Spectroscopy, Ultraviolet

The ultraviolet spectra of coals, examined as suspensions in potassium bromide, show au absorption band at 2650 A which becomes more pronounced with increasing rank of the coal. This band has been assigned to aromatic nuclei and, on the basis of data obtained from comparison between the specific extinction coefficients of coal and those of standard condensed aromatic compounds, it has been concluded that the concentration of aromatic systems in coal is lower than had previously been believed. 

The ultraviolet spectrum of 3,6,6-trimethylcyclohex-2-en-l-one, is shown below. The concentration is 1.486 x 10 g ml in ethanol and the path length is 1.0 cm. Calculate e and compare with the value predicted by 

Empirical Parameters for ir — ir Transitions of Conjugated Systems in Ethanol (Woodward s Rules) 

Xmax specific degree of absorbance associated with it. The absorbance at a particular wavelength is 

For each alkyl group substituted on the a, 3- insaturated ketone, a correction of + 10 nm must be made according to Woodward s Rules. Therefore, the calculated X 

The molar absorptivity (e) may be calculated by using the aforementioned equation 

Ultraviolet spectroscopy tells us about conjugation in a molecule. It uses the highest energy radiation of any of the techniques described in this book radiation at 200 nanometres is equivalent to 595 kilojoules per mole. Absorption of this energy raises an electron from a bonding orbital to an antibonding orbital. 

Almost all ultraviolet spectroscopy involves radiation from 200 nm to 400 nm (1 nm = 10 cm = 10 A = 1 m/x). The region is often extended from 400 to 800 nm to cover the visible spectrum. The region below 200 nm, often referred to as the far ultraviolet, requires special equipment, since oxygen absorbs just below 200 nm. 

Diagram 4.1 Orbital changes when a C atom and an H atom form a C H bond 

When we consider a molecule which contains oxygen, we have to consider the effect of lone pairs of electrons, as well as cr and n bonding electrons. Thus, for the C=0 bond of formaldehyde, H2C=0, we can construct the diagram shown in Diagram 4.2. The n—transition results from the absorption of energy at 270 nm, so this transition is found in the ultraviolet spectroscopy range. It is the only transition not involving d orbitals found in the ultraviolet 

Ultraviolet-visible spectroscopy gives information on the extent, shape, and substituents of r-conjugation in molecules [2,3]. It is a measure of the energy gaps between the electronic ground and excited states. Intensity of the peaks is most often used to quantitate changes in concentration and this technique is used to track the progress of a reaction, rather than to identify structural features in molecules. 

Prelab Exercise Predict the appearance of the ultraviolet spectrum of 1 -bufylamine in acid, of methoxybenzene in acid and base, and of benzoic acid in acid and base. 

By comparison with infrared spectra and nmr spectra, uv spectra are fairly featureless (Fig. 2). This condition results as molecules in a number of different vibrational states undergo the same electronic transition, to produce a band spectrum instead of a line spectrum. 

U nlike ir spectroscopy, ultraviolet spectroscopy lends itself to precise quantitative analysis of substances. The intensity of an absorption band is usually given by the molar extinction coefficient e, which, according to the Beer-Lambert Law, is equal to the absorbance A, divided by the product of the molar concentration c, and the path length /, in centimeters. 

The wavelength of maximum absorption (the tip of the peak) is given by A ,ax- Because uv spectra are so featureless it is common practice to describe a spectrum like that of cholesta-3,5-diene (Fig. 2) as A ax 234 nm (e = 20,000), and not bother to reproduce the actual spectrum. 

The extinction coefficients of conjugated dienes and enones are in the range 10,000-20,000, so only very dilute solutions are needed for spectra. In the example of Fig. 2 the absorbance at the tip of the peak. A, is 1.2, and the path length is the usual 1 cm so the molar concentration needed for this spectrum is 6 x 10 mole per liter. 

One of the simplest measurements is the plot of ultraviolet absorbance of a system versus temperature. The UV absorbance of both double and single stranded polynucleotides increases with temperature for a double helix this hyperchromicity is attributed to disruption of the ordered state and elimination of the stacking interactions of the base pair chromo-phores. 

A useful and important parameter, readily obtained from these measurements, is the helix-to-coil transition temperature, the temperature at which half of the helical structure is lost or altered. The transition, also called melting or denaturation, is highly cooperative and is also pH dependent. Indeed, the transition occurs within 0.1 pH unit instead of several units as might be expected from a consideration of the titration values of the individual bases. Below the transition temperature separated complementary strands automatically reassociate (renaturation). 

