X-center

Belt length = Pitch circumference - - (2 x Center distance)... 

Figure 6. Schematic projection and tunnel structure of BaTi40g(a) and NagTieOiaCb). O.oxygen atom .titeinium atom barium or sodium atom X. center of gravity of six oxygen ions. The arrows indicate the dipole moments.

R X cent <- scale (X, center = TRUE, scale = FALSE)... 

The very strong polarization of the Te=X —> Te+ - X (X = C,N,0) bonds accounts for the enhanced electrophilicity of the tellurium center in 7r-telluranes, which markedly exceeds that of other chalcogen centers in respective 7r-chalcogenuranes (ylides, imides, oxides). Such a polarization provides also for the highest nucleophilcity and basicity of the X-centers in ir-telluranes. 

One can only speculate as to the exact identity of the Z q or Zl species, but the presence or formation of (0 )x (e )x centers in the clay structure should be investigated. Ideas advanced by Coyne (43) and Freund and Batllo (44) regarding (0 ) and (02 ) centers in clay minerals could, therefore, play an important part in the development of a complete understanding of the Fe reduction mechanisms in smectites ... 

Organocatalysis has led to the development of new methods for the asymmetric a-halogenation of carbonyl compounds leading to the formation of stereogenic C-X centers. Hence, details of direct enantioselective fluorination, chlorination, and bromination reactions will be presented in the following sections. 

The study of the radiative recombination of excitons makes it possible to investigate the influence of the radiation-stimulated destruction of C60 fullerenes (partially of C70) on changes of the singlet states within the energy gap. It is known that the emission of excitons in this case is the result of the presence of own dimeric traps [11] and X-centers, caused by the chemically bound with fullerenes and intercalated impurities [8], and also of taking into account the corresponding phonon states. 

The use of prochiral alkenes such as propene, 2-butene, tran -dideutereoethylene, dialkyl fumarates, and trani-dimethoxyethylenes, have allowed detailed structural and mechanistic studies of alkene complexes. The diastereomers produced upon binding of prochiral alkenes to CpM(CO)X centers provided the key complexes to prove that interconversions occurred by rotations about the metal-(C=C) axis. Thus observations that neither the chirality at the metal nor at the alkene is changed in the rearrangement of a trans-substituted alkene provided proof that the nature of the dynamic process was a propeller rotation. Note that in equation (9) the equilibrium between (20) and (21) averages enviromnents b and d separately from a and c. A key feature is that olefin rotation does not alter the chirahty at the olefin because of the olefin-metal bond. 

X= (center-to-center spaclng)/(outside tube diameter)... 

Elimination can also occur at C(sp3)-X centers under acidic conditions. The mechanism is called El. The leaving group leaves, sometimes after protonation, to give a carbocation. The carbocation then fragments (one of the three reactions... 

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