On the use of a peak deconvolution procedure for the determination of energy barrier to enantiomerization in dynamic chromatography

Abstract

The term enantiomerization characterizes a process in which the enantiomers undergo inversion of their respective conformations during a chromatographic separation [1]. Enantiomerization of enantiomers A and B can be described by a scheme:where k 1 and k -1 are the rate constants.The A→B enantiomerization is the first order kinetic reaction which is described by following differential equation:(1) The B→A enantiomerization is described by the similar equation:where c is concentration of a considered enantiomer and t is time.Equations 1 and 2 describe interconversion of individual enantiomers in a static system where enantiomers A and B are in a contact. Direct chromatographic separation of enantiomers is, however, a dynamic system, and therefore interconverted enantiomers A* and B* will be separated from the "original ones". A peak cluster composed of a peak of interconverted enantiomers (A*B*) between the two peaks of enantiomers is formed as a consequence of this separation [2].Energy barriers to enantiomerization of enantiomers can be determined by several methods. While the isolation of single enantiomers followed by classical racemization kinetics using chiroptical methods is cumbersome and time-consuming, dynamic [1,[3][4][5] and stopped-flow [6,[8][9][10] chromatographic techniques are more straightforward since enantiomers are separated and analyzed on-line, thus requiring only minute amounts of the racemic or enriched mixture of enantiomers. The lower energy barrier to interconversion of enantiomers (around 70-100 kJ mol -1 ) can be determined by dynamic HPLC (DHPLC) [11]. Higher energy barriers to interconversion of enantiomers (around 70-200 kJ mol -1 ) can be determined by dynamic gas chromatography (DGC) [9]. DHPLC and DGC has been shown to produce rate constants and energy barriers to enantiomerization in good agreement with those obtained by independent techniques [12] except for the rare case when the stationary phase shows high catalytic activity [13]. Mannschreck with co-workers have used simultaneous photometric ad polarimetric detectors for aOn the use of a peak deconvolution procedure for the determination of energy barrier to enantiomerization in dynamic chromatography Abstract. Manual or computer assisted peak deconvolution on chromatograms of the racemate of 1-chloro-2,2-dimethylaziridine enantiomers GC was used to determine the peak areas of enantiomers in the racemate prior (A A,0 , A B,0 ) and after the separation (A A, A B ). These peak areas were used in the determination of apparent rate constants and apparent energy barrier to enantiomerization. Comparison of apparent energy barriers determined using deconvolution of chromatograms with data published in literature showed differences within 7 % rel.

Investigation of the stereodynamics of molecules and catalyzed reactions by CE

“…Krupcik et al [8,54,55] have developed computer-assisted strategies to deconvolute such elution profiles based on the concept, that the non-interconverted enantiomers and the converted part can be fitted by Gaussian/ Lorentzian as well as exponentially modified Gaussian functions to approximate the peak shapes in the deconvolution procedure. Advantage of this method is that essentially no knowledge about the separation mechanism is necessary.…”

Determination of enantiomerization barriers by dynamic and stopped‐flow chromatographic methods

“…19,32 In contrast to these ab initio type simulations, semiempirical peak deconvolution methods have also been developed to estimate rate constants of enantiomerization k 1 . [33][34][35][36][37][38] All these methods can consider peak tailing and fronting, yet the ab initio type simulations (TPM, SM) require significantly longer calculation times because of the exponentially modified Gaussian function [39][40] and the semiempirical peak deconvolution methods become less accurate due to the multitude of assumptions needed.…”

Chiroptical detection during liquid chromatography: Deconvolution of overlapping peaks of enantiomers and its applications

“…Thus, the configurations of the molecules at the time of detection are looked at. Some other articles 36,[39][40][41][42][43][44] use the same word "deconvolution" with a different meaning: A transformation of an experimental curve of overlapped peaks into more than two curves, namely, the ones of the two nonconverted enantiomers and the ones of converted enantiomers. Thus, the fates of the molecules before detection are looked at in this case.…”