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 ¹Σ⁺_g and ¹Π_u states of C₃, the 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 ²Σ⁺ and 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 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.