Solvent-induced symmetry breaking of nitrate ion in aqueous clusters: A quantum-classical simulation study

Abstract

A hybrid quantum-classical computational algorithm, which couples a density functional Hamiltonian to a classical bath, is applied to investigate symmetry breaking and the vibrational spectrum of [NO3]− in aqueous clusters. The nitrate ion was modeled using density functional theory with a Gaussian basis set; two different force fields for the classical bath were investigated: the TIP4P-FQ fluctuating charge and the TIP4P mean-field potentials. The choice of basis sets, functionals, and force field parameters has been validated by performing calculations on small complexes [NO3(H2O)n]− (n=1,2) at 0 K. We have found different asymmetrical configurations, mostly of Cs symmetry, with characteristic lifetimes in the picosecond range in a molecular dynamics (MD) simulation of [NO3 (H2O)124]− using the TIP4P potential. The vibrational density of states (DOS), computed by calculating the Fourier transform of the velocity autocorrelation function, shows two distinctive peaks corresponding to the antisymmetric N–O stretching (around 1500 cm−1) for each configuration, in contrast with the degenerate peak observed in the isolated solute. The DOS corresponding to the whole simulation, in which several configurations were visited, is similar to the broad band observed experimentally in aqueous solution. The structural and DOS results obtained for a TIP4P simulation of [NO3]− solvated with 256 water molecules do not differ significantly from those obtained with the smaller cluster, confirming that the main features of solvation are already present in the smaller system. In order to assess the influence of solvent polarization, we have performed a hybrid simulation employing the fluctuating charge TIP4P-FQ water potential. We obtain similar results to those obtained using the mean-field potential, except that residence times of each asymmetric configuration are larger than in the TIP4P case.