Neutral Oxygen Species

The ground state of the oxygen molecule is a triplet 3 g state with two unpaired electrons, and at slightly higher energy there are two low-lying electronically excited states, the singlet Ag and Sg+ levels (6) (Fig. 1). 

Busca (18b) has evaluated the literature values for infrared bands attributed to coordinated and adsorbed dioxygen species. He concludes that it is very difficult to deduce the nature of the dioxygen coordination from measurements of the frequency shift, Av00, with respect to the stretching frequency of the free molecule. It needs to be stressed that it is also difficult to distinguish between mononuclear and molecular species from measurements of v00, and this can only be achieved by careful interpretation of experiments using 160/180 isotopic mixtures. The absence of such experiments very often accounts for the conflicting attributions in the literature which are discussed in later sections. 

Eberhardt et al. (22a) have studied the photoemission of oxygen physi-sorbed on graphite at 10 K. The photoemission spectra exhibit vibrational structure in the 2n band. From calculations based on Franck-Condon factors, the authors conclude that on the graphite surface the equilibrium distance of the oxygen nuclei is decreased by 0.065 A relative to the gas phase. This would also be consistent with a partial electron withdrawal from oxygen antibonding orbitals into available orbitals in the graphite. 

The presence of singlet oxygen was shown by a specific reaction with a sulfonic acid (26g). It might be expected that surface oxygen would show the same reaction chemistry as singlet oxygen does in homogeneous media. 

The proposals that singlet oxygen is involved in heterogeneous catalytic reactions have not yet been explored fully. The methods used for the detection of the excited singlet state of oxygen need to be improved, and an approach based on the detection of the emission from ( Ag)02 as observed by Khan (23b) will present a significant advance if it can be applied to heterogeneous systems. It is clear that more quantitative work is required and in this respect the evidence that 02 can react with water to form ( Ag)02 (23b) could have considerable mechanistic interest. 

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The proposed diamagnetic species include the neutral oxygen vacancy, 6i—S , the doubly coordinated siUcon, —O—Si—O— and the peroxyl bond,... 

Two types of nuclei with an oxygen and three nitrogen atoms are possible, namely 1,2,3,4-oxatriazoles (1) and 1,2,3,5-oxatriazoles (7). The neutral aromatic species have not yet been reported but 1,2,3,4-oxatriazolium salts (2) and mesoionic species (3)-(6) are known. 1,2,3,5-Oxatriazolium salts (8) and mesoionic compounds (9)-(12) are not yet known but 1,2,3,5-oxatriazolines (13) and (14) have been reported. The two types of oxatriazoles will be discussed in two separate sections. 

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