Temperature Dependence of ¹ H Paramagnetic Chemical Shifts in Actinide Complexes, Beyond Bleaney's Theory: The An^VI O₂ ²⁺ -Dipicolinic Acid Complexes (An=Np, Pu) as an Example

Actinide +VI complexes ( ${An}^{VI}$ = $U^{VI}$ , ${Np}^{VI}$ and ${Pu}^{VI}$ ) with dipicolinic acid derivatives were synthesized and characterized by powder XRD, SQUID magnetometry and NMR spectroscopy. In addition, ${Np}^{VI}$ and ${Pu}^{VI}$ complexes were described by first principles CAS based and two-component spin-restricted DFT methods. The analysis of the ¹ H paramagnetic NMR chemical shifts for all protons of the ligands according to the X-rays structures shows that the Fermi contact contribution is negligible in agreement with spin density determined by unrestricted DFT. The magnetic susceptibility tensor is determined by combining SQUID, pNMR shifts and Evans' method. The SO-RASPT2 results fit well the experimental magnetic susceptibility and pNMR chemical shifts. The role of the counterions in the solid phase is pointed out; their presence impacts the magnetic properties of the ${Np}^{VI}$ complex. The temperature dependence of the pNMR chemical shifts has a strong $1/T$ contribution, contrarily to Bleaney's theory for lanthanide complexes. The fitting of the temperature dependence of the pNMR chemical shifts and SQUID magnetic susceptibility by a two-Kramers-doublet model for the ${Np}^{VI}$ complex and a non-Kramers-doublet model for the ${Pu}^{VI}$ complex allows for the experimental evaluation of energy gaps and magnetic moments of the paramagnetic center.