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.