6533b7cffe1ef96bd12597af

RESEARCH PRODUCT

1H nmr chemical shifts of [14]annulenes and their dianion salts

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Abstract The bridged (14]annulene 2 was reduced into its dianion and the structure of the dianion salt was proven by chemical and spectroscopic means. The 1H NMR spectroscopic properties of 2 are similar to those of the structurally related bridged [14]annulene 1 , whereas the corresponding dianions 2 2- and 1 2- are strikingly different. The geometries of 1 and 2 and their dianions were studied by means of a modified π-SCF force field approach. Almost planar perimeters without significant bond length alternation are obtained for the neutral annulenes 1 and 2 , whereas their dianions are characterized by structures with reduced symmetry. The 1H chemical shifts of the bridged [14]annulenes and of the related non-cyclic conjugated compounds 3 and 4 were calculated by taking into account ring current, local anisotropic, and charge density contributions. It is shown that the transformation of the diatropic neutral systems 1 and 2 into the paratropic dianions 1 2- /2K+ and 2 2- /2K+ is due to the sign inversion of the ring current contribution. Furthermore we demonstrate that a satisfactory assessment of 1H NMR chemical shifts in the paratropic systems 1 2- /2K+ and 2 2- /2K+ can be achieved only if the interaction between the p-systems of the annulenes and the counterions is explicitely taken into account.