Characterization of Pi-donor Chiral Stationary Phases Through a Thermodynamic Study of Analyte Adsorption

Download or read book Characterization of Pi-donor Chiral Stationary Phases Through a Thermodynamic Study of Analyte Adsorption written by Robin Shane Readnour and published by . This book was released on 1991 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Liquid chromatography on several $pi$-donor chiral stationary phases (CSP's) at different temperatures allows determination of the values of $Delta$H and $Delta$S of adsorption. A homologous series of N,N$spprime$-di(2,4-dinitrophenyl)$alpha$,$omega$-diaminoalkanes (di DNP amines) was investigated on several $pi$-donor CSP's with a maximum $Delta$H occurring for the homolog having five methylene groups. Analytes, such as the di DNP amines, are called di-analytes since they are functionalized in such a way as to interact with two adjacent strands of bonded phase simultaneously. The di DNP amine having five methylene groups corresponds most closely in dimensions to the most probable interstrand spacing of the bonded phase. Since the maximum in $Delta$H occurs for the di DNP amine having five methylene groups regardless of the extent of surface coverage, the strands of bonded phases are likely inhomogeneously spaced forming "patches" on the surface. "Patches" of bonded phase, the sizes of which change with surface coverage, are found to influence $Delta$H and $Delta$S by influencing the extent of solvation of the bonded phase. Another homologous series of $alpha$,$omega$-di(2,4-dinitrophenoxy)alkanes (di DNP ethers) was investigated on the same CSP's as the di DNP amines. The di DNP ethers are better probes of most probable interstrand spacing because they have fewer possible ways in which to interact with the stationary phase. Changes in the "observed" most probable strand spacing were found to be dependent upon surface coverages to some extent. These results are consistent with the "patch" model since changes in the most probable interstrand distance are much smaller than would be expected if the strands were uniformly spaced on the surface. Additionally, thermodynamic studies were carried out on a homologous series of N-(3,5-dinitrobenzoyl)1-phenylalkylamines using various $pi$-donor CSP's. Results suggest competing chiral recognition mechanisms, one of which involves intercalation of the alkyl group between neighboring strands. Lastly, a study of end-capping reagents was carried out in an attempt to understand the effect end-capping has upon the performance of chiral stationary phases.