Low-energy electronic transitions of alkylamines

We report ab-initio quantum chemical calculations on the electronic transitions of three model alkylamines: trimethylamine, azabicyclo[2.2.2]octane (Abco), and 1,4-diazabicyclo[2.2.2]octane (Dabco) along with NH3 for comparison purposes. Our calculations indicate that the low-lying transitions of these amines are Rydberg excitations with no significant valence state mixing, at least at the ground-state equilibrium geometry. Very good agreement is obtained between calculated excitation energies and observed absorption bands. The changes in the excitation energies, term values, and oscillator strengths of the Rydberg transitions of NH3 upon symmetric alkylation are rationalized at the MO level. Special emphasis is placed on elucidating the ordering of the excited states of the symmetric diamine Dabco. The through-bond and through-space interaction of the Rydberg orbitals on the equivalent nitrogens of Dabco result in a ∼0.5-eV splitting of the resulting states with the symmetric combination states lying at lower energies. The one- and two-photon spectra of Dabco are assigned and, hopefully, some of the existing spectroscopic ambiguities regarding this molecule are resolved. © 1981 American Chemical Society.