Topochemical transformation

In the crystal structure of the polymer phase (Fig. 17a), the polymer chains are aligned along the c-axis and the distance (3.71 A) between the centres of adjacent cyclobutane and pyrazine rings corresponds to half the c-axis repeat of the unit cell. For comparison between the monomer and polymer structures, an overlay plot of these structures is shown in Fig. 17b. It is clear that the solid-state reaction is associated with only very small atomic displacements at the site of the [2-1-2] photocyclization reaction (the displacement of the carbon atoms of the C=C double bonds of monomer molecules on forming the cyclobutane ring of the polymer is only ca. 0.8 A for one pair of carbon atoms and ca. 1.6 A for the other pair). Such small displacements are completely in accord with the assignment of this solid-state reaction as a topochemical transformation [124—127] (in which the crystal structure of the reactant monomer phase imposes geometric control on the pathway of the... 

H2 O for the topochemical transformation and are very helpful in the assignment of the different modes. It is beyond the scope of this brief review to discuss the large number of different local vibrational modes in detail. Instead, we would like to briefly discuss two particularly interesting issues which can be addressed by vibrational spectroscopy of sheet polymers the question of bonding between the polymer sheets and the use of these polymers as model substances for Si surfaces. 

Knowledge of the topochemistry of the Zintl phases of Ca with Si and Ge known is still rather limited. New approaches could include the use of different sources of chlorine or of protic solvents in the topochemical transformation of the layered Zintl phases. A first attempt in this direction has involved the use of aqueous solutions of C0CI2 for the removal of A different approach to broaden the topochemistry is the transformation of Zintl phases with different structures of the group IV polyanions, leading to Si or Ge compounds with other backbone structures.An example of this is the reaction of CaSi with HCl, which leads to the formation of linear polysilane chain polymers. However, the Zintl phases of Si and Ge have an even richer structural chemistry, containing polyanions in the form of tetrahedra or planar Si rings, which could lead to Si compounds of great interest from both a chemical and a physical point of view. ° ... 

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