Roles of the Ether Oxygen in Hydration of Tetrahydrofuran Studied by IR, NMR, and DFT Calculation Methods

We studied the concentration dependence of ν(C−H)’s in IR and <sup>1</sup><i>J</i>(C,H) in NMR for binary water−tetrahydrofuran (THF) mixtures and found different trends for the two types of CH<sub>2</sub> groups in the five-membered ring. The changes of the ν(C−O) spectra showed that complexes of THF associated with water are formed, in which the number of water molecules increases with the water concentration. We suggested that hydration proceeds through the formation of 1:1, and 1:2 complexes of [THF:water] up to <i>X</i><sub>H<sub>2</sub>O</sub> ≈ 0.9, where <i>X</i><sub>H</sub><sub><sub>2</sub></sub><sub>O</sub> is the mole fraction of the water in the mixtures. We carried out ab initio MO and DFT calculations to optimize the geometries of a THF dimer as a model of THF molecules in pure liquid, and 1:1 and 1:2 complexes of [THF:water] to simulate observed concentration dependence of ν(C−H)’s in IR and <sup>1</sup><i>J</i>(C,H) in NMR. The changes of the calculated ν(C−H) spectra and <sup>1</sup><i>J</i>(C,H) values for the optimized complexes are in agreement with those observed with varying <i>X</i><sub>H</sub><sub><sub>2</sub></sub><sub>O</sub>, supporting our proposal. From the vibrational and NBO analyses of the optimized complexes, the observed blue shift of ν(C−H)’s and the increase of <sup>1</sup><i>J</i>(C,H) for the CH<sub>2</sub> groups neighboring to the ether oxygen were explained in terms of the changes in the stereoelectronic effect, resulting from HO−H···O< hydrogen bonding. The optimized 1:2-complex contains two weak C−H···OH<sub>2</sub> hydrogen bonds, and blue shift of ν(C−H)’s and increase of <sup>1</sup><i>J</i>(C,H) were demonstrated from the same analyses of the complexes. This result of simulation also supports that the blue shift of ν(C−H)’s and increase of <sup>1</sup><i>J</i>(C,H) observed for both the type of CH<sub>2</sub> groups at 0.6 <i>X</i><sub>H</sub><sub><sub>2</sub></sub><sub>O</sub> < 0.9 are attributed to these interactions. On the basis of all these results, we propose that the formation of the 1:2-complex involving weak C−H···OH<sub>2</sub> hydrogen bonds is responsible dominantly for the hydrophobic hydration of THF.