Analysis of the 1400 Å electronic transition of the ethane molecule

• Published in: Journal of molecular spectroscopy Vol. 30; no. 1; pp. 232 - 240
• Main Authors: Pearson, E.F., Innes, K.K.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 1969
• ISSN: 0022-2852; 1096-083X
• DOI: 10.1016/0022-2852(69)90259-8

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 C 2H 6. However, the 0-0 band of C 2D 6, 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 1 E u - 1 A 1 g , in agreement with majority theory. Vibrational analysis shows that the transition effects a large reduction in the CH stretching frequency ( a 1 g ), 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 CH bond. Electronic angular momentum is detected in the excited state. Finally, it is deduced that the lifetime of the excited state of C 2D 6 is about 10 −11 seconds.