The equilibrium geometry, harmonic vibrational frequencies, and estimated ab initio limit for the barrier to planarity of the ethylene radical cation

The equilibrium geometry, barrier to planarity, and harmonic vibrational frequencies were determined theoretically for the ground state of the ethylene radical cation using several quantum mechanical methods and basis sets. The minimum-energy structure is a nonplanar D-2 conformer separated from its symmetry equivalent by a planar transition state. The CCSD(T)/cc-pVTZ level of theory obtained an equilibrium C-C bond length and torsion angle of 1.4004 Angstrom and 21.0degrees, respectively, which are 0.005 Angstrom and 4.0degrees less than the experimentally derived values of Koppel et al. [J. Chem. Phys. 1978, 69, 4252]. The documented reliability of CCSD(T)/cc-pVTZ equilibrium geometries might call into question the experimentally derived geometry. In addition, the barrier to planarity was determined using a series of basis sets and methods aimed at reaching the complete-basis-set limit. The final vibrationless barrier was determined to be 116 +/- 35 cm(-1). Also, to aid in the interpretation of a recent infrared cavity-ring-down experiment, the harmonic vibrational frequencies were determined at the CCSD(T)/TZ2P level of theory. After the harmonic frequencies were scaled by a factor to account for incompleteness in the basis set and electron correlation treatment, the difference between the theoretically and experimentally deduced omega(7)(b(1)) frequencies was a mere 1.4%.