Oxygen ultraviolet transitions/spectrum

The electronic spectrum (36) of the pol5uner is dominated by a very broad ultraviolet band, with shoulders at 280 and 470 m/t, which tails into the visible region and is responsible for the deep brown color of the polymer. Very weak crystal field excitations are found at 640 and 880 m. From the latter transition one can estimate that for high-spin Fe +, Dq = 1100 cm i. This value is typical of Fe3+ in octahedral coordination with oxygen ligands, but the X-ray evidence (see below) indicates that the coordination is tetrahedral, so that Dq seems anomalously high. However, the coordination symmetry is actually lower than tetrahedral, since both hydroxide and oxide ligands are involved. 

Similar sorts of results may be found with the nitrate anion. In this case, the nitrate ion itself has a characteristic absorption in the ultraviolet. When paired with a transition-element cation, in alcoholic solution, this absorption is markedly altered (2). It also shows alterations with other cations. In certain ketone and ether solutions, it has been possible to demonstrate further that the vibrational spectrum of the nitrate ion has been altered in such a pattern as to be consistent with a binding of one of the nitrate oxygens to the cation (2), so that major vibration now occurs between this oxygen and the rest of the bound nitrate group. 

We note that the induced absorption bands mentioned are associated with single or double electronic transitions. Such induced bands are not limited to oxygen similar bands of a few other systems (benzene, for example) were known for some time. However, most of the common diatomic molecules have electronic states that are many eV above the ground state. As a consequence, for those molecules, electronic induced transitions commonly occur in the ultraviolet region of the spectrum where interference from allowed electronic spectra may be strong. Electronic induction is not nearly as common as rovibro-translational induced absorption. 

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