Nuclear quantum effects in aqueous systems

This thesis is concerned with the influence of nuclear quantum effects (NQEs) on the properties of aqueous systems. Although such systems have been studied ever since methods to account for NQEs within atomistic simulations were first developed, it has been shown in recent years that a full accounting for the flexibility of O–H bonds leads to a cancellation between intermolecular and intramolecular quantum effects, meaning that the total effect is often much smaller than was originally believed to be the case. Several examples of systems are studied in which NQEs give rise to a small, though detectable, effect. Path integral molecular dynamics simulations are used to explain the results of sum-frequency generation spectroscopy experiments that observe a small preference for hydrogen over deuterium atoms in the dangling O–H bonds at the surface of isotopically substituted liquid water. These simulations are able to give a free energy of isotope fractionation that agrees quite well with that obtained from experiments. The effect of quantum mechanics on dynamical properties is studied by using ring polymer molecular dynamics to look at two types of process: the exchange of water around monatomic, monovalent ions and the reorientation of hydrogen bonds in the pure liquid. In both cases, NQEs speed up the process by a small degree, but leave the mechanism of the process unchanged. This suggests that a classical treatment of these processes will give an excellent qualitative description, and will give a quantitatively reasonable result. The results of second-harmonic generation scattering experiments on aqueous solutions of ions are also considered: the ions are shown to give rise to long-ranged orientational correlations between solvent molecules. Replacement of light water with heavy water causes a significant change in the scattered signal intensity, a hallmark of NQEs. A preliminary, classical study is performed using both Debye-Hückel theory and molecular dynamics simulations to explain the microscopic origin of these effects.