Energy, configurational orientation

Because of the orientational freedom, plastic crystals usually crystallize in cubic structures (Table 4.2). It is significant that cubic structures are adopted even when the molecular symmetry is incompatible with the cubic crystal symmetry. For example, t-butyl chloride in the plastic crystalline state has a fee structure even though the isolated molecule has a three-fold rotation axis which is incompatible with the cubic structure. Such apparent discrepancies between the lattice symmetry and molecular symmetry provide clear indications of the rotational disorder in the plastic crystalline state. It should, however, be remarked that molecular rotation in plastic crystals is rarely free rather it appears that there is more than one minimum potential energy configuration which allows the molecules to tumble rapidly from one orientation to another, the different orientations being random in the plastic crystal. 

Energies of the lowest lying sextet, quartet and doublet states were calculated for each of the heme units studied. The geometries of the complexes were taken from crystal structures and simplified to unsubstituted porphyrins. The orientations of the porphyrin macrocycles were such that the pyrrole nitrogens were on the x- and y-axes. The choice of the lowest energy configurations for each state was as follows ... 

Polarity may also be healed by the removal of a certain percentage of atoms in the outer layers. When the vacancies order, most of the time, this leads to surface reconstructions. The surface concentrations compatible with a vanishing max roscopic dipole moment can be correctly estimated within a fully ionic picture. Low energy configurations are expected to be insulating, and indeed, on non-polar oxide surfaces, a correlation has been found between the stability of a surface orientation and the surface gap value [58,59]. 

High surface tension Water molecules on the surface can readily orient themselves into the lowest energy configuration, which is the strongly bonded state. This phenomenon has been observed in the INS spectrum of vapour deposited ice [51]. Porosity in these ices is very high and a large number of water molecules are on surfaces. The INS spectrum shows a single, dominant. 

The hybridisation theory states that atomic orbitals can be mixed, i.e. hybridised, to form new atomic orbitals that are degenerate, i.e. equal in energy and orientation. Thus, for example, one of the 2s electrons in the ground state of carbon is promoted to give the electron configuration of 2s1, 2pr ,... 

If two dipoles are free to rotate relative to each other, they will take up the head-to-tail (lowest energy) configuration as seen in Figure 2.2 b. For the same r12, when the two dipoles align parallel to each other, that is V (r12, 90°, 90°, 180°) then the interaction potential energy is half of the maximum value. However, when r12 is not constant, contrary to the expectations that dipole-dipole attraction will prefer to orientate themselves in line, most dipolar molecules prefer to align in parallel so as to become significantly closer to each... 

When such a permanent dipole is placed in an electric field, orientation takes place as the molecule attempts to align with the field in order to adopt a low-energy configuration. This orientation is clearly time dependent and gives rise to Pd(t), the time-dependent macroscopic polarization discussed previously. 

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