A thesis submitted in partial fulfilment of the requirements for the degree of master of science in Computational Physics at Stockholms Universitet.

Abstract

The nitrate anion in solution functions as a probe of its environment through the spectroscopy of its vibrational modes. In water, dynamic hydrogen-bonding with the hydration shell is manifested in the symmetry breaking of nitrate’s asymmetric stretches.

This computational study aims to support recent experimental evidence of frequency dependence in the reorientation of nitrate in water and provide a direct molecular picture of the mechanisms involved. I run molecular dynamics simulations of a potassium nitrate pair in water to analyse structural properties and dynamic behaviour of the system. I make quantum chemistry calculations on small complexes of nitrate with up to two water molecules and on full hydration shell structures cut out from the molecular dynamics trajectories.

I find that the reorientation is driven by two mechanisms: an ultrafast (sub-picosecond) libration and a fast (picosecond) rotation. Based on direct observation of the simulated trajectories and recent theoretical study, the picosecond rotation is attributed to a sudden jump mechanism. In both motions, reorientation is faster with higher temperature. No anisotropy is found within time correlation functions of the molecular vector reorientation nor of the angular velocity.

The asymmetric stretch symmetry breaking in hydration shells is consistent: the lowest vibrational frequency corresponds to the longest internal nitrogen-oxygen bond and hence to that oxygen being the most strongly hydrogen-bonded. This direct picture of the fluctuating split in the asymmetric stretch modes offers an explanation as to why the frequency dependence is not observed as an anisotropy in molecular vector reorientation, and suggests a next move toward detecting it in an extension to this work.