Local mode behavior in the acetylene bending system

The bending eigenfunctions of the acetylene (X) over tilde(1)Sigma(g)(+) st ate, as represented by our recently reported effective Hamiltonian [J. Chem . Phys. 109, 121 (1998)], are analyzed up to E-vib = 15 000 cm(-1). A trans ition from normal to local mode behavior is observed around 8000-10 000 cm( -1), such that above these energies, the eigenstates are better described i n terms of local mode quantum numbers. The local mode behavior in the bend degrees of freedom of acetylene that is described here is in many ways anal ogous to the local mode behavior that has been observed in the stretching d egrees of freedom of many ABA molecules. However, the local mode behavior i n the acetylene bend degrees of freedom, because it involves two two-dimens ional rather than two one-dimensional vibrational modes, encompasses a rich er range of motions. Specifically, in the "local" limit, the bending eigenf unctions are describable in terms of a continuum of motions ranging from lo cal bend (one hydrogen bending) to counter-rotation (the two hydrogens exec uting rotations in opposite directions). (C) 1999 American Institute of Phy sics. [S0021-9606(99)00702-3].