Dienes and trienes If the compound is suspected to be a conjugated or substituted diene, its wavelength of maximum absorption can be predicted with the help of Table 1. 1. To be able to use this table, one must first learn to recognize different types of dienes, conjugations, double bonds, etc. These are as follows  

A cyclic diene for example, cyclohexadiene, cyclohepta 1,3- diene, etc. 

A semicyclic diene one of the double bonds forms part of a ring and the other is exocyclic, or outside the ring. When only one of of the two sp hybridized carbons of a double bond is a part of the ring under 

A homoannular diene is one in which the two double bonds are conjugated and are in a single ring. 

Note that both double bonds are exocyclic to ring B. 

The UV spectra of several 5-aminoimidazoles (180) have been examined in ethanol or acetonitrile solution (Table IX). A change of pH from neutral to either basic or acidic pH results in a shift of maxima. For example, compound (180 R = R2 = Me) in ethanolic solution with 1 M HC1 gave two maxima at 210 nm (e 1470) and 243 nm (e 2040), and in ethanolic solution with 1 M NaOH gave one maxima at 216 nm (e 10720) (87TH1). Also, AIRs (180 R1 = ribonucleoside, R2 = H) displayed a single maxima 

Thiazole shows a first absorption band assigned to a type 

The ultraviolet spectra of these compounds are similar to those of trans stilbene or of 2- and 4-stilbazole. The effect on the ultraviolet spectrum of various substituents have been found to parallel in many respects the efiects produced by the corresponding group in derivatives of aromatic hydrocarbons (142). 

Arylthiazoles substituted by functional groups follow the same pattern as aromatic hydrocarbons. 

For nonsubstituted phenyl thiazoles or for alkylarylthiazoles, one of the problems investigated is the determination of the angle between the aryl and the thiazole rings. In the case of 4,5-diphenylthiazole the problem is complicated by the interaction of the two phenyl rings (126). 

Calculations show that the deviation from planarity leads to greater conformational stability for the phenylthiazoles (143, 145). In particular, the potential energy minimum is achieved at a twist angle of about 30° for 4-phenylthiazole, 40° for 2-phenylthiazole, and 45° for 5-phenylthiazole. 

Structure of Five-membered Rings with One Heteroatom 

Solvent EtOH EtOH, MeOH EtOH EtOH n-hexane 

71PMH(3)79 71PMH(3)79, 71T245 58AK(13)239, 58SA350 58G453 72JCS(P1)199 

Annelation increases the complexity of the spectra just as it does in the carbocyclic series, and the spectra are not unlike those of the aromatic carbocycle obtained by formally replacing the heteroatom by two aromatic carbon atoms (—CH=CH—). Although quantitatively less marked, the same trend for the longest wavelength band to undergo a bathochromic shift in the heteroatom sequence O NH S Se Te is discernible in the spectra of the benzo[Z ] heterocycles (Table 17). As might perhaps have been anticipated, the effect of the fusion of a second benzenoid ring on to these heterocycles is to reduce further the differences in their spectroscopic properties (cf. Table 18). The absorption of the benzo[c] 

Tandem mass spectrometry is also developing into an important analytical method for application to coal-derived materials (Wood, 1987). The analysis of heteroatom ring species and hydrocarbon species in coal-derived liquids offers indications of the location of the heteroatoms in, or on, ring systems, as well as indications of the hydrocarbon systems. 

Application of gas chromatographic/mass spectrometric analysis to acidic/basic subfractions of coal-derived asphaltenes has led to the conclusion that the asphaltenes are made up of one-ring and/or two-ring aromatic units that are linked by methylene chains as well as by functional groups (Koplick et al., 1984). Projection of this finding to coal itself is of interest only if it can be assumed that the intemuclear bonds withstood the high temperatures and were not formed as a result of secondary and tertiary (etc.) reaction. In short, the question relates to the relationship of the structural types in the asphaltenes to those in the original coal. 

in the absence of specific test methods for coal, ultraviolet spectroscopic investigations must rely on investigations applied to other substances with the criteria of sample handling and sample preparation followed assiduously. The practices to be used for recording spectra (ASTM E-169) provide general information on the techniques most often used in ultraviolet and visible quantitative analysis. The purpose is to render unnecessary the repetition of these descriptions of techniques in individual methods for quantitative analysis. 

