Chemical hydrodynamics of nuclear spin states

TL;DR

This paper introduces a numerically stable quantum mechanical formalism for modeling complex nuclear spin dynamics involving diffusion, flow, and second-order reactions, with applications in microfluidics and magnetic resonance imaging.

Contribution

It presents a novel formalism integrated into the Spinach library for simulating complex nuclear spin systems under realistic conditions.

Findings

Successfully modeled Diels-Alder reaction with diffusion and flow

Demonstrated stability of the numerical formalism

Applied to microfluidic NMR detection chamber

Abstract

Quantum mechanical equations of motion are strictly linear in state descriptors, such as wavefunctions and density matrices, but equations describing chemical kinetics and hydrodynamics may be non-linear in concentrations. This incompatibility is fundamental, but special cases can be handled - for example, in magnetic resonance where nuclear spin interactions may be too weak influence concentration dynamics. For processes involving single spins and first-order chemical reactions, this is a well-researched topic, but time evolution of complex nuclear spin systems in the presence of second-order kinetics, diffusion, and flow has so far remained intractable. This creates obstacles in microfluidics, homogeneous catalysis, and magnetic resonance imaging of metabolic processes. In this communication we report a numerically stable formalism for time-domain quantum mechanical description of nuclear spin dynamics and decoherence in the simultaneous presence of diffusion, flow, and second-order chemical reactions. The formalism is implemented in versions 2.11 and later of the open-source Spinach library. As an illustration, we use Diels-Alder cycloaddition of acrylonitrile to cyclopentadiene, yielding endo- and exo-norbornene carbonitrile, in the presence of diffusion and flow in the detection chamber of a microfluidic NMR probe (a finite element model with thousands of Voronoi cells) with a spatially localised stripline radiofrequency coil.