Enantiomer thermodynamic principle

It is significant to mention here that below T so enantioseparation is enthalpy controlled, and the 7 -enantiomer is eluted first followed by the -enantiomer, while above T[ o enantioseparation is entropy controlled and the reverse order of elution is observed. Therefore, the reverse order of elution occurs due to enthalpy and entropy compensation. Only when the van t Hoff plot is linear over a wide range of temperatures, including Tisoj is the thermodynamic principle of entropy/enthalpy compensation established. In principle, temperature-dependent multimodal recognition mechanisms that oppose each other may also account for the observation of an isoenantioselective temperature [87, 135, 136]. 

The overall observed retention of the enantiomers, and thus the elution order, is based on several kinetically and thermodynamically controlled parameters concerned with stereorecognition nonstereoselective interactions of all partners SO(R), SA(R S), and particularly of the [SO(RI-SA(KI] and [SO(K)-SA(Si] complexes with the achiral stationary phase, also play a role (Figure 21). Therefore the retention order may be reversed for a specific pair of enantiomers depending on whether a covalently bound CSP or a CMPA is applied, but using the same chiral molecule (part) as chiral selector. These general principles, shown schematically for a CLEC system, are further complicated by the complexity of the entire system, hence they are difficult to anticipate and each case must be studied individually. 

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