[CrimpJiggler]

In an IR spectrum, the wavenumber is plotted against absorbance (well actually its transmittance but lets just pretend its absorbance for this thread). The absorbance axis represents the magnitude of the absorbance, therefore it represents the number of photons of that particular wavenumber that are absorbed. From what I've read, a particular bond will absorbs only a photon that has the exact amount of energy that the bond needs to jump to its higher energy level. Why then, do some functional groups absorb more photons than others? For example, the carbonyl group products an intense, sharp peak while a different functional group might product a very weak peak. Why do carbonyl groups absorb more photons than other functional groups?

[wolfzhchk]

In an IR spectrum, the wavenumber is plotted against absorbance (well actually its transmittance but lets just pretend its absorbance for this thread). The absorbance axis represents the magnitude of the absorbance, therefore it represents the number of photons of that particular wavenumber that are absorbed. From what I've read, a particular bond will absorbs only a photon that has the exact amount of energy that the bond needs to jump to its higher energy level. Why then, do some functional groups absorb more photons than others? For example, the carbonyl group products an intense, sharp peak while a different functional group might product a very weak peak. Why do carbonyl groups absorb more photons than other functional groups?

The strength of molecular vibration of different functional groups is variable, and the change of electric dipole moment of different molecules, caused by the absorption of IR radiation, is strikingly different.

a photon=hν

'h' is Planck's constant. 'ν' is frequency of light, which varies with specific wavelength of radiation. Because the different vibration types of functional groups, different wavelength radiation can be absorbed, which means they may absorb same number of photons but of different energy (different frequency).

The intensity of the peak depends on:
a) how large the change of dipole moment when compounds absorb radiation. (e.g. the dipole moment of carbon dioxide is always 0, because of its symmetrical structure, so it has no peak) The Strength and direction of dipole moment of a compound depends on the negativity of each atom which consists the compound, and the degree of its symmetry.

b) the probability of energy level transition, which means how probable the absorption will occur to cause ground-exicited level jump of electrons.