Vibrational analysis of crystalline diketopiperazine--II. Normal mode calculations

Abstract

Based on our Raman and i.r. [1] data and assignments for DKP and five of its isotopic derivatives, we have refined an intramolecular force field in a non-redundant basis. This analysis shows that the cis peptide group may be best differentiated from the trans by the presence of an NH out-of-plane bend mode in the 800 cm-1 region. Several atom--atom potentials have been evaluated for their ability to explain observed lattice frequencies and internal mode splittings. Observed splittings in CO stretch modes can be accounted for by dynamical charge transfer and transition dipole coupling interactions.