Water as a molecular hinge in amide-like structures

The authors have studied the reorientational dynamics of isolated water molecules in a solution of N,N-dimethylacetamide (DMA). From linear spectra, the authors find that the water in this solution forms double hydrogen bond connections to the DMA molecules, resulting in the formation of DMA-water-DMA complexes. The authors use polarization-resolved mid-infrared pump-probe spectroscopy on the water in these complexes to measure the depolarization of three distinct transition dipole moments, each with a different directionality relative to the molecular frame (OH stretch in HDO, symmetric and asymmetric stretch normal modes in H₂O). By combining these measurements, the authors find that the system exhibits bimodal rotational dynamics with two distinct time scales: a slow (7±1 ps) reorientation of the entire DMA-water complex and a fast (0.5±0.2 ps) “hinging” motion of the water molecule around the axis parallel to the connecting hydrogen bonds. Additionally, the authors observe an exchange of energy between the two normal modes of H₂O at a time scale of 0.8±0.1 ps and find that the vibrational excitation decays through the symmetric stretch normal mode with a time constant of 0.8±0.2 ps.