Vibrational Electronic Coupling and a Close Look at a Severe Case

Abstract

AbstractElectronic spectroscopy, at once the oldest and most intricate branch of spectroscopy, has seen intense activity over the past decade. The difference between small‐molecule and large‐molecule spectroscopy is pointed out. The emphasis here is on larger polyatomics. Certain developments of note, especially of the interpretation of unresolved rotational fine structure, are mentioned. No point of principle has remained more tantalizing than the issue of the integrity of an electronic state: to what extent are electronic and nuclear motions really separable, in the Born‐Oppenheimer sense? There is much new theory, yet quantitative applications of theory to real cases are few, especially in polyatomic molecules. A straightforward computational approach to vibronic (= vibrational‐electronic) coupling in molecules such as naphthalene has proved to be quantitatively successful. Physical insight is harder to obtain, however, but a fairly straightforward explanation (based on a polyatomic version of Walsh's rules) can be offered for the presence of exceptionally strong vibronic coupling between two excited electronic states of azulene (C10H8). The effects of this coupling are so profound that the electronic spectrum is highly sensitive to such minor influences as partial deuteration or changes of solvent. These effects can be modelled rather successfully.