Question
The structure of a molecule greatly influences its ability to absorb heat (thus, infrared radiation). For a molecule to absorb into the infrared, it must have a permanent dipolar moment, or a temporary dipolar moment created by the vibrations between the bound atoms. For example, the CO2 molecule, which is linear and non-polar, still absorbs IR rays because it vibrates in 3 ways on the image below:
The vibration shown in (a) is called symmetrical elongation; since both springs stretch at the same time, but in opposite directions, this elongation (also called vibration mode) does not generate a temporary dipolar moment, and is said to be inactive in IR. However, the elongation mode in (b), called antisymmetric elongation, is active in IR since it generates a temporary dipolar moment (the right link becomes longer while the left one shortens, creating a polarity with the negative side to the right).
The elongation mode shown in (c) is called deformation (or shearing), and also generates a temporary dipolar moment (depending on the arrows on the figure, one would have the negative pole upwards). Thus, it is said that CO2 has 2 active vibration modes in IR, of the 3 possible modes. Using this information, explain
a) why the N2 molecule can never be active in IR;
b) why the water molecule has 3 active IR modes out of the 3 possible.
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