A computational method to discover the existence of vibrations (promoting vibrations) symmetrically coupled to the reaction coordinate is presented in the context of chemical reactions in condensed phases. Using the Zwanzig Hamiltonian as a theoretical model and molecular dynamics simulations of a model, linear triatomic in a Lennard-Jones liquid, it is shown that such a coupled motion leaves a unique signature on the spectral density computed from the autocorrelated force on the reaction coordinate. The spectral density is shown to have a peak at the effective frequency of the promoting vibration whose height increases with the reaction coordinate and vanishes at the transition state. (C) 2001 American Institute of Physics.