Mirror, atomic

Surface modifications and surfiice roughness Cu, Mo, and Be laser mirrors atomic oxygen modified (corroded) surfaces and films, and chemically etched surfaces. 

The main effects of laser-controlled atomic-beam optics are as follows (1) colli-mation of atomic beams, (2) reflection, (3) focusing, (4) guiding in hollow fibers, and (5) interferometry. The collimation of an atomic beam by making use of 2D laser cooling has already been considered in Section 5.4.2, and the laser guiding of atoms in a hollow optical fiber in Section 6.1.3. Therefore, we shall briefly consider the atomic reflection (atomic mirrors), atomic-beam focusing, and interferometry effects. In the latter case, laser radiation is used to produce the atomic beams necessary for the observation of the interference of atoms. 

A molecule is chiral if it cannot be superimposed on its mirror image (or if it does not possess an alternating axis of symmetry) and would exhibit optical activity, i.e. lead to the rotation of the plane of polarization of polarized light. Lactic acid, which has the structure (2 mirror images) shown exhibits molecular chirality. In this the central carbon atom is said to be chiral but strictly it is the environment which is chiral. 

In certain crystals, e.g. in quartz, there is chirality in the crystal structure. Molecular chirality is possible in compounds which have no chiral carbon atoms and yet possess non-superimposable mirror image structures. Restricted rotation about the C=C = C bonds in an allene abC = C = Cba causes chirality and the existence of two optically active forms (i)... 

Figure B3.3.13. Intersecting stacking faults in a fee crystal at the impact plane induced by collision with a momentum mirror for a square cross section of side 100 unit cells. The shock wave has advanced half way to the rear ( 250 planes). Atom shading indicates potential energy. Thanks are due to B Holian for tliis figure.

If the tctravalcnt carbon atom has three difFcrcnt substituents, the molecule is chiral and it is not possible to superimpose it onto its mirror image. Our feet are also chiral objects the right foot is a mirror image of the left one and does not ftt into the left shoe. 

We can view this reaction as the replacement of one or the other of the two methylene protons at C 2 of butane These protons are prochiral atoms and as the red and blue protons m the Newman projection indicate occupy mirror image environments... 

Diastereotopic (Section 7 13) Descnbing two atoms or groups in a molecule that are attached to the same atom but are in stereochemically different environments that are not mirror images of each other The two protons shown in bold in H2C=CHC1 for example are diastereotopic One is cis to chlonne the other is trans... 

Conformational Isomers. A molecule in a conformation into which its atoms return spontaneously after small displacements is termed a conformer. Different arrangements of atoms that can be converted into one another by rotation about single bonds are called conformational isomers (see Fig. 1.1). A pair of conformational isomers can be but do not have to be mirror images of each other. When they are not mirror images, they are called diastereomers. 

Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image it has an internal mirror plane. This is an example of a diaster-eomer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound. 

Chiral separations are concerned with separating molecules that can exist as nonsupetimposable mirror images. Examples of these types of molecules, called enantiomers or optical isomers are illustrated in Figure 1. Although chirahty is often associated with compounds containing a tetrahedral carbon with four different substituents, other atoms, such as phosphoms or sulfur, may also be chiral. In addition, molecules containing a center of asymmetry, such as hexahehcene, tetrasubstituted adamantanes, and substituted aHenes or molecules with hindered rotation, such as some 2,2 disubstituted binaphthyls, may also be chiral. Compounds exhibiting a center of asymmetry are called atropisomers. An extensive review of stereochemistry may be found under Pharmaceuticals, Chiral. 

Sihcon is a Group 14 (IV) element of the Periodic Table. This column iacludes C, Si, Ge, Sn, and Pb and displays a remarkable transition from iasulatiag to metallic behavior with increasing atomic weight. Carbon, ia the form of diamond, is a transparent iasulator, whereas tin and lead are metals ia fact, they are superconductors. SiUcon and germanium are semiconductors, ie, they look metaUic, so that a poHshed siUcon wafer is a reasonable gray-toned mirror, but they conduct poorly. Traditionally, semiconductors have been defined as materials whose resistance rises with decreasiag temperature, unlike metals whose resistance falls. 

Figure 4 Interference pettern created when regularly spaced atoms scatter an incident plane wave. A spherical wave emanates from each atom diffracted beams form at the directions of constructive interference between these waves. The mirror reflection—the (00) beam—and the first- and second-order diffracted beams are shown.

Compounds in which one or more carbon atoms have four nonidentical substituents are the largest class of chiral molecules. Carbon atoms with four nonidentical ligands are referred to as asymmetric carbon atoms because the molecular environment at such a carbon atom possesses no element of symmetry. Asymmetric carbons are a specific example of a stereogenic center. A stereogenic center is any structural feature that gives rise to chirality in a molecule. 2-Butanol is an example of a chiral molecule and exists as two nonsuperimposable mirror images. Carbon-2 is a stereogenic center. 

Enantiotopic (Section 7.9) Describing two atoms or groups in a molecule whose environments are nonsuperimposable mirror images of each other. The two protons shown in bold in CH3CH2CI, for example, are enantiotopic. Replacement of first one, then the other, by some arbitrary test group yields compounds that are enantiomers of each other. 

FIGURE 4.12 Enantiomeric molecules based on a chiral carbon atom. Enantiomers are nonsuperimposable mirror images of each other. 

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