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(PO−2)) and asymmetric (𝜈𝐴𝑆(PO−2)) PO−2 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(PO−2) and 𝜈𝐴𝑆(PO−2) transition frequencies with larger frequency excursions for 𝜈𝐴𝑆(PO−2). The calculated frequency-time correlation function is in good agreement with the experiment. The 𝜈(PO−2) 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.