A study of the infrared and Raman spectra of previously synthesized phthalimide-¹⁵N-H and phthalimide-¹⁵N-D has been carried out. With the new data obtained, the vibrational spectra of the phthalimide molecule in a C_{2 v}* symmetry has been reassigned, but taking into account that the molecule in the solid state forms dimers bonded by intermolecular hydrogen bonds; these dimers have a C_i symmetry. On the other hand, a semi-empiric calculation of the force fields of this molecule has been carried out by the MINDO/3-FORCE method, which required a prior optimization of the molecular geometry; the force field was transformed to the space of internal coordinates.