The isomerization of acetylene to vinylidene is examined theoretically in full dimensionality (six degrees of freedom), using a new ab initio potential energy surface [S. Zou and J. M. Bowman, Chem. Phys. Lett. 368, 421 (2003)]. Eigenfunctions and eigenvalues of the exact Hamiltonian, for zero total angular momentum, are obtained using a series of novel truncation/recoupling procedures that permits calculations up to very high energies. The Hamiltonian is given in diatom-diatom Jacobi coordinates, with the choice H-2-C-2 for the two diatoms in order to exploit the full permutational symmetry of the problem. By examining expectation values of the eigenfunctions, a number of states are definitely identified with vinylidenelike characteristics. Corresponding calculations are also done for C2D2. Full dimensional simulations of the photodetachment spectra of C2H2- and C2D2- are done (within the Franck-Condon approximation) and compared to the experimental ones. For this calculation the ground vibrational state wave function of the anion is obtained using a new force field, based on high quality ab initio calculations, which are also briefly reported. (C) 2003 American Institute of Physics.