Ion-water hydrogen-bond switching observed with 2D IR vibrational echo chemical exchange spectroscopy

The exchange of water hydroxyl hydrogen bonds between anions and water oxygens is observed directly with ultrafast 2D IR vibrational echo chemical exchange spectroscopy (CES). The OD hydroxyl stretch of dilute HOD in H2O in concentrated (5.5 M) aqueous solutions of sodium tetrafluoroborate (NaBF4) displays a spectrum with a broad water-like band (hydroxyl bound to water oxygen) and a resolved, blue shifted band (hydroxyl bound to BF4-). At short time (200 fs), the 2D IR vibrational echo spectrum has 4 peaks, 2 on the diagonal and 2 off-diagonal. The 2 diagonal peaks are the 0-1 transitions of the water-like band and the hydroxylanion band. Vibrational echo emissions at the 1-2 transition frequencies give rise to 2 off-diagonal peaks. On a picosecond time scale, additional off-diagonal peaks grow in. These new peaks arise from chemical exchange between water hydroxyls bound to anions and hydroxyls bound to water oxygens. The growth of the chemical exchange peaks yields the time dependence of anion-water hydroxyl hydrogen bond switching under thermal equilibrium conditions as T-aw = 7 +/- 1 ps. Pump-probe measurements of the orientational relaxation rates and vibrational lifetimes are used in the CES data analysis. The pump-probe measurements are shown to have the correct functional form for a system undergoing exchange.