Solvents for Spectroscopy

Solvents for UV Spectroscopy. Because the transitions of diagnostic interest generally occur above 200 nm, solvents that absorb UV in the region of interest should be avoided. Fortunately, both very polar (e.g., water and methanol) and nonpolar (e.g., hexane) solvents are available so that a wide variety of solutes can be accommodated. This is aided by the sensitivity of the technique. 

it is clear that the choice of solvent has derived from the particular purpose to which it was put, and it remains necessary to find a solvent in which the solute has suitable solubility. Frequently, the literature of organic chemistry will hold examples of compounds similar to the one of interest and the results of investigators who have already undertaken solubility studies will be available. However, lacking that, the traditional rule of thumb like dissolves like is a reasonable place to start. 

Problem 5.12. If ethanol dissolves in gasoline (for the commercial product), what is wrong with using it as a solvent in the laboratory  

Problem 5.13. The all-too-frequent crude oil spills resulting from drilling, shipping, and so on operations and accidents common to our current way of life demonstrate that oil and water do not mix. Yet, ways are found to remove grease and oil stains from fabrics and contaminated surfaces. Why is spilt oil such a problem then  

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The more powerful the solvent-solute interaction, the more pronounced will solvent broadening be for this reason, saturated hydrocarbons are preferred as solvents for spectroscopy, and such strongly interacting media as methylene chloride and chloroform are to be avoided. It is obvious that the requirements of spectroscopy and those of solubility are in direct conflict. Carbon tetrachloride and carbon disulphide are often used as compromise solvents ) (although both of these react thermally or photochemically with many carbonyl complexes) but are generally inferior spectroscopically to alkanes. 

Tlris overview addresses these shortcomings. Specifically, its focus is not on tire synthesis of novel ILs or the investigations of their chemical properties but ratlrer on what ILs can uniquely do, as solvents for spectroscopy, and what spectroscopic techniques can do for ILs. That is, it will summarize studies which focus on (1) the exploitation of unique properties of ILs as solvents to enhance sensitivity and selectivity of spectroscopic measurements, and (2) the development of novel spectroscopic methods which are nondestructive, require only a small amount of samples and have high sensitivity, accuracy for the determination of thermal physical properties of ILs. 

The common solvents used in UV/VIS spectroscopy are listed in Table 5.5, along with their low wavelength cutoff. At wavelengths shorter than the cutoff wavelength, the solvent absorbs too strongly to be used in a standard 1 cm sample cell. The cutoff is affected by the purity of the solvent. For spectroscopy, the solvents should be of spectral or spectro-chemical grade, conforming to purity requirements set by the American Chemical Society. 

It is also possible to use NMR spectroscopy in acidic solvent for analytical purposes. The difference in chemical shift induced by protonation will allow in some cases the identification of the compound [e.g., phenyl or arylthiazoles (109)]. 

Discussion. Because of the specific nature of atomic absorption spectroscopy (AAS) as a measuring technique, non-selective reagents such as ammonium pyrollidine dithiocarbamate (APDC) may be used for the liquid-liquid extraction of metal ions. Complexes formed with APDC are soluble in a number of ketones such as methyl isobutyl ketone which is a recommended solvent for use in atomic absorption and allows a concentration factor of ten times. The experiment described illustrates the use of APDC as a general extracting reagent for heavy metal ions. 

Liquefied Xe and Kr have two features which make them particularly attractive as low-temperature solvents for examining the spectra of unstable organometallics. These solvents are inert, and this is important as the significant role of even innocuous solvents such as cyclohexane is more readily recognised in contrast to conventional solvents for IR spectroscopy they have no absorptions over a wide range of the spectrum, which also permits the use of special long path cells to overcome problems of low solubility. 

We have also investigated the kinetics of free radical initiation using azobisisobutyronitrile (AIBN) as the initiator [24]. Using high pressure ultraviolet spectroscopy, it was shown that AIBN decomposes slower in C02 than in a traditional hydrocarbon liquid solvent such as benzene, but with much greater efficiency due to the decreased solvent cage effect in the low viscosity supercritical medium. The conclusion of this work was that C02 is inert to free radicals and therefore represents an excellent solvent for conducting free radical polymerizations. 

Similarly, organic liquids have a variety of applications. For example, hexane, which frequently contains impurities such as aromatic compounds, is used in a variety of applications for extracting non-polar chemicals from samples. The presence of impurities in the hexane may or may not be important for such applications. If, however, the hexane is to be used as a solvent for ultraviolet spectroscopy or for HPLC analysis with UV absorbance or fluorescence detection, the presence of aromatic impurities will render the hexane less transparent in the UV region. It is important to select the appropriate grade for the task you have. As an example, three different specifications for n-hexane ( Distol F , Certified HPLC and Certified AR ), available from Fisher Scientific UK, are shown in Figure 5.5 [10]. You will see that the suppliers provide extra, valuable information in their catalogue. 

The advantages of the study of the NMR spectroscopy of solutes dissolved in liquid crystal solvents for structural studies have been exploited extensively despite the well-known limitations of the technique, and many compounds have been studied89,90. 

Solvents for NMR Spectroscopy, NMR spectra are almost invariably obtained in... 

Generally solvents chosen for NMR spectroscopy do not associate with the solute. However, solvents which are capable of both association and inducing differential chemical shifts in the solute are sometimes deliberately used to remove accidental chemical equivalence. The most useful solvents for the purpose of inducing solvent-shifts are aromatic solvents, in particular hexadeuterobenzene (CgDg), and the effect is called aromatic solvent induced shift (ASIS). The numerieal values of ASIS are usually of the order of 0.1 - 0.5 ppm and they vary with the moleeule studied depending mainly on the geometry of the complexation. 

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