The intramolecular activated nonradiative decay of the first excited singlet of the Rhodamine 3B cation is studied in a large number of polar solvents covering a wide range of viscosities. The photophysics are interpreted in terms of an isomerization process occurring at the diethyl amino group with adiabatic barrier crossing. The experimental data show that friction effects determine the reaction rates, which are modeled in the framework of Kramers and Grote-Hynes theories. The reactant well wavenumber (omega (0)/2 pic) is estimated as 30 cm(-1) and coincides with the wavenumber of the barrier top (omega (b)/2 pic) for the less viscous solvents. The former value is invariant with the solvent and is a good approximation to the reactive molecular mode involved in the nonradiative deactivation. The increase in dipole moment at the barrier top relative to the bottom of the reactant well is inferred from intrinsic barrier heights whose stabilization energies vary according to Onsager's theory of dielectrics.