Space group selection

Birman, J. L. (1962) Space group selection rules, diamond and zinc blende. Phys. Rev. 127, 1093-106. 

To derive factor group (space group) selection rules, it is necessary to utilize X-ray data for a molecule from a literature source or from Wyckoffs (54) Crystal Structures. The factor group and site symmetries of the ion, molecule, or atoms must be available, as well as the number of molecules per unit cell reduced to a primitive unit cell. 

In a crystalline lattice, the true complex group symmetry is different from but often near to the theoretical D4ft one. It can reach at minimum the symmetry of the site occupied by the metal atom in each space group. Selection rules are modified in consequence, but generally their influence on the spectra is limited to the appearance of forbidden bands and the splitting of degenerate ones. The analysis of those spectral features is valuable as it provides information in each particular ceise on the structure of the complex group. 

F. TauleUe, NMR crystallography crystaUochemical formula and space group selection, Thematic Sect. NMR CrystaUogr. 6 (10) (2004) 1053—1057. 

R 520 F. Taulelle, NMR Crystallography. Crystallochemical Formula and Space Group Selection , Solid State Sci., 2004,6,1053 R 521 R.E. Taylor, Setting up C CP/MAS Experiments , Concepts Magn.Reson., A, 2004,22A,37... 

Compound Ref. Crystal Phase Space group Z Dihedral angle between the phenyl ringsV° Selected intermolecular distances [A]... 

Compound Ref. Space group Z Dihedral angle Selected intermolecular... 

The compounds crystallise in noncentrosymmetric space groups namely PI, P2i, C2, and P2i2i2i (but with priority of P2i) due to the chirality of the molecules. Most of the compounds have a tilted layer structure in the crystalline state. The tilt angle of the long molecular axes with respect to the layer normal in the crystal phase of the compounds is also presented in Table 18. Some compounds show larger tilt angles in the crystalline state than in the smectic phase. In the following only the crystal structures of some selected chiral liquid crystals will be discussed. 

---