Electronic excitation and pi-electron interaction in borazine.

The extent of pi-electron interaction in borazine has been investigated by comparing its spectral properties with those of a typical unsaturated molecule, benzene, and benzene's saturated counterpart, cyclohexane. The valence-shell electron energy loss spectrum of borazine is analyzed to reveal the pattern of valence and Rydberg excitations which is characteristic of a multicenter pi-electron chromophore. Though the electronic manifolds of borazine and benzene share a common pattern of Rydberg and valence excitations, the electronic states of borazine do not stand necessarily in a one-to-one relationship with those of benzene. Nonetheless, the magnetic circular dichroism spectra do show that the 7.6-eV band of borazine is closely related to that at 7.0 eV in benzene, both involving doubly degenerate pi-electron excitations. In support of the description of borazine as an unsaturated pi-electron system, the B ls and N ls inner-shell energy loss spectra of borazine are shown to far more closely resemble the C ls spectrum of benzene than that of cyclohexane. The benzene-like splitting of the pi M0's of borazine deduced from the inner-shell spectra is abundantly confirmed by comparisons of the electron transmission spectrum of borazine with those of benzene and cyclohexane. It is concluded that borazine has a planar D(3h) structure and a delocalized pi-electron system much like that of benzene.