79 Magnetism and Structure

Among the many classes of chiral molecules, helical systems are particularly fascinating. Their structure is relevant to proposed mechanisms of handedness induction in relation to chiral amplification [76], Helicenes ([A]-H) are helical molecules formed from A-ortho-fused benzene rings (Fig. 8) which display considerable rotatory power [77]. Helicenes are presently the subject of intense synthesis efforts that try to functionalize these molecules in order to attain enhanced electric, magnetic, and optical properties [78, 79]. Phenylenes ([A]-P), or heliphenes, constitute another class of helical aromatic compounds for which syntheses have recently been reported [80, 81]. They are made up of N benzene rings fused together with N - 1 cyclobutadiene rings (Fig. 8). 

Wol] Wolfers, P., Obbade, S., Frachart, D., Verhoe R., Precise Crystal and Magnetic Stracture Determinations. Part I. A Neutron Diffiaction Study of Nd2Fci4B at 20 K , J. Alloys Compd., 242, 74-79 (1996) (Crys. Structure, Experimental, 31)... 

Shimakawa Y, Kubo Y, Hamada N, Jorgensen JD, Hu Z, Short S, Nohara M, Takagi H (1999) Crystal structure, magnetic and transport properties, and electronic band structure of A2Mn207 pyroehlores (A = Y, In, Lu, and Tl). Phys Rev B 59 1249-1254... 

Figure 11 Character of the arrangement of the spin magnetic moments, and average magnetic moment, in seven-atom clusters with a pentagonal bipyramid structure and interatomic distances ranging from <4ulk to 80% <4ulk- Reproduced with permission from Ref. 79.

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