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 H-1, C-13, and P-31 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 beta = 0-2.0 mol/mol show binding between TOPO and HP. Self-diffusion measurements using H-1 PFG NMR demonstrate that larger complexes with higher content of TOPO are generally formed at beta < 0.75. Analyzing the dependence of P-31 NMR chemical shifts on beta by the use of program LETAGROP, we obtained very good fitting for the assumed coexistence of three complexes (TOPO)(i)center dot 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 P-31 NMR chemical shifts Delta delta(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 alpha(i) =[C-i]/[TOPO](0) were utilized in the analysis of the system dynamics. This was done by measuring the transverse P-31 NMR relaxation by the CPMG sequence with varying delays t(p) between the pi pulses in the mixtures with beta = 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 C-1 <-> TOPO, C-2 <-> C-1, and C-3 <-> C-2. Using the slopes of the R-2-t(p)(-1) dependences in the above three mixtures, the following correlation times were obtained: tau(10) = 2.5 x 10(-6), tau(21) = 7.4 x 10(-5), tau(32) = 11.3 x 10(-5) s. The DFT calculations support the hypothesis that complexes C-1 to C-3 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 C-1 and C-2. Schematically, the compositions of the three stable complexes is [3TOPO center dot H3O](+), [2TOPO center dot H3O center dot H2O](+), and [TOPO center dot H3O center dot 2H(2)O](+). The relative P-31 NMR shifts calculated for the optimized structures of C-1, C-2, and C-3 are in very good agreement with the experimentally observed values.