DOUBLE AROMATICITY IN THE 3,5-DEHYDROPHENYL CATION AND IN CYCLO[6]CARBON

The presence of two (4n+2)-electron conjugated systems in perpendicula r planes results in considerable aromatic stabilization. Despite havin g two fewer hydrogens, the 6 pi e-2 sigma e 3,5-dehydrophenyl cation ( C6H3+, 1) is 32.7 (CCSD(T)/6-31G*) and 35.2 kcal/mol (RMP4sdtq/6-3iG* //RMP2(fu)/6-31G) more stable than the phenyl cation (evaluated by an isodesmic reaction involving benzene and m-dehydrobenzene (4)). Catio n 1, the global C6H3+ minimum, is 11.7,24.2, 11.8, and 30.4 kcal/mol l ower in energy than the 2,6- (11) and 3,4-dehydrophenyl (12) cations a s well as the open-chain isomers 13 and 14 (RMP4sdtq/6-31G//RMP2(fu)/ 6-31G + ZPE(RMP2(fu)/6-31G*)). The stability of 1 is increased hyperc onjugatively by 2,4,6-trisilyl substitution. The double aromaticity of 1 is indicated by the computed magnetic susceptibility exaltations (I GLO/II//RMP2(fu)/6-31G) of -5.2, -6.8, -15, and -23.2 relative to 11, 12, 13, and 14, respectively. Thus, 1 fulfills the geometric, energet ic, and magnetic criteria of aromaticity. The double aromaticity of th e D-6h cyclo[6]carbon is apparent from the same criteria.