Solid-state NMR characterization of amylose tris(3,5-dimethylphenylcarbamate) chiral stationary-phase structure as a function of mobile-phase composition

Solid-state NMR (H-1/C-13 CPMAS) was utilized to identify structural differ ences in amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phase (Chiralpak AD), as a function of mobile-phase composition. Dry Chiralpak AD stationary phase displayed an amorphous CPMAS NMR spectrum. However, CPMAS spectra of Chiralpak AD flushed,with organic mobile phases clearly display ed evidence of solvent complexes. Chiralpak AD flushed with nonpolar hexane exhibited solvent complexes with minimal structural perturbation. For Chir alpak AD flushed with hexane containing alcohol modifiers, however, solvent incorporation caused significant difference in conformation distribution a s evidenced by increased resolution of C-13 peaks in the CPMAS spectrum of the stationary phase. 2-Propanol modifier displayed more efficient displace ment of incorporated hexane while forming relatively more distinct/ordered solvent complexes with Chiralpak AD in comparison to ethanol modifier. Reve rsed elution order and unusual retention behavior on Chiralpak AD as a func tion of mobile-phase modifier was reported earlier. These chromatographic b ehaviors are believed to be due to different alterations of the steric envi ronment of the chiral cavities in the CSP by the different mobile-phase mod ifiers. In addition, on the basis of the chemical shift of C-1 carbon on th e amylose backbone, it is possible that Chiralpak AD's structure is a helix with a number of fold less than six.