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Extension of Fourier Transform Vibrational Circular Dichroism into the Near-Infrared Region: Continuous Spectral Coverage from 800 to 10 000 cm-1
Volume 58, Number 9 (Sept. 2004) Page 1057-1064
Cao, Xiaolin; Shah, Rekha D.; Dukor, Rina K.; Guo, Changning; Freedman, Teresa B.; Nafie, Laurence A.
We report the first vibrational circular dichroism (VCD) spectra with continuous coverage from 800 cm-1 in the mid-infrared (MIR) region to 10 000 cm-1 in the near-infrared (NIR) region. This coverage is illustrated with MIR and NIR absorbance and VCD spectra of 2,2-dimethyl-dioxolane-4-methanol (DDM), α-pinene, and camphor that serve as calibration samples over this entire region. Commercially available, dual-source Fourier transform (FT) MIR and NIR VCD spectrometers were equipped with appropriate light sources, optics, and detectors, and were modified for dual-polarization-modulation (DPM) operation. The combination of liquid-nitrogen- and thermoelectric-cooled HgCdTe (MCT) detectors, as well as InGaAs and Germanium (Ge) detectors operating at room temperature, permitted collection of the desired absorbance and VCD spectra across the range of vibrational fundamental, combination band, and overtone frequencies. The spectra of DDM and α-pinene were measured as neat liquids and recorded for both enantiomers in the various spectral regions. Spectra for camphor were all measured in CCl4 solution at a concentration of 0.6 M, except for the carbonyl-stretching region, where a more dilute concentration was used. The typical anisotropy ratios (g) of the three molecules were estimated with respect to their strongest VCD bands in each spectral region. It was found that for all three molecules in the spectral regions above 2000 cm-1, anisotropy ratios are approximately the same order (10-5) of magnitude. However, in the MIR region, the typical anisotropy ratios are significantly different for the three molecules. This study demonstrates that with modern FT-VCD spectrometers modified for DPM operation, VCD spectra can be measured continuously across a wide spectral range from the MIR to nearly the visible region with an unsurpassed combination of signal-to-noise ratio and spectral resolution.