Mirror plane, virtual

In the search for new organic conducting materials, x-ray structure determinations of fused 1,2,5-selenadiazoles and 1,2,5-thiadiazoles have been carried out. Compounds (25a) and (25b) are planar molecules possessing a two-fold axis and mirror plane. Short C=N S and C N Se contacts connect each molecule in the crystal with four neighbors, forming a virtually planar sheet network (Figure 7), The sheets stack to form infinite layers with face-to-face overlapping between... 

A point, such as a center of inversion, can hardly be said to bisect a bond, except perhaps when it lies at its midpoint - as it does here. However, inversion can quite generally be regarded as an improper rotation of order 2 i = S2, and - as such - has a virtual mirror plane that can be placed so as to bisect any bond that we choose. 

Mirrors. For reflection from a plane mirror, an observer situated at a distance u from it sees a "virtual image" of himself/herself at a distance... 

The symmetry operations that we have encountered are either proper or improper. Proper symmetry elements are rotations, also including the unit element. The improper rotations comprise planes of symmetry, rotation-reflection axes, and spatial inversion. All improper elements can be written as the product of spatial inversion and a proper rotation (see, e.g.. Fig. 1.1). The difference between the two kinds of symmetry elements is that proper rotations can be carried out in real space, while improper elements require the inversion of space and thus a mapping of every point onto its antipode. This can only be done in a virtual way by looking at the structure via a mirror. From a mathematical point of view, this difference is manifested... 

A familiar example of specular reflectance is provided by a plane mirror, in which the angles of light incidence and reflectance are equal. An ideal mirror reflects all incident light nonselectively with wavelength. If free of dust and suitably framed, the surface of an even less than ideal real mirror is not perceived at all instead, the virtual image of an object located physically in front of the mirror is seen as if positioned behind. 

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