Principles and applications of infrared spectroscopy

To better understand the nature and features of these vibrations, bonds can be considered as springs. Given this analogy, the behaviour of these molecular springs approximately follows Hooke s law of elasticity. In physics, Hooke s law relates the strain on a body (spring) to the force (load or mass) causing the strain . In essence, molecular bonds follow this linear relationship, where the 

Reduced mass relates to the relationship of the masses (atoms) at either end of the molecular spring, termed mi and m2, where reduced mass is defined as (mi x m2)/(mi + m2). As an example, in a C-H bond the masses in question are C = mi and H = m2. 

Considering the above equation and remembering that energy also relates to frequency (E = hv), a number of important points should be noted  

When exciting vibration in bonds, single bonds require (and absorb) less energy than double bonds, which require (and absorb) less energy than triple bonds. 

Energy is inversely proportional to reduced mass, and thus, the smaller the reduced mass of a bond the greater the energy (and frequency) required for vibration. For example, C-H has a smaller reduced mass than C-C and therefore stretching is induced at a higher frequency. 

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