Shortened triple bonds

It is the electrons in a crystal that interact with the X-ray photons, and the electron density distribution is the quantity actually measured in crystallography. When taking the step from the electron density map to the atomic model, the simple assumption is that locations of high electron density correspond to atomic positions. Albeit a very simple and maybe a little naive assumption, the models derived with its help are mostly very sensible and accurate. There are cases, however, when this assumption does not hold. 

Triple bonds between light atoms, that is carbon-carbon or carbon-nitrogen triple bonds, show relatively high electron density between the atoms, when compared to 

As long as atoms are not described as electronic multipoles (Bader, 1990), there is no way to overcome or to correct for this effect. It is important to always bear in mind that bond lengths determined by X-ray diffrachon are distances between electron density maxima and not between the true posihons of the nuclei. 

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Double and triple bonds are made by adding n-bonds to a sigma bond. Each additional bond shortens the distance between the bonding atoms. 

To explain the bond shortening, Pauling estimates that the two triple-bonded structures are about as important as the double-bonded one. Now it is not easy to estimate the energy of a structure such as B or <7, but the estimates that have been made indicate that both B and C are far less stable (100 kilocalories per mole) than A (chiefly because of the reluctance of oxygen to assume a positive charge). That there is a difference between the C—O bonds in CO2 and those in acetone is indisputable whether this difference is best expressed by heavy contributions of high-energy triple-bonded structures is doubtful. 

Some bonds, classically represented as single bonds, are found to be abnormally short. Using the above interpretation, such bonds are considered to have partial double-bond charaeter and extra resonance pictures are thus drawn which are said to contribute to the structure of the molecule. Almost always, such shortening occurs in a single bond adjacent to a double or triple bond if the bond is between two multiple bonds, the shortening is even more marked ... 

Since the distance of 1.15 A is smaller than is to be expected for the first configuration, namely 1.22 A, the other two also contribute to a considerable extent. The shortening by the contribution of the triple bond is then larger than the lengthening by that of the single bond. 

This species is a cyclic form of Cs and may be considered as the interaction between a triple bond and a carbene center. We assumed Czv symmetry and obtained the local optimum geometry. This shows a lengthening of the triple bond and a shortening of the single bonds compared with cycloprop5me. 

Metal-metal multiple bonding can have dramatic effects on bond distances, as measured by X-ray crystallography. One way of describing the shortening of interatomic distances by multiple bonds is by comparing the bond distances in multiple bonds to the distances for single bonds. The ratios of these distances is sometimes called the formal shortness ratio. Values of this ratio are compared below for main group triple bonds and for some of the shortest of the measured transition metal quadruple bonds ... 

Saturated and Unsaturated Lipids Not all lipids are the same. Recall the difference between saturated and unsaturated hydrocarbons. Unsaturated molecules have one or more double or triple bonds between carbon atoms. Lipid molecules also can be saturated or unsaturated. As you can see in Figure 23, when a lipid is saturated, the acid chains are straight because aU the bonds are single bonds. They are able to pack together closely. A compact arrangement of the molecules is typical of a solid such as margarine or shortening. These solid lipids consist mainly of saturated fats. 

A) CJI., has a triple bond between the two carbons, while has double bonds. This allows for increased overlap and the formation of an additional pi bond. This has the effect of shortening the bond length. 

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