ANALYSIS OF 1400-A ELECTRONIC TRANSITION OF ETHANE MOLECULE

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 Å, exhibits a distinctive rotational contour. Computer simulation of the contour shows that the transition moment for the 0-0 band lies perpen-dicular to the carbon-carbon bond: The electronic transition is 1Eu-1A1g, in agreement with majority theory. Vibrational analysis shows that the transition effects a large reduction in the CH stretching frequency (a1g), consistent with a 731 cm-1 shift of the origin band toward higher frequencies on deuterium substitution, and with the promotion of an electron out of the CH bond. Electronic angular momentum is detected in the excited state. Finally, it is deduced that the lifetime of the excited state of C2D6 is about 10-11 seconds. © 1969.