Using a newly constructed FT-IR vibrational circular dichroism (VCD) instrument, we have found that elimination of the ellipsoidal collection mirror before the detector and its replacement by a lens leads to a significant improvement in the absorption artifact problem seen previously in FT-IR/VCD. In the mid-IR region, we have been able to measure VCD of a single enantiomer for molecules such as α-pinene, 3-methylcyclohexanone, and dimethyltartrate. More importantly, this reduction in artifact level brings the FT-IR/VCD band shape of some particularly-difficult-to-measure bands, such as carbonyl stretches, into better agreement with those found in dispersive measurements. These results imply that the dispersive results are reliable, though of lower resolution than those obtained with the use of FT-IR/VCD.
The bark of the roots of Piscidia carthagenensis afforded the known insecticides rotenone (1) and millettone (2), as well as the new rotenoid piscicartone (3). The structure of 3 followed from nuclear magnetic resonance studies, while its absolute configuration (AC) was determined by vibrational circular dichroism (VCD) measurements in comparison with discrete Fourier transform B3LYP/DGDZVPcalculated spectra using the Compare VOA software. In addition, the AC of 1 and 2 was verified using the same VCD methodology.
Fourier transform infrared (FT-IR) spectrometers are commonly designed with small-area detectors and tight focusing mirrors. Vibrational circular dichroism (VCD) measurements made with such FT-IR instruments contain polarization artifacts, and VCD measurements on both enantiomers (or one enantiomer and racemic mixture) are required in order to reduce these artifacts. This restriction limits the VCD measurements to only those samples for which both enantiomers (or one enantiomer and racemic mixture) are available. Recently a modified design was reported in the literature where the mirrors between sample and detector were replaced with a BaF2 lens, and a larger-area detector was substituted for the smaller-area counterpart. These modifications successfully alleviated some of the artifact problems. This design, however, is not suitable for polarizing interferometers, where polarizations exiting the interferometer are to be preserved to a high degree of purity. In addition, it is not clear whether the throughput enhancement advantage realized with a larger-area detector completely offsets the disadvantage from increased noise with detector area. Furthermore, BaF2 lenses reduce the broad range routinely available on an FT-IR instrument. Here we report a new design that replaces all the mirrors at the exit port of the interferometer with two KBr lenses and retains the full spectral range (4000–400 cm−1) of mid-infrared FT-IR spectrometers. VCD measurements obtained with small- (1 × 1 mm) and large- (4 × 4 mm) area detectors are found to have similar signal quality.
Piscicartone, a Rotenoid From Piscidia carthagenensis