One particular test method (ASTM D-2008) covers measurement of the ultraviolet absorption of a variety of petroleum products covers, or the absorbtivity of liquids and solids, or both, at wavelengths in the region 220 to 400 nm. Use of this test method implies that the conditions of measurement (wavelength, solvent if used, sample path length, and sample concentration) are specified by reference to one of the examples of the application of this test method or by a statement of other conditions of measurement. 

The rule of thumb for UV Is 30 to 40 nm increase for each additional conjugated double bond, and a 5 nm increase for each additional alkyl group. 

The molar absorptivity is a measure of how strongly the sample absorbs light at a particular wavelength. It is probably easiest to think of it mathematically as c = A/cl. 

UV spectra lack detail. Samples must be extremely pure or the spectrum is obscured. To the right is a UV spectrum of 2-mcthyl-1,3-butadiene dissolved in methanol. The methyl group increases the absorption wavelength slightly. The methanol solvent makes no contribution to the spectrum. Spcetra are typically not printed, but instead given as lists. The spectrum to the right would be listed as  

Carbonyls, compounds with carbon-oxygen double bonds, also absorb light in the UV region. For instance, acetone has a broad absorption peak at 280 nm. In this example, the electron can be excited from an unshared pair into a nonbonding -orbital, (ji 

A heteroannular diene is a conjugated system in which the two double bonds belong to two different rings. However, these double bonds are also exocyclic, one of them being exo-to ring A and the other exo-to ring B. 

Polyenes The above rules (Table 1.1) holds fairly well for unsaturated compounds containing up to four conjugated double bonds. However, for systems of extended conjugation, such as those found in carotenoid pigments, Fieser and Kuhn have suggested equations to calculate the basic Xmax and e of UV absorption. 

The exact amount of UV light absorbed at a particular wavelength is expressed as the compound s molar absorptivity or molar extinction coefficient (e). This is a good estimate of the efficiency of light absorption and is calculated from the absorbance of light, which is derived from the Beer-Lambert law. 

A = absorbance W Iq = intensity of incident light striking sample 

7T Antibonding (double bonds) n Non-bonding (e.g. lone pair) 7T Bonding (double bonds) 

The 3//-pyrroles generally absorb at approximately 20 nm longer wavelength than the 2H analogues (Tables 29 and 30). Both the 2H- and 3//-pyrrole nuclei show bathochromic shifts of 20 nm when acidified, accompanied in the 2H series by a substantial increase in log e b-90MI 201-02 . 

The absorption spectra of a number of 3//-pyrrolizines have been collected in Table 31 84AHC(37)l . The parent compound and some simple derivatives show two major bands at 

Pyrroles and their Benzo Derivatives Structure Table 26 UV spectra for pyrrole and mono-substituted derivatives. 

UV spectral data for a variety of isoindoles is collected in Table 35. Most of the reported isoindoles have at least two common absorption bands the first around 320-370 nm, which arises owing to the orthoquinoid-like structure 87KGS1629 , and the second a more intense band anywhere from 210 to 230 nm. 

This technique has found very limited applications in the direct analysis of additives in polymers. 

Soucek and Jelinkova [27] have also used this differential principle to determine in polypropylene (PP) two antioxidants (2,6-di- er -butyl 4 methylphenol and 4-substituted 2,6-xylenol) which have virtually identical UV absorption spectra in the absence of alkali. The antioxidants can be distinguished in alkaline medium, where 4-substituted 2,6-xylenol forms phosphonate readily, thus allowing the ntilisation of the bathochromic shift for its determination. The use of derivative spectroscopy reduces light scattering and matrix interferences when extracts from PP samples are measured. 

Lutzen and co-workers [28] describe an in-line monitoring, UV method for the determination of polymer additives such as thermal and UV stabilisers and antioxidants in polymers. 

Thermal UV spectroscopy has been used to identify and determine organic and inorganic pigments in polymers. 

Albarino [30] demonstrated the feasibility of quantitative UV analysis of additives in PE at temperatures above the polymer melting point where the crystallites, which account for much of the scattering, are eliminated. Greater sample thickness and analytical sensitivity are possible compared to analysis of solid samples at room temperature. In this work, sample thickness was controlled by brass shims held between Suprasil grade silica windows (Heraeus Amersil, Inc.) by a faceplate bolted to the cell body. 

Further details of instrumentation are given in Section 1.12 and Appendix 1. 

