DISTINGUISHING KETO AND ENOL STRUCTURES BY INNER-SHELL SPECTROSCOPY

Oscillator strengths for C 1s and O 1s excitation of gas-phase 1,2-cyc lohexanedione, 1,3-cyclohexanedione, 1,4-cyclohexanedione, and cyclohe xanone have been derived from electron energy loss spectra recorded un der conditions of electric dipole scattering. Spectral assignments hav e been aided by extended Huckel (EHMO) and HAM/3 calculations. Spectra l features characteristic of diketo and monoenol structures are identi fied. The C 1s and O 1s spectra clearly indicate that 1,2-cyclohexaned ione has a monoenol structure in the gas phase while 1,3-cyclohexanedi one is predominantly in the diketonic structure in the gas phase. The latter-result resolves an existing literature discrepancy concerning t he preferred gas-phase tautomer of 1,3-cyclohexanedione.