Theoretical modeling of paramagnetic chemical shifts in actinide complexes (Ashraful Islam / LCPQ / Thesis). – 24/11, 14H.

Ashraful Islam, LCPQ, seminary room, 3rd floor -3r1B4

Abstract:

Paramagnetic NMR(pNMR) shifts are the extra induced chemical shifts in a paramagnetic complex compared to its diamagnetic counterpart. pNMR shift of a nucleus can be divided into two terms: the pseudocontact shift which is a “through-space” magnetic dipole-dipole interaction and the contact shift arises from the presence of spin density at the nuclear position. The information obtained from the study of the pNMR shifts lies in the separation of the two terms.
First-principles computation of the pNMR shifts are at present a challenge for quantum chemists because the pseudocontact shifts originate from the magnetic anisotropy of the paramagnetic center, and hence demand the multireference ab initio methods to properly address the electronic structure of an open shell system. But the multireference methods at present suffer from a poor description of spin polarization which results in large contact contribution to the actinide induced shifts. On other hand, single determinant unrestricted DFT-based approaches are reasonably good in addressing the metal-ligand spin polarization and delocalization but fail to properly describe the multiconfiguration nature of the paramagnetic center. To make progress, in my thesis, I chose to follow a hybrid approach to interpret both the contributions in the actinide-induced chemical shifts.
And ironically also, the ab initio calculations of electronic structures and magnetic properties are not straightforward as the strong interweaving of many-electron effects, spin-orbit coupling, J − J coupling, intricate bonding interactions with the ligands need to be properly addressed. SO-CAS (Spin-orbit-CAS) based methods give a balanced description of all the underlying physics. I will present the actinide-induced shifts measured by our experimental colleagues in different actinide complexes and theoretical interpretation of the two contributions from combined SO-CAS and unrestricted DFT-based methods.