Shape selectivity solute mixtures

SRM 869a Column Selectivity Test Mixture for Liquid Chromatography [44] is composed of three shape-constrained PAHs (phenanthro[3,4-c]phenanthrene, PhPh l,2 3,4 5,6 7,8-tetrabenzonaphthalene, TBN and benzo[a]pyrene, BaP) and is routinely employed to evaluate the shape selectivity of stationary phases. The retention differences between the nonplanar TBN and planar BaP solutes (expressed as a selectivity factor axEN/BaP = provide a numerical assessment of... 

FIGURE 5.3 (See color insert following page 280.) Solute test mixtures used for shape-selectivity characterization. 

Reversed-phase liquid chromatography shape-recognition processes are distinctly limited to describe the enhanced separation of geometric isomers or structurally related compounds that result primarily from the differences between molecular shapes rather than from additional interactions within the stationary-phase and/or silica support. For example, residual silanol activity of the base silica on nonend-capped polymeric Cis phases was found to enhance the separation of the polar carotenoids lutein and zeaxanthin [29]. In contrast, the separations of both the nonpolar carotenoid probes (a- and P-carotene and lycopene) and the SRM 869 column test mixture on endcapped and nonendcapped polymeric Cig phases exhibited no appreciable difference in retention. The nonpolar probes are subject to shape-selective interactions with the alkyl component of the stationary-phase (irrespective of endcapping), whereas the polar carotenoids containing hydroxyl moieties are subject to an additional level of retentive interactions via H-bonding with the surface silanols. Therefore, a direct comparison between the retention behavior of nonpolar and polar carotenoid solutes of similar shape and size that vary by the addition of polar substituents (e.g., dl-trans P-carotene vs. dll-trans P-cryptoxanthin) may not always be appropriate in the context of shape selectivity. 

Shape Selectivity Molecular recognition of the solute by the stationary phase with respect to its geometrical dimension is called shape selectivity. For this test, one can employ aromatic components with identical hydrophobicity that differ only in their three-dimensional shape. The chromatographic selectivity of o-terphe-nyl/triphenylene or tetra-benzonaphthalene/benzo[a]pyrene are commonly used and show dependencies on several features of the phase, for example, pore structure, ligand type, and density. Figure 3.6 shows a chromatogram of a test mixture of uracil (to marker), n-butylbenzene, and w-pentylbenzene (to assess hydrophobic properties and efficiency), and o-terphenyl and triphenylene (to assess shape selectivity). The test mixture was chosen to provide a short analysis time and to facilitate calculation of parameters from baseline-separated peaks. 

Zeolites can facilitate the shape-selective bromination of olefins. When a mixture of cyclohexene and oct-2-ene are reacted with bromine the two alkenes are brominated to a similar extent. However, when the zeolite catalyst silicalite-1 is present the reaction becomes selective [143]. This selectivity depends upon the order in which the reactants are introduced to the catalyst. If the alkene mixture is stirred with the zeolite prior to the addition of bromine then the straight chain octene enters the zeolite pores. The more bulky cyclohexene remains in solution and is halogenated in preference to the octene when the bromine is introduced. If the bromine is pre-absorbed on the zeolite before the alkene mixture is added then the selectivity of the process is reversed [144]. 

Another informative test mixture is that described by Tanaka [43], in which selectivity between triphenylene (TRI) and o-terphenyl (o-TER) is used to characterize the shape recognition capability of LC stationary phases. The primary difference between these two solutes is their planarity TRI is a planar PAH and o-TER possesses... 

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