THERMAL TRANSFORMATION SYSTEMS

One may now consider how changes can be made in a system across an adiabatic wall. The first law of thermodynamics can now be stated as another generalization of experimental observation, but in an unfamiliar form the M/ork required to transform an adiabatic (thermally insulated) system, from a completely specified initial state to a completely specifiedfinal state is independent of the source of the work (mechanical, electrical, etc.) and independent of the nature of the adiabatic path. This is exactly what Joule observed the same amount of work, mechanical or electrical, was always required to bring an adiabatically enclosed volume of water from one temperature 0 to another 02. 

The thermal transformations observed with these systems can be rationalized in terms of an equilibration of the 1-azirine with a transient vinylnitrene which subsequently rearranges to the 2,5-disubstituted pyrrole (56). 

The cycloaddition reaction of compound 6 with N-aryl- and N-aralkylazides 23 was also investigated (967(52)7183). Thiadiazabicyclo[3.1.0]hexene derivatives 25 were obtained from the labile triazoline intermediate 24 through nitrogen elimination. This bicyclic system underwent thermal transformation, producing thiadiazine dioxides 26 as the main product together with thiazete dioxides 27 and pyrazoles 28. 

It has long been known that unsaturated compounds containing a delocalized system of 71 electrons can rearrange into cyclic compounds or other n systems. Such reactions were only incidentally studied until 1930. 0. Diels and K. Alder published their first paper on diene synthesis (which was later given the name Diels-Alder reaction) in 1928. Subsequent work of K. Alder and G. Stein (1933 and 1937) proved the generality of the reaction and its high regio and stereo selectivity. This led to the interest on thermal transformations in unsaturated compounds. 

Semiclassical techniques like the instanton approach [211] can be applied to tunneling splittings. Finally, one can exploit the close correspondence between the classical and the quantum treatment of a harmonic oscillator and treat the nuclear dynamics classically. From the classical trajectories, correlation functions can be extracted and transformed into spectra. The particular charm of this method rests in the option to carry out the dynamics on the fly, using Born Oppenheimer or fictitious Car Parrinello dynamics [212]. Furthermore, multiple minima on the hypersurface can be treated together as they are accessed by thermal excitation. This makes these methods particularly useful for liquid state or other thermally excited system simulations. Nevertheless, molecular dynamics and Monte Carlo simulations can also provide insights into cold gas-phase cluster formation [213], if a reliable force field is available [189]. 

Prinzbach and co-workers <2000EJ0743> carried out photochemical and thermal transformations of several diazetine derivatives in a strained cyclic system. They reported the formation of diazetidine 80 by photolysis of diazenelene 79 in acetonitrile. 

The thermal transformation reaction of the aziridine derivative 208 has been rationalized by a mechanism involving cycloreversion of the bicyclic system into an open-chain intermediate which cyclizes to the 4//-thiazctc 1,1-dioxides 17 (Scheme 69) <1996T7183>. 

Finally, the preparation method has a pronounced influence on the obtained products. In fact, the preparation of mixed oxides by traditional techniques (such as the pH controlled co-precipitation) does not result in single phases but several multi-component phases with various atomic ratios are obtained. In the system under investigation it seems to be impossible to obtain a contemporary precipitation of two components, like hydroxides or non soluble salts, instead non uniform precipitates are always obtained [11]. The thermal transformation of these precipitates gives compounds with a variable atomic ratio as a function of their local concentration in the precipitate. On the contrary, the sol-gel method can get to the formation of a truly homogeneous gel (atomic interdispersion), that is the precursor of a very well interdispersed final material. In all cases the calculated metal amounts and a good metal interdispersion have been verified by SEM-EDS microanalyses. 

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