Pyrazine ultraviolet spectra

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. ... 

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... 

The ultraviolet spectrum of pyrazine in cyclohexane shows maxima at 260 nm (corresponding to a tt-tt transition) and 328 nm (corresponding to a n-7r transition) in each case with vibrational fine structure the coefficients of molecular extinction are 5600 and 1040, respectively.78,79 Substitution of halogen has a bathochromic effect on the ultraviolet spectrum of pyrazine.80 A useful index of the ultraviolet and visible spectra of pyrazine derivatives is available for the period from 1955 to 1963.81 The far-ultraviolet spectrum of pyrazine... 

The 1,2,5-thiadiazole nucleus is extremely weakly basic and exhibits an ultraviolet spectrum in concentrated hydrochloric acid identical to that in water. A bathochromic shift of 9 m a in the spectrum taken in 96 % sulfuric acid indicates some protonation of the ring in that solvent. 1,2,5-Thiadiazole is a much weaker base than pyrazine (piTa 0.6) and probably has a pAa well below zero. The very low basicity of pyrazine was explained by Albert et ai. as being related to the... 

Aminopyrazine is a yellowish crystalline solid with m.p. 118-120 (420). It is a weak base with reported pAj, values of 3.14 (123) and 2.96 (821) (others are recorded in Chapter X). The similarity in the ultraviolet spectra (in water) of the neutral molecules of 2-amino-, 2-methylamino-, and 2-dimethylaminopyrazines, and of their monocations, and their similar basic strengths (2.96,3.42,3.27) (821) supports the conclusion that in aqueous solution 2-aminopyrazine exists mainly in the amino form (20) (821). The ultraviolet spectrum of 2-aminopyrazine cation differs from that of the neutral molecule of pyrazine, thus indicating that protonation does not take place at the extranuclear nitrogen atom (821) 2-aminopyrazine in fact protonates at Ni (see below). 

The spectra of pyrazine, pyrimidine, and pyridazine, all in cyclohexane, are compared by Albert (1462). They show two bands with associated fine structure, the peaks for pyrazine center around 260 and 328 nm. The near-ultraviolet spectrum of pyrazine has been measured in several solvents [and at various pH values (1463)] and the transitions assigned (1474, 1482). The diffuse system at 260 nm has been attributed to rr->-7r transitions whereas the sharp system at 328 nm has been ascribed to n rr transitions (1467, 1474). Semiempirical calculations have been made on the electronic structure of pyrazine with reference to its -> rr transition (1483-1486), calculations have been made of transition energies in N-... 

Molecular orbital calculations have been performed on pyrido[3,4-fojpyrazine using several approaches. Along with pyrido[2,3- )]pyrazine and other azanaphthalenes, these calculations provide good estimates of the transition energies and intensities of the ultraviolet spectrum of the parent heterocycle. The infrared spectra of the parent and other azanaphthalenes have been determined and discussed. Infrared data for several a- and y-hydroxy compounds confirm their existence, in solution and in the solid state, as oxo compounds. The visible and ultraviolet spectrum of the parent has been analyzed. In 95% ethanol the principal peaks are at 232 and 309 nm. The spectrum is substantially the same as that of quinoxaline, and analogous derivatives of the two heterocycles also possess similar spectra. Comparative ultraviolet data have been used to differentiate between amino derivatives of pyrido[3,4-b]pyrazine and pyrido[2,3-fc]pyrazine and between isomeric dihydro and tetrahydro derivatives of the [3,4-b]system. PMR data are available for certain... 

LCAO calculations have been performed on this heterocycle, and the resulting electron density map suggests that the 4-position is the most susceptible to nucleophilic substitution. The ultraviolet spectrum in methanol of imidazo[l,2-a]quinoxaline has a maximum at 315 nm and a minimum at 266 nm. The compound is a moderately weak base with a pKa (50% ethanol) of 4.6. No evidence for the site of protonation has been published, although the 3-position seems favored both from the theoretical calculations and by comparison with imidazo[l,2-a]pyrazines (see Chapter XXI). 

Amino-pyrazines and -pyridazines have been shown to exist predominantly in the amino form by infrared spectroscopic studies (cf. Table VI). Ultraviolet spectral data have been interpreted to indicate that 4-aminocinnoline exists predominantly in the imino form 256, but this conclusion, which was based on comparison of its spectrum with those of cinnolin-4-one and 4-ethoxycinnoline, is probably incorrect. Ultraviolet spectroscopic data strongly support the predominance of amino structures for 2-aminopyrazine (257) and 2-aminoquin-oxaline how ever, the former compound was at first erroneously concluded to exist in the imino form from ultraviolet spectral evidence. Isolation of two isomers of 2-amino-8-dimethylamino-3-methylphenazine, assigned the amino and imino structures 258 and 259, respectively, has been claimed, but it is very unlikely that these assignments are correct. 

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. 

Ultraviolet spectra of numerous pyrazines have been recorded, but in many cases without regard to the effects of ionization, and in various solvents. All pyrazines are basic and thus have both neutral (e.g., 5) and cationic (e.g., 6) forms. Pyrazines with a substituent containing an ionizable hydrogen, such as a carboxy, hydroxy, or mercapto group, may also exist in the anionic form (e.g., 7), the tautomeric neutral form (e.g., 2, R = H), a potentially zwitterionic form (8, R = H) or an isomeric cationic form (e.g., 9, R = H). Many published spectra are in fact of mixed ionic species to determine the spectrum of each ionic form it is necessary to measure its spectrum in a solution buffered at least two units above or below the pAa value (or values) of the substance. In nonaqueous solvents, the neutral (uncharged) species are favored. The ultraviolet spectra of pure species may then serve to characterize the pyrazine, may permit the correlation of spectra with structure, and may be used in quantitative determinations. 

PMR spectrum (in CDCI3) of imidazo[l,5-a]pyrazine and its 7-oxide are give in Figure 1. Ultraviolet and PMR details have also been reported for the 3-methyl, the 8-amino, and the 3-(2-pyrazinyl) ° compounds. More recently PMR data have been published for a variety of 3-substituted compounds. Several substituted imidazo[l,5-a]pyrazines have been studied using NMR spectroscopy. " ... 

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... 

The Pariser-Parr-Pople method, self-consistent molecular orbital calculations, and the Huckel approach have been applied to the theoretical calculation of transition energies and intensities in the ultraviolet spectra of pyrido[2,3-ft]pyrazine and other azanaphthalenes. Good agreement with experimental data was obtained. The infrared spectra of the parent heterocycle" and its 8-hydroxy derivative have been discussed the 0X0 form is preferred to the hydroxy form both in chloroform solution and in the solid state. As with other a-hydroxyazines, the oxo form is also favored in the 2-, 3-, and 6-hydroxy compounds. Ultraviolet spectra have been measured for the parent heterocycle and various substituted compounds. The spectrum of the parent is substantially similar to other azanaphthalenes. In ethanol the principal bands are at ca. 260,300, and 350 nm. Both the infrared and ultraviolet spectra of about 60 derivatives of the ring system have been assembled. ... 

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