The structure of the triethylenediamine molecule in an excited electronic state

Ultraviolet absorption spectrum has been observed of triethylenediamine (1,4 diazabicyclo[2.2.2]octane) ($\text{D}{}_{\mathrm{<mtext>3</mtext><mtext>A</mtext><mtext>?</mtext>}}$) vapor in the 2200–2600 Å region, and an analysis has been made of its vibrational structure. The vibronic 0-0 transition was determined to be at 2513.65 Å (39782.8 cm^?1), and in the 2540–2590 Å region there were four hot bands found: two from the a₁′-type vibrational levels and two from the a₂″-type vibrational levels of the ground electronic state. The fine structures of these hot bands were examined with 0.49 Å/mm dispersion (slit width = 50 μm). For each of the two a₂″-type hot bands, a progression with 30 ～ 40 cm^?1 spacings was observed; whereas, no such progressions were found for the a₁′-type hot bands, in which absorption peaks are much more concentrated within narrow ranges. These progressions were attributed to a large-amplitude twisting motion of triethylene-diamine molecule. For explaining the whole energy-level structure, another large-amplitude motion, has been postulated. That is a double-minimum deformation motion along the molecular axis; in each minimum, one of the two NC₃ umbrellas is half-way open and the other NC₃ umbrella is half-way closed. The height of the potential barrier between these two minima has been estimated to be 1586 cm^?1.