Spectroscopic investigation of the generation of “isomerization” states: Eigenvector analysis of the bend-CP stretch polyad

The highly excited vibrational levels of HCP exhibit a regular energy level and intensity pattern characteristic of 2:1 bend-CP stretch polyads. Stimulated by the experimental observation of vibrational levels with rotational constants $\mathrm{(B}$ -values) 5%–10% larger than other observed levels, Schinke and co-workers noticed that these large-$B$ levels were characterized by atypical nodal structures indicative of large amplitude motion along the minimum energy HCP↔CPH isomerization path [J. Chem. Phys. 107, 9818 (1997)]. In this paper, we show that the transition from “normal-mode-type” to “isomerization” vibrational states arises naturally out of a traditional spectroscopic (algebraic) effective Hamiltonian polyad model. A global least squares fit, based on this polyad ${{H}}^{\mathrm{eff}}$ model, shows that all of the observed “isomerization” states belong to polyads and that the eigenvectors of this ${{H}}^{\mathrm{eff}}$ model have the qualitatively distinct nodal structure first noticed by Schinke and co-workers. The “isomerization” states are not indicative of a breakdown of the polyad model; rather they are a natural consequence of this traditional spectroscopic model.