Electronic Structure of Cage Amines: Absorption Spectra of Triethylenediamine and Quinuclidine

Publisher Website
Google Scholar
Abstract
The gas‐phase electronic absorption spectra of triethylenediamine and quinuclidine have been measured. Each compound shows two strong bands in the regions 1650–2500 Å and 1650–2300 Å, respectively, which have well‐resolved vibrational structure, as well as very weak, structured absorption at longer wavelengths (2560–2700 Å for triethylenediamine and 2300–2500 Å for quinuclidine). An essentially complete vibrational analysis of all but one of these bands has been accomplished. The results indicate a substantial change in the equilibrium positions of the nuclei in the excited states relative to the positions in the ground state. For quinuclidine, the most prominent excited state vibrational mode can be correlated with the ground‐state mode which corresponds to an approach to planarity of the nitrogen‐bearing end of the molecule. For triethylenediamine, ambiguities in the ground‐state assignments preclude conclusions about the upper state geometry. By comparisons with the spectra of other amines, the strong electronic bands of triethylenediamine and quinuclidine are assigned as n → p Rydberg transitions. Excitonmodel calculations of the shift of the first strong band of triethylenediamine relative to the corresponding band in quinculidine predict changes of about 3000 cm−1 to lower energy for an n → 3p z transition and about 1500 cm−1 to higher energy for an n → p x y transition. The observed shift of the 0–0 band is 3920 cm−1 to lower energy, which is clearly in accord with z polarization. The weak, longer‐wavelength absorption of the cage amines is tentatively considered to arise from sub‐Rydberg transitions.