Results of spectrographic measurements

The most detailed spectroscopic investigations have been carried out with diatomic molecules and at the present time the characteristic constants, obtained from vibrational-rotational spectra arc known for more than 250 diatomic molecules. Several of these compounds are not sufficiently stable to exist in the pure state and have only been observed and studied by spectroscopic methods. Such compounds as CH, NH etc are of great interest as they undoubtedly occur as intermediate products in many chemical reactions. 

It should be pointed out, that in spite of the very great importance of spectroscopic methods, the interpretation of the data is not always unambiguous. For example, the dissociation energy of nitrogen has at various times been given as 268, 220, 206, 180 and 169 7 kcals. The latter figure, 

Tabu XXIX. IntemucUar Distances The agreement between the data obtained 

Some relationship must exist between the energy of dissociation, the equilibrium internuclear distance /, and the frequency of vibration, since the greater the value of the / dissociation energy, the greater will be the frequency of vibra- / tion and the smaller the internuclear distance. Attempts to relate these quantities by an empirical or semi-empirical formula, however, have so far failed. 

It is particularly interesting in this connection to notice that the dissociation energies of F. and Ng are very nearly in the ratio i 2 3. If we compare the properties of Lig and Fg each of which have a single effective bond we should perhaps expect that owing to the increased electronic repulsion in F. the dissociation energy of Lig might be greater than 

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