SUPEREXCHANGE PATHWAY CALCULATION OF ELECTRONIC COUPLING THROUGH CYCLOHEXANE SPACERS

The nature of the electronic coupling in donor/acceptor systems with cyclohexane-type spacers, including trans-1,4-dimethylenecyclohexane, cis-1,3-dimethylenecyclohexane, 2,6-dimethylenedecalin, 2,7-dimethylenedecalin, and 3,16-dimethyleneandrostane, is investigated using a superexchange (SE) pathway method based on ab initio molecular orbital theory with natural bond orbitals (NBOs). The pi couplings in anions and cations are examined. The magnitudes of the couplings calculated from the SE pathways method are in reasonable agreement with those from Koopmans' theorem, the Delta SCF method, and the Delta MP2 method. Paths involving hops which skip over bonds make the largest contributions to the total coupling, in agreement with previous studies. The dominant pathway in every case is through CC antibonds. The SE method is used to examine the dependence of the coupling on rotation of the CH2 groups in the dimethylenecyclohexane and dimethylenedecalin donor/acceptor systems. Rotation of the CH2 groups from the (0,0) to the (90,90) conformation decreases couplings by factors of 1.5-10. The (0,0) couplings are larger because the magnitudes of most paths are larger in (0,0) than in (90,90). This is because interaction of the donor/acceptor groups with CC bonds or CC* antibonds in the bridges is generally larger in (0,0). In 1,3-dimethylenecyclohcxane the coupling decrease is the smallest (factor of about 1.5) because axial CH bond paths make a significant contribution in the (90,90) conformation. Finally, the calculated couplings are compared to experimentally derived couplings from molecules containing the same spacers, but with biphenyl and naphthyl donor/acceptor groups. Reasonable agreement is found between theory and experiment on the distance dependence.