``**The Equilibrium Geometry, Harmonic Vibrational Frequencies, and
Estimated Ab Initio Limit for the Barrier to Planarity of the
Ethylene Radical Cation**,'' M. L. Abrams, E. F. Valeev,
C. D. Sherrill, and T. D. Crawford, *J. Phys. Chem. A*
**106**, 2671-2675 (2002).
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
Å and 21.0^{o}, respectively, which are 0.005 Å and
4.0^{o} less than the experimentally derived values of Köppel
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
*w*_{7} (b1) frequencies was a mere 1.4%.