Straightforward UV spectroscopy is liable to be in error owing to interference by other highly absorbing impurities that may be present in the sample [11-13]. Interference by such impurities in direct UV spectroscopy has been overcome or minimised by selective solvent extraction or by chromatography. Flowever, within prescribed limits UV spectroscopy is of use and, as an example [14-18], procedures have been developed for the determination of lonol and of Santonox R in polyolefins. 

Organic and inorganic pigments are used for coloration of polymers, polymer films, and polymer coatings on metal containers. Vapour-phase UV absorption spectrometry at 200 nm has been used [19] to identify such pigments. In this method powdered samples are directly vaporised in a heated graphite atomiser. Thermal UV (TUV) profiles of 

Where might the following compound have IR absorptions  

On irradiation with ultraviolet light [hv], buta-1,3-diene absorbs energy and a v electron is promoted from the highest occupied molecular orbital, or HOMO, to the lowest unoccupied molecular orbital, or LUMO. Since the electron is promoted from a bonding tt molecular orbital to an antibonding 

FIGURE 10.19 Ultraviolet irradiation of buta-i,3-diene results in promotion of an electron from ip2, the highest occupied molecular orbital (HOMO), to 1//3, the lowest unoccupied molecular orbital (LUMO). 

The amount of UV light ahsorhed is expressed as the sample s molar absorptivity (e), defined hy the equation 

Molar absorptivity is a physical constant, characteristic of the particular substance being observed and thus characteristic of the particular it electron system in the molecule. Typical values for conjugated dienes are in the range e = 10,000 to 25,000. Note that the units are usually dropped. 

1 Stability of Conjugated Dienes Molecular Orbital Theory 

2 Electrophilic Additions to Conjugated Dienes Allylic Carbocations 

7 Structure Determination in Conjugated Systems Ultraviolet Spectroscopy 

The unsaturated compounds we looked at in Chapters 7 and 8 had only one double bond, but many compounds have numerous sites of imsaturation. If the different unsaturations are well separated in a molecule, they react independently, but if they re close together, they may interact with one another. In particular, compounds that have alternating single and double bonds—so-called conjugated compoimds—have some distinctive characteristics. The conjugated diene 1,3-butadiene, for instance, has some properties quite different from those of the nonconjugated 1,4-pentadiene. 

3-Butadiene (conjugated alternating double and single bonds) 

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As the medium is still further diluted, until nitronium ion is not detectable, the second-order rate coefficient decreases by a factor of about 10 for each decrease of 10% in the concentration of the sulphuric acid (figs. 2.1, 2.3, 2.4). The active electrophile under these conditions is not molecular nitric acid because the variation in the rate is not similar to the correspondii chaise in the concentration of this species, determined by ultraviolet spectroscopy or measurements of the vapour pressure. " ... 

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),... 

Quantitative determination is also possible by ultraviolet spectroscopy with the intense absorption at 320 nm (94). They may also be characterized electrochemically with a mercury electrode (95),... 

Physical properties of A-4-thiazoline-2-one and derivatives have received less attention than those of A-4-thiazoline-2-thiones. For the protomeric equilibrium, data obtained by infrared spectroscopy favors fbrm 51a in chloroform (55, 96, 887) and in the solid state (36. 97. 98) (Scheme 23). The same structural preference is suggested by the ultraviolet spectroscopy studies of Sheinker (98), despite the fact that previous studie.s in methanol (36) suggested the presence of both 51a and... 

The pK s of some 2-substituted 4-hydroxythiazoles have been determined by ultraviolet spectroscopy (403) and by potentiometry (419). They range between 6.65 and 6-85 pK. units. 

Humphlett and Lamon (522) have recently studied the intermediary compounds of this reaction and have shown with the help of infrared and ultraviolet spectroscopy that 176 was not present in the reaction mixture (Scheme 90) instead, a compound containing an hydroxyl radical and not a carbonyl function was present (Scheme 91). 

Chloroacetone, phenacylbromide, a-bromoisobutyrophenone, 3-bromo-3-methyl-2-butanone, 1 -alkylsulfonyl-3-bromo-2-propanone, and ethyl-y-chloroacetoacetate give with ammonium dithiocarbamate the corresponding 4-hydroxythiazolidine-2-thiones (177), which have a characteristic absorption between 273 and 279 nm. Dehydration by heating with dilute HCl can be followed by ultraviolet spectroscopy because the products formed (175) absorb at 315 to 340 nm. 

