Knowledge Bank

Ethane is the only alkane which exhibits structure in its molecular electronic absorption spectrum. High resolution photographs reveal that this is diffuse vibrational structure in the case of $C2H6$. However, the 0-0 band of $C2D6$, near $1406\backslash AA$, exhibits a distinctive rotational contour. Computer-simulated band contours show that the transition moment for the 0-0 band lies perpendicular to the carbon-carbon bond. Moreover, electronic angular momentum is detected in the excited state. These results are consistent with a $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}E2u\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}A1g$ electronic transition, an identification made, on other grounds, in recent work by Sandorfy and his co-workers.