Group Frequencies of Units Where the Mass Factor Is Important

Group Frequencies of Units Where the Mass Factor Is Important 

If one atom has a mass very much different from the masses of the atoms in the remainder of the molecule, then the structural groups with this atom will have characteristic group frequencies. An example would be a unit such as —C— As—, i.e., the case where isolated arsenic atoms are found in a hydrocarbon. The C— As stretching frequency is fairly distinct since the vibration is not likely to couple with another. Other examples are C—Cl, C— Br, C—metal, etc. A C—group would be expected to give less satisfactory group frequencies since the masses of C and N are similar and both are involved in other chemical bonds. 

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