Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Study of the inclusion processes of styrene and α-methyl-styrene in β-cyclodextrin
Introduction
Cyclodextrins (CDs) are cyclic oligosaccharides consisting of d-glucopyranose units (generally six (α-CD), seven (β-CD), or eight (γ-CD)) connected by α-1,4-indican bonds. These compounds are truncated cone-shaped molecules with a hydrophobic cavity (Fig. 1) [1]. The secondary hydroxyl groups (2- and 3-OH) of the respective glucose units are located at the wider side of the rim (the secondary face), and the primary hydroxyl groups (6-OH) are located at the narrower side of the rim (the primary face). CDs act as molecular hosts toward a wide range of guests ranging from ions and very polar molecules to non-polar molecules such as hydrocarbons [2], [3] in order to improve guest’s stability. The binding forces presented in some complexes are recognized as van der Waals attractions, ion pairing, hydrogen bonding along with hydrophobic and solvent liberation driving force [4].
Intensive theoretical works have been performed over the past few years on CDs [5], [6], [7], [8], [9]. Most computational studies of CDs involve host–guest complex, their shapes, energies, preferred bonding orientations and so on, have been typically computed [10]. Early quantum calculations were performed with semi-empirical CNDO method [11], [12], followed by several semi-empirical quantum calculations, e.g. molecular mechanics (MM) [13], [14], [15], [16], [17] and molecular dynamics (MD) [18], [19] with various force fields approaches. Recently, at higher level of quantum calculations, ab initio methods [20], [21] at the Hartree–Fork or the density functional theory (DFT) levels with a minimal basis set, were carried out. Calculations are useful for better understanding of such inclusion processes of CDs. The calculations can strengthen and supplement the conclusion from experiment and vice versa.
In the present research, the solid inclusion complexes of styrene and α-methyl-styrene with β-CDs were prepared, and studied by using elemental analysis (EA), X-ray diffraction (XRD) analysis, thermo-gravimetric analysis (TGA). The association constants of inclusion complexes in solution were investigated by using [] NMR titration method followed by non-linear least square regression. The binding processes were also performed by using PM3 method.
Section snippets
Materials
β-CD was obtained from Aldrich Co. All the other chemicals were reagent grade and used without further purification.
Preparation of inclusion complexes in the solid state
The solid inclusion complexes were prepared by mixing styrene or α-methyl-styrene (0.5 mmol) and β-CD (0.5 mmol) in 100 ml water. The mixture was stirred for 24 h at room temperature. The precipitate of the inclusion complexes was filtered off using a sintered glass filter G-4, after washed several times with small portions of diethyl ether and dried in air.
X-ray diffraction analysis
X-ray diffraction (XRD)
X-ray diffraction analysis
XRD patterns of the solid state inclusion complexes and their starting compounds are presented in Fig. 2. A comparison of XRD patterns of the complexes with those of the pure parent materials revealed the formation of a new crystal lattice of the inclusion complexes. The inter-planar distances in the XRD patterns of the β-CD–styrene and β-CD–α-methyl-styrene inclusion complex were characterized by values which did not occur in the X-ray diffraction patterns of β-CD (e.g. 5.05, 4.75° for
Conclusion
The formation and composition of the two inclusion complexes in the solid state were determined by elemental analysis, X-ray diffraction analysis and TGA–DSC analysis. TGA–DSC results show that the thermal stabilities of the two inclusion complexes are different. Inclusion complexes of β-CD–α-methyl-styrene is stable than β-CD–styrene, because of large increase of the temperature of the endothermic peak in the TGS–DSC cure for β-CD–α-methyl-styrene. The association constants of two complexes in
Acknowledgements
We are grateful to the NSFC for the financial support (No. 29873062).
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