Aromatization of fulvene by complexation with lithium

none4siAt the B3LYP/6-311++G(d,p) level, approach of a lithium atom to a face of the fulvene molecule leads to formation of a complex with binding energy 41 kcal/mol and significant ion-pair character. The fulvene moiety gains a delocalized aromatic cyclic π system, documented by the geometry-based aromaticity index HOMA, and a strong diatropic ring current, visualized by ipsocentric calculation of the π current-density, which leads to an "aromatic" NICS value of -11 ppm. © 2010 American Chemical Society.noneOziminski W.P.; Krygowski T.M.; Fowler P.W.; Soncini A.Oziminski, W. P.; Krygowski, T. M.; Fowler, P. W.; Soncini, A

Capturing the elusive aromaticity of bicalicene

<p>The ring-current aromaticity of the bicalicene molecule arises, in spite of the 16 p carbon perimeter, from strong local diatropic circulations on the two pentagonal rings, as shown by current-density maps computed at the ipsocentric RHF/6-311G** and DFT/6-311G** levels of theory. Conjugated-circuit models cannot capture this pattern of circulation as it arises from 'ionic' contributions in a valence-bond picture. Canonical molecular-orbital analysis reveals a cancellation of paratropic and diatropic frontier-orbital contributions, which explains the difficulties that Huckel-based models have in producing qualitatively correct current-density maps for this molecule. Other measures of aromaticity reflect, to different extents, the dominance of the 'tetraionic' contribution to the aromaticity of this species.</p>

Switchable Mesomeric Betaines Derived from Pyridinium‐Phenolates and Bis(thienyl)ethane

Syntheses of push–pull substituted non-symmetric bis(thienyl)ethenes (BTEs) possessing a central perfluorocyclopentene core are described. The substituent effects of anisole, phenole, and phenolate as well as pyridine, pyridinium, and N-methylpyridinium substituents, joined through their 3- or 4-positions to the central BTE core, respectively, cover the range from very strongly electron-donating [σ(4-phenolate)=−1.00] to extremely strongly electron-withdrawing [σ(pyridinium-4-yl)=+2.57] in the title mesomeric betaines. The different isomers possessing 4-yl/4-yl, 4-yl/3-yl and 3-yl/3-yl substituents represent different combinations of conjugated and cross-conjugated partial structures and cause different spectroscopic properties. In addition, through-space conjugation between the 2- and 2′-position of the thiophenes can be observed which circumvents the charge-separation of through-bond cross-conjugation. The BTE possessing the push–pull chromophore consisting of 3-anisole and 4-pyridinium substituents (24) displays the best extinction coefficients within the series of compounds described here (ϵ=33.8/15.7 L/mol ⋅ cm), while the mesomeric betaine possessing an N-methylpyridinium-4-yl and a 4-phenolate substituent (29) displays considerable bathochromic shifts to λmax=724 nm in its closed form