Bond distances individual molecules

Two important applications of radiation to determine molecular structure—X-ray crystallography and magnetic resonance—were discussed in Chapters 3 and 5. In this chapter we will discuss a variety of other techniques. Microwave absorption usually forces molecules to rotate more rapidly, and the frequencies of these absorptions provide a direct measure of bond distances. Individual bonds in a molecule can vibrate, as discussed classically in Chapter 3. Here we will do the quantum description, which explains why the greenhouse effect, which overheats the atmosphere of Venus and may be starting to affect the Earth s climate, is a direct result of infrared radiation inducing vibrations in molecules such as carbon dioxide. 

One of the most important problems connected to the carbon— carbon distances of aromatic rings is the determination of the lengths of the different individual bonds in condensed aromatic systems. Extensive X-ray work - has been carried through in order to give accurate values for the bond distances in molecules like naphthalene, anthracene, and coronene. These three molecules have also been studied in this institute using the electron diffraction method. Elaborate studies including least-squares calculations have led to the result presented in third column of Table VI. The X-ray data are also included. As for naphthalene, the correspondence between the electron diffraction results and those obtained from X-ray studies is satisfactory. For the two other compounds there is a deviation that has to be explained. There are a series of... 

Of the various geometric parameters associated with molecular shape, the one most nearly constant from molecule to molecule and most nearly independent of substituent effects is bond length. Bond lengths to carbon depend strongly on the hybridization of the carbon involved but are little influenced by other factors. Table 1.2 lists the interatomic distances for some of the most common bonds in organic molecules. The near constancy of bond lengths from molecule to molecule reflects the fact that the properties of individual bonds are, to a good approximation, independent of the remainder of the molecule. 

Three years ago it was pointed out2 that observed values of interatomic distances provide useful information regarding the electronic structures of molecules and especially regarding resonance between two or more valence bond structures. On the basis of the available information it was concluded that resonance between two or more structures leads to interatomic distances nearly as small Us the smallest of those for the individual structures. For example, in benzene each carbon-carbon bond resonates about equally between a single bond and a double bond (as given by the two Kekul6 structures) the observed carbon-carbon distance, 1.39 A., is much closer to the carbon-carbon double bond distance, 1.38 A., than to the shrgle bond distance, 1.54 A. 

We underline these results and the implied concepts quoting from a comprehensive review on this subject (Simon 1983). We remember indeed that, ever since it was experimentally possible to determine atomic distances in molecules and crystals, efforts have been made to draw conclusions about the nature of the chemical bonding, and to compare interatomic distances (dimensions) in the compounds with those in the chemical elements. Distances between atoms in an element can be measured with high precision. As such, however, they cannot be simply used in predicting interatomic distances in the compounds. In a rational procedure, reference values (atomic radii) have to be extracted from the individual (interatomic distances) measured values. Various functions have been suggested for this purpose. In the specific case of the metals it has been pointed out that interatomic distances depend primarily on the number of ligands and on the number of valence electrons of the atoms (Pearson 1972). 

A secondary bond , as defined by Alcock [6-8], is an interaction between two atoms characterized by a distance longer than the sum of the covalent radii but shorter than the sum of the van der Waals radii of the corresponding atoms. Such secondary interactions are weaker than normal covalent or dative bonds, but strong enough to connect individual molecules and to modify the coordination geometry of the atoms involved. They are often present in a crystal, thus resulting in self-assembled supermolecules or supramolecular architectures. For gold complexes,... 

Here X is the ratio of the probability of an end bond breaking to the probability of a bond breaking in the middle of the molecule. Examples of the distribution of bond weights for an individual molecule is shown in Figure 18.8 for the case where the likelihood of bonds breaking increases with distance away from the molecule centre. 

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