The C9H9– potential energy surface studied by MINDO/3, MNDO, and ab initio-SCF(STO-3G) calculations

Abstract

Parts of the C₉H₉^–potential energy surface have been studied by MINDO/3, MNDO, and ab initio-SCF(STO-3G) methods. The following structures are found to correspond to energy minima: bicyclo[3.2.2]nona-3,6,8-trienyl anion (1), barbaral-9-yl (tricyclo[3.3.1.0^2,8] nona-3,6-dien-9-yl)anion (2), and the two structures (5) and (6) which both have C_2v symmetry. As expected the D_3h structure (3) is presumably an energy maximum. Due to Jahn–Teller distortions it assumes structures (5) and (6). The following energy ordering (MNDO) was obtained (1) < (2) < (5) < (6). That ion (1) is more stable than (2) is in agreement with experimental results. However, ions (5) and (6) which both could be intermediates in the Cope rearrangement of (2), are found to have much higher energy than (2). This indicates a very slow Cope rearrangement of (2) contrary to earlier predictions. Comparison with results from experimental and theoretical studies of the C₉H₉⁺ potential energy surface is also presented.