Point group classification scheme

Point group classification scheme. (Adapted from reference 31.)... 

In the above classification scheme I have indicated point groups that are "unusual , that is, that are rarely encountered. While these are not impossible, special care should be taken to check the identification procedure if a molecule appears to belong to one of these point groups. 

Similar problems arise when we deal with possible reference structures that can be interconverted by internal rotations. Consider, for example, the diphenylmethane molecule (Scheme 2.2), where conformations with the planes of both phenyl groups either parallel or perpendicular to the C-CH2-C plane have C2V symmetry. Rotation of a phenyl group through 71 produces a conformation that is isometric with the initial one, but such a rotation is not a symmetry operation of the C2V point group. However, it is clear that such operations have to be included in the group of all operations that leave the reference structure invariant. We shall not attempt to review here the work that has been done on the classification of such symmetry operations on non-rigid molecules [23], nor the controversies that have ensued from it. Instead we present four representative examples of such molecules. 

The structure of a given crystal might be assigned to one of the seven crystal systems or to one of the 14 Bravais lattices. In those classification schemes, one can state with certainty the crystal system as well as the type of lattice. On a deeper level, one could deduce the molecular point group of the molecules in the unit cell, which would then pinpoint to which of the 32 point groups the crystal belongs. This latter classification would yield both the crystal system and the relationships between the tensors that describe the macroscopic behavior and the properties of the crystal. 

However, the ability to classify elements and compounds into groups according to similarity in properties (reaction behaviour) allows us also to classify reactions themselves into useful groups. There are many specific named reactions in more advanced chemistry (such as the Wittig and Diels-Alder reactions), but even in introductory chemistry it is useftil to set out some common types of reaction that students will meet in their studies. Every particiilar reaction (that is, every different combination of reacting substances) will have some distinct, unique features, but a classification scheme provides a useful starting point for dealing with what would otherwise be an overload of information for the chemist (let alone the school student). 

One can see that the classification scheme (1.11) points out the separate group of chemical film systems where the process does not involve electric current across the interface. Let us briefiy overlook the main features of such systems. 

CHECKPOINT Crystals are analyzed according to a dizzying number of different classification schemes because different levels of stmcture are relevant to different fields. For example, certain optical properties depend on the point group symmetry of the crystal, whereas the identiricalion of a particular crystalline compound under a microscope may rely partly on the shape given by the lattice system. 

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