4.5. Isolated-Molecule Corrections

DFT NMR shieldings for the isolated molecules (in vacuum) were calculated using the Gaussian16 program [47]. The gas-phase molecule-input geometries were taken from the periodic DFT geometry-optimized structures and were not further optimized. The PBE functional together with the 6-31+G(d,p) basis set were used for the calculations. NMR shieldings at the CCSD [48,49,50,51] level and the 6-31+G(d,p) basis set were calculated with the CFOUR program package, which is suitable for performing high-level quantum-chemical calculations on atoms and molecules [52,53]. The CCSD correction was obtained as the difference between the CCSD and PBE chemical shieldings.