Probing Intramolecular Vibrational Coupling between the C═C Ring and Amide-I Modes in Quinolinone Derivatives: Insights from DFT and Molecular Dynamics Simulations

Vibrational coupling is an intrinsic component of IR spectroscopy. Coupling between vibrating modes is crucial for energy transfer and may result in alterations in peak position, intensity, and perturbations in molecular dynamics. In most biomolecules, the amide-I mode (C═O) has been highly studied. The data obtained regarding the observed mode may not be precise due to other perturbing modes present in a closer frequency region, such as the C═C vibrating mode. The C═C and amide-I modes can engage in inter- and intramolecular vibrational coupling. Intramolecular vibrational coupling is an inherent property of molecules. To investigate the influence of intramolecular vibrational coupling between symmetric/asymmetric C═C rings and amide-I modes, we have performed theoretical calculations using DFT on three quinolinone-based fused ring derivatives. We observed that vibrational coupling is present between both symmetric/asymmetric rings and the amide-I mode for one of the systems, but only the asymmetric vibrational ring and amide-I modes couple in the case of the others. This observation is also validated through a molecular dynamics simulation performed using a normal-mode Hamiltonian, where the energies in the amide-I mode are monitored along with the symmetric and asymmetric C═C ring modes. This disparity in the coupling behavior is due to the difference in the positional orientations of the respective modes.