Gadolinium Spin Decoherence Mechanisms at High Magnetic Fields

[Image: see text] Favorable relaxation processes, high-field spectral properties, and biological compatibility have made spin-7/2 Gd(3+)-based spin labels an increasingly popular choice for protein structure studies using high-field electron paramagnetic resonance. However, high-field relaxation and decoherence in ensembles of half-integer high-spin systems, such as Gd(3+), remain poorly understood. We report spin–lattice (T(1)) and phase memory (T(M)) relaxation times at 8.6 T (240 GHz), and we present the first comprehensive model of high-field, high-spin decoherence accounting for both the electron spin concentration and temperature. The model includes four principal mechanisms driving decoherence: energy-conserving electron spin flip-flops, direct “T(1)” spin–lattice relaxation-driven electron spin flip processes, indirect T(1)-driven flips of nearby electron spins, and nuclear spin flip-flops. Mechanistic insight into decoherence can inform the design of experiments making use of Gd(3+) as spin probes or relaxivity agents and can be used to measure local average interspin distances as long as 17 nm.