Effects of π* ← n excitation in 4-cyclopentene-1,3-dione

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) <-- X(1)A(1), <pi>* <-- n transition with origin at 22 000 cm(-1). A few weak bands near 28 465 cm(-1) have been assigned to a second (1)A(2) <pi>* <-- 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, (C) 2000 Academic Press.