Our studies of the alanine dipeptide (ADP) vibrational frequency map (J. Phys. Chem. A 2002, 106, 3391) reveal that the (CHalpha)-H-alpha stretch mode, and particularly its (CDalpha)-D-alpha isotope-edited form, is a potentially powerful tool for determining the phi, psi conformation in peptide systems. In this communication, we present ab initio (CDalpha)-D-alpha and (CHalpha)-H-alpha stretch frequencies of the alpha(R), beta, and polyproline II conformations of the ADP, showing that they give rise to well-separated frequencies, and discuss the kinds of experimental data needed to establish the desired structural information.