Acrylonitrile has been characterized using infrared, Raman, and ultraviolet spectroscopies, electron diffraction, and mass spectroscopy (10—18). 

Microscopy (qv) plays a key role in examining trace evidence owing to the small size of the evidence and a desire to use nondestmctive testing (qv) techniques whenever possible. Polarizing light microscopy (43,44) is a method of choice for crystalline materials. Microscopy and microchemical analysis techniques (45,46) work well on small samples, are relatively nondestmctive, and are fast. Evidence such as sod, minerals, synthetic fibers, explosive debris, foodstuff, cosmetics (qv), and the like, lend themselves to this technique as do comparison microscopy, refractive index, and density comparisons with known specimens. Other microscopic procedures involving infrared, visible, and ultraviolet spectroscopy (qv) also are used to examine many types of trace evidence. 

Ref. 277 unless otherwise noted gc = gas chromatography hplc = high pressure Hquid chromatography ir = infrared spectroscopy uv = ultraviolet spectroscopy glc = ga sliquid chromatography eia = enzyme immunoassay vis = visible spectroscopy. 

J. A. R. Sampson, Techniques of Vacuum Ultraviolet Spectroscopy,JohnWHey 8c Sons, Inc., New York, 1967. 

A. N. Zaidef and E. Ya. Shreider, Vacuum Ultraviolet Spectroscopy, Humphrey Science PubHshers, Ann Arbor, Mich., 1970. 

The most powerful method for stmcture elucidation of steroid compounds during the classical period of steroid chemistry (- 1940 1950s) was ir-spectroscopy. As with the ultraviolet spectra, data collected on the infrared spectra of steroids are available in several books, spectmm atiases, and review articles (265,266). Unlike ultraviolet spectroscopy, even the least substituted steroid derivatives are relatively rich in characteristic absorption bands in infrared spectroscopy (264). 

Modem analytical techniques have been developed for complete characteri2ation and evaluation of a wide variety of sulfonic acids and sulfonates. The analytical methods for free sulfonic acids and sulfonate salts have been compiled (28). Titration is the most straightforward method of evaluating sulfonic acids produced on either a laboratory or an iadustrial scale (29,30). Spectroscopic methods for sulfonic acid analysis iaclude ultraviolet spectroscopy, iafrared spectroscopy, and and nmr spectroscopy (31). Chromatographic separation techniques, such as gc and gc/ms, are not used for free... 

Ubichromenol synthesis, 3, 752 Ugi reaction, 5, 830 Uliginosin B, bromo-molecular dimensions, 3, 621 Ullman and Fetvadjian synthesis quinolines, 2, 477 Ullman synthesis acridines, 2, 470-benzacridines, 2, 470 Ultraviolet light absorbers in polymers, 1, 397-398 Ultraviolet spectroscopy heterocyclic compounds reviews, 1, 78... 

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. 

Greater range of detection systems to which the desorbed gas can be subjected (e.g. chromatography, infra-red and ultraviolet spectroscopy, colorimetry) Limitations Certain resins undergo degradation even below 250°C Test sample may be thermally unstable Not all compounds readily desorb ... 

Further aspects of the reaction of aromatic tertiary hydroxyl amines have been examined by more sophisticated techniques [49]. 2-Methyl-2-nitrosopropane was used as a radical trap, and the endgroups on PMMA resulting from its addition were detectable by ultraviolet spectroscopy. Electron spin resonance results on the same system have also been reported [50]. 

The presence of iminium salts can be detected by chemical means or by spectroscopic methods. The chemical means of detecting iminium salts are reactions with nucleophiles and are the subject of this review. The spectroscopic methods are more useful for rapid identification because with the large number of model compounds available now the spectroscopic methods are fast and reliable. The two methods that are used primarily are infrared and nuclear magnetic resonance spectroscopy. Some attempts have been made to determine the presence of iminium salts by ultraviolet spectroscopy, but these are not definitive as yet (14,25). 

These structural problems are also insoluble by physical methods alone. The infrared spectrum often gives an unambiguous decision about the structure in the solid state the characteristic bands of the carbonyl or the hydroxyl group decided whether the compound in question is a carbinolamine or an amino-aldehyde. However, tautomeric equilibria occur only in solution or in the liquid or gaseous states. Neither infrared nor ultraviolet spectroscopy are sufficiently sensitive to investigate equilibria in which the concentration of one of the isomers is very small but is still not negligible with respect to the chemical reaction. 

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