Vibrational and rotational spectroscopy of the first electronically allowed transition of α-dicarbonyls

Laser induced fluorescence excitation spectra of the series glyoxal, methylglyoxal, biacetyl, and 2,3-pentanedione at high resolution show that except for glyoxal these molecules undergo a characteristic structural deformation during the transition. Our incomplete assignment of the methylglyoxal spectrum suggests that the methyl internal rotation and carbonyl skeletal motions (probably out of plane) are involved. The positions of the 0-0 bands shift to the blue as a methyl group is added to glyoxal (22 000 cm ^-1) and methylglyoxal (22 125 cm^-1) but not as the side groups are extended from biacetyl (22 336 cm^-1) to 2,3-pentandedione (22 340 cm^-1). We cannot precisely identify the interaction between the carbonyls and methyl groups that cause these effects. As the size of the carbon skeleton increases, the number of observed low frequency vibronic excitations increases and the energy at which the spectra break off decreases. These observations are consistent with the known radiationless behavior of these molecules.