Effects of π* ← n Excitation in 4-Cyclopentene-1,3-Dione

✍ Scribed by Robert A. Back; Robert D. Gordon

• Publisher: Elsevier Science
• Year: 2000
• Tongue: English
• Weight: 189 KB
• Volume: 204
• Category: Article
• ISSN: 0022-2852
• DOI: 10.1006/jmsp.2000.8191

✦ Synopsis

The weak visible absorption spectrum of 4-cyclopentene-1,3-dione has been photographed near room temperature and assigned to the A 1 B 1 4 X 1 A 1 , * 4 n transition with origin at 22 000 cm Ϫ1 . A few weak bands near 28 465 cm Ϫ1 have been assigned to a second 1 A 2 * 4 n transition. The effects of excitation are delocalized over the highly conjugated molecule, the most important geometrical change upon excitation being an in-plane distortion of the five-membered ring. Many sequences are observed in the ring-puckering and twisting modes, both of which increase substantially in wavenumber upon excitation. Possible reasons for this dramatic increase in the forces favoring ring planarity are discussed.