1P105

Hydration of short-chain poly(oxyethylene)s:
A quantum chemical study{ Shaheda A. Wahab, Takanori Harada, Toshiaki Matsubara and Misako Aida
Center for Quantum Life Sciences, and Graduate School of Science, Hiroshima University
Introduction
Poly(oxyethylene) (POE) is an excellent candidate for both experimental and computational
studies aimed at gaining a better understanding of polymer-water interactions. POE is an
important polymer for the various biotechnical and biomedical applications. Because of its
significant behavior, a number of studies have been performed so far on the peculiarproperties of POE in water. Hydration is in particular a key factor responsible for theemergence of the hydrophilic nature. We therefore focused on the hydration of theoxyethylene (–O−CH2−CH2−O–) unit of POE and studied the role of a water molecule on thestabilization of the POE. We have investigated the stability of POE with the variousconformations and the interaction of the short-chain POE with a water molecule in the gas
phase and in the several solvents with the various dielectric constants using the quantum
chemical methods. We used CH3(−OCH2CH2−)mOCH3 (C1EmC1) as a model of POE.
Computational Methods
All the calculations have been carried out using the GAUSSIAN 03 program. The
geometry optimizations were performed by the second-order Møller-Plesset perturbation(MP2) method using the basis set at the 6-311G** level. The quadratic configurationinteraction method with single and double substitutions (QCISD) in addition to the MP4
method were used to obtain more reliable energies. The thermochemical parameters, enthalpy,
entropy and Gibbs free energy, were also calculated by the MP2 method with a scale factor of0.9496 for calculated vibrational frequencies at the temperature of 298.15K. The solvent
effects were investigated by the polarized-continuum-model (PCM) approximation using thesolvents, water (H2O), dimethylsulfoxide ((CH3)2SO), methanol (CH3OH), dichloromethane(CH2Cl2), chloroform (CH3Cl), and carbon tetrachloride (CCl4) with the dielectric constants, ε= 78.4, 46.7, 32.6, 8.93, 4.9, and 2.2.
Results and Discussion
We have found the monodentate and the bidentate H-bonding interation of a water in thehydration, which leads to three hydration modes, type I, II, and III, as shown in Figure 1. In
type I, one hydrogen of water is attached to one of the POE oxygens. On the other hand, intype II and III, two hydrogens of water are attached to the two oxygens of POE that are in the1,2- and 1,3-position. We investigated the stability of POE with three hydration modes thatare closely related to its solubility, using the model of POE, C1E1C1 and C1E2C1.

C1E1C1: The stability of the conformers of C1E1C1 in the gas phase decreased in the order,TTT (0.0 kcal/mol) > TGG' (0.02 kcal/mol) > TGT (0.52 kcal/mol). When one watermolecule is added, the conformer TGT is especially largely stabilized by the H-bonding withthe water with the bidentate mode. As a result, this tendency is reversed. The tendency in thestability with a water molecule, TGT > TGG' > TTT were not changed, even if the solvent
effect is taken into account.
C1E2C1: For C1E2C1, we calculated three essentially important conformers, i.e., TTTTTT,TGTTGT and TGTTG'T. One water molecule interacts with the bidentate mode in bothconformers, TGTTGT and TGTTG'T. In contrast, it interacts with the monodentate mode inthe conformer, TTTTTT.
H26O24O24C1C1C3O2O5C4Type I

H26O2H25C7C6C9O8O24C9H25C3O5C4C1H25O8O2C7C3C6Type II

C9H26O8O5C4C7C6Type III

Fig.1. Three types of hydration modes of one water molecule on the POE chain of C1E2C1 with the variousconformations. Type I: monodentate, Type II: bidentate, Type III: bidentate.

The H-bonding with type III more stabilized the conformers compared to that with type II inboth gas phase and solution. The O(POE)−H(water) distances of the H-bonding are in therange from 2.05 Å to 2.48 Å. In type III, especially two dihedral angles ∠O2−C3−C4−O5and ∠O5−C6−C7−O8 were reduced due to the strong H-bonding. Without a water molecule,
TTTTTT was more stable in energy than TGTTGT and TGTTG'T. However, this tendency
was reversed with a water molecule, because both conformers, TGTTGT and TGTTG'T, aresignificantly stabilized by the H-bonding with a water molecule. These computational results
are in good agreement with the experimentally observed results1 that the H-bonding with typeIII obviously more stabilizes the conformers compared to that with type II. The H-bonding
with the bidentate mode is further strengthened in the solution. The H-bonding becomes
weaker with the increase in the dielectric constant of the solvents. TGTTG'T with type III was
more stable in energy compared to TGTTGT with type III in the solution, which is inaccordance with the experimental result.2 It is also suggested by the natural bond orbital(NBO) analysis that type III is more stable in energy than type II. Thus, we successfully
verified by the computations the hydration mode speculated experimentally.
References
1. H. Matsuura, Md. R. Matin, and K. Okura, Chem. Lett., 32, 122−123 (2003).
2. M. R. Matin, S. A. Wahab, Y. Katsumoto, H. Matsuura, and K. Ohno, Chem. Lett., 34,502-503 (2005).