My chemistry course requires us to know which molecules absorb infrared, and under which vibrational modes that they do. My professor provides us the following rule, which is very confusing for me and I still don't know how to start.

Rules for IR absorption: (1) The frequency of the IR radiation matches the frequency of the vibrational mode. AND (2) There is a net change in the symmetry of electrical charges during the vibrational mode.

So, say we have a molecule such as CF2Cl2. This absorbs infrared under all vibrational modes: symmetric stretching, asymmetric stretching, and bending. How do you get to that answer?

Thank you very much.


1 Answer 1


All forms of radiation match the difference in two energy levels, electronic, vibrational, rotational & nmr etc. For IR transitions we refer to molecular vibrations. The molecule must have a permanent dipole or produce a transient dipole as a result of vibrating in one of its normal modes.

So homonuclear diatomics have no IR spectrum, they of course have energy levels, but the radiation cannot 'couple' to these and so be absorbed. Heteronuclear molecule have a dipole and so also have IR spectra. The classical idea is that the electrons oscillate as the nuclei move in a vibration, this oscillating charge can interact with electromagnetic radiation of the correct energy and cause a transition from a lower energy level to a higher one.

Molecules such as benzene vibrate in many ($3N-6$) normal modes, some such as asymmetric stretches produce transient dipoles and have IR transitions. Others do not and so have no transitions. What determines the are called selection rules.

The only secure way is to look at the symmetry of the molecules using a point group table, its very easy when you know how, but a bit daunting at first sight. Have a look at this answer Understanding group theory easily and quickly for more details.

(The molecule you mention belongs to the $\ce{C_{2v}}$ point group, see molecule-viewer.com for a 3D model with symmetry elements.)

  • $\begingroup$ Thank you for your input, but some of it is hard to understand as I'm only a beginner at chemistry. We will not be needed to know group theory, nor have we discussed selection rules or anything like that. I just need to be able to tell whether a given molecule will absorb IR, and under which vibrational modes. How can I do that? $\endgroup$ Commented Oct 30, 2016 at 22:13
  • $\begingroup$ Then the simplest 'rule of thumb' until you learn more is to see if there is any change in the molecules dipole as the molecules bonds vibrate, if there is then there is an IR absorption. This is presumably the 'net change in symmetry' in your question. All heteronuclear diatomics have IR absorption due to vibration, none of the homonuclear diatomics do. $\endgroup$
    – porphyrin
    Commented Oct 31, 2016 at 8:30

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