UNUSUAL CASE OF DIRECT RELATIONSHIP BETWEEN TEMPERATURE AND RETENTION IN REVERSED-PHASE LIQUID CHROMATOGRAPHY

Abstract

The effect of temperature on RP-LC retention of s-triazine derivatives containing amino amide substituents was studied. The slope of van't Hoff plots (ln(k′) vs 1/T) became less positive as the hydrophobicity of the amino amide R-groups increased. For derivatives containing two amino amide substituents, L-L diastereomers showed substantially less positive van't Hoff slopes compared to the corresponding L-D diastereomers. For the most hydrophobic R-group studied (isopropyl), a negative van't Hoff slope was found for the L-L diastereomer. The tendency towards less positive/more negative van't Hoff slopes for the L-L diastereomers was attributed to the existence of a particular conformational isomer having poor RP-LC retention, which was stabilized at low temperatures via an intramolecular hydrogen bond. Another observation in this study was the significant peak broadening as temperature was lowered. The free energy barrier to bond rotation was estimated based on the effect of temperature on the magnitude of peak broadening. Calculated values were in the range of 65–66 kJ/mol for the L-L diastereomers, which is very close to literature values for similar compounds. The accuracy of values obtained by this approach is discussed.

Keywords:

• bond rotation
• conformational isomer
• diastereomer
• s-triazine
• van't hoff