Configuration of molecules

Molecules. The electronic configurations of molecules can be built up by direct addition of atomic orbitals (LCAO method) or by considering molecular orbitals which occupy all of the space around the atoms of the molecule (molecular orbital method). 

L. E. Sutton, "Tables of Interatomic Distances and Configurations of Molecules, The Chemical Society, London, 1958. 

Just as for atoms, excited configurations of molecules are likely to give rise to more than one state. For example the excited configuration... 

The newer (R,S) system of nomenclature is superior to the older d,l system in one important way. The configuration of molecules with more than one... 

In chemical reactions, when the atomic configurations of molecules are changed, matter is neither created nor destroyed (Law of Conservation of Matter). The identity and number of atoms remain unchanged. When methane gas (Cl L) is burned, its atoms don t disappear they combine with oxygen (O,) in the air and are transformed into carbon dioxide (CO,) and water vapor (H,0) ... 

Recently the subject of conformational analysis20 has acquired some importance as a branch of organic chemistry. This is the study of the preferred configurations of molecules involving one or more possibilities of internal rotation. A better understanding of the... 

Fig. 4.2a, b. Configurations of molecules which are unfavourable to further growth, a The growth face of a lamella is shown, on which a molecule has deposited but is prevented from reaching the length required for stability by other attachments elsewhere, b Two examples of possible cross-sections perpendicular to the growth front. The outermost depositions must be removed before further growth of the stable crystal... 

Electron configurations of transition metal complexes are governed by the principles described in Chapters. The Pauli exclusion principle states that no two electrons can have identical descriptions, and Hund s rule requires that all unpaired electrons have the same spin orientation. These concepts are used in Chapter 8 for atomic configurations and in Chapters 9 and 10 to describe the electron configurations of molecules. They also determine the electron configurations of transition metal complexes. 

However, one of the few sources of information regarding the configurations of molecules in their electronically excited states is electronic spectra. Consequently the excited state geometries of relatively few molecules are known to date. The most accurate method for the determination of excited... 

Strictly speaking, the two models are not mutually compatible, and the concentrations of different cluster species are not equivalent to the weights associated with different basic lattice section occupancies. Nevertheless there exists the similarity that thermodynamic weights are associated, in each calculation, with various kinds of hydrogen bonded configurations of molecules. How these are subsequently used differs profoundly. I have deliberately used the very loose language of the text to extract for the reader the concept most nearly common to the two approaches. 

With more than one monocentric unit the overall configuration of molecules with inhibited internal rotations may also be determined by the dihedral angles of the bond system between monocentric units. The latter may no longer be treated as independent. A representation by polycentric configurations is advisable. 

For each configuration of molecules at the surface, there are a number of possible events. The events occur randomly with a characteristic rate for each type of configuration. This model is the kinetic Monte Carlo simulation. 

Another system of specifying configuration around a chiral center is the RS system, which is used in the systematic nomenclature of organic chemistry and describes more precisely the configuration of molecules with more than one chiral center (see p. 18). 

FIGURE 7-2 Three ways to represent the two stereoisomers of glyc-eraldehyde. The stereoisomers are mirror images of each other. Ball-and-stick models show the actual configuration of molecules. By convention, in Fischer projection formulas, horizontal bonds project out of the plane of the paper, toward the reader vertical bonds project behind the plane of the paper, away from the reader. Recall (see Fig. 1-17) that in perspective formulas, solid wedge-shaped bonds point toward the reader, dashed wedges point away. 

Modem structural chemistry differs from classical structural chemistry with respect to the detailed picture of molecules and crystals that it presents. By various physical methods, including the study of the structure of crystals by the diffraction of x-rays and of gas molecules by the diffraction of electron waves, the measurement of electric and magnetic dipole moments, the interpretation of band spectra, Raman spectra, microwave spectra, and nuclear magnetic resonance spectra, and the determination of entropy values, a great amount of information has been obtained about the atomic configurations of molecules and crystals and even their electronic structures a discussion of valence and the chemical bond now must take into account this information as well as the facts of chemistry. 

We conclude from the foregoing examples that in general the configurations of molecules with unshared electron pairs are similar to those of molecules with only shared pairs in the valence shell, except that the unshared pairs occupy a larger volume than the shared pairs, thus causing a decrease in the values of the bond angles. This effect has been discussed in detail in Section 4-3 for the simple case of sp hybridization. 

For many molecules it is possible to formulate valence-bond structures that are so reasonable and that account so satisfactorily for the properties of the substances that they are accepted by everyone without hesitation. The structures given on the next page may be shown for illustration. The physical and chemical properties of substances and the configurations of molecules associated with structures of this type are well understood, and this understanding forms the basis for a large part of chemical reasoning. 

The question might be asked Are there similar mechanisms for changing the configuration of molecules without breaking bonds in molecules with coordination numbers other than 3 and 5 The answer is yes. One of the most important series of inorganic compounds consists of six-coordinate chelate compounds exemplified by the tris(ethylenediamine)cobalt(lIl) ion. Because of the presence of the three chelate rings, the ion is chiral and racemization can take place by a mechanism that is closely related to atomic inversion or Berry pseudorotaiion (the mechanism for six-coordination is termed the Bailar twist see Chapter 13k... 

Figure 18. Fitting of the epitaxial layers planar defect (left) and local configuration of molecules along the planar defect (right)...

In the examination of crystalline borates, caution has been exercised over the structural interpretations of spectroscopic studies. Certain features pertaining to the boron coordination and water/hydroxyl entities can be deduced, but absolute configurations of molecules require more exacting techniques. 

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