Photoelectron spectra of polynuclear aromatics. V. Correlations with ultraviolet absorption spectra in the catacondensed series

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Abstract
IP’s derived from the high‐resolution PE spectra of 61 catacondensed hydrocarbons are presented, most of them for the first time, and correlated with the energies of the four main singlet–singlet transitions. For the p and β′ bands (1La and 1Ba) a linear correlation with IP1 and IP2, respectively, is established. The α and β band energies (1Lb and 1Bb) correlate linearly with the mean of IP1 and IP2. The regressions encompass planar as well as nonplanar hydrocarbons of the helicene type; standard deviations range from 0.070 to 0.126 eV. Comparison with Hückel and PPP theory shows the empirical correlations to be superior; their potential in disentangling diffuse uv spectra is demonstrated. Many β′ bands are assigned for the first time, and estimates are given for the position of the α band in anthracene, tetracene, and pentacene. From the slopes and intercepts of the regressions, important conclusions regarding the wavefunctions of the excited states and the role of electron repulsion and CI can be drawn. In some hydrocarbons the energies of the four uv transitions are shown to correlate with IP1 and IP3 rather than with IP1 and IP2. An analysis of this finding in terms of perimeter orbitals is given, and a novel classification of PE bands based on vibrational fine structure and correlation with perimeter states is presented. Correlations between the IP’s and chemical reactivity are pointed out.

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