Transient versus Static Electron Spin Relaxation in Mn(2+) Complexes Relevant as MRI Contrast Agents

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Molecular Dynamics

Background:

• Zero-field splitting (ZFS) is crucial for understanding electron spin dynamics in transition metal complexes.
• Accurate estimation of ZFS parameters is essential for predicting magnetic properties and relaxation times.
• Previous studies have explored ZFS in various manganese complexes, but detailed analysis of dynamic contributions is ongoing.

Purpose of the Study:

• To computationally estimate the zero-field splitting (ZFS) parameters for [Mn(EDTA)(H2O)](2-)·2H2O and [Mn(MeNO2A)(H2O)]·2H2O complexes.
• To investigate the contributions of transient and static ZFS to electron spin relaxation.
• To determine the correlation times governing the modulation of ZFS by molecular motion.

Main Methods:

• Density Functional Theory (DFT) and ab initio CASSCF/NEVPT2 calculations were employed to determine ZFS parameters.
• Molecular dynamics (MD) simulations using the atom-centered density matrix propagation (ADMP) approach were performed.
• Calculated ZFS parameters were compared with experimental data derived from (1)H relaxation measurements.

Main Results:

• Calculated ZFS parameters showed good agreement with experimental values.
• Transient ZFS correlation times (τc) were found to be very short (∼0.02-0.05 ps), indicating modulation by molecular vibrations.
• Rotational correlation times (τR) for the complexes were longer (∼35-60 ps), indicating static ZFS modulation by molecular rotation.

Conclusions:

• Electron spin relaxation in small Mn(2+) complexes is predominantly determined by the static ZFS component.
• Molecular vibrations significantly influence transient ZFS, while molecular rotation modulates static ZFS.
• The study provides valuable insights into the factors controlling spin relaxation mechanisms in paramagnetic manganese complexes.