title: Ultrafast phosphate hydration dynamics in bulk H2O.
abstract: Phosphate vibrations serve as local probes of hydrogen bonding and structural fluctuations of hydration shells around ions. Interactions of H2PO4(-) ions and their aqueous environment are studied combining femtosecond 2D infrared spectroscopy, ab-initio calculations, and hybrid quantum-classical molecular dynamics (MD) simulations. Two-dimensional infrared spectra of the symmetric (νS(PO2(-))) and asymmetric (νAS(PO2(-))) PO2(-) stretching vibrations display nearly homogeneous lineshapes and pronounced anharmonic couplings between the two modes and with the δ(P-(OH)2) bending modes. The frequency-time correlation function derived from the 2D spectra consists of a predominant 50 fs decay and a weak constant component accounting for a residual inhomogeneous broadening. MD simulations show that the fluctuating electric field of the aqueous environment induces strong fluctuations of the νS(PO2(-)) and νAS(PO2(-)) transition frequencies with larger frequency excursions for νAS(PO2(-)). The calculated frequency-time correlation function is in good agreement with the experiment. The ν(PO2(-)) frequencies are mainly determined by polarization contributions induced by electrostatic phosphate-water interactions. H2PO4(-)/H2O cluster calculations reveal substantial frequency shifts and mode mixing with increasing hydration. Predicted phosphate-water hydrogen bond (HB) lifetimes have values on the order of 10 ps, substantially longer than water-water HB lifetimes. The ultrafast phosphate-water interactions observed here are in marked contrast to hydration dynamics of phospholipids where a quasi-static inhomogeneous broadening of phosphate vibrations suggests minor structural fluctuations of interfacial water.
authors.affiliation: Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, D-12489 Berlin, Germany.
authors.forename: Rene
authors.initials: R
authors.lastname: Costard
authors.affiliation: Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, D-12489 Berlin, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Tyborski
authors.affiliation: Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, D-12489 Berlin, Germany.
authors.forename: Benjamin P
authors.initials: BP
authors.lastname: Fingerhut
authors.affiliation: Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, D-12489 Berlin, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Elsaesser
mesh_terms: D006860:Hydrogen Bonding; D056004:Molecular Dynamics Simulation; D010710:Phosphates / Q000737:chemistry*; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared; D014867:Water / Q000737:chemistry*
chemical_list: D010710:Phosphates; D014867:Water