Step-Scan Fourier Transform Vibrational Circular Dichroism Measurements in the Vibrational Region above 2000 cm⁻¹

Abstract

We present the results of a new Fourier transform infrared vibrational circular dichroism (FT-IR-VCD) spectrometer optimized for performance in the near-infrared region above 2000 cm−1. The instrument is capable of operating in either the conventional rapidscan mode or the step-scan mode and is designed around the Bruker IFS 55 FT-IR spectrometer. We find that in the region of hydrogen stretching, the instrument performs as well as or better than (depending on the spectral coverage and resolution desired) a corresponding dispersive grating VCD spectrometer, with noise levels at approximately 10−6 Δ A units for a two-hour collection at 8-cm−1 resolution. The performance is approximately a factor-of-2 better in signal-to-noise ratio in optimized step-scan operation in comparison to the corresponding results in rapid-scan operation. The reasons are traced to advantages in electronic filtering and undersampling efficiency in step-scan mode. It is expected that the advantages of step-scan will increase with increasing optical transition frequency, such as the region of overtone and combination bands between 4000 and 8000 cm−1.

Mini review: Instrumentation for vibrational circular dichroism spectroscopy, still a role for dispersive instruments

“…The I mod / I trans values are subsequently used to determine the VCD spectrum . (Near‐IR VCD can be measured with good results, especially with an InSb detector, but require higher frequency digitization or slower mirror scan speeds .…”

“…23,39,51,60,61,68 (Near-IR VCD can be measured with good results, especially with an InSb detector, but require higher frequency digitization or slower mirror scan speeds. 40,42,43,61,69,71,76 ) Vibrational circular dichroism can also be measured with step-scan designs, but these have shown little advantage other than allowing for different modulation techniques. 50,59,61,69 Digital signal processing can eliminate the need for a lockin, 59,77 and the newer BioTools instrument does this with rapid-scan, simplifying the instrumentation and presumably improving S/N.…”

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 .…”

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.…”