# Spectra from Dipole Moments [closed]

If there is a change in dipole moments such as absorption spectroscopy, there are infrared absorptions (and/or transmittance) corresponding to them. Is the reason change in dipole moments are in infrared frequency simply because of the size of the molecules or scale of wavelength? Or are the dipole moments producing particular frequencies simply because of their sizes just like in antenna? If not, why are they mostly in IR?

(Note: image from https://en.wikipedia.org/wiki/Electromagnetic_spectrum)

## closed as unclear what you're asking by Buck Thorn, Mithoron, Mathew Mahindaratne, Karsten Theis, Todd MinehardtOct 14 at 16:51

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

• possible duplicate: chemistry.stackexchange.com/questions/73565/… – Buck Thorn Oct 14 at 9:27
• The energy of IR radiation matches the energy of the transitions between vibrational states. So it is a match in energy, not in length. – Karsten Theis Oct 14 at 15:34
• Could you edit your question to make sure it is clear what you are asking. If you look at the answers and the comments, folks are guessing what you want to ask. – Karsten Theis Oct 14 at 16:03
• Karsten. You answered it directly, I thought the energy of IR radiation is a match in length. You rightly answered it is a match in energy. – Jtl Oct 14 at 22:43

The transition energy, or wavelength if you prefer, are governed by the nature of the molecules involved. Whole body free rotational motion (in the gas phase) gives rise to microwave spectra ( fractions of wavenumbers cm$$^{-1}$$ or THz frequencies). The frequency is determined by the molecules moment of inertia. At higher energy comes vibrational motion, stretching and bending of chemical bonds, a few hundred to a few thousand cm$$^{-1}$$. These bond frequencies are approximately given by $$\sqrt{k/\mu}$$ where $$k$$ is force constant which relates force to bond extension as in Hook's law, and $$\mu$$ is the reduced mass. There are 3N-6 types of vibration for N atoms, (3N-5 for linear molecules). At higher frequencies are electronic transitions where and electron gets promoted from a bonding to anti-bonding orbital. Typically these transitions are in the visible and uv regions.