The resonant excitation of 13 vibrational states of the gauche and the trans forms of CH2D-CH2D isolated in a xenon matrix has been performed with the laser radiation provided by the free electron laser CLIO and by an optical parametric oscillates configuration. The determination of the sample composition by infrared spectroscopy gives the energy dependence of the conformational inversion quantum yield in the 1000-3000 cm(-1) domain. This reaction proves the coupling of the molecular internal rotation with the laser excited vibrational mode. We observe a global increase of the isomerization efficiency with the excitation energy, but with large fluctuations in the energy dependence. The increase is interpreted as due to the addition of new coupling channels when higher states of the vibrational ladder are excited, whereas the fluctuations are a signature of specific molecular couplings. The influence of the temperature and of the nature of the rare gas matrix on the isomerization rate emphasizes the key role of the intermolecular interactions along the reaction path. (C) 1997 American Institute of Physics.