Electron-impact spectroscopy of various diketone compounds

The spectra of the diketone compounds biacetyl, acetylacetone, acetonylacetone, 1,2-cyclohexanedione, and 1,4-cyclohexanedione have been investigated by the technique of low-energy variable-angle electron energy-loss spectroscopy. With this method low-lying, spin-forbidden transitions have been observed. The energy difference between the lowest spin-allowed and spin-forbidden n→π* excitations in the acyclic diketones is found to be 0.35 eV, on average, which is nearly the same as that of comparable acyclic monoketone compounds; in 1,2-cyclohexanedione, however, this energy difference is 0.84 eV, more than twice as large. This discrepancy in the magnitude of the n→π* singlet--triplet splittings may be attributed to differing amounts of overlap between the initial and final orbitals