SUPEREXCHANGE PATHWAY CALCULATION OF ELECTRONIC COUPLING THROUGH CYCLOHEXANE SPACERS

The nature of the electronic coupling in donor/acceptor systems with c yclohexane-type spacers, including trans-1,4-dimethylenecyclohexane, c is-1,3-dimethylenecyclohexane, 2,6-dimethylenedecalin, 2,7-dimethylene decalin, and 3,16-dimethyleneandrostane, is investigated using a super exchange (SE) pathway method based on ab initio molecular orbital theo ry with natural bond orbitals (NBOs). The pi couplings in anions and c ations are examined. The magnitudes of the couplings calculated from t he SE pathways method are in reasonable agreement with those from Koop mans' theorem, the Delta SCF method, and the Delta MP2 method. Paths i nvolving hops which skip over bonds make the largest contributions to the total coupling, in agreement with previous studies. The dominant p athway 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 sy stems. Rotation of the CH2 groups from the (0,0) to the (90,90) confor mation decreases couplings by factors of 1.5-10. The (0,0) couplings a re larger because the magnitudes of most paths are larger in (0,0) tha n 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 smal lest (factor of about 1.5) because axial CH bond paths make a signific ant contribution in the (90,90) conformation. Finally, the calculated couplings are compared to experimentally derived couplings from molecu les containing the same spacers, but with biphenyl and naphthyl donor/ acceptor groups. Reasonable agreement is found between theory and expe riment on the distance dependence.