Abstract : Interactions of electroactive species and adsorbates with the solvent, electrolyte and electrode are observed in the situ infrared spectrum of the interface. The analysis of these spectra is usually difficult, and conventional normal coordinate analysis, while useful for simple molecular systems which are undisturbed by the environment, is not well suited for systems whose vibrational structure is perturbed by external forces. We have recently formulated a Taylor series in symmetry-adaptable spherical harmonic functions which has been applied successfully to account for spectral perturbations observed in an electromechanical system. In the method, an adsorbed molecule is described as the sum of pair-wise potential energy functions which operate between the bonded and non-bonded atoms. Perturbations are introduced by additional pair-wise terms operating between the perturbing force center and the specific atoms in the molecule. In many systems, Morse or Lennard-Jones potential energy functions may be used. Cartesian force constants, from which the vibrations of the system are determined, are calculated from expansion of the pair-wise potential functions to second order using the symmetry-adapted from the Taylor series. The elements of the cartesian force constant matrix have been derived for the general case. Keywords: Infrared spectroelectrochemistry.