Crystals polar

Zheludev, A., Grand, A., Ressouche, E. et al. (1994) Experimental determination of the spin density in the tetracyanoethenide free radical, [TCNE]-, by single-crystal polarized neutron diffraction, a view of a 71 orbital, J. Am. Chem. Soc., 116,7243-7249. 

We report herein a single crystal polarized neutron investigation of the spin density of two purely organic nitronyl nitroxide free radicals which present ferromagnetic interactions the 2-(6-ethynyl-2-pyridyl)-4,4,5,5-tetramethylimidazoline-l-oxyl-... 

Theoretical model porous electrode solid reagent dissolution electrochemical reaction crystallization polarization characteristic chloranile carbon black. 

In reference to Figure 5 for MNA crystals, the polar axes of the individual molecular sites are aligned with one another along the crystal polar axis. The microscopic co mo nents gx add resulting in the large macroscopic x i i i following equation 7. 

The unique piezoelectric and pyroelectric properties of semicrystalline films of PVDF arise from changes in the polarization imparted to the overall film by the crystalline P-phase. The polar nature of the P-phase is, in turn, a direct result of the parallel alignment of the dipole moment of the repeat units in the unit cell (Figure 11.1). The crystal polarization is defined as the dipole moment density of the crystal ... 

Equation (1) expresses the crystal polarization (P, C/iiF). as a function of the dipole moment (p, Cm) and the unit cell volume (V, iif). In PVDF, it suffices to express Eq. (1) in scalar form, where it is miderstood that P and p represent the components of the polarization and dipole moment vectors parallel to the ( -crystal axis. This arrangement of dipoles produces a significant local electric field in the... 

From Eq, (1) it is clear that a model of crystal polarization that is adequate for the description of the piezoelectric and pyroelectric properties of the P-phase of PVDF must include an accurate description of both the dipole moment of the repeat unit and the unit cell volume as functions of temperature and applied mechanical stress or strain. The dipole moment of the repeat unit includes contributions from the intrinsic polarity of chemical bonds (primarily carbon-fluorine) owing to differences in electron affinity, induced dipole moments owing to atomic and electronic polarizability, and attenuation owing to the thermal oscillations of the dipole. Previous modeling efforts have emphasized the importance of one more of these effects electronic polarizability based on continuum dielectric theory" or Lorentz field sums of dipole lattices" static, atomic level modeling of the intrinsic bond polarity" atomic level modeling of bond polarity and electronic and atomic polarizability in the absence of thermal motion. " The unit cell volume is responsive to the effects of temperature and stress and therefore requires a model based on an expression of the free energy of the crystal. 

Equation (2) expresses the model of the crystal polarization used in the molecular modeling of PVDF reported in this chapter, where is the dipole of each repeat unit of the single chain in vacuum, Ap is the change in dipole moment of the repeat unit of the chain in going from the vacuum environment to the environment of the packed crystal and (cos tp) is the attenuation of the dipole moment of the repeat unit along the fe-axis due to thermally stimulated oscillations about thec-axis. Ap is directly related to the local electric field (Eioc, V/m) through the repeat unit polarizability (ot, m ) ... 

Investigations of the electronic structure of quadruply bound dimers have relied heavily on electronic spectra to reveal the nature of the excited-state configurations. The availability of quantitative molecular orbital calculations coupled with single-crystal polarized electronic absorption spectral studies of quadruply bound dimers at low temperature has firmly established certain features of the excited electronic states of these compounds. A discussion of electronic spectra follows and a tabulation of energies associated with 6 - 6 transitions in quadruply bound dimers is given in Table 111. 

Additional information has been obtained from single crystal, polarized optical and ESR spectroscopic studies924 on poplar plastocyanin, which have allowed a correlation of the electronic structure of the blue copper active site with its geometric structure. In summary, the three dominant absorption bands at 13 350, 16 490 and 17 870 cm-1 were assigned to CysS- Cu (d 2-,2 charge-transfer transitions. The methionine makes only a small contribution, due to the long Cu—S(Met) bond (2.9 A) and the poor overlap of the methionine sulfur orbitals with the dx y orbital of copper. Histidine-Cu charge transfer contributes to the weaker absorptions at 21 390 and... 

Redy RA, Tschierske C (2006) Bent-core liquid crystals polar order, superstructural chirality and spontaneous desymmetrisation in soft matter systems. J Mater Chem 16 907-961... 

Figure 7.19 Possible guest arrangements in PHTP inclusion channels. While the. .. DA... interaction is by far the most likely, the occurrence of a greater number of. ..DD... defects than. ..AA... results in overall crystal polarity.

The random directions of the crystallographic axes of the crystallites of a ceramic limit the extent to which spontaneous polarization can be developed. It has been calculated that the fractions of the single-crystal polarization value that can be attained in a ceramic in which the polar axes take all possible alignments... 

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