Equilibrium geometry of the ethynyl (CCH) radical

The equilibrium geometry of the ethynyl (CCH) radical has been obtained using the results of high-level quantum chemical calculations and the available experimental data. In a purely quantum chemical approach, the best theoretical estimates (1.208 Angstrom for r(CC) and 1.061-1.063 Angstrom for r(CH)) have been obtained from CCSD-(T), CCSDT, MR-AQCC, and full CI calculations with basis sets up to core-polarized pentuple-zeta quality. In a mixed theoretical-experimental approach, empirical equilibrium geometrical parameters (1.207 Angstrom for r(CC) and 1.069 Angstrom for r(CH)) have been obtained from a least-squares fit to the experimental rotational constants of four isotopomers of CCH which have been corrected for vibrational effects using computed vibration-interaction constants. These geometrical parameters lead to a consistent picture with remaining discrepancies between theory and experiment of 0.001 Angstrom for the CC and 0.006-0.008 Angstrom for the CH distances, respectively. The corresponding r(s) and r(0) geometries are shown not to be representative for the true equilibrium structure of CCH.