Pseudo-centrosymmetric

A great number of chiral TTFs have been prepared [31,32], and some of their charge transfer salts have been crystallised. The 7r-donor 1 can be elec-trocrystallised to give a salt 12 PF6, which has a conductivity of 5 2 1 cm 1 at room temperature and metallic behaviour when cooled down [33]. While the structure of the crystals is chiral, the structure of this salt and other related ones [34] has an essentially achiral stack of donor molecules, which are pseudo-centrosymmetric [35]. The methyl groups at the periphery of the molecule are apparently not sufficient to cause a truly chiral stack of donors, which prefer to stack in parallel arrangements with partial overlap of the tr-systems in the solid state, a situation which is true for the majority of the efforts to prepare salts of this type (even if the salts are metallic). 

The bicycUc polyethers give rise to complex structures. With lanthanum chloride and Bicy(4,2B,2B), the adduct corresponds approximately to a 1 1 stoichiometry, but a crystalline product is obtained only in the presence of water. The crystal structure analysis shows that partial hydrolysis takes place. In this, the behaviour is reminiscent of the 2 1 TEA complexes with 15C5. Indeed, the unit cell contains a pseudo-centrosymmetric dimer [LaCl(Cl,OH)L]2 with an OH Cl hydrogen bond across the centre of symmetry one site is occupied by Cl in one half and by OH in the other half. The ligand in this structure has an approximate twofold axis of symmetry. The O atoms form an end-capped trigonal prism around La(III) and the metal ion lies in the mean plane formed by the O atoms of the aliphatic unsubstituted chain. The other bicyclic polyether, Bicy(4B,2B,2B), forms 3 2 complexes with lanthanum nitrate. The asymmetric unit contains two dinitrato cations [La(N03)2L] and one pentanitrato anion [La(N03)5(Me0H)] . The La(III) ion is 11-coordinate in the anion and one of the cations, in which there is one bidentate and one monodentate nitrate ion. It is 12-coordinate in the other cation. 

Although it is possible to determine the complete electron density distribution using the Fourier transform of the observed structure factors, Eq. (1), the errors inherent in the structure factor amplitudes and, in the case of non-centrosymmetric structures, the errors in their phases introduce significant noise and bias into the result. Because of this, it has become normal practice to model the electron density by a series of pseudo-atoms consisting of a frozen, spherical core and an atom centered multipole expansion to represent the valence electron density [2,17]. 

In order to pin-point the properties of order-disorder we concentrated on molecules whose racemates are centrosymmetric and the resolved enantiomorphs, therefore, quasi-centrosymmetric. These systems are advantageous since the enantiomeric crystal contains two independent molecules related to each other by a pseudo centre of inversion, and replaced in the racemate by a centrosymmetric pair. 

Of particular relevance to the measurability of crystal-field effects is a recent study that analyzed the experimental BCP properties of 9-ethynyl-9-fluorenol (crystallizes in the centrosymmetric space group C2/c with two molecules in the asymmetric unit) in relation to the extent to which constraints (local pseudo-symmetry and chemical similarity) were imposed on the pseudoatom model... 

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