Solvents for ultraviolet spectroscopy

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. 

Ethanol is commonly obtained in a 95 per cent and an absolute grade. The former has the composition of the azeotrope with water (bp 78.2°) and except for the water is quite pure. If absolute ethanol (bp 78.3°) is required, it may be prepared by heating the 95 per cent ethanol to reflux with calcium oxide for several hours and then distilling. However, absolute ethanol is available at a reasonable cost and is rarely prepared in a laboratory. The commercial absolute ethanol often contains a small amount of benzene, since it is prepared from 95 per cent ethanol by removing the water through the ternary azeotrope of benzene-ethanol-water (bp 65°). The absolute ethanol is therefore not suitable for use as a solvent for ultraviolet spectroscopy, and the 95 per cent ethanol is usually used. 

Acetonitrile (bp 82°) is another polar solvent, which promotes the ionization of compounds such as the triphenylmethyl halides. It is also a very useful solvent for the recrystallization of polar compounds such as dicarboxylic acids. It is easily purified by heating to reflux with and then distilling from phosphorous pentoxide. Much of the commercial acetonitrile is dried by azeotropic distillation with benzene. If it is to be used as the solvent for ultraviolet spectroscopy, either one must obtain material which has not been treated with benzene, or the benzene may be removed by azeotropic distillation with water and drying with phosphorus pentoxide. The presence of benzene is indicated by weak absorption around 260 m/ and strong tail absorption beginning at about 220 m/i.. 

Solvents and substances that are specified as pure for a particular purpose may, in fact, be quite impure for other uses. Absolute ethanol may contain traces of benzene, which makes it unsuitable for ultraviolet spectroscopy, or plasticizers which make it unsuitable for use in solvent extraction. 

If the solvent is to be used for ultraviolet spectroscopy, it is necessary to remove all the aromatic compounds. This may be done by shaking the hydrocarbon with a mi.xture of concentrated sulfuric and nitric acids, which will nitrate the aromatic compounds. The hydrocarbon is separated, washed with water, distilled, and passed through a column of activated alumina which will remove any residual unsaturated or nonhydrocarbon materials. The spectrum of the solvent is monitored as it passes from the column, and when significant absorption at 210 m/i is observed, the alumina is replaced. 

A solvent for ultraviolet/visible spectroscopy must be transparent in the region of the spectrum where the solute absorbs and should dissolve a sufficient quantity of the sample to give a well-defined analyte spectrum. In addition, we must consider possible interactions of the solvent with the absorbing species. For example, polar solvents, such as water, alcohols, esters, and ketones, tend to obliterate vibration spectra and should thus be avoided to preserve spectral detail. Nonpolar solvents, such as cyclohexane, often provide spectra that more closely approach that of a gas (compare, for example, the three spectra in Figure 24-14). In addition, the polarity of the solvent often influences the position of absorption maxima. For qualitative analysis, it is therefore important to compare analyte spectra with spectra of known compounds measured in the same solvent. 

Fig. 10.18. Ultraviolet cut-off points of spectrally pure solvents for absorption spectroscopy.

These methods are now obsolete in comparison with spectroscopic methods. Werbel has shown that the structures of these isomers are easily determined by NMR (125) (see also Table VI-5). Furthermore. 2-imino-4-thiazoline derivatives are characterized by their stretching C=N vibration at 1580 cm , absent in their 2-aminothiazole isomers, and by the stretching NH vibration that appears in the range of 3250 to 3310 cm for the former and between 3250 to 3340 cm" for the latter (131). Ultraviolet spectroscopy also differentiates these isomers (200). They can be separated by boiling in ethanol the thiazoline isomer is usually far less soluble in this solvent (131),... 

Table 7.9 Electronic Absorption Bands for Representative Chromophores Table 7.10 Ultraviolet Cutoffs of Spectrograde Solvents Table 7.11 Absorption Wavelength of Dienes Table 7.12 Absorption Wavelength of Enones and Dienones Table 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy Table 7.14 Primary Bands of Substituted Benzene and Heteroaromatics Table 7.15 Wavelength Calculation of the Principal Band of Substituted Benzene Derivatives...

TABLE 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy... 

Many of the properties oj -hydroxypyridines are typical of phenols. It was long assumed that they existed exclusively in the hydroxy form, and early physical measurements seemed to confirm this. For example, the ultraviolet spectrum of a methanolic solution of 3-hydroxypyridine is very similar to that of the 3-methoxy analog, and the value of the dipole moment of 3-hydroxypyridine obtained in dioxane indicates little, if any, zwitterion formation. However, it has now become clear that the hydroxy form is greatly predominant only in solvents of low dielectric constant. Comparison of the pK values of 3-hydroxypyridine with those of the alternative methylated forms indicated that the two tautomeric forms are of comparable stability in aqueous solution (Table II), and this was confirmed using ultraviolet spectroscopy. The ratios calculated from the ultraviolet spectral data are in good agreement with those de-... 

