Knowledge Bank

The electronic absorption spectra in the range from 40 000 to 77 000 $cm-1$ and the photoelectron spectra (PES) up to 20 eV have been recorded for the carboranes $1,5-C2B3H5,1,6-C2B4H6,2,4-C2B5H7$ and $4,5-C2B4H8$. In 1,6 - $C2B4H6$, three Rydberg series having quantum defects of 0.54, 0.09 and 0.03 can be discerned. These all lead to the first ionization potential of 78 720 $\pm$20 $cm-1$. The PES yields the same value for the first ionization potential. The optical spectrum of $4,5-C2B4H8$ is diffuse except for one structured transition. This has been shown to be a Rydberg transition by the effect of a high pressure blanket of nitrogen upon it. Its term value with respect to the $0-0$ band of the PES ($IP=75800cm-1$) leads to a quantum defect of 0.51, Suggesting assignment as a p-Rydberg. In $1,5-C2B3H5$, two possible Rydberg series leading to the first ionization potential of 73 880 $cm-1$ can be located. The PES assignments will be discussed with the aid of an all electron Gaussian orbital ab initio calculation and comparison to the alkane analog bicyclo-[1.1.1]-pentane will be made. The optical spectrum of $2,4-C2B5H7$ is completely diffuse and no assignments are possible. The PES yields a first ionization potential of 10.66 $\pm$ 0.03 eV (86 000 $cm-1$), the highest by 0.9 eV of the carboranes studied.