Pyridine, ultraviolet spectrum

Further evidence for the presence of l-(4-pyridyl) pyridinium ion in the reaction mixtures was provided by a separation technique based on the precipitation of the l-(4-pyridyl)pyridinium ion in the presence of pyridine with Ph4B in a solution with a pH of 8. This procedure was necessary, since the ultraviolet spectrum of l-(4-pyridyl)pyridinium ion in a solution prepared by acid hydrolysis of the crude reaction products was completely obscured by the intense spectrum of pyridinium ion, which also was formed by the hydrolysis of the reaction products. However, as shown in Figure 3, the l-(4-pyridyl) pyridinium ion was easily identified after its separation as the tetraphenylborate(III) salt... 

Interatomic distances as calculated from the analysis of the rotational fine structure of the ultraviolet spectrum are C-C, 1.395 A C-N, 1.341A and C-H, 1.085 A.66 These are very similar to the bond lengths for pyridine which are C-2-C-3, 1.3945 A C-3-C-4, 1.3944 A and C-2-N, 1.3402 A. The C-N-C bond angle in pyrazine is 115° and the C-C-N bond angle 122.5°.56,67 A delocalization energy for pyrazine of ca. 18 kcal/mole is indicated from heats of combustion data.68 The C=N bond energy in 2,2,5,5-tetramethyl-2,5-dihydropyrazine has been calculated to be 130.3 kcal.58a... 

One other pyridine alkaloid has been detected in dendrobatid frogs. The structure of this minor alkaloid, noranabasamine (XIII), was established by proton and carbon-13 magnetic resonance spectroscopy (14). The ultraviolet spectrum was as follows X ,ax (CH3OH) 244 nm, e 11,000, 275 nm, e 10,000. The optical rotation, [a]o, was -14.4° (CH3OH). Anabasamine, a plant alkaloid, also is levorotatory, but it is unknown whether noranabasamine, now given a code number 239J, has the same 2S configuration. 

Compound 215 may be acetylated with acetic anhydride in pyridine to tetra-O-acetyl-5-thio-a-D-xylofuranose. Acetylation with sodium acetate and acetic anhydride gives, besides the a-form a smaller quantity of the levorotatory /3-D-tetraacetate also. Neither acetate shows an S-acetyl band at 230 nm in the ultraviolet spectrum, showing that the compounds have the thiopyranoid ring. ... 

However, in 2,3,5-trifluoro-6-hydroxypyrazine (47), the infrared spectrum of the solid was reported to show no absorption attributable to the carbonyl group, and its ultraviolet spectrum (in ethanol) was similar to that of 23,5-trifluoro-6-methoxypyrazine, thus indicating that it was a true hydroxy compound (851). A similar phenomenon has been observed in the pyridine (1087-1089) and pyrimidine (1090) series, but differs from observations in the pyridazine series (1091). 

As in the case of pyridine (185), the quaternization of thiazole induces a bathochromic shift of the ultraviolet absorption spectrum in ethanol the long wavelength maximum at 232.3 nm (3900) for thiazole moves to 240 nm (4200) for 3-methylthiazolium tosylate (186) (Table 1-19). 

Samples of the niobium (V) chloride or niobium (V) bromide-pyridine reaction mixtures were hydrolyzed in concentrated hydrochloric acid. Aliquots were diluted and neutralized with sodium carbonate to a pH of approximately 8. Sodium tetraphenylborate(III) solution then was added and a precipitate of l-(4-pyridyl) pyridinium tetraphenylborate(III) was produced. The precipitate was filtered and extracted with concentrated hydrochloric acid. The ultraviolet absorption spectrum of the extract is shown in Figure 3 for comparison with the spectrum of a known sample of l-(4-pyridyl) pyridinium dichloride in dilute hydrochloric acid. 

In the ultraviolet photoelectron spectrum, the most readily ionized level of pyridine is the nonbonding orbital (with contributions from the o -framework). The three diazines show two lone-pair levels, with the greatest splitting in the case of pyridazine but considerable also in pyrimidine and pyrazine. These long-distance splittings are attributed to both through-space and through-bond interactions, particularly the latter. 

Infrared spectroscopy is an important technique for studying acidity. Acidic OH groups can be studied directly. Probe molecules such as pyridine may be used to study both Bronsted and Lewis acidity since two forms of adsorbed probes are easily distinguished by their infrared spectra. Quantitative infrared spectroscopy may be performed by measuring the spectrum of acidic OH or probes adsorbed on thin, self-supporting wafers of the acidic solid. Other spectroscopic methods which may provide information in specific cases include Fourier Transform Raman spectroscopy, electron spin resonance spectroscopy, ultraviolet spectroscopy, and nuclear magnetic resonance spectroscopy. 

Bis(4-imino-2-pentanonato)nickel(II) crystallizes from a benzene-petroleum ether mixture as dark red needles or as fine red-orange needles. The two forms have identical melting points. The compound is very soluble in chloroform, but less soluble in benzene, pyridine, and carbon tetrachloride, and very insoluble in water. The compound crystallizes from pyridine without adduct formation. The compound is diamagnetic and apparently has the trans configuration. Partial resolution in optically active fractions has been achieved by means of a chromatographic technique. Molecular weight determinations indicate that the compound is monomeric in chloroform and benzene solution. The visible absorption spectrum of this compound in chloroform is characterized by a band centered at 552 m/i (e = 43). The ultraviolet absorption maxima for solutions in 1 1 benzene-petroleum ether occur at 298, 348, and 364 m x (e = 4150, 4760, and 4460, respectively). ... 

Ultraviolet absorption data in either ethanol or cyclohexane have been tabulated for ten derivatives of the ring system. In ethanol, s-triazolo[4,3-a]pyrazine has three main bands, at 206, 253, and 292 nm, and the spectrum is very similar to that of s-triazolo[4,3-a]pyrimidine and the corresponding fused pyridine. Replacement of CH by N in the pyridine ring of s-triazolo[4,3-a]pyridine results in only small shifts in the... 

Figure 7.20 includes the ultraviolet spectra of pyridine, quinoline, and isoquinoHne. You may wish to compare the spectrum of pyridine with that of benzene (Fig. 7.18) and the spectra of quinoline and isoquinoHne with the spectrum of naphthalene (Fig. 7.19). 

Other Spectral Properties of Pyridine. Also used in characterizing heterocycles are ultraviolet (UV) and infrared (IR) spectroscopy. The UV spectrum of pyridine is much like that of benzene... 

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