Why does urea not alter the vibrational spectroscopic signatures of water?

Usually, external solutes alter the local structural arrangement of the water hydrogen bond (H-bond) network. However, urea has a negligible effect on the local structure of water, irrespective of its concentration. In this work, we aim to find the microscopic origin of the inertness of urea toward water's local structure by using theoretical isotope dilute IR spectroscopy. To get a microscopic resolution of different local environments, we investigate frequency distributions, population of structurally different ensembles, H-bond configurations, and H-bond strength. We find that urea has a negligible effect on the vibrational spectroscopic signatures of water. The solvation shell-wise decomposition of frequency distribution further reveals that the urea has almost zero perturbation to the local structure of any of its solvation shells. However, the frequency distribution of waters near the C=O group and NH2 group reveals a non-negligible impact by urea on water. The frequency distribution of the CO-region is red-shifted and narrower than the bulk water response, indicating a stronger H-bond and enhanced structural ordering of the waters in the CO-region; however, an opposite response is observed for the NH-region. The H-bond strength outcome further supplements this finding. We infer that the O-H oscillators in the CO-region and NH-region have non-negligible counteracting perturbation from the urea molecule. Yet, the overall response is free from any such effects. The insignificant populations and counteracting responses of these two ensembles play a major role.