Unravelling CdSe Nanocrystal Surface Structures with Relativistic DFT Calculations of Solid-State NMR Spectra

Semiconductor nanocrystals (NCs) offer novel tunable electronic and optical properties depending on their size, shape and surface passivation. Solid-state nuclear magnetic resonance (SSNMR) is a powerful tool to determine the structure of NC surfaces. We performed relativistic DFT calculations of cadmium and selenium magnetic shielding tensors with (i) the 4-component Dirac-Kohn-Sham (DKS) Hamiltonian, and (ii) the scalar and (iii) spin-orbit levels within the ZORA Hamiltonian. Molecular clusters with Cd and Se sites in varying bonding environments were used to model CdSe (100) and CdSe (111) nanocrystal surfaces capped with carboxylic acid ligands. Our calculations identify the observed ¹¹³Cd isotropic chemical shifts d(iso) of –465 ppm, –318 ppm and –146 ppm arising from CdSeO₃, CdSe₂O₂, and CdSe₃O surface groups respectively, with very good agreement with experiment. The ¹¹³Cd chemical shifts linearly decrease with the number of O-neighbors. The calculations predicted a one-bond ¹¹³Cd-⁷⁷Se scalar coupling of 258 Hz, in good agreement with the experimental values of 250 Hz. Relativistic DFT simulations aid in interpretation of NMR spectra of nanomaterials, and offer new insights into the complex nanocrystal surface structures.