Knowledge Bank

The photoelectron spectrum of quinoline has been reported by several authors and closely resembles that of napthalene. To determine the location of the n-orbital ionization potential Heilbronner measured the photoelectron spectra of a large number of azanapthalenes, and then used regression analysis to correlate observed bands to their calculated H\”{u}ckle energies. This procedure showed the n-orbital ionization potential to be nearly degenerate with the second $\pi$-ionization potential. This means that the second band in the photoelectron spectrum must consist of two bands, although it is not possible to determine this from the spectrum itself. In the present work we reexamine the photoelectron spectra of quinoline, 2,3-dimethyl quinoline, dihydrocyclopenta[b]quinoline, and the strained system dihydrocyclobuta[b]quinoline (see structural formula). A comparison of these photoelectron spectra to those of the analogous naphthalene compounds (some of which are also reported for the first time) shows that the second band is Indeed split in the case of dihydrocyclobuta[b]quinoline, presumably because the n-ionization potential is now shifted to higher energy. A discussion of the magnitude and direction of the n-ionization potential, and its Implications for through-bond interactions will be given.