3.9.1. NMR Calculations

Conformation searches based on molecular mechanics with MMFF force fields were performed for stereoisomers to get stable conformers within 20 kJ/mol. All conformers were further optimized by the density functional theory method at the B3LYP/6-31G(d) level by the Gaussian 16 program package. Gauge Independent Atomic Orbital (GIAO) calculations of their ¹H and ¹³C NMR chemical shifts using density functional theory (DFT) at the mPW1PW91/6-311+G(d,p) level with the PCM model in DMSO. The calculated NMR data of these conformers were averaged according to the Boltzmann distribution theory and their relative Gibbs free energy. The ¹H and ¹³C NMR chemical shifts for TMS were also calculated by the same procedures and used as the reference. After calculation, the experimental and calculated data were evaluated by linear correlation coefficients (R²) and the improved probability DP4+ method [40].