Final answer:
Not all molecular vibrations result in the absorption of IR energy because only infrared active vibrations, which cause a change in dipole moment, can absorb IR. Symmetrical bond stretching that doesn't change dipole moments is infrared inactive, whereas polar bonds like the carbonyl bond absorb IR strongly. This principle forms the basis of infrared spectroscopy.
Step-by-step explanation:
Not all molecular vibrations result in absorption of IR energy because only vibrations that result in a change in dipole moment are infrared active. For example, symmetrical stretching of certain bonds like completely symmetrical double and triple bonds does not change the overall dipole moment of a molecule and therefore does not lead to IR absorption. However, other modes such as bending may still be IR active.
The energy of molecular vibration is quantized, and when exposed to electromagnetic radiation that matches one of its vibrational frequencies, a molecule can absorb energy and transition to a higher vibrational state. Infrared radiation matches the vibrational frequencies of organic bonds, and absorption occurs when there is a periodic change in the dipole moment of the molecule, much like in the case of a polar carbonyl bond, which absorbs IR strongly.
Bonds with greater polarity absorb IR more efficiently. The vibrational modes of molecules, including stretching and bending of bonds such as carbon-carbon and carbon-hydrogen bonds, can be complex and not all will result in IR absorption. This specificity allows for the usage of infrared spectroscopy to identify certain bonds and molecular structures.