Viscosity Dependence of the Ultrafast Vibrational Dynamics of Borohydride in NaOH Solutions: Crowding Effect on Dihydrogen Bonds

Two-dimensional infrared spectroscopy (2D-IR) probes the local solvent structure and dynamics of the nu₃ mode (BH antisymmetric stretch) of borohydride (BH₄^- ) in aqueous solution. The 2D-IR spectra of the BH stretches have broad and overlapping bands. Vibrational energy relaxation occurs on a 2 ps timescale. Energy that is initially deposited in BH stretching modes and directly in the solvent generates a long lived hot ground state. Before the hot ground state appears, some indications of intramolecular vibrational energy redistribution are observed. The solvent viscosity was varied (1.5–6 cP) by increasing the hydroxide concentration (0.1–7 M). Within the vibrational lifetime of the BH stretching modes, the rate of structural relaxation slows proportionally to the viscosity due to the overlap of the ions’s solvation shells.