Spectroscopy: Assignment of the IR spectrum of propanoic acid

abbreviations: ν(nu)=stretching; δ(delta)=in-plane bending or deformation; ω(omega)=wagging; ρ(rho)=rocking; τ(tau)=twisting; π(pi)=out-of-plane bending
Subscripts as, s, and d denote antisymmetric, symmetric, and degenerate modes

More information about infrared spectroscopy on Wikipedia. In relation with the IR or Raman activity of a mode, see also the character tables for the more common molecular point groups. You can also have a look on a scheme that summarizes the origin of overtones, combination bands and Fermi resonances.