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I've a very straight forward question, I'm confused because I've been told that a atom which has a oscillating transient/permanent dipole is IR active. But there are few asymmetric stretching/bending modes in acetylene? So, why does my teacher say it's IR inactive?

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    $\begingroup$ IR activity is not a property of an atom, nor that of a molecule. It is a property of a particular oscillation. $\endgroup$ – Ivan Neretin Sep 8 '20 at 20:55
  • $\begingroup$ @IvanNeretin This should have been the answer. And its clear, concise, to the point, too. $\endgroup$ – Buttonwood Sep 8 '20 at 21:03
  • $\begingroup$ Clear as it may be, my comment is way too short for an answer. $\endgroup$ – Ivan Neretin Sep 8 '20 at 21:16
  • $\begingroup$ You may find some vibration frequencies of acetylene here. $\endgroup$ – Mathew Mahindaratne Sep 8 '20 at 21:42
  • $\begingroup$ Also, vibration patterns (active and inactive) here with animations. $\endgroup$ – Mathew Mahindaratne Sep 8 '20 at 22:00
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Looks like it has an IR spectra to me:

enter image description here

Source: https://webbook.nist.gov/cgi/cbook.cgi?ID=C74862&Type=IR-SPEC&Index=1

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