High energy vibrational states of cyanoacetylene: Variational and perturbative analysis of an anharmonic normal mode.

The high energy vibrational eigenstates of cyanoacetylene are calculated with variational and perturbation theory, using a new empirical potential energy function. A detailed analysis of the eigenstates reveals that the HC bond in HCCCN is not a good local mode, even as a zeroth-order description, due predominantly to the linear geometry of the molecule. A unique anharmonic normal mode description is developed and applied to calculate accurately the eigenenergy spectrum with second-order perturbation theory. An indirect coupling scheme is presented which accounts for mixing of nearly degenerate but uncoupled high energy zeroth-order states. These results are used to account for spectral congestion in the experimental high energy spectrum of cyanoacetylene. [ABSTRACT FROM AUTHOR]