Hydrophobic hydration of tert-butyl alcohol probed by NMR and IR

Abstract

The hydrophobic hydration in aqueous tert-butyl alcohol(TBA) mixtures was studied by examining the concentration and the temperature dependences of 1H and 13C chemical shifts in NMR, together with the concentration dependence of the frequency for the CH stretching vibration band, νCH. The chemical shifts were measured with the external double reference method, with which the bulk volume magnetic susceptibility for every sample mixture was also obtained to be used for the correction of the chemical shifts in situ. The polarization of the water molecules in the mixture probed by the 1H chemical shift, ∗δH2O, varies with concentration; δH2O increases at 23.3°C from 3.5 ppm in the TBA-rich region to the value for pure water at the mole fraction of water χW=0.84 and reaches to the maximum at χW=0.95. In the region of 0.84<χW<1.0, δH2O is larger than that for pure water, indicating clearly the presence of highly polarized or strongly hydrogen-bonding water molecules compared with pure water. The νCH also continues to increase with increasing χW from 2973.6cm−1 for pure TBA gently up to χW=0.84 and then steeply up to χW=0.95 to reach 2980.3cm−1, then remains constant up to the measurable concentration limit, χW=0.99. The polarization of TBA molecule was also found to increase in the same way as νCH. These results not only indicate the formation of associated complexes of TBA with water molecules, but also confirm the formation of polyhedron or cluster so far presented for mixing state in water-rich region. In addition to this the results show that the hydration of CH groups occurs through a kind of dispersion interaction between CH group and water oxygen, in which contraction of the CH bond occurs as demonstrated by the blueshifts of the CH stretching vibration band. The new mechanism of CH hydration, termed “push ball” hydration, is proposed.