Phosphates feature prominently in the energetics of metabolism and are important solvation sites of DNA and phospholipids. Here we investigate the ion H2PO4- in aqueous solution combining 2D IR spectroscopy of phosphate stretching vibrations in the range from 900-1300 cm(-1) with ab initio calculations and hybrid quantum-classical molecular dynamics based simulations of the non-linear signal. While the line shapes of diagonal peaks reveal ultrafast frequency fluctuations on a sub-100 fs timescale caused by the fluctuating hydration shell, an analysis of the diagonal and cross-peak frequency positions allows for extracting inter-mode couplings and anharmonicities of 5-10 cm(-1). The excitation with spectrally broad pulses generates a coherent superposition of symmetric and asymmetric PO2- stretching modes resulting in the observation of a quantum beat in aqueous solution. We follow its time evolution through the time-dependent amplitude and the shape of the cross peaks. The results provide a complete characterization of the H2PO4- vibrational Hamiltonian including fluctuations induced by the native water environment.
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