Title

Enantiomeric excess determination and reaction monitoring of chiral molecules using near-infrared and mid-infrared vibrational circular dichroism

Date of Award

2004

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor(s)

Laurence A. Nafie

Second Advisor

Teresa B. Freedman

Keywords

Enantiomeric excess, Chiral, Near-infrared, Vibrational circular dichroism

Subject Categories

Analytical Chemistry | Chemistry | Physical Sciences and Mathematics

Abstract

Fourier transform vibrational circular dichroism (FT-VCD) spectrometers equipped with various types of light sources and detectors and modified to dual polarization modulation (DPM) were set up in our laboratory on the basis of commercial instruments--the Chiral IR from BioTools, Inc. VCD spectra of nine terpenes and derivatives were recorded with the DPM-FT-VCD spectrometers in Near-infrared (NIR) regions from 10000 to 4000 cm -1 and Mid-infrared (MIR) region from 4000 to 800 cm -1 . The NIR-VCD spectra were the first measured by FT-VCD instruments, with unsurpassed quality in spectral precision, signal-to-noise ratio, spectral resolution and simplicity of instrumentation.

The first use of FT-VCD in conjunction with partial least squares (PLS) chemometric analysis to follow changes in the percent enantiomeric excess (%EE) of chiral sample molecules with time using a flow-cell sampling apparatus is reported. The flow-cell sampling simulates the progress of the chemical reaction from a chiral reactant to a chiral product where the %EE of both molecules can change with time. For the molecules studied, α-pinene, camphor and borneol, %EE changes were successfully followed by MIR- and NIR-VCD spectroscopy, and predicted by PLS models with decent accuracy. These findings demonstrate the potential for VCD to be used for real-time monitoring of the composition and %EE of chemical reactions involving the synthesis of chiral molecules.

NIR-VCD spectroscopy and the PLS method have been applied to follow real reactions involving chiral molecules. The epimerization reactions of (S)-(+)-2,2-Dimethyl-1,3-dioxolane-4-methanol (S-DDM) in different solvents were successfully monitored by following the NIR-VCD spectral changes of the O-H combination bands of S-DDM in the spectral range of 5050 to 4700 cm -1 . An interesting solvent effect of the DDM epimerization reaction was discovered. A preliminary NIR-VCD investigation of an SN2 substitution reaction, preparation of 2-chlorobutane, is also reported.

To obtain high signal-to-noise ratio and better quantitative analysis accuracy, the optimal instrument settings and data collection parameters for VCD measurements, such as measurement time, resolution, aperture size, optical filters and preamplifier feedback resistor were explored.The best spectral resolutions for VCD measurements have been recommended.

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