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 Ã 1Πu states of C3, the Ã 1Πu 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 Ã 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 Ã 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.
- Physical chemistry