I am not sure if this question is based on a correct understanding of molecular vibrations. Let me first describe what’s in my head.

Start from a molecule with 3N coordinates,

(i) one converts them to internal coordinate, e.g., S = Dq, where q and S are Cartesian and internal coordiantes respectively, D is the transformation matrix.

(ii) then one diagonalizes the Hessian in internal coordinate to obtain the normal modes, the reduced masses are typically printed out in each normal mode.

My question is in step (i), is that possible to print out the reduced mass in each internal coordinate? I thought about two-body systems as the simplest example, e.g., a hydrogen atom or diatomics, the reduced mass comes from step (i). Since there is only one degree of freedom, the diagonalization step (ii) is unnecessary.