Application of nuclear quadrupole resonance to organic chemistry : Probing delocalization effects in tetrachlopentafulvenes

The ³⁵Cl quadrupole resonance frequencies of a series of 6 - aryl - 1,2,3,4 - tetrachloropentafulvenes (2, R₁ = H, R₂ = Aryl) and related substances were determined at 77°K The results indicate the negligible contnbution of the delocalized ‘aromatic’ cyclopentadienide structures in the ground-state. In the relatively dipolar p-dimethylaminophenyl and triapentafulvalene derivatives (9 and 11, respectively), the partial negative-charge is not distributed evenly in the five-membered ring but is localized mostly in the vicinity of the fulvenic double bonds. In most of the 6 - aryl -1,2,3,4 - tetrachloropentafulvenes under study, the NQR absorptions of the chlorine nuclei Z to the aryl groups are considerably shifted to higher frequencies. The effect, which is most pronounced in the 6,6-diphenyl derivative (10), is explained in terms of the spatial structures of the compounds and their substantial deviations from planarity.