Azides, band structures

Treatment of thionhydrazides, RC(==S)—NH—NH2 with nitrous acid should lead to thiocarbonyl azides, RC(=S)—N3, and it seems likely that these azides are indeed formed as short-lived intermediates. The isolated products are 1,2,3,4-thiatriazoles. When thiosemi-carbazides are nitrosated, 5-amino-l,2,3,4-thiatriazoles or/and 5-mercaptotetrazoles are obtained. Freundformulated his products as such in 1895, while Oliveri-Mandala considered them thiocarbamoyl azides on the basis of some chemical evidence. Lieber resolved this controversy by recording the infrared spectra of the compounds and demonstrating the absence of an azide band . He did the same for the 5-alkylmercapto-l,2,3,4-thiatriazoles , which had earlier been reported as azido-dithiocarbonates , and for the products from azide ion and thiophosgene and carbon disulphide , all of which possess thiatriazole structures. Lieber also demon-... 

Eloy has pointed out that many compounds described in the older literature as tetrazoles are really imidoyl azides, and vice versa. A few such instances had been clearly recognized before , but in general the structures were sorted out only after infrared spectroscopy became available, and the presence or absence of an azide band was used as the decisive criterion. 

The optical properties of many metal azides permit the determination of the band structures of the solids. From these, the energy requirements for the generation of various types of defects can be estimated, and quantitative consideration can be given to the roles of these identified defects in formulating decomposition mechanisms. 

Deb and Yoffe [66] compared the photochemical decomposition of mercury(I) azide with that of triphenylmethyl azide. The first step in the decomposition is suggested to be the fission of the longer N-N bond of the azide group. Results were compared with data for reactions of inorganic azides and it was concluded that there is no clear distinction between the energy requirements of the two classes of azides, covalent and ionic. Deb [67] has determined the electron energy levels of several azides and correlated the band structures with observed stabilities. 

In the alkali metal pseudohalides the contribution of cationic wave functions to the valence band structure can be neglected. The optical absorption spectra can therefore be correlated to transitions involving excited states of the anions. However, one can see solid state effects like the superposition of vibronic structure on the molecular symmetry forbidden transition at 5.39 eV in the crystal spectra of the alkali metal azides (76). In the more complex heavy metal and divalent azides, a whole range of optical transitions can occur both due to crystal field effects and the enhanced contributions from cationic states to the valence band. Detailed spectral measurements on a-PbNe (80), TIN3 (57), AgNs (52), Hg(CNO)2 (72) and AgCNO (72) have been made but the level assignments can at best be described as tentative since band structure calculations on these materials are not available at present. 

Figure 7. Schematic band structure for a monovalent azide.

The ideas just reviewed are used below to discuss the electronic structure of specific azides. Such features of the band structure as the magnitudes of the effective masses and the positions of the band extrema in momentum space are - not sufficiently understood to warrant discussion. It should also be noted that electrons and holes tend to polarize the medium in which they move. If they are not particularly mobile, ionic nuclei have time to adjust to their presence by moving toward or away from them. An electron or hole is then dressed by this polarization and either carries it about (as an electron or hole polaron) or is trapped by it (self-trapping). Young [64] suggested that exciton models in the alkali azides should take the polaron character of electrons and holes into account. No one has yet attempted such a description. While it may be that polaron effects are necessary for a proper understanding of electronic structure, the discussion here is limited to a model involving undressed holes and electrons. 

An important distinction between the alkali azides and ionic materials, such as the halides, is the susceptibility of the azides to point-defect production by UV radiation. There has been little research concerned with understanding how UV radiation produces individual point defects in the azides. Clearly the presence of the molecular anion, NJ, is important, and many of the defects are dissociation products of the azide ion. However, just the presence of a molecular anion is not sufficient. Cyanates, for example, do not have defects produced in them by UV light even though the NCO is isoelectronic with N3 [35]. It is important to consider the detailed electronic structure of the azide ion as well as the electronic structure of the lattice. Such factors as the energy of the first excited state with respect to the ground state, the proximity of an unbound state of the NJ to the lower excited state, the electron affinity and the ionization potential, as well as the band structures of individual azides, are important. 

Recent research with the II-VI semiconductor material ZnO (band gap = 3.8 eV, similar to those of the heavy-metal azides) has revealed that the ability of the specimen to retain stable ions formed by electrostatic charging is a critical function of the degree of surface order A highly disordered surface allows the formation of stable adsorbed ions a highly ordered surface does not [43]. A semiquantitative theory to account for this has been proposed [44]. Further, with disordered surfaces, the location of the electronic state of the adsorbed ions relative to the band structure of the specimen can be probed by an optical discharge technique to yield information about the electronic properties of the surface [43]. These potentialities, coupled with the field-assisted initiation capabilities of an azide specimen, argue that the electrostatic charging technique should be applied quantitatively to explosive azides. 

In the absence of conjugation N3 absorption bands of organic azides occur in the 2100-2110 cm 1 region (Ref 9). Conjugation or presence of electron acceptor groups shift the bands to 2135-2166 cm"1. It is claimed that for organic azide measurement of band intensity is a more sensitive indication of compound structure than measurement of band position. Electron donor groups in the molecule increase band intensity and electron acceptors lower it... 

Compound (159) has an IR band at 1070 cm-1, in neutral solution or the solid state, consistent with the tetrazole structure. In trifluoroacetic acid this band is replaced by a typical azide absorption at 2140 cm-1. Only in this solvent will photolysis give a good yield (91%) of the pyrimidinoindole (160) (72JOC3216). 

The complex ion [Au(N3)4] reacts with alkyl isocyanides according to equation (69) 291 and with CO to give [Au(NCO)2]. 294 Azide-bridged complexes such as [Au2(Ju-N3)2Me4] are also known. The ligand field and charge transfer bands in the UV-visible spectra of [Au(N3)4] have been assigned and the structure determined.567... 

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