Spectroscopic properties of polyenes. II. The vacuum ultraviolet spectra of cis- and trans-1,3,5-hexatriene

Abstract

Absorption spectra are reported for cis‐ and trans‐1,3,5‐hexatriene in the region between 47 000 and 69 000 cm−1. The observed transitions are compared to those predicted from Pople‐Pariser‐Parr pi‐electron calculations using configuration interaction including both single and double excitations. The ordering of states predicted from calculations based on single excitations appears to be more plausible than that of the more extensive single‐plus‐double excitation calculations. Vibrational progressions associated with the higher energy ππ* transitions of the trans isomer are almost identical to those reported previously for the lowest energy allowed transition (39 000–47 000 cm−1). The linewidths of the trans isomer transitions are considerably smaller than those for the cis isomer transitions throughout the entire vacuum ultraviolet region. An ionization petential of 8.27 eV is obtained for the trans isomer from a Rydberg series extrapolation. This value compares favorably with the value (8.29 eV) obtained recently from photoelectron spectroscopy. Strong bands which could be assigned unambiguously as Rydberg transitions were not found for cis‐hexatriene. However, the shape of the absorption curve in the region 62 000–68 000 cm−1 implies that the ionization potential for cis‐hexatriene is probably within 0.1 eV of that for trans‐hexatriene.