UV, EPR, X‐ray and Related Spectroscopic Techniques

Abstract

The techniques described in Chapters 3.1 and 3.2 provide information on a molecular level about solute structure, solution dynamics and chemistries in SCFs. The goal of this chapter is to briefly discuss other spectroscopic techniques applied to SCFs which can be used to determine solution structure or the effects of pressure and temperature on reactions in these solvent systems. An overview of this type, by its nature, can not be considered inclusive, but it is hoped to give future practitioners in the field a reasonable expectation of the type of molecular-level information that different spectroscopic techniques provide under SCF conditions. UV-VisUV-Vis spectroscopy of compounds dissolved in SCFs has been used to determine solvent structure [l-61, solute solubilities [7,8], solid matrix changes [9, lo], and reaction kinetics [ll-161. UV-Vis is often the spectroscopic technique of first choice because many solute molecules of interest have an observable absorption band which can be used for identification and quantitation.In the simplest UV-Vis SCF experiment a pressure vessel (stainless steel or other high pressure metal alloy) is fitted with an inlet (and an outlet) connection for high-pressure tubing and high pressure windows (quartz, sapphire, or diamond). The relative strength of the window materials is diamond > sapphire > quartz. The pressure limits of the cell design depend on the window material and the area of the unsupported window exposed to the fluid pressure. The typical UV cutoff wavelengths for these materials are: sapphire -1500A, quartz -2100 A, and diamond type I1 -2600 A. Many different cell designs have been reported in the literature. A typical high pressure cell design is shown in Figure 3.3-1. The pressure seal between the sapphire window and the cell body is made with a gold plated metal V-ring. The path length is typically 2.5 cm and the cell volume is 3-4 mL. The sapphire windows are 1 cm thick and 2.5 cm in diameter. Common, high-pressure UV-Vis spectroscopy