A theoretical study of the acetylide union, HCC-

The results of various ab initio calculations are reported for the electron ic ground state of the acetylide anion. An "Eyring's lake" in the T-shaped configuration is identified with six different methods (SCF, MP2, CCSD, CCS D-T, CCSD(T), and CEPA-1). The equilibrium bond lengths of HCC- are estimat ed to be r(e)(CH) = 1.0689(3) Angstrom and R-e(CC) = 1.2464(2) Angstrom, an d the ground-state rotational constant is predicted to be B-0 = 41636(20) M Hz. The large permanent dipole moment of mu(0) = -3.093 D should facilitate detection of the anion by microwave spectroscopy. The band centers are pre dicted to be 3211.3 cm(-1)(v(1)), 511.1 cm(-1)(v(2)), and 1805.0 cm(-1)(v(3 )). A large transition dipole moment of 0.477 D is calculated for the v(2) band. Rovibrational levels of HCC- up to approximately 20000 cm(-1) above e quilibrium are calculated with DVR-DGB and FBR methods on the basis of a pr evious CEPA(-1) potential energy surface. Different energy patterns are fou nd and discussed, for which anharmonic and Coriolis resonances are shown to play an important role.