Coherent three-pulse spectroscopy of coupled vibrations in a rigid dipeptide: Density functional theory simulations

The full quartic six-mode force field for the NH, CO, and CN stretches of the peptide bonds of a cyclic dipeptide rigidly held by a bridge (2,5-diazabicyclo[2,2,2]octane-3,6-dione (DABCODO)) is computed at the ab initio level and used for the simulation of two-dimensional three-pulse femtosecond infrared measurements. Analysis of the complete set of one- and two-color signals generated at all four possible wavevectors (-k(1) + k(2) + k(3), k(1) - k(2) + k(3), k(1) + k(2) - k(3), k(1) + k(2) + k(3)) shows distinct signatures of anharmonicities, mode couplings, Fermi resonances, and relative transition dipole orientations.