A new way of analyzing vibrational spectra. IV. Application and testing of adiabatic modes within the concept of the characterization of normal modes

The CNM (characterization of normal modes) method for extracting chemical information out of vibrational spectra is tested for vibrational spectra of molecules with relatively strong or relatively weak coupling between internal vibrational modes. Symmetry, parameter set stability, and frequency uncertainty tests are applied to check whether internal vibrational modes, internal mode frequencies, and amplitudes 𝒜nμ comply with symmetry, are independent of the set of internal parameters ζn used to describe molecular geometry or fulfill a Lorentzian correlation between amplitudes 𝒜nμ and frequency differences Δωnμ=ωn−ωμ. In all cases considered, amplitudes 𝒜nμ based on adiabatic internal modes and mass or force constant matrices as metric O are superior to any other definition of amplitude. They represent the basic elements of the new CNM method that leads to chemically reasonable results and presents a new way of extracting chemical information out of vibrational spectra. A number of deficiencies of the potential energy distribution (PED) analysis is discussed.