Pyrazine ionization

Protonation of pyrazine A-oxides takes place at the unsubstituted ring nitrogen as revealed by examination of their UV spectra and ionization constants in water. The same holds for unsubstituted quinoxaline A-oxide and the 3-amino derivative. Pyrazine and quinoxaline di-A-oxides are protonated at one A-oxide oxygen atom (74KGS1554). 

Example The ionization spectra at the first ionization threshold of pyrazine as obtained by PI, MATI and ZEKE-PES are clearly different (Fig. 2.21). [92] The... 

Figure 19 Ionization energies of the upper orbitals of benzene, pyridine and pyridazine. The assignments and correlations are tentative. The point group of pyrazine is D2h and that of pyridine is C2 (Reproduced with permission from (b-78mi204io ...

With heterocyclic compounds the determination of the first ionization potential, corresponding to excitation of an electron from the highest occupied 77-molecular orbital, is complicated considerably by the fact that excitation from an n-orbital often precedes that from a 7T-orbital the difference in the energies of these two orbitals is small in the case of pyridine and pyrazine.83 There are two pieces of evidence which indicate that for pyridine-like heterocycles and their aza analogues the excitation in question is from an w-orbital first, the parallelism between ionization potentials and basicities (in agreement with an SCF treatment84-860), and, second, the very small differences... 

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. 

Binding Study. Concentrations of soy and diffusate were equivalent to ground beef fortified with 5-20% hydrated soy protein. The diffusate/soy mixtures were heated under the same conditions and generated pyrazines were separated and quantitated by gas chromatography with an alkali flame ionization detector (afid). Triplicate results were averaged for each level of vegetable protein and the concentration of pyrazines generated in the presence of soy proteins was determined relative to the control, diffusate heated neat. 

The lowest ionization potential of pyrazine and related nitrogen-containing heteroaromatic compounds has been measured. The experimental value of 9.29 eV is very similar to that of benzene (9.25 eV) and pyrimidine (9.35 eV) and is thought to be due to ionization from the highest occupied 77- orbital. Subsequent calculations have supported this assignment.04,65 (For a discussion of the relative ordering of 7r, a, and n orbitals, see Turner et ol.65a)... 

Comparison of the ultraviolet absorption and ionization constants of aminopyrazine, methylaminopyrazine, and dimethylaminopyra-zine indicates that aminopyrazine exists as such and not in the tautomeric imino form.147 Aminopyrazines, like their pyridine analogs, form diazonium salts, which readily decompose to the corresponding pyrazinones. For example, nitrous acid treatment of aminopyrazine318 and 2-aminopyrazine-5-carboxylic acid210 gives the corresponding pyrazinones in 30 and 59% yield, respectively. The diazonium