Why does the carbonyl group in an acid anhydride have two stretching frequencies?

Question

The carbonyl stretching frequencies for an acid anhydride are approximately $1820~\\mathrm{cm^{-1}}$ and $1760~\\mathrm{cm^{-1}}$. These are both higher frequencies than a simple ketone owing to the electron withdrawing capability of the adjacent group. However, why are there two distinct infrared stretching frequencies?

Answer 1

Both frequencies refer to the type of stretching acidic anhydrides exhibit. The 1760 frequency is symmetric, while the 1820 is asymmetric. Think about it like this, when one carbonyl shortens, the other one stretches further, altering the frequency. It's a phenomenon you'll only usually see in anhydrides given that their 2 carbonyl carbons are almost right next to each other.