Hydration of the CH Groups in 1,4-Dioxane Probed by NMR and IR:  Contribution of Blue-Shifting CH···OH₂ Hydrogen Bonds

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.

A Study of the Solvation Structure of l‐Leucine in Alcohol–Water Binary Solvents through Molecular Dynamics Simulations and FTIR and NMR Spectroscopy

“…The interaction between the alkyl CÀHb onds of Leu and the water oxygen atoms may be key to the IR and NMR features of the alkyl CÀH bonds in the aliphatic alcohol solutions. As shown in the previous reports, [31,32,40] the oxygen atom of water may also act as ah ydrogen acceptorf or the blue-shifting hydrogen bonds, due to the high electron density of the lone pairs.H ence, the blue-shifting hydrogen bonds between the alkyl CÀHb onds and water mayb ef ormed in the aqueous solution,b ecause the Leu alkyl group is surrounded by water molecules as shown in the SDF.I nf act, as shown in Figure 11,t he interaction between the H d and H d' atoms of the Leu methyl groups and the water oxygen atoms is observed as ah ump at % 2.9 in the g(r) HC Leu -O W for the Leu-water system obtained by the MD simulation. The interaction between the Leu alkyl group and water is gradually weakened in aliphatic alcohol-water solutions, as ar esult of the replacement of water molecules by the alcohola lkyl groups around the Leu alkyl group with increasing x A .C onsequently,t he length of the contracted Leu alkyl CÀ Hb onds is recovered in the aliphatic alcohol solutions with the increasing x A .I tr esults in ar edshift of the Leu CÀHv ibrations with increasing x A .T his also leads to the deshielding of both carbon and hydrogen atoms of the CÀHb onds because of the decrease in the electron densities for both atoms.…”

“…[33][34][35][36][37][38][39] However,e xperimental results on blue-shifting hydrogen bonds are much scarcer than the theoretical ones. [40][41][42] In particular,b lue-shifting hydrogen bonds have not been observed in the promotion of the secondary structure of biomolecules by TFE and HFIP at the microscopic level, despite the NOE observation of the interactions between the hydrophobic moieties of biomolecules and fluorinated alcohols. [21][22][23][24][25][26] We aimed to clarifyt he solvation structure of the amino acid Leu in aliphatic-alcohol-water andf luorinated-alcohol-water solvents as the first stage of our investigations, because the structures of proteins and peptides are too complex to extract the interactions between their hydrophobic moiety and alcohol molecules by using spectroscopic techniques.…”

Theoretical and Experimental Studies of Enflurane. Infrared Spectra in Solution, in Low-Temperature Argon Matrix and Blue Shifts Resulting from Dimerization

“…24 It has also been observed that the frequencies of the νCH stretching modes of 1,4-dioxane in water solution increase and the infrared intensities of the modes systematically decrease with increasing concentration of water. 25 This difference has been accounted for by a strong polarization of the water molecules and by the formation of larger hydration clusters formed cooperatively. In the present case, increasing the concentration of enflurane in carbon tetrachloride results in an increase of the polarity of the medium.…”

Investigating the formation of an adduct of formamide with dioxane by means of Raman spectroscopy

“…However, these small variations have been interpreted in terms of a direct intermolecular interaction, involving the CH 2 group and water, 19 and more recently, like a nonconventional hydrogen bond. 20 Thus, we shall discuss our results as well as the interpretations made in those works in the next section.…”

“…Such evidence must be emphasized because the works presented so far, involving DX and water (acting as a Lewis acid), reveal only wavenumber shifts of the C-H and C-O stretching vibrations of DX. 19,20 The second objective is to investigate the influence of the two oxygen atoms of DX on the adduct stoichiometry, and then compare it with that determined for the FA-THF adduct.…”