Fig. 4: Local electrostatic field determines water molecule reorientation motions in PBBNB⁺Br^-.

a, b Selected 2D IR spectra at 55% RH at waiting times \({t}_{2}\,\)= 0.20 and 10.20 ps from experiments and simulations, respectively. The 0−1 transitions are shown with yellow-red contours, and the 1–2 transitions are shown with blue contours. The light green lines indicate the center lines used to extract the CLS decays. Additional 2D IR spectra can be found in Supplementary Fig. 35. c Center Line Slope (CLS) extracted from 2D IR spectra. d Frequency-frequency correlation function (FFCF) simulated by MD. The inset presents an enlarged view of the FFCF curves on a shorter timescale for clarity. e, f Anisotropy decay r(t) from Ultra-fast IR and MD simulations, respectively. g Schematic illustration detailing reorientation motions from two initial states: State I (water molecule sits in the 1st solvation shell and interacts with the bromide ion through H-bond) and State W (water molecule sits in the 2nd shell and interacts with the adjacent water molecule through H-bond). One type of water reorientation includes jumps through breakage and formation of H-bonds, as indicated by a purple arrow. The associated relaxation times are \({\tau }^{{Ex}}\) (water molecule jumps from one solvation shell to another) and \({\tau }^{{Jump}}\) (water molecule jump within the same solvation shell). The other type of water reorientation includes co-rotations of a water molecule with intact H-bonds, as indicated by a green arrow. The associated relaxation time is \({\tau }^{{Frame}}\). The black dashed line corresponds to H-bond, and light blue and gray spheres correspond to initial and new H-bond acceptors, respectively.