The use of trifluoroethanol as solvent or absorption of the dienone on silica gel promotes the photoconversion of dienones into bicyclic ketenes.<47) For the photolysis<48 60) of (63) it has been shown by low-temperature infrared and ultraviolet spectroscopy that the initial photolysis gives a ketene which can be efficiently trapped by cyclohexylamine or, in the absence of a good nucleophile, thermally rearranges by a OA, + 20) allowed process to a bicyclic ketone (64) ... 

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. 

For oxirane rings an IR absorption around 890 cm-1 is characteristic. This is also observed in the case of K-region epoxides and can be used for diagnostic purposes, but it is not sensitive enough to provide detailed structural information. The oxepins ordinarily do not show this band. Ultraviolet spectroscopy has been invaluable in studying the dynamic equilibrium between the arene oxides and oxepins. The solvent variation of UV spectra has also been exploited very effectively.8... 

Solvents used for visible-ultraviolet spectroscopy may be used only for wavelengths greater than some ultraviolet cutoff wavelength Xc, below which the solvent absorbs strongly. These cutoff wavelengths /,c are listed with some other useful data in Tables 11.3 and 11.4. 

Proper selection of a solvent for preparing lignin solutions is of great importance in ultraviolet spectroscopy since only true solutions yield maximum absorptivity... 

ULTRAVIOLET SPECTROSCOPY 12.2.1 Ultraviolet Cutoff Limits for Solvents... 

This effect was interpreted in terms of an inhibiting role of the monomer upon initiation. Selfionisation of the catalyst was mled out because it led to an expected initia-ti i efficiency wWch disagreed with the experimental relationship [R ] = K[C]o/[M]o, where [R ] is the concentration of active species (carbenium ions), and [C]q and [M]q are the initial catalyst and monomer concentrations. Particular care was taken to purify the monomer and the above relationship is not an artefact. Solvent cocatalysis (CH2CI2) was proved not to be operative in this system and the same applies to initiation throu electron transfer. The very interesting mechanism proposed to account for all experimental observations assumed that a complex TiCl4- M2 must form. In this complex the TiCl4 is inactive, but can be liberated by in vacuo ev oration. The search for that complex was successfully carried out using ultraviolet spectroscopy ... 

Solvents used for spectroscopy, especially nmr and uv, should be of high purity. Many suppliers provide spectroscopic grade solvents which are particularly suitable for uv spectroscopy because ultraviolet absorbing impurities have been removed. 

Ultraviolet spectroscopy is only of use if your compound has a characteristic chromophore. There is little point trying to measure weak bands which will provide no information. However, it is of considerable value in several areas of research for example, natural product isolation, heteroaromatic chemistry, porphyrin and related chemistry, and in the study of dyestuffs. The amount of material required is usually very small (fractions of a mg) since the extinction coefficients are usually large. The sample must be as pure as possible and is dissolved in the solvent of choice (usually spectroscopically pure ethanol). The concentration must be known accurately before extinction coefficients can be calculated, and will vary depending upon the type of chromophore. An estimate of the concentration to used can be made if the extinction coefficients of compounds similar to that being studied are available. If this data is not available make up a solution accurately and dilute it (accurately ) until a reasonable spectrum is obtained. 

A major difference between infrared and ultraviolet spectroscopy is in the concentrations required for assay In infrared spectroscopy as much as a 10% w/v solution of sample must be prepared. This means that the path length of the cells used in infrared must be very short, usually 0.025-0.1 mm (otherwise absorbance values would be too high). Another problem with infrared spectra is that the solvent used in the assay (usually chloroform or dichloromethane) also possesses chemical bonds that will absorb infrared radiation in some part of the spectrum, obscuring the absorption by the sample at these wavelengths. Samples are prepared in solution, in a mull or paste made with liquid paraffin (Nujol), or in a solid disc prepared by trituration with dry potassium bromide followed by compression in a hydraulic press. 

Rhenium dinitrogen phosphine and phosphite complexes give products through intramolecular C-H activation upon ultraviolet (UV) irradiation in hydrocarbon solvents. For example, the agostic complex 88 identified by IR spectroscopy decays to stereoisomeric cyclometallated products [Eq. (6.68)]. Analogous transformations were observed for phenoxy and phenyl derivatives. 

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