EXCITED ELECTRONIC STATES OF ADAMANTANE, BICYCLO [2.2.2]-OCTANE, CYCLOHEXANE AND OTHER CYCLIC ALKANES

Abstract

Alkanes are generally thought to be highly dissociative in their excited electronic states. However, many cyclic alkanes possess stable excited states characterized by sharp vibronic structure. The spectrum of adamantane provides an especially interesting example of such sharp structure. Adamantane is a spherical top in the point symmetry group $T_{d}$. Thus all of its excited states to which transitions are dipole-allowed are triply degenerate and instability due to Jahn-Teller effects is to be expected, in keeping with what is found for many other tetrahedral molecules such as methane, neopentane and silane. We find a number of very sharp bands along with some more diffuse vibronic bands which can be organized into a Rydberg series with quantum defect a=0.50 and leading to an ionization potential of about 9.31 eV. This is about 0.06 eV higher than a recent photoelectron spectroscopic value of Dewar and Worley. Possible reasons for this discrepancy involving mild Jahn-Teller distortions in the ground state ion will be discussed. The spectra of bicyclo-[2.2.2]-octane and cyclohexane as well as several other cyclic alkanes also display sharp vibronic structure. Rotational band envelopes are resolved in some cases. Tentative interpretations of some of these spectra in terms of Herzberg-Teller vibronic borrowing will be presented.