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 ΔA 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.