Methods for studying reaction kinetics in gas chromatography, exemplified by using the 1-chloro-2,2-dimethylaziridine interconversion reaction.

In this paper, methods are described that are used for studying first-order reaction kinetics by gas chromatography. Basic theory is summarized and illustrated using the interconversion of 1-chloro-2,2-dimethylaziridine enantiomers as a representative example. For the determination of the kinetic and thermodynamic activation data of interconversion the following methods are reviewed: (i) classical kinetic methods where samples of batch-wise kinetic studies are analyzed by enantioselective gas chromatography, (ii) stopped-flow methods performed on one chiral column, (iii) stopped-flow methods performed on an achiral column or empty capillary coupled in series with two chiral columns, (iv) on-flow method performed on an achiral column coupled in series with two chiral columns, and (v) reaction gas chromatography, known as a dynamic gas chromatography, where the interconversion is performed on chiral column during the separation process. The determination of kinetic and thermodynamic activation data by methods (i) through (iv) is straightforward as the experimental data needed for the evaluation (particularly the concentration of reaction constituents) are accessible from the chromatograms. The evaluation of experiments from reaction chromatography method (v) is complex as the concentration bands of reaction constituents are overlapped. The following procedures have been developed to determination peak areas of reaction constituents in such complex chromatograms: (i) methods based on computer-assisted simulations of chromatograms where the kinetic activation parameters for the interconversion of enantiomers are obtained by iterative comparison of experimental and simulated chromatograms, (ii) stochastic methods based on the simulation of Gaussian distribution functions and using a time-dependent probability density function, (iii) approximation function and unified equation, (iv) computer-assisted peak deconvolution methods. Evaluation of the experimental data permits the calculation of apparent rate constants for both the interconversion of the first eluted (k (A-->B)(app)) as well as the second eluted (k(B-->A)(app)) enantiomer. The mean value for all the rate constants (from all the reviewed methods) was found for 1-chloro-2,2-dimethylaziridine A-->B enantiomer interconversion at 100 degrees C: k (A-->B)(app)=21.2 x 10(-4)s(-1) with a standard deviation sigma=10.7 x 10(-4). Evaluating data for reaction chromatography at 100 degrees C {k (app)=k(A-->B)(app)=k(B-->A)(app)=13.9 x 10(-4)s(-1), sigma=3.0 x 10(-4)s(-1)} shows that differences between k(A-->B)(app) and k(B-->A)(app) are the same within experimental error. It was shown both theoretically and experimentally that the Arrhenius activation energy (E(a)) calculated from Arrhenius plots (lnk(app) versus 1/T) is proportional to the enthalpy of activation {E(a)=DeltaH+RT}. Statistical treatment of Gibbs activation energy values gave: DeltaG (app)=110.5kJmol(-1), sigma=2.4kJmol(-1), DeltaG (A-->B)(app)=110.5kJmol(-1), sigma=2.2kJmol(-1), DeltaG (B-->A)(app)=110.3kJmol(-1), sigma=2.8kJmol(-1). This shows that the apparent Gibbs energy barriers for the interconversion of 1-chloro-2,2-dimethylaziridine enantiomers are equal DeltaG (app)=DeltaG(A-->B)(app)=DeltaG(B-->A)(app) and within the given precision of measurement independent of the experimental method used.

On the use of computer assisted resolution of non-separable peaks in a congener specific polybrominated diphenyl ether capillary gas chromatographic analysis.

The use of computer assisted deconvolution for chromatographically not separated peaks in the analysis of polybrominated diphenyl ethers (PBDEs) by capillary gas chromatography (CGC) was studied. Twenty-two not separated clusters containing 48 overlapped PBDEs were registered in the separation of a sample containing 122 PBDE congeners on a semipolar poly(8%-phenyl-92%-dimethyl)siloxane column. There were only two clusters in which overlapped PBDEs differ in the number of bromine atoms {PBDE 126(5Br) co-elutes with 154(6Br) and PBDE 105(5Br) co-elutes with 144(6Br)} and therefore their mass spectra could be successfully used for deconvolution purposes. In 22 other clusters 46 isomeric PBDEs with identical mass spectra overlapped and for their resolution a computer assisted deconvolution procedure using a commercial available program was used. A published procedure for the estimation of minimum number of peaks in a peak cluster for which the data found by deconvolution are reliable, has been adapted. Using this procedure, for eight overlapped PBDE full peak data (single peak retention times and peak areas) were extracted.

