Interaction of Hydrated Protons with Trioctylphosphine Oxide: NMR and Theoretical Study

Interaction of trioctylphosphine oxide (TOPO) with fully ionized hydrated protons (HP) was studied in acetonitrile-d 3 and nitrobenzene-d 5 using 1H, 13C, and 31P NMR, PFG NMR, and magnetic relaxation, and the experimental results were confronted with high-precision ab initio DFT calculations. Relative chemical shifts of NMR signals of TOPO (0.02 mol/L) under the presence of HP in the molar ratio β = 0−2.0 mol/mol show binding between TOPO and HP. Self-diffusion measurements using 1H PFG NMR demonstrate that larger complexes with higher content of TOPO are generally formed at β < 0.75. Analyzing the dependence of 31P NMR chemical shifts on β by the use of program LETAGROP, we obtained very good fitting for the assumed coexistence of three complexes (TOPO) i ·HP (named C i ), where i = 1, 2, 3. The logarithms of the respective stabilization constants log K i were found to be 3.63, 4.67, and 7.23 in acetonitrile and 3.91, 6.04, and 7.92 in nitrobenzene. The 31P NMR chemical shifts Δδ i corresponding to these complexes are 39.35, 29.51, and 19.72 ppm in acetonitrile and 38.37, 28.47, and 18.63 ppm in nitrobenzene. These values and the calculated values of α i =[C i ]/[TOPO]0 were utilized in the analysis of the system dynamics. This was done by measuring the transverse 31P NMR relaxation by the CPMG sequence with varying delays t p between the π pulses in the mixtures with β = 0.5, 1.25, and 1.5. Calculating the probabilities of imaginable exchange processes shows that only three of them can have significant influence on relaxation rate R 2, namely C1 ↔ TOPO, C2 ↔ C1, and C3 ↔ C2. Using the slopes of the R 2−t p −1 dependences in the above three mixtures, the following correlation times were obtained: τ10 = 2.5 × 10−6, τ21 = 7.4 × 10−5, τ32 = 11.3 × 10−5 s. The DFT calculations support the hypothesis that complexes C1 to C3 are the main species in the mixtures of TOPO with HP, with the only exception that additional water molecules are bound to the complexes in the case of C1 and C2. Schematically, the compositions of the three stable complexes is [3TOPO·H3O]+, [2TOPO·H3O·H2O]+, and [TOPO·H3O·2H2O]+. The relative 31P NMR shifts calculated for the optimized structures of C1, C2, and C3 are in very good agreement with the experimentally observed values.