Bending Vibrational Levels in the Electronic Spectra of Small Radicals

The role played by bending vibrations in the spectroscopy of small carbon-containing radicals is illustrated by the patterns and effects shown by C₃, CCH, and C₃Ar. Because of the large change in the bending frequency between the (Formula presented.) ¹Σ⁺ _g and à ¹Π_u states of C₃, the à ¹Π_u state provides one of the best known examples of the coupling of electronic and vibrational motion in linear molecules (the Renner–Teller effect). The (Formula presented.) ²Σ⁺ and à ²Π states of CCH provide a classic instance of vibronic coupling between two close-lying electronic states, which leads very rapidly to a chaotic pattern of mixed-state vibrational energy levels, which can only be understood by extensive high-quality ab initio calculations. C₃Ar is an approximately T-shaped molecule with no less than four large-amplitude vibrations. Its à state provides a beautiful example of what happens to the angular momentum of a Π state of C₃ when the symmetry is lowered by complex formation.