fiRelax – Novel fitting platform for relaxivity data of paramagnetic systems

Fast-Field-Cycling (FFC) NMR relaxometry probes the molecular dynamics of paramagnetic systems through Nuclear Magnetic Relaxation Dispersion (NMRD) profiles, from which structural, dynamic and electronic parameters can be extracted by model fitting. No existing software combines all paramagnetic relaxation models, from Solomon-Bloembergen-Morgan (SBM) to the (modified) Florence models for systems with static Zero-Field-Splitting. fiRelax (https://firelax.cerm.unifi.it) is a free, web-based fitting application that fills this gap. Its intuitive interface requires no programming or theoretical expertise, and it runs on the open-source One-Fit-Engine (OFE), which other programs have also adopted. The platform handles inner-sphere, outer-sphere and fast internal mobility contributions. It also supports simultaneous multi-dataset fitting through shared parameters or Arrhenius-based temperature dependencies.