Semi‐empirical calculations concerning the geometry of the cycloheptatrienyl anion

Semi‐empirical calculations (mostly MINDO/3) have been performed on singlet C 7 H 7 − . A preference for planar structures is evident from the calculations. Two different structures (an allylic structure and a pentadienylic structure, respectively) are found as absolute minima on the potential surface. They were found to be second‐order Jahn‐Teller deformations of the highly symmetrical (D 7h ) structure, the energy lowering on these distortions being about 9 kcal/mole. A first‐order Jahn‐Teller effect was found to be negligible. The related C 5 H 5 + system is considered briefly for comparison.