Spectra, ultraviolet

Determination of structural features. The ultraviolet spectrum has been of value in the determination of the structure of several vitamins. Thus the presence of an a-naphthoquinone system in vitamin K was first detected by this means. Also the 4-methylthiazole and the 2 5-dimethyl-6-aminopyridine system was first identified in vitamin Bj (thiamine), a- and /3-Ionones can be distinguished since the former contains two conjugated chromophores and the latter three conjugated chromophores. 

Davies and Warren" found that when 1,4-dimethylnaphthalene was treated with nitric acid in acetic anhydride, and the mixture was quenched after 34 hr, a pale yellow solid with an ultraviolet spectrum similar to that of a-nitro-naphthalene was produced. However, if the mixture was allowed to stand for 5 days, the product was i-methyl-4 nitromethylnaphthalene, in agreement with earlier findings. Davies and Warren suggested that the intermediate was 1,4-dimethyl-5 nitronaphthalene, which underwent acid catalysed rearrangement to the final product. Robinson pointed out that this is improbable, and suggested an alternative structure (iv) for the intermediate, together with a scheme for its formation from an adduct (ill) (analogous to l above) and its subsequent decomposition to the observed product. 

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). 

IP, isolated pure MI, matrix isolated GP, data from pure gas phase material CE, chemical evidence for existence TH, theoretical calculation XR, X-ray structure MW, microwave structure UV ultraviolet spectrum. 

The infrared spectrum of the l-methoxy-l,4-cyclohexadiene shows the absence of strong aromatic absorption at 1600 cm.the ultraviolet spectrum shows absence of absorption at 270 nm., indicating absence of the conjugated isomer. 

The purity of the 2-cyclohexenone may be assayed by gas chromatography on an 8 mm. x 215 cm. column heated to 125° and packed with di-(2-ethylhexyl) sebacate suspended on ground firebrick. This method of analysis indicates that the 3-cyclo-hexenone in the product amounts to no more than 3%. The fore-run from this fractional distillation contains substantial amounts of 2-cyclohexenone accompanied by ether, ethanol, and minor amounts of other lower-boiling impurities. Additional quantities of pure 2-cyclohexenone can be recovered by redistillation of this fore-run. The preparation of 2-cyclohexenone has been run on twice the scale described with no loss in yield. The ultraviolet spectrum of an ethanol solution of the 2-cyclohexenone obtained has a maximum at 226 m/i (s = 10,400). 

The product may be analyzed by gas chromatography on an 8 mm. x21S cm. column heated to 220-240° and packed with Dow-Corning Silicone Fluid No. 550 suspended on 50-80 mesh ground firebrick. The chromatogram obtained with this column exhibits a single major peak. The ultraviolet spectrum of an ethanol solution of the product has a maxium at 250 m>i (s = 17,200). 

P-Hydroxy-A-norpregn-3(5)-en-2-one (7) A solution of the hydroxy-methylene steroid (5) (24.8 g) dissolved in 240 ml of acetic acid and 240 ml of ethyl acetate is ozonized at — 10° with one molar equivalent of ozone. The resulting solution is diluted with 240 ml. of water and 60 ml of 30 % hydrogen peroxide and allowed to stand overnight. The solution is diluted with 1.5 liters of water and extracted with 3 x 700 ml portions of ethyl acetate. The combined extracts are washed with water, saturated sodium chloride solution, dried over sodium sulfate and concentrated to dryness under vacuum, leaving 23.4 g of a colorless amorphous residue of crude diacid. This material shows a maximum in the ultraviolet spectrum at 224 mp (s 6,400) indicating a 53 % yield of unsaturated acid (6). It is used without further purification. 

The simplest examples of this type of compound are enamines derived from the quinuclidine skeleton (67). The formulation of enamines of qflmuclidine in a inesomeric form would violate Bredt s rule. Actually, the ultraviolet spectrum of 2,3-benzoquinuclidine shows that there exists no interaction of aromatic ring tt electrons and the nitrogen-free electron pair (160,169). The overlap of the olefinic tt orbital and the lone pair orbital on nitrogen is precluded. 

