Thermal Stability of Complex Gaseous Molecules

This discussion would be incomplete if we did not consider one of the arguments against extending the CDV mechanism to thermal decomposition of any compound. This is the allegedly low probability of the existence in the gaseous phase of complex molecules, in particular, of those metal salts produced in the thermal decomposition of crystalline hydrates. The thermal stability of these compounds is believed to be so low as to make the presence of their vapours at the decomposition temperatures hardly probable. This belief is, however, dispelled by MS observations accumulated in the recent three decades [50-57]. 

In his recently published monograph, Vaporization Thermodynamics of Double Oxides , Kazenas [57] presents a wealth of collected, systematized, and generalized material on the vapour pressure and composition of metal borates, aluminates, carbonates, silicates, nitrates, sulphates, phosphates, chromates, and other double oxides. This material is based on studies by Kazenas and his group and the results reported in the literature. In a foreword to this book, Kazenas [57] claims Observation of new t3rpes of molecules completely disproved the view on the high-temperature vapour as a medium which is poor in molecular forms. It has been established, in particular, that the molecular composition of the vapour phase for many chemical compounds is more complex and diverse than it was assumed earlier. By the use of effusion MS, 

MS with free-surface vaporization (after Langmuir) is undoubtedly the most efficient way for direct study of the composition of the gaseous phase, particularly, in the decomposition of melts, in the absence of a layer of solid phase interfering with the escape of vapours of low-volatility products from the reactant surface. As an illustration, we present in Fig. 2.10 a quadrupole 

MS recording of the vapour composition obtained in the process of dehydration of Cu(N03)2 3H2O [f9]. 

It is easy to see that two-stage decomposition of the hydrate with a melting point of 387 K is accompanied by the appearance of Cu(N03)2 molecules, as well as of their fragmentation products (CUNO3, CuO, and Cu). It is a pity that this variant of MS is applied for these purposes on much fewer occasions than it certainly should.  

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