Comparison of Step-Scan and Rapid-Scan Approaches to the Measurement of Mid-Infrared Fourier Transform Vibrational Circular Dichroism

Abstract

We report mid-infrared Fourier transform vibrational circular dichroism (FT-VCD) spectra obtained by using three different measurement schemes: the traditional sequential rapid-scan method, a new simultaneous rapid-scan method, and the step-scan method. We also provide a description of the optical setup used to obtain the VCD spectra. Using this optical layout, we are able to measure FT-VCD spectra for single enantiomers of the chiral sample with low levels of interfering artifacts and for collection times as fast as one minute. For approximately 20 min of spectral collection, the three FT-VCD measurements approaches tested here yield nearly the same signal quality. The approach with the highest quality, by a small margin, was the simultaneous rapid-scan method. Next was the sequential rapid-scan approach, followed closely by the step-scan method. VCD spectra and noise curves obtained with these different measurement techniques are presented and compared, and their relative advantages and disadvantages are discussed.

Solution conformations of cyclosporins and magnesium‐cyclosporin complexes determined by vibrational circular dichroism

“…32 In the NH/ OH-stretching region, the Magna 850 with a modified accessory VCD bench was used, 33 which included the addition of a CaF 2 photoelastic modulator (Hinds Instruments PEM-90) oscillating at 57 kHz, located between the sample and the focusing lens placed before a liquid nitrogen-cooled 2 mm diameter InSb detector (D* ϭ 3.2 ϫ 10 11 ) with a BaF 2 window. Amide I spectra of the free cyclosporins (20 mM in CDCl 3 ) and magnesium complexes of the cyclosporins [20 mM in CD 3 CN with 10-fold molar excess of Mg(ClO 4 ) 2 ] were measured in a 69 m fixed path length cell equipped with BaF 2 windows, at a resolution of 4 cm Ϫ1 .…”

“…VCD spectra were collected in simultaneous rapid-scan mode 32 and were corrected for baseline artifacts by subtraction of the solvent VCD spectra. The total collection times for each VCD spectrum in the amide I and NH/OH-stretching regions were 3.75 and 12 h, respectively.…”

INFRARED AND RAMAN VIBRATIONAL OPTICAL ACTIVITY: Theoretical and Experimental Aspects

“…Niemeyer et al's (131) report of this approach to VCD appeared in the same year. More recently, two studies of new step-scan FT-VCD instruments, one dedicated to the mid-infrared (132) and the other to the near-infrared (133), have appeared. In the near-infrared paper, high-quality spectra in the CH-stretching region were obtained that surpass the quality of the corresponding spectra obtained with a dispersive spectrometer in the same laboratory.…”

Measurements of concentration dependence and enantiomeric purity of terpene solutions as a test of a new commercial VCD spectrometer

“…The performance of the VCD spectrometers is usually tested by measurements of chiral terpenes, especially of (+)-or (−)-␣-pinene (see, for example, excellent reviews on VCD spectroscopy 1,2 ). 3,4 Using ␣-pinene, resolution, time stability of the VCD setup, and the baseline distortion were tested. Terpenes provide a rich structure of vibrational absorption bands in the mid-infrared region.…”

“…The terpenes are often available in satisfactory enantiomer purity as mirror-image stereoisomers and can be easily measured either as a neat fluid or solutions. [3][4][5] If enantiomeric pairs of terpenes are measured at the same conditions, the quality of the baseline can be checked by comparing both spectra, which should be mutually sign-inverse in respect of the baseline. The VCD spectra of ␣-pinene enantiomers in the region of 1,400-900 cm −1 exhibit about 10 well-distinguished bands of both signs with intensities higher than 1 и 10 −4 ⌬A units, if measured under usual conditions.…”