Similar behavior can be observed even in the case of substituted quinuclideines 170). Neostrychnine (68) serves as an example of more complex compounds which show spectra differing from those of other enamines. The ultraviolet spectrum of this compound exhibits no batho-chromic shift and its basicity is considerably decreased 159,171,172) (pK in methylcellosolve at 20° is 3.8, whereas the analogous saturated compound has a pK under the same conditions of 7.45, and a compound with the double bond further removed, strychnine, has a pK of 7.37). As another example, the ultraviolet spectrum of trimethyl conkurchine (69) shows the same absorption maxima as a saturated tertiary amine (A in ether, about 213 m/i). 

The fugitive species SO was first identified by its ultraviolet spectrum in 1929 but it is thermodynamically unstable and decomposes completely in the gas phase in less than I s. It is formed by reduction of SOn with sulfur vapour in a glow discharge and its spectroscopic properties... 

According to Dobbie et the ultraviolet spectrum of cotarnine in dilute aqueous or alcoholic solution is identical with that of cotarnine chloride [(1), Ch instead of OH"], but in nonpolar solvents it is identical with that of hydrocotarnine (10a), 1-ethoxy-hydrocotarnine (10b), and cotarnine pseudocyanide (10c). This is in agreement with Decker s view of the structure of cotarnine and with the conclusions of Hantzsch and Kalb. Measurement of electrical conductivity in-... 

Fig, 1. Ultraviolet spectrum of quiuazoline in water. Solid line, neutral molecule dotted line, cation. 

The stabilizing influence in the hydrated cation is the amidinium resonance. If a solution of the cation is neutralized, a short-lived hydrated neutral molecule (4) (half-life 9 sec at pH 10) is obtained with an ultraviolet spectrum similar to that of the hydrated cation but shifted to longer wavelengths (5 m/ ). Supporting evidence can be derived from the anhydrous nature of the cation of 4-nitroiso-quinoline (pK 1.35), in which the nitro group has a similar electronic influence to that of the ring nitrogen atom N-I in quinazoline and where amidinium resonance is not possible. 

The structure of the unstable hydrated neutral molecule (4) was deduced from the similarity of its ultraviolet spectrum with that of tlie pseudo base (5), derived from (6) of known structure. This... 

The hydrated cation of quinazoline in dilute acid solution becomes dehydrated when the acidity of the solution is progressively increased. At Ho —4.3, the solution consists predominantly of the anhydrous cation with some anhydrous dication ( 7%). The ultraviolet spectrum of the anhydrous cation is similar to that of the neutral molecule (there is a small bathochromic shift) and it is also similar to that of quinazoline in anhydrous dichloroacetic acid. When the acid strength is further increased to Ho —9.4, the quinazoline dication is formed (pKa —5.5). 

Tetrahydroquinazolines are cyclic methylene diamines and are, therefore, readily hydrolyzed by acid to the corresponding o-aminobenzy]amines, 1,2,3,4-Tetrahydroquinazoline is a strong base (pi a 7.65), though it is not as strong as 3,4-dihydroquinazoline because of the absence of amidinium resonance in the cation. The ultraviolet spectrum is only slightly altered on protonation clearly indicating that it takes place on N-3. ... 

The electronic spectrum of a compound arises from its 7r-electron system which, to a first approximation, is unaffected by substitution of an alkyl group for a hydrogen atom. Thus, comparison of the ultraviolet spectrum of a potentially tautomeric compound with the spectra of both alkylated forms often indicates which tautomer predominates. For example, Fig. 1 shows that 4-mercaptopyridine exists predominantly as pyrid-4-thione. In favorable cases, i.e., when the spectra of the two alkylated forms are very different and/or there are appreciable amounts of both forms present at equilibrium, the tautomeric constant can be evaluated. By using this method, it was shown, for example, that 6-hydroxyquinoline exists essentially as such in ethanol but that it is in equilibrium with about 1% of the zwitterion form in aqueous solution (Fig. 2). 

