No abstract

H NMR and IR studies were carried out for 1,4-dioxane/H 2 O mixtures over the whole concentration range at temperatures of 1, 25, and 50 °C. 1 H NMR spectra were also acquired for 1,3-dioxane/H 2 O and 4-methyl-1,3-dioxane/H 2 O mixtures. All the chemical shift data were measured by referring to an external reference and corrected using in situ bulk susceptibilities as determined on a unified scale. The results are compared with those previously obtained for dimethyl sulfoxide/H 2 O and acetone/H 2 O mixtures in order to determine the influence of the polar group on the hydrogen bond formation of water at the polar and hydrophobic groups. It was found that an anomalous polarization of the water molecules, which had been observed in the dimethyl sulfoxide/H 2 O and acetone/H 2 O mixtures, did not occur in the three different aqueous dioxane mixtures. Thus, we have concluded that the hydrophobic moiety in an organic solute having a polar group does not play a role in anomalously high polarization of the water molecules. We have also concluded that the hydrogen-bonding basicity of the polar group is an important factor for the anomalous polarization of the water in the water-rich region and that the basicities of the ether oxygens in the three dioxanes are not strong enough to cause the anomalous polarization of water. It was found that the frequencies of IR C-H stretching vibration modes of 1,4-dioxane increase and the absorption intensities of the modes decrease with increasing water concentration. Since these spectroscopic features correspond well to the formation of the blue-shifting C-H‚‚‚OH 2 hydrogen bonds obtained from ab initio calculations for complexations in gas phases, we can categorize the solvation of the C-H groups in the dioxanes in aqueous solutions as blue-shifting C-H‚‚‚O hydrogen bonding. Thus, we propose for the first time that the hydration of CH groups in organic solutes having a polar group is due to the formation of blue-shifting C-H‚‚‚OH 2 hydrogen bonds. As a picture of the hydration of the C-H groups in 1,4-dioxane, we propose the formation of a bifunctional hydrogen-bonded hydration complex in which each water molecule plays a role as both a proton donor in the conventional O-H‚‚‚O hydrogen bonding with the ether oxygen and a proton acceptor in the blue-shifting C-H‚‚‚O hydrogen bonding simultaneously.

We present the principle of an external reference method of NMR, named the external double reference method, to study concentration and temperature dependences of chemical shifts. By the method, we refer chemical shifts of various samples at different temperatures to the peak of a reference substance at a fixed referring temperature. Thus, we can determine the chemical shifts in a variety of sample conditions on a unified scale. Results of the application of the method to 1H- and 13C-chemical shifts of water and methanol are presented. We also report new data of volume magnetic susceptibilities for several deuterated solvents useful for the calibration in the method.