A vibrational frequency map of the amide III mode of the alanine dipeptide has been obtained at the B3LYP/6-314+G* level. It shows that, contrary to an earlier speculation by Lord, the frequency of this mode is a function of phi and psi, rather than psi, alone. We note that an analysis of the H-alpha bend mode, which independently depends on phi and psi, together with amide III may permit determination of these two torsion angles. Preliminary results on the alanine tripeptide indicate that such studies will be needed to establish the exact relationships in polypeptide chains.