Ultraviolet spectra have long been used to study systems of this type. In 1889, comparison of the ultraviolet spectrum of 2-hydroxy-quinoline with those of its O- and A -methylated derivatives led... 

Attempts have been made to deduce the structure of the predominant form of a potentially tautomeric compound from the shifts which occur in the ultraviolet spectrum of the compound in question on passing from neutral to basic or acidic solutions. The fact that no bathochromic shifts were observed for 2- and 4-hydroxy quinoline and 1-hydroxyisoquinoline under these conditions was taken as evidence that they existed in the oxo form [similar work on substituted quinol-4-ones led to no definite conclusions ]. A knowledge of the dissociation constants is essential to studies of this type, and the conclusions can, in any case, be only very tentative. A further dif-... 

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 ultraviolet spectrum of vitamin Be, or pyridoxine, measured in aqueous ethanol varies with the composition of the solvent indicating that this compound is in equilibrium with the zwitterion form 38. The equilibrium constant in pure water was obtained by extrapolation. Prior to this, equilibria which involved tautomers of type 39 had been suggested for vitamin Be, but see Section VI,A. In the case of pyridoxal, an additional equilibrium, 40 41, occurs (cf. Section VIII) other pyridoxal analogs have also been studied (Table II). 

Between 1951 and 1953 investigations by three English groups clearly demonstrated the preponderance of the oxo forms of pyrimidin-2- and -4-ones by comparing the ultraviolet spectra of these compounds with those of the N and 0-alkylated derivatives, The o-quinonoid form 91 (R = H) is favored by the evidence that A -methylation of the 6-methyl derivative of 89a does not cause a bathochromic shift in the ultraviolet spectrum (A -methylation of pyrid-4-one causes a bathochromic shift, but this is not observed for pyrid-2-one) The isomeric A -methyl derivatives of pyrimidin-4-ones [e.g., 91 (R = Me) and 90 (R = Me)] form similar cations (e.g., 92 and 93), and hence the equilibrium constant between... 

Shugar and FoxS " reported that 4-ethoxypyrimidin-2-one exists in the 0X0 form 102 since its ultraviolet spectrum is different from that of 103. They further claimed that the isomeric compound, 2-ethoxy-4-hydroxypyrimidine, existed in the hydroxy form (104) however, reexamination of the ultraviolet spectral data suggests that this unlikely conclusion may be incorrect, and the infrared spectrum of 104 does, indeed, show a carbonyl absorption band. 2-Methylthiopyrim-idin-4-one has been reported to exist in the hydroxy form, but this to appears unlikely. 

Early investigators adduced various kinds of chemical evidence in support of a monohydroxy-dioxo structure for barbituric acid (112) (a) reaction with diazomethane afforded a mono-O-methyl deriva- iye,i59,i6o barbituric acid and its 5-alkyl derivatives are much stronger acids than the 5,5-dialkyl derivatives, and (c) the 5-bromo and 5,5-dibromo derivatives have different chemical properties. - The early physical evidence also appeared to substantiate the monoenol structure, this formulation having been suggested for barbituric acid in 1926 on the basis of its ultraviolet spectrum and again in 1934, In the 1940 s, ultraviolet spectroscopic studies led to the suggestion of other monohydroxy and dihydroxy structures for barbituric acid, whereas its monoanion was assigned structure 113 (a clear distinction between ionization and tautomerism was not made in these papers). 

Pyrazin-2-one (124) has been shown to exist predominantly as such by comparison of its ultraviolet spectrum with those of the fixed alkylated derivatives and by its infrared spectrum. The pK measurements support this conclusion but cannot yield quantitative results since cations of a common type are not formed. ... 

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