Distances and Force Constants of Bonds

The usefulness of the multiple-bond radii in the discussion of the electronic structure of molecules will be illustrated in later sections. 

When the available spectroscopic values for simple molecules are examined, it is seen that the force constants of the bonds, which, to- 

Here k is the force constant, De the equilibrium internuclear distance, and an and / are constants, with values determined by the nature of the bonded atoms, as given in Table 7-7. 

The equation applies to both the normal states and excited states of molecules. 

As an illustration of the usefulness of the equation we may mention the calculation by Eyster22 in 1938, when no very reliable value of C==C distance in ethylene was available, of the value 1.325 0.005 A from the spectroscopic value for the carbon-carbon force constant. (The accepted value is now 1.334 A). 

---