Group Frequencies of M—H Structural Units

The hydrogen is also a terminal atom therefore, there is only one bond. This reduces the number of force constants that will determine the vibration. Finally, hydrogen is monovalent, so that the force constant is similar for various M atoms. M—H structural units, therefore show characteristic group frequencies unless the vibration is perturbed by another. As a general rule, whenever a terminal group has atoms that differ in mass, a characteristic group frequency is likely to exist. We shall see examples of this below. 

Terminal Structural Groups such as C=0 and Multiple-Bond Groups 

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