Electronic signatures of large amplitude motions: Dipole moments of vibrationally excited local-bend and local-stretch states of [S.sub.0] acetylene

A one-dimensional local bend model is used to describe the variation of electronic properties of acetylene in vibrational levels that embody large amplitude local motion on the [S.sub.] potential energy surface. The use of a one-dimensional model for the local bend is justified by comparison to the well-established polyad model which reveals a decoupling of the large amplitude bending from other degrees of freedom in the range of [N.sub.bend] = 14-22.