Anisotropy in the magnetic susceptibility strongly influences
the
paramagnetic shifts seen in nuclear magnetic resonance (NMR) and magnetic
resonance imaging (MRI) experiments. A previous study on a series
of C3-symmetric prototype MRI contrast agents showed that
their magnetic anisotropy was highly sensitive to changes in molecular
geometry and concluded that changes in the average angle between the
lanthanide–oxygen (Ln–O) bonds and the molecular C3axis due to solvent interactions had a significant impact
on the magnetic anisotropy and, consequently, the paramagnetic shift.
However, this study, like many others, was predicated on an idealized
C3-symmetric structural model, which may not be representative
of the dynamic structure in solution at the single-molecule level.
Here, we address this by using ab initio molecular dynamics simulations
to simulate how the molecular geometry, in particular the angles between
the Ln–O bonds and the pseudo-C3axis, evolves over
time in the solution, mimicking typical experimental conditions. We
observe large-amplitude oscillations in the O–Ln–C̃3angles, and complete active space self-consistent field spin–orbit
calculations show that this leads to similarly large oscillations
in the pseudocontact (dipolar) paramagnetic NMR shifts. The time-averaged
shifts show good agreement with experimental measurements, while the
large fluctuations suggest that an idealized structure provides an
incomplete description of the solution dynamics. Our observations
have significant implications for modeling the electronic and nuclear
relaxation times in this and other systems where the magnetic susceptibility
is exquisitely sensitive to the molecular structure.