[fsci19]

Can someone explain why exactly the intensity of a signal is a function of the dipole moment of the bond? Why is the C=O absorption so strong compared to C=C?
I was thinking that because a stronger dipole may correlate with a stronger bond (Ie a C=O bond is stronger than a C=C bond because the electronegativity difference accounts for an extra attractive force), that means the bond vibrates quicker and absorbs at a higher frequency... Since the bond absorbs at a higher frequency, the energy it is absorbing results in a larger change in dipole moment and therefore a more intense signal. Is there another reason I am missing? I feel like this answer is kind of repeating itself...!
Thanks!

[Irlanur]

You are confusing quite many concepts here. First, you can't generalise that the dipole moment of a bond (whatever that is) correlates with the bond strength (http://en.wikipedia.org/wiki/Bond-dissociation_energy). Then the bond strength has no direct connections to the vibration frequency. The vibrational frequency is determined by the force constant of the bond and the reduced mass of the two parts forming the bond.

Since the bond absorbs at a higher frequency, the energy it is absorbing results in a larger change in dipole moment and therefore a more intense signal

That doesn't make any sense. That would mean that the higher the wavenumber, the more intense the band in the spectrum, which is obviously not the case.

[Corribus]

Why is the C=O absorption so strong compared to C=C?

It takes a fair amount of math to show explicitly why this is the case, but I have given fairly detailed explanations with minimal use of equations in the following two posts:

http://www.chemicalforums.com/index.php?topic=68049.msg245230#msg245230

http://www.chemicalforums.com/index.php?topic=76737.msg279617#msg279617