Electron and Nuclear Spin Dynamics of a Dysprosium Complex in Solution

The electron spin dynamics of lanthanide complexes in solution and their influence on the nuclear spin dynamics underpin key applications including their use as highly sensitive MRI contrast agents. However, due to the complex time dependence of the ligand field potential and the failure of standard assumptions, this is an exceptionally difficult problem. In fact, the electron spin dynamics in solution have never been approached using fully ab initio methods and hence remain poorly understood. In this work, we perform time-domain electron spin dynamics simulations for a dysprosium complex in solution to tackle this difficult problem and directly predict nuclear spin relaxation rates. The system in question is a PARASHIFT agent, a type of contrast agent for which tuning the electron spin dynamics has the potential to improve signal intensity. The variation of the ligand field is described with a 90 ps DFT molecular dynamics trajectory with a CASSCF-SO calculation performed at each time step. Our simulations show excellent agreement with experimental nuclear spin relaxation rates and provide unprecedented detail on the electron spin dynamics in this challenging system, which deviate qualitatively from the traditional Solomon-Bloembergen-Morgan theory. At low magnetic field, nuclear spin relaxation is primarily driven by the electron spin dynamics and can potentially be tuned via the vibrational dynamics of the complex. At high magnetic fields, the relaxation it is dominated by the Curie mechanism and is instead sensitive to the rotational motion of the molecule.