Role of Non-Covalent Interactions on Acid Strength Measurement of Liquid Acids using Phosphine Oxide NMR Probe

The 31P NMR Chemical Shifts(δP) of Trimethylphosphine Oxide (TMPO) conjugated with ten common liquid molecular acids have been calculated employing the Density Functional Theory (DFT) based computational method. These shifts have been correlated with the intrinsic acid strength parameter, Deprotonation Energy (DPE), of the underlying acids and the complexation energy of TMPO (ΔΕ). The acids, anions, and conjugated complexes were modeled with PW91 functional and 6-31++G(d,p) basis set. The NMR calculations were performed on the relaxed structures as single-point calculations using the Gauge Including Atomic Orbital (GIAO) method at the MP2 level and TZVP basis set. Poor linearity was observed in the correlation of δP with respect to DPE (R2=0.867), while strong linearity was seen with the complexation energy ΔΕ (R2=0.967). We have hypothesized the unreliable modeling of anions and Non-Covalent Interactions (NCI) to be the factors affecting the linear trend of δP with respect to DPE. To support our hypothesis, we have presented Reduced Density Gradient (RDG) Maps and 31P NMR Tensor spatial orientation data of the Acid-TMPO conjugates.