Anharmonicities and coherent vibrational dynamics of phosphate ions in bulk H2O

Abstract

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.