STRUCTURES, ELECTRONIC-PROPERTIES, AND ISOMERIZATION OF THE HCCCO RADICAL

The free radical HCCCO exhibits two distinct minima on the potential s urface of the X(2)A' ground electronic state, corresponding to two fav orable; nonequivalent canonical structures: propynonyl (acetylenic) an d propadienonyl (cumulenic). The geometries at these minima and the is omerization coordinate that couples them are characterized by ab initi o calculations at Hartree-Fock and configuration interaction (CISD and QCISD) levels. At the QCISD/6-311G* level, the propynonyl structure has parameters r(HCa) = 1.067 Angstrom, r(CaCb) = 1.213 Angstrom, r(Ca Cc) = 1.435 Angstrom, r(CcO) = 1.183 Angstrom, theta(HCC) = 176.9, the ta(CCC) = 168.5 degrees, and theta(CCO) = 133.3 degrees. Coupled clust er [CCSD and CCSD(T)] calculations of the relative energies were carri ed out at the QCISD optimized geometries. The propynonyl structure is consistently found to be the most stable, in qualitative agreement wit h recent experiments [J. Chem. Phys. 1994, 101, 178]. At the highest l evel of theory used, the propadienonyl structure is predicted to lie 1 2.5 kJ mol(-1) higher in stabilization energy. The (2)A'' excited Renn er-Teller state is found to be linear, with adiabatic and vertical sta bilization energies of 25.6 and 165 kJ mol(-1), respectively, relative to the X(2)A' equilibrium structure. Discrepancies between the experi mental geometry and predicted equilibrium geometry are tentatively asc ribed to vibrational averaging on the extremely anharmonic bending pot ential surface. Predicted harmonic vibrational frequencies, permanent dipole moments, and Fermi contact terms are reported, with comparison to experimental results when possible.