[reaction: see text] Perturbing influences, such as strain and the presence of heteroatoms, complicate the evaluation of aromatic stabilization energies (ASE). These complications are overcome easily by employing the "isomerization method". This is based on the differences (ISE) between total energies computed for only two species: a methyl derivative of the aromatic system and its nonaromatic exocyclic methylene isomer. The ISEs of benzene, pyridine, and naphthalene as well as the strained [5]paracyclophane, methano[10]annulene, and octadehydro[14]annulene are evaluated.

An improved procedure for the synthesis of (N-isocyanimino)-triphenylphosphorane, CϵN-N=PPh 3 (3), is described. The X-ray structure analysis reveals an unusually small N-N=P angle [115.2(2)°] and an N-N bond order of only about 1.5, which indicates considerable CϵN-N --P + participation and electronically more-isolated functional groups (CN, P=N) in the isocyanide than, for example, in the isomeric NϵC-N=PPh 3 (4) [C-N=P = 123.0(4)°, C-N bond order = 2.0]. In order to gain insight into the stereochemical consequences of metal coordination of 3, an X-ray structural study of [Cr-(CO) 5 CϵN-N=PPh 3 ] (5) was also undertaken. Surprisingly, the central bond lengths (C-N, N-N) and angles (C-N-N) remain practically unchanged with noticeable coordination effects occurring only at the periphery of 5, with the N-N-P angle [112.3(2)°] further decreased by 15σ, the elongated (by 7σ) P-N bond, the somewhat shortened (by 4σ) P-C(Ph) bonds and even shorter C-H(Ph) bonds on the one side, and the well-known Cr-C(O) trans contraction on the other. Treatment of 5 or its tungsten derivative with anhydrous Brønstedt and Lewis acids such as CF 3 COOH, HCl, COS, phosgene or,

Despite having six highly electronegative F’s, perfluorobenzene C6F6 is as aromatic as benzene. Ab initio block-localized wave function (BLW) computations reveal that both C6F6 and benzene have essentially the same extra cyclic resonance energies (ECREs). Localized molecular orbital (LMO)-nucleus-independent chemical shifts (NICS) grids demonstrates that the F’s induce only local paratropic contributions that are not related to aromaticity. Thus, all of the fluorinated benzenes (C6F n H(6−n), n = 1−6) have similar ring-LMO-NICSπzz values. However, 1,3-difluorobenzene 2b and 1,3,5-trifluorobenzene 3c are slightly less aromatic than their isomers due to a greater degree of ring charge alternation. Isoelectronic C5H5Y heterocycles (Y = BH−, N, NH+) are as aromatic as benzene, based on their ring-LMO-NICSπzz and ECRE values, unless extremely electronegative heteroatoms (e.g., Y = O+) are involved.