Measurement of Line Width and Anisotropy in C ₃ / C ₄ -Symmetric Gd(III) Complexes

Gadolinium(III) complexes are frequently used as spin labels in high-field electron paramagnetic resonance spectroscopy and dynamic nuclear polarization nuclear magnetic resonance spectroscopy due to their high spin ( S = 7/2) ground state and chemical robustness. A primary goal in their design is to minimize the magnetic resonance line width by engineering the zero-field splitting (ZFS). Here, we present a comprehensive experimental study of a series of C 3 -symmetric TPATCN and C 4 -symmetric DOTA derivatives. Using multifrequency EPR spectroscopy (34 to 390 GHz) here we try to understand trends in their ZFS parameters ( D, E ) and their distributions ( D strain, E strain ). We find that functionalization of the TPATCN scaffold with an electron-withdrawing nitro group (NO 2 ) minimizes the axial ZFS to D = 0.0110 cm –1 ∥0.330 GHz, but this is coupled with a significant increase in conformational flexibility, resulting in a large D strain . Conversely, amide-functionalized DOTA compounds demonstrate superior structural rigidity, evidenced by near-zero rhombic ZFS and small E strain . Many complexes studied herein show considerably smaller resonance line widths than existing tags, suggesting there is scope for the design of improved Gd(III) spin labels.