Why $\ce{H2O}$ shows three and $\ce{CO2}$ shows two fundamental bands in IR while they both are triatomic molecules?
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4Maybe something to do with symmetry and degenerate modes? – Todd Minehardt Sep 30 '21 at 18:38
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1Welcome to the site. Note, chemical information may be advantageously formatted using on ChemSE with mhchem. Take moment to familiarize with this. You are encouraged to use it in the body of questions, answers, and comments. Because it is something special not all web browsers understand well, do not use it in the title of questions or answers. – Buttonwood Sep 30 '21 at 21:14
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Related, see "Is carbon dioxide IR inactive?" – ron Sep 30 '21 at 22:28
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Molecules vibrate. Some of the vibrations alter the polarizability of the electron cloud of a chemical bond, which renders them suitable for a characterization by RAMAN spectroscopy. Some of the vibrations alter the dipole moment of the molecule; these are the vibrations characterized by infrared spectroscopy. Both techniques are complementary to each other, and especially in larger molecules, there are vibrations which may be observed by either one.
For $\ce{CO2}$, a linear molecule, there is one vibration which does not alter the net dipole moment. Two others coalesce upon observation since they match in energy. Chemistry LibreTexts contains an animation of all of them:
For water, already bent in the ground state, the vibrations IR spectroscopy may characterize are these:
Both animations are a verbatim copy from the entry on Chemistry LibreTexts about this topic.
Buttonwood
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