Fitting the rovibrational spectra of symmetric tops with unitary equivalent sets of parameters: The ν₅ band of D₃⁷⁰GeH

The high-resolution FTIR spectra of the ν₅ band of monoisotopic D₃⁷⁰GeH have been fitted by two sets of parameters corresponding to different reductions of the effective rotational Hamiltonian for the doubly degenerate vibrational state of a molecule of C_3ν symmetry. The unitary equivalence of both sets of parameters has been verified by using relations which follow from the theory of the reduction of the effective rotational Hamiltonians of C_3ν molecules. This made it possible to determine “true” values of the parameters of the Δl = ±2, Δk = ∓1 and Δl = 0, Δk = ±3 interactions, as well as of the higher-order terms η_J and η_K in the z-Coriolis coupling. Relations between unitary equivalent sets of spectroscopical parameters are compared for the ν₁₀) = 1 vibrational state of CF₃CCH, ν₆ = 1 of CDF₃, and ν₅) = 1 of D₃⁷⁰GeH.