Point Groups and Molecular Symmetry

Molecular symmetry, and point groups, 53-59 Molecules reactivity of. 203 stereochemically nonrigid, 723-730... 

T2.6 The planar from of NHj molecule would have a trigonal planar molecular geometiy with the lone pair residing in nitrogen s p orbital that is perpendicular to the plane of the molecule. This structure belongs to a Djh point group (for discussion of molecular symmetry and point groups refer to Chapter 6). Now we can consult Resource Section... 

Molecular symmetry and point groups Complexes electronic spectra and magnetism (C3) (H8) ... 

Molecules that are not superimposable on their mirror images are termed dissymmetric. This term is used rather than asymmetric, since the latter means, literally, have no symmetry that is, it is applicable only to a molecule belonging to point group Cx. All asymmetric molecules are dissymmetric, but the converse is not true. Dissymmetric molecules can and often do possess some symmetry. It is possible to give a very simple, compact rule expressing the relation between molecular symmetry and dissymmetric character ... 

The most important and frequent use for projection operators is to determine the proper way to combine atomic wave functions on individual atoms in a molecule into MOs that correspond to the molecular symmetry. As pointed out in Chapter 5, it is essential that valid MOs form bases for irreducible representations of the molecular point group, we encounter the problem of writing SALCs when we deal with molecules having sets of symmetry-equiv-... 

The two characteristic features of normal modes of vibration that have been stated and discussed above lead directly to a simple and straightforward method of determining how many of the normal modes of vibration of any molecule will belong to each of the irreducible representations of the point group of the molecule. This information may be obtained entirely from knowledge of the molecular symmetry and does not require any knowledge, or by itself provide any information, concerning the frequencies or detailed forms of the normal modes. 

When correlating the number of infrared-active CO-stretching modes with molecular structure, the phase in which measurements are made has some bearing on the effective molecular symmetry and must therefore be considered. Measurement of the infrared spectra of compounds in the gas phase is ideal because mtermolecular interactions can be neglected the selection rules which determine the number and activity of the CO-stretching modes are those associated with the point group of the isolated molecule. Because of the limited volatility of many carbonyl complexes and their tendency to decompose at higher temperatures, gas-phase measurements have been limited chiefly to the binary carbonyls (44). It... 

Usually in the study of the electric properties of molecules, it is assumed that the electron and nuclear dynamics cannot destroy the molecular symmetry, and hence the individual characteristics of molecules are conceptually determined by the symmetry of the nuclear framework only. This statement is based on the assumption that the charge distribution (either classical or quantic) is totally symmetric (with respect to the nuclear configuration) i.e., it transforms into itself under the operations of the point symmetry group of the molecule. In cases in which the elec-... 

In this chapter we have introduced the matrix as a means of handling sets of objects and discussed the key aspects of matrix algebra. A great deal of this chapter has involved a cataloguing of the properties and types of matrices, but we have also tried to emphasize the chemical importance of matrices, in particular in the vital role they play in the classification of molecular symmetry and the development of group theory. The key points discussed include ... 

The thermal stability of the novel compounds DCNDBQT and CNBTPA were compared to two other thiophene molecules widely used in literature P,P -dibutylquaterthiophene (DBQT) and p,P -dibutylsexithiophene (DBST). As a first characterisation we used DSC and TGA. DCNDBQT has a higher melting point compared to DBQT and a lower one than DBST. This can be explained by different steric influences of the a,p-substituted groups and the number of thiophene units to molecular symmetry and steric stracture of the thiophene derivatives. 

This appendix briefly summarizes molecular symmetry (symmetry elements, point groups and character tables) of a molecule so as to facilitate readers understanding of the molecular structures and molecular orbitals presented in this chapter. We use some speciflc examples such as NH3 with Csv symmetry, CH3 radical with >3 symmetry (see Fig. 5.18) and c-C4Fg radical with D4h symmetry (Fig. 5.3) to introduce and illustrate some important aspects of the molecular symmetry. 

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