Ultraviolet Spectra of Pyrazines

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. 

Published ultraviolet spectra of some simple pyrazines, measured in the solvent specified, are recorded in Table X.2. References for the table are 86, 89, 98,108, 

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- 

The connection between electronic structure and u.v. spectra of pyrazine and protonated pyrazine has been examined the electronic distributions were calculated using a Pariser-Parr-Pople method employing an electrostatic model (proton bounded to the unshared electron pair of the nitrogen atom by electrostatic forces) (1493). The electronic absorption of a number of alkyl-substituted pyrazines in neutral and in acid solvents have also been measured (1476). 

Whereas pyrazine and alkylpyrazines exhibit an absorption at about 260-270 nm in water, their mono A -oxides show two peaks, one about 215 nm, and the other about 260 nm, which are characteristic of the presence of an A -oxide function (838). The u.v. absorption maxima of pyrazine A -oxide moved to shorter wavelengths from that in heptane as the polarity was increased in ethanol and water (1481). Band analyses of pyrazine mono- and di-A -oxides have been carried out (627). 

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