  Magnetic resonance is a prime method for the study of chemical and biological
structures and their dynamical processes. The interpretation of these
experiments relies on considering the spin of electrons as the sole relevant
degree of freedom. By applying ab inito open quantum systems theory to the full
electronic wavefunction, here we show that contrary to this widespread
framework the thermalization of the unpaired electron spin of two Cr(V)
coordination compounds is driven by virtual transitions to excited states with
energy higher than 20,000 1/cm instead of solely involving low-energy spin
interactions such as Zeeman and hyperfine ones. Moreover, we found that a
window of low-energy THz phonons contributes to thermalization, rather than a
small number of high-energy vibrations. This work provides a drastic
reinterpretation of relaxation in spin-1/2 systems and its chemical control
strategies, and ultimately exemplifies the urgency of further advancing an ab
initio approach to relaxometry.
