Barrier Crossing Reaction of Electronically Excited DBMBF2 in n-Nitriles: The Role of Solvent Polarity on Activation Energy

Abstract

Solvent viscosity and polarity effects on the picosecond barrier crossing reaction of (dibenzylmethine)boron difluoride in n-nitrile and n-alcohol solutions have been studied. The hydrodynamic Kramers theory yields a poor description of the barrier crossing rates in n-nitriles. Nonlinear isoviscosity plots, solvent-dependent effective potential barrier height, and the anomalous Kramers behaviour in long-chain n-nitriles suggest that the transition state is highly polar and the subsequent solvation effects contribute to the reaction rates. An excellent description of the kinetics is obtained by taking into account the contribution of the solvation energy through solute−solvent dipole−dipole interactions. The results suggest that the effective potential barrier of the reaction is higher in more polar n-nitriles than in less polar n-nitriles. The dipole−dipole model yields a lower effective potential barrier in n-nitriles than in n-alcohols due to equilibrium solvation caused by a fast dielectric relaxation of nitriles. The novel approach provides an excellent understanding of the barrier crossing kinetics of DBMBF2 in n-nitriles and n-alcohols since both dynamic and static solvation effects are taken into account.