Polarimetry cells

Two modem polarimetry cells are shown in Figure 23.6. In the first case, the cell is filled until the liquid completely fills the bore and a small portion of the center stem. Then, if one gently rocks the cell back and forth along its axis, bubbles will rise and collect in the stem where they are above the light path. A stopper is placed in the stem when you are finished. In the second case, the cell is filled vertically, and the end is screwed on. Bubbles are trapped at the raised end when the cell is turned horizontally. 

The basic experiment consisted of simply treating an l-UNCA with a base and monitoring the formation of the d-UNCA. The most straightforward analysis is accomplished by polarimetry. For example, Boc-Phe-NCA was dissolved in THF at a concentration of 0.33 M, and 1.5 equivalents of TEA were added. The resulting solution was placed in a polarimeter cell and the optical rotation was monitored over time (Figure 1). 

Periodic or continuous spectral readings. In many cases the reaction can be carried out in the cell while it is in the instrument. Then all that is necessary is that the instrument be read, periodically or continuously. Among the methods used are ir and uv spectroscopy, polarimetry, nmr, and esr.43... 

Differences in enantiomers become apparent in their interactions with other chiral molecules, such as enzymes. Still, we need a simple method to distinguish between enantiomers and measure their purity in the laboratory. Polarimetry is a common method used to distinguish between enantiomers, based on their ability to rotate the plane of polarized light in opposite directions. For example, the two enantiomers of thyroid hormone are shown below. The (5) enantiomer has a powerful effect on the metabolic rate of all the cells in the body. The (R) enantiomer is useless. In the laboratory, we distinguish between the enantiomers by observing that the active one rotates the plane of polarized light to the left. 

The Standard cell length is 1 decimeter (1 dm or 10 cm). Concentration is expressed in grams per milliliter (g/mL). Because specific rotation depends on temperature (T, in degrees Celsius) and wavelength A of light, these variables are designated, respectively, as superscript and subscript. The light source most commonly used in polarimetry is the sodium D line (A = 589 nm), the line responsible for the yellow color of sodium-vapor lamps. 

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