The isomerization of acetylene to vinylidene is examined in four degrees of freedom using a full dimensional potential. Fifteen hundred vibrational wave functions and eigenvalues that extend roughly 1500 cm(-1) above the isomerization threshold are calculated using C-2-H-2 diatom-diatom Jacobi coordinates. Eigenfunctions of the four degree-of-freedom Hamiltonian are obtained using a series of truncation/recoupling procedures that begins with the eigenfunctions of a two degree-of-freedom Hamiltonian. The double-well nature of the isomerization is clear in these calculations and is exploited to divide the eigenfunctions into symmetric and antisymmetric groups. The lowest energy doublet states of vinylidene at two levels of dimensionality reduction indicate a splitting of the order of a wave number. Franck-Condon factors between these molecular eigenstates and a model, ground state vibrational wave function for the vinylidene anion are calculated to simulate the photodetachment spectrum of the vinylidene anion.