The benchmark of 31P NMR parameters in phosphate: a case study on structurally constrained and flexible phosphate

Abstract

A benchmark for structural interpretation of the ³¹P NMR shift and the ²J_P,C NMR spin–spin coupling in the phosphate group was obtained by means of theoretical calculations and NMR measurements in diethylphosphate (DEP) and 5,5-dimethyl-2-hydroxy-1,3,2-dioxaphosphinane 2-oxide (cDEP). The NMR parameters were calculated employing the B3LYP, BP86, BPW91, M06-2X, PBE0, KT2, KT3, MP2, and HF methods, and the 6-31+G(d), Iglo-n (n = II, III), cc-pVnZ (n = D, T, Q, 5), aug-cc-pVnZ (n = D, T and Q), and pcS-n and pcJ-n (n = 1, 2, 3, 4) bases, including the solvent effects described with explicit water molecules and/or the implicit Polarizable Continuum Model (PCM). The effect of molecular dynamics (MD) on NMR parameters was MD-calculated using the GAFF force field inclusive of explicit hydration with TIP3P water molecules. Both the optimal geometries and the dynamic behaviors of the DEP and cDEP phosphates differed notably, which allowed a reliable theoretical benchmark of the ³¹P NMR parameters for highly flexible and structurally constrained phosphate in a one-to-one relationship with the corresponding experiment. The calculated ³¹P NMR shifts were referenced employing three different NMR reference schemes to highlight the effect of the ³¹P NMR reference on the accuracy of the calculated ³¹P NMR shift. The relative Δδ(³¹P) NMR shift calculated employing the MD/B3LYP/Iglo-III/PCM method differed from the experiment by 0.16 ppm while the NMR shifts referenced to H₃PO₄ and/or PH₃ deviated from the experiment notably more, which illustrated the superior applicability of the relative NMR reference scheme. The ²J_P,C coupling in DEP and cDEP calculated employing the MD/B3LYP/Iglo-III(DSO,PSO,SD)/cc-PV5Z(FC)/PCM method inclusive of correction due to explicit hydration differed from the experiment by 0.32 Hz and 0.15 Hz, respectively. The NMR calculations demonstrated that reliable structural interpretation of the ³¹P NMR parameters in phosphate must involve both the structural and the dynamical components.