BARRIER CROSSING REACTION OF ELECTRONICALLY EXCITED DBMBF(2) IN N-NITRILES - THE ROLE OF SOLVENT POLARITY ON ACTIVATION-ENERGY

Solvent viscosity and polarity effects on the picosecond barrier cross ing reaction of (dibenzylmethine)-boron difluoride in n-nitrile and n- alcohol solutions have been studied. The hydrodynamic Kramers theory y ields a poor description of the barrier crossing rates in n-nitriles. Nonlinear isoviscosity plots, solvent-dependent effective potential ba rrier height, and the anomalous Kramers behaviour in long-chain n-nitr iles suggest that the transition state is highly polar and the subsequ ent solvation effects contribute to the reaction rates, An excellent d escription of the kinetics is obtained by taking into account the cont ribution 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 potentia l barrier in n-nitriles than in n-alcohols due to equilibrium solvatio n caused by a fast dielectric relaxation of nitriles, The novel approa ch provides an excellent understanding of the barrier crossing kinetic s of DBMBF(2) in n-nitriles and n-alcohols since both dynamic and stat ic solvation effects are taken into account.