Ab initio study of the amino group twisting and wagging reaction paths in the intramolecular charge transfer of 4-(N,N-dimethylamino)benzonitrile

The behaviour of 4-(N,N-dimethylamino)benzonitrile in its locally-excited and charge-transfer singlet states following the twisting and wagging displacements of the amino group has been investigated theoretically. These reaction paths are expected to play a decisive role in the intramolecular charge transfer observed for this compound in polar solvents. Minimum-energy paths for the lowest excited singlet states have been determined at the CIS level. The corresponding potential-energy profiles have been calculated with the CASSCF and CASPT2 methods. The potential-energy functions depend strongly on the amount of electron correlation included. It can be concluded that, except for large twist angles, wagging leads only to minor changes in the potential-energy curves and dipole moments. The results suggest that the wagging angle can be excluded as a reaction coordinate for the intramolecular charge-transfer process as well as a coupling coordinate of the two lowest excited singlet states. Twisting of the amino group appears to be the relevant reaction coordinate for the intramolecular charge transfer in 4-(N,N-dimethylamino)benzonitrile.