HOMOTROPENYLIUM CATION - STRUCTURE, STABILITY, AND MAGNETIC-PROPERTIES

The equilibrium geometry of the homotropenylium cation (1) and its planar form (5) have been calculated at the MP2/6-31G(d) level of theory. The potential surface of 1 in the direction of the 1,7 coordinate R(1,7) has been explored by MP2 and MP4(SDQ) calculations. In addition, C-13 NMR chemical shifts, H-1 chemical shifts, and the magnetic susceptibility of 1 have been determined for a continuous change from R(1,7) = 1.5 to R(1,7) = 2.5 angstrom using IGLO/6-31G(d,p). All calculations performed predict a single minimum potential curve for 1 with the minimum being located at 2.03 angstrom. At this distance, there is a maximum equalization of positive charge and bond lengths in the seven-membered ring C1-C7, which indicates efficient 6-pi-electron delocalization involving through-space 1,7 interactions. Electron delocalization is also responsible that C-13 chemical shifts in the ring become similar for 1 and that both the magnetic susceptibility and the shift difference between endo and exo proton at C8 reach maximum values. Using 5 as an appropriate reference state, the relative values of magnetic, geometric, and energetic properties of 1 establish its homoaromatic character.