TEMPO molecules, electron paramagnetic

Another useful method for investigations of water/C02 emulsions and microemulsions is electron paramagnetic resonance (EPR) spectroscopy, because no transparent samples are necessary [9,13]. Furthermore, data from EPR experiments can provide information about the polarity of the local environment of the EPR-active compound. The diagnostic unpaired electron(s) can be introduced either through stable free radicals or by using transition metal ions such as Mn. The active moieties may be incorporated directly in the surfactant [17,18] or added as a soluble probe molecule such as TEMPO (4-hydroxy-2,2,6,6-tetramethylpiperidin-Toxyl) [9]. 

Electron spin resonance (ESR), often called electron paramagnetic resonance (EPR), requires the presence of unpaired electrons in the sample [5, 15, 16, 135]. Since most organic molecules do not carry free radieals, the most common method is to simply dissolve stable free radieals as spin probes in the sample. Alternatively, free radical groups can be ehemically bonded to the molecule of interest providing a spin label. Probably the most frequently used spin probes are nitroxide free radicals like 2,2,5,5-tetra-methyl-piperidinoxide (TEMPO). 

A spin label is a stable paramagnetic molecule that contains an atom or group of atoms with an unpaired electron spin that can be bonded to another molecule. In this way, one can detect molecules that otherwise would not give an ESR spectrum. Most spin labels are nitroxide compounds. For example, 2,2,6,6-tetramethylpiperidine-A-oxyl (known as TEMPO) is a common spin label with a characteristic three-line ESR spectrum, seen in Figure 3.87. 

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