INNER-SHELL SPECTROSCOPY OF PARA-BENZOQUINONE, HYDROQUINONE, AND PHENOL - DISTINGUISHING QUINOID AND BENZENOID STRUCTURES

Oscillator strengths for C Is and 0 ls excitation of gas-phase p-benzoquinone, hydroquinone, and phenol have been derived from electron energy loss spectra recorded under electric dipole scattering conditions. Spectral assignments have been aided by extended Huckel (EHMO) calculations carried out within the equivalent core analogy, as well as by comparisons to previously recorded spectra of benzene, acrylic acid, 1,4-cyclohexanedione, and acetone. Two methods of relating EHMO results to experimental core excitation spectra are explored. That derived from the complete charge matrix is found to be preferable. Several features of the C 1s spectrum of p-benzoquinone can be associated with the quinoid structure of this species, namely a second pi* resonance more intense than the first, and a shift of the C ls(C=C) --> pi*LUMO(b2g) transition to 1.6 eV below that of the lowest C ls --> pi* transition in benzenoid species. This shift is associated with the loss of aromatic stabilization. EHMO calculations of p-benzoquinone in nonequilibrium geometries and of related molecules provide further insight into spectral assignments.