Molecular simulation of rotational diffusion in ideal liquids

Overview

Rotational tumbling of molecules in a liquid is an important phenomenon in Magnetic Resonance Imaging (MRI) because it determines the spin-relaxation rates of the resident nuclei which can determine MRI contrast.

For a relatively simple molecular process, the theoretical description of rotational motion of molecules in liquids remains controversial. The most commonly used model, the Debye model, assumes that:

• the rotational diffusion propagator of a tumbling molecule is a solution of the diffusion equation on a spherical surface
• this solution is described by a single rotational diffusion coefficient.

However, recently it has been suggested that molecular tumbling is described more accurately as a discrete jump-like reorientational process than as continuous-time rotational diffusion.

Furthermore, recent results from our group suggest that, even for simple molecules within the limit of validity of the continuous-time random-walk model, a single rotational diffusion coefficient can be inadequate for quantitative description of the rotational dynamics of molecules.

Research activities

This project will use first-principles molecular simulations in order to test the limits of validity of the Debye rotational-diffusion model.

To do this, you'll set up and run molecular dynamics simulations in an ideal Lennard-Jones liquid under a range of conditions, such as:

• 2D vs 3D liquid
• interactions between nearest neighbours only vs interactions between all neighbours
• different densities.

You'll then analyse the resulting rotational diffusion propagators, evaluate the suitability of different empirical models for describing the rotational diffusion process,and analyse the implications of the findings for the commonly used models of magnetic-resonance spin relaxation in liquids.

Outcomes

This research will contribute to the development of Nuclear Magnetic Resonance spin-relaxation theory through advancing the quantitative understanding of molecular mobility in liquids.

In this project, you'll learn about molecular simulations, aspects of molecular physics and fundamentals of Magnetic Resonance.

You will also deepen your quantitative data analysis and programming skills.

Skills and experience

This project will involve the development of custom-made code for the simulations and the data analysis. Therefore, familiarity with Mathematica and/or common programming languages is required.

Contact

Contact the supervisor for more information.