Decomposition of the inorganic cyanamides

Abstract

A detailed study has been made of the thermal decomposition of silver cyanamide crystals and powder, and also on the effect of pre-irradiation with y-rays, neutrons and fission fragments. The decomposition is a complex process, and involves the formation of an intermediate. The physical properties and decomposition behaviour of silver cyanamide show that it is a covalent solid, and it is suggested that the initial step in the decomposition involves bond rupture (possibly the silver-nitrogen bond), the energy required being ccl. 44 kcal/mole. The decomposition of sodium cyanamide, thallous cyanamide, and lead cyanamide have been examined in outline. Where possible, infra-red analysis, X-ray diffraction, and electron microscopy and diffraction, have been used to supplement kinetic measurements. In many respects the decomposition of sodium cyanamide resembles that of sodium azide. Colours are produced wfithin the solid, and the decomposition is dependent on impurities produced by hydrolysis, mainly at the surface. The basic mechanism, however, involves electron transfer, where two adjacent cyanamide ions react probably via an excited state. With lead cyanamide, it is found that cyanogen is given off preferentially. Lead cyanamide is also a covalent solid and bond fission occurs during decomposition. It is suggested that the first step is rupture of the weakest X—C bond. The differences in behaviour of the silver, thallium, and lead salts can be understood in terms of the nature of the bonding orbitals between the metal atom and the cyanamide radical.