Electron - impact spectroscopy of various diketone compoundsa >

The spectra ofthe diketone compounds biacetyl, acetylacetone, acetonylacetone, 1,2cyclohexanedione, and 1,4-cyclohexanedione have been investigated by the technique oflowenergy variable-angle electron energy-loss spectroscopy. With this method low-lying, spinforbidden transitions have been observed. The energy difference between the lowest spin-allowed and spin-forbidden n-+ 1r* 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 then-+ 1r* singlet-triplet splittings may be attributed to differing amounts of overlap between the initial and final orbitals.