Bond Lengths Depend on Method Used to Determine Them

BOND LENGTHS DEPEND ON METHOD USED TO DETERMINE THEM 

Chemistry is dehned as a study of the elements and their properties. Most of the time chemists are interested not so much in the elements themselves but rather in molecules, which are combinations of elements into discreet units. The organic chemist normally thinks of a molecule by reference to a rather simple model, usually of the ball-and-stick type. Such models are extremely useful for aiding in the visualization of three-dimensional interactions between molecules and between parts of molecules. But they are also a little bit misleading. A real molecule is not static, the way the usual mechanical model is. Rather, it is undergoing rapid motions, both internally and externally. Molecular structure can be defined at different levels, which are useful for different kinds of purposes. As we go past the ball-and-stick model, to the hard sphere model (space-filling model), and then to the soft sphere model (computational chemistry), we obtain an increasingly accurate description of the molecule and its properties. 

Most orgaific chemists are more interested in chanical reactions than they are in molecular structures. But the two are intimately related. The molecular structure of a molecule is more simple than the chemical reaction of two molecules, so let us start with the more simple case. What, exactly, is a molecular structure and how do we determine it  

A molecular structure, at a minimum, consists of the molecular formula of the molecule, together with a list of the coordinates of each atom in the molecule. The structure for a given molecule can be determined by a variety of ways, some of them experimental and some of them computational. But it is important to note at the outset that the structure that is determined for a particular molecule depends to a considerable extent on just what method is used to determine it. The reason for this is that the molecules undergo an internal vibrational motion. What that means is that the atoms do not occupy fixed points, as in the mechanical model, but they are moving about some 

It is also perhaps of interest that the small bond length difference obtained by going from C-H to C-D is snfficient to indnce other changes in physical properties. For example, molecnles having the type of structure shown in A and B are nonplanar because of the puckered cyclohexadiene ring in the center  

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