Transmission vs. Diffuse Transmission in Circular Dichroism: What to Choose for Probing Solid-State Samples?

Recent advances in equipment enabled the collection of solid-state electronic circular dichroism (ECD) spectra using the commercially available integrating sphere attachment for a regular ECD spectrometer. This accessory was designed to reduce negative factors occurring in solid-state ECD measurements, and is, thereby, very useful for recording diffuse transmittance CD (DTCD) spectra using the pellet technique. In the present article, the operating principle of the integrating sphere and utility of the DTCD method in recording solid-state ECD spectra is demonstrated. Based on illustrative examples, i.e., 10-camphorsulfonic acid ammonium, cholest-4-en-3-one, (3R,4R,5S)-oseltamivir, and (S)-linezolid, ECD solid-state measurements were performed by means of both transmission and diffusion methods and later compared. Selection of these compounds as models for comparative studies was made in view of their different chromophoric systems and the profound importance in the pharmaceutical industry. During the course of this work the benefits and limitations of the use of integrating sphere are presented. The final conclusion is that more relevant solid-state spectra can be obtained by means of the DTCD method.