Direct Probing of Sorbent−Solvent Interactions for Amylose Tris(3,5-dimethylphenylcarbamate) Using Infrared Spectroscopy, X-ray Diffraction, Solid-state NMR, and DFT Modeling

Abstract

The sorbent-solvent interactions for amylose tris(3, 5-dimethylphenylcarbamate) (ADMPC) with five commonly used solvents, hexane, methanol, ethanol, 2-propanol (IPA), and acetonitrile (ACN), are studied using attenuated total reflection infrared spectroscopy (ATR-IR) of thin sorbent films, X-ray diffraction (XRD) of thin films, (13)C cross polarization/magic angle spinning (CP/MAS) and MAS solid state NMR of polymer-coated silica beads (commercially termed "Chiralpak AD"), and DFT modeling. The ADMPC-polymer-coated silica beads are used commercially for analytical and preparative scale separations of chiral enantiomers. The polymer forms helical rods with intra- and inter-rod hydrogen bonds (H-bonds). There are various nm-sized cavities formed between the polymer side-chains and rods. The changes in the H-bonding states of the C=O and NH groups of the polymer upon absorption of each of the five solvents at 25 degrees C are determined with ATR-IR. The IR wavenumbers, the H-bonding interaction energies, and the H-bonding distances of the polymer side-chains with each of the solvent molecules are predicted using the DFT/B3LYP/6-311+g(d,p) level of theory. The changes in the polymer crystallinity upon absorption of each solvent are characterized with XRD. The changes in the polymer crystallinity and the H-bonding states of C=O groups are also probed with (13)C CP/MAS solid-state NMR. The changes in the polymer side-chain mobility are detected using (13)C MAS solid-state NMR. The H-bonding states of the polymer change upon absorption of each polar solvent and usually result in an increase in the polymer crystallinity and the side-chain mobility. The polymer rods are reorganized upon solvent absorption, and the distance between the rods increases with the increase in the solvent molecular size. These results have implications for understanding the role of the solvent in modifying the structure and behavior of the polymer sorbents.

Solution Properties of Amylose Tris(Phenylcarbamate): Local Conformation and Chain Stiffness in 1,4-Dioxane and 2-Ethoxyethanol

“…The values of M w determined in the two solvents agree with each other within AE3%, so that their averages are presented in Table I, along with the A 2 data. The table also includes M w and M z =M w from sedimentation equilibrium as well as M w =M n determined by SEC-LS in tetrahydrofuran at 30 C. The angular dependence of PðkÞ À1=2 is shown in Figure 3, in which the initial slopes indicated by the dashed lines and hence the radii of gyration for the respective samples are seen to be larger in DIOX than in 2EE. The values of hS 2 i z 1=2 determined are listed in Table I.…”

“…Figure 5a illustrates IR spectra for ATPC300K in mixtures of DIOX and 2EE with indicated volume fractions x of 2EE. We assigned the double peaks at 1706 and 1754 cm À1 to the amide I band 30 which depends remarkably on x while the other peaks are almost independent of x. The amide I band reflects C=O stretching, and the double peaks indicate the presence of, at least, two different C=O groups in solution, that is, intramolecularly hydrogen bonding C=O with NH (1706 cm À1 ; roughly 40% in amount) and virtually no or weakly interacting C=O with some species (1754 cm À1 ).…”

Separation of Chiral Compounds on Polysaccharide Columns

“…These studies begin with a model of an ADMPC 8-mer rod, with a fourfold helix, in which about half of the C=O and NH groups are expected to form intrarod hydrogen bonds. Further, this rod structure shows the presence of various cavities of dimension 0.4 to 0.7 nm, in which solvents and solutes may penetrate and interact with the sorbent [91]. From ATR-IR measurements it was concluded that all solvents, polar and nonpolar, are incorporated into the polymer structure.…”

“…In a recent series of papers, Kasat et al undertook systematic studies to better understand the mechanism of interaction on several polysaccharide phases [90][91][92][93][94]. In their approach, they studied sorbent-solute and sorbent-solvent interactions separately before bringing them together to develop chiral recognition models based on the combined sorbent-solute-solvent interactions.…”

“…Later, Kasat et al studied sorbent-solvent interaction between ADMPC and hexane, methanol, ethanol, 2-propanol, and acetonitrile [91]. In addition to ATR-IR, HPLC, and DFT, these studies included x-ray diffraction (XRD) of thin films and 13 C cross-polarization/magic-angle spinning (CP/MAS) and MAS solid-state NMR of polymer-coated silica beads.…”