“Forbidden” character in the 3200-Å transitions of pyrazine-h 4 and -d 4 vapors

• Published in: Journal of molecular spectroscopy Vol. 11; no. 1; pp. 257 - 261
• Main Authors: Innes, K.K., Simmons, J.D., Tilford, S.G.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 1963
• ISSN: 0022-2852; 1096-083X
• DOI: 10.1016/0022-2852(63)90025-0

It is shown that all strong vibronic bands of the 3200-Å absorption region of pyrazine (1,4-diazine) must be assigned to one electronic transition of mixed polarization. The allowed bands show polarization perpendicular to the plane of the molecule and the somewhat weaker “forbidden” bands show polarization along the line of the nitrogen atoms. A second allowed electronic transition, polarized along the line of the nitrogen atoms, loans the intensity, while an out-of-plane hydrogen-bending motion is active in the “borrowing.” Mass dependence of the “forbidden” intensity and the Raman intensity of the active vibration offer unique manifestations of the Herzberg-Teller theory of vibronic transitions.