Noncentrosymmetric crystal structure

Second harmonic generation is an effect of the interaction of electromagnetic fields with materials having high molecular and bulk polarizability. After the interaction, the frequency of the output field is twice that of the incident field. Pure p-nitroaniline microcrystals do not generate second harmonic radiation when excited by a Nd-Yag laser [261]. The phenomenon was displayed by / -CD-p-nitroaniline microcrystals, as shown by the formation of noncentrosymmetric crystal structures. 

Table 1.2 Results of a query from CSDSymmetry, showing the ten molecular point groups (with more than 30 occurrences in the CSD) that give rise most often to noncentrosymmetric crystal structures. Reproduced from [28] by permission of the International Union of Crystallography...

A condition for second-order nonlinear organic materials to double the frequency of an incident laser light is a noncentrosymmetric configuration on the molecular and macroscopic level. The term macroscopic implies that the compounds must have a noncentrosymmetric crystal structure, because only materials without a center of symmetry have anisotropic polarizability. 

The nonmesogenic compound CB2 is described here, because it shows a reversible distortive solid-solid phase transition at 290.8 K (transition enthalpy 0.9 kj/mol) from the centrosymmetric low temperature phase I to the noncentrosymmetric high temperature phase II. The crystal structures of both solid phases I and II are very similar [45] as demonstrated in Fig. 2. The molecules are arranged in layers. The distances between the cyano groups of adjacent molecules are 3.50 A Ncyano-Ncyano and 3.35 A Ncyano-C ano for phase I and 3.55 A Ncyano-Ncyano and 3.43 A Ncyano-Ccyano for phase II. In the two... 

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. 

The EFISHG technique has also been applied to some pyridyl carbonyls of Rh1 or Ir1 by Bruce et al72,13 These complexes (e.g., (9) and (10)) contain stilbazole ligands bearing resolved 2-methylbutyloxy substituents, incorporated with the aim of encouraging noncentrosymmetric crystal packing structures. The observed 1.5- to 3-fold enhancements of (3 of the free stilbazole on complexation are attributable to the er-electron-withdrawing abilities of the metal centers.72,73... 

The crystal structure of Compound 4, which was found to possess a centro-symmetric triclinic unit cell containing six tetraphenylarsonium cations, two [Nii2(CO)2iH]3 trianions, and two solvent acetone molecules, also was solved by the combined Patterson-Fourier method, based on the assumption of the dodecanickel core of the trianion being analogous to that of the dianion. Peaks corresponding to all of the nickel and arsenic atoms were first located from successive Fourier syntheses in the unit cell under noncentrosymmetric PI symmetry, after which initial atomic coordinates for the 12 independent nickel and three independent arsenic atoms were obtained by an origin shift to an approximate center of symmetry relating pairs of these peaks to one another. 

Following the experimental results of Werner and his colleagues, a number of theoretical attempts were made to correlate the sign and magnitude of the optical rotation to the exact chiral molecular structure of metal complexes. These approaches have been summarized by Richardson.3 The term absolute structure is sometimes limited to a discussion of the structure of noncentrosymmetric crystals, and the term... 

The strengthening of H-bonds that occurs when one of the two partners is charged can be sufficient to form H-bonds between C atoms. For example, a recent crystal structure revealed that a proton can bind together the C atoms of a pair of imidazole species. The H-bond is apparently noncentrosymmetric with r(C—H) distances of 2.03 and 1.16 A the bond is within 8° of being linear. 

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