Complexation of phenol and thiophenol by amine N-oxides: isothermal titration calorimetry and ab initio calculations.

To develop a new solvent-impregnated resin (SIR) system for removal of phenols from water, the complex formation of dimethyldodecylamine N-oxide (DMDAO), trioctylamine N-oxide (TOAO), and tris(2-ethylhexyl)amine N-oxide (TEHAO) with phenol (PhOH) and thiophenol (PhSH) is studied. To this end we use isothermal titration calorimetry (ITC) and quantum chemical modeling (on B3LYP/6-311G(d,p)-optimized geometries: B3LYP/6-311+G(d,p), B3LYP/6-311++G(2d,2p), MP2/6-311+G(d,p), and spin component scaled (SCS) MP2/6-311+G(d,p); M06-2X/6-311+G(d,p)//M06-2X/6-311G(d,p), MP2 with an extrapolation to the complete basis set limit (MP2/CBS), as well as CBS-Q). The complexes are analyzed in terms of structural (e.g., bond lengths) and electronic elements (e.g., charges). Furthermore, complexation and solvent effects (in benzene, toluene, and mesitylene) are investigated by ITC measurements, yielding binding constants K, enthalpies ΔH(0), Gibbs fre energies ΔG(0), and entropies ΔS(0) of complex formation, and stoichiometry N. The ITC measurements revealed strong 1:1 complex formation between both DMDAO-PhOH and TOAO-PhOH. The binding constant (K=1.7-5.7×10(4) M(-1)) drops markedly when water-saturated toluene was used (K=5.8×10(3) M(-1)), and π-π interaction with the solvent is shown to be relevant. Quantum mechanical modeling confirms formation of stable 1:1 complexes with linear hydrogen bonds that weaken on attachment of electron-withdrawing groups to the amine N-oxide moiety. Modeling also showed that complexes with PhSH are much weaker than those with PhOH, and in fact too weak for ITC determination. CBS-Q incorrectly predicts equal or even higher binding enthalpies for PhSH than for PhOH, which invalidates it as a benchmark for other calculations. Data from the straightforward SCS-MP2 method without counterpoise correction show very good agreement with the MP2/CBS values.