometries and bond orders, chemical hardness, and NICS values gave no definite indication of aromatic or antiaromatic character. However, homodesmotic ring-opening reactions to give acyclic analogues indicated that 7a and 7b are nonaromatic (resonance energies –11 and 5 kJ mol–1) while 7c and 7d are antiaromatic (resonance energies –83 and –54 kJ mol–1). The resonance energies were obtained with the aid of an estimate of the strain energy of the molecules 7 (86 kJ mol–1) by a novel extrapolation procedure on hydropentalenes. Calculated pBP/DN* activation energies for Diels–Alder reactions with ethyne and ethene placed 7a and 7b in an "unreactive" class similar to 1,3-butadiene and fulvene, and 7c and 7d in a "reactive" class, similar to cyclopentadienone.Key words: aromaticity, pentalenoquinones, DFT, hardness, NICS, homodesmotic, resonance energy, bicyclo[3.3.0]octatrienediones.