BOND ALTERNATION AND UNPAIRED SPIN DISTRIBUTIONS IN THE RADICAL-ANIONS OF CYCLOOCTATETRAENE AND MONOSUBSTITUTED DERIVATIVES - AN ABINITIO STUDY

Ab initio UHF, ROHF, and CI calculations have been performed on the radical anion of 1,3,5,7-cyclooctatetraene (COT.-) with the 3-21G and 6-31G* basis sets. At the UHF level, a D4h structure with alternating bond angles was spuriously found to be lower in energy than a D4h structure with alternating bond lengths. However, CI calculations showed that the true energy minima on the potential surface for COT.- pseudorotation are a pair of equivalent, bond-alternated D4h structures, which are connected by a pair of angle-alternated D4h transition states, lying 4-5 kcal/mol higher in energy. At this level of theory, the calculated hyperfine coupling constant in COT.- is in excellent agreement with that measured by EPR. Calculations on methyl-, fluoro-, and cyano-COT.- have also been performed, and the computed hyperfine coupling constants for these derivatives of COT.- are reported. These three substituents are calculated to have little effect on the degree of bond alternation in COT'-, but fluoro and cyano are each found to reduce the barrier to bond equalization by more than 50%.