Gas chromatographic determination of the interconversion energy barrier for dialkyl 2,3-pentadienedioate enantiomers.

The enantiomers of dialkyl 2,3-pentadienedioate undergo interconversion during gas chromatographic separation on chiral stationary phases. In this paper the on-column apparent interconversion kinetic and thermodynamic activation data were determined for dimethyl, diethyl, propylbutyl and dibutyl 2,3-pentadienedioate enantiomers by gas chromatographic separation of the racemic mixtures on a capillary column containing a polydimethylsiloxane stationary phase coupled to 2,3-di-O-methyl-6-O-tertbutyldimethylsilyl-beta-cyclodextrin. A deconvolution method was used to determine the individual enantiomer peak areas and retention times that are needed to calculate the interconversion rate constants and the energy barriers. The apparent rate constants and interconversion energy barriers decrease slightly with an increase in the alkyl chain length of the dialkyl 2,3-pentadienedioate esters. The optimum conformation of the dialkyl 2,3-pentadienedioate molecules, their separation selectivity factors and apparent interconversion enthalpy and entropy data changes with the alkyl chain length. The dependence of the apparent interconversion energy barrier (deltaG(app)(a-->b), deltaG(app)(b-->a)) on temperature was used to determine the apparent activation enthalpy (deltaH(app)(a-->b), deltaH(app)(b-->a)) and apparent entropy (deltaS(app)(a-->b), deltaS(app)(a-->b)) (where a denotes the first and b second eluted enantiomer). The comparison of the activation enthalpy and entropy (deltaS(app)(a-->b), deltaS(app)(a-->b)) indicated that the interconversion of dialkyl 2,3-pentadienedioate enantiomers on the HP-5+Chiraldex B-DM column series is an entropy driven process at 160 degrees C. Data obtained for dimethyl 2,3-pentadienedioate enantiomers on the HP-5+Chiraldex B-DM column series at 120 degrees C (deltaG(app)(a-->b) = 123.3 and deltaG(app)(b-->a) = 124.4 kJ mol(-1)) corresponds (at the 95% confidence interval) with the value of deltaG(#) = 128+/-1 kJ mol(-1) found at this temperature by gas chromatography using a two-dimensional stop flow technique on an empty capillary column [V. Schurig, F. Keller, S. Reich, M. Fluck, Tetrahedron: Asymmetry 8 (1997) 3475].

Computerized separation of chromatographically unresolved peaks.

A computerized peak deconvolution software and mass spectra were successfully applied for the deconvolution of overlapped peak cluster in the chromatogram obtained separating the complex mixture of pesticides by retention time locking gas chromatography-mass spectroscopy. The method based on the unique fragment ions in the spectra can be used for deconvolution of peak clusters if mass spectra of overlapped peaks differ. This method allows determining actual retention times of overlapped peaks. Peak areas found by this method however, cannot be used naturally for the quantitative purposes as the abundance of fragment ions used for this deconvolution procedure can dramatically differ. Computer assisted deconvolution of peaks in the peak clusters gives more realistic peak area ratios as at this method it is supposed equal response for all peaks overlapped in a cluster.

On-flow gas chromatographic method for the determination of the enantiomer interconversion energy barrier.

A novel on-flow gas chromatographic (GC) method is developed for the determination of the kinetic rate constants and interconversion energy barrier of thermally labile enantiomers. The validity of the developed method is approved by the study of interconversion of 1-chloro-2,2-dimethylaziridine enantiomers on an achiral column. The overall experiments are performed in a series of three columns placed in two independently heated GC ovens. The racemate of the 1-chloro-2,2-dimethylaziridine is injected and separated in the first chiral column at 60 degrees C in which the interconversion of enantiomers is suppressed. Separated enantiomers are then transferred into the achiral column, where the enantiomers are interconverted at a selected temperature under the current carrier gas flow. Effluent from this column is transferred into the second chiral column, where the native enantiomers and those originated by the on-flow interconversion on an achiral column are again separated at 60 degrees C. Chromatograms obtained by monitoring the effluents from the second chiral column are used to determine the peak areas of the original and the newly interconverted enantiomers. The corresponding peak areas and the interconversion times are used to calculate the interconversion rate constants and energy barriers of the 1-chloro-2,2-dimethylaziridine enantiomers. The apparent energy barriers of the enantiomers of 1-chloro-2,2-dimethylaziridine are equal for both enantiomers within a 95% confidence interval and independent of the polarity of the stationary phase of the column in which the interconversion of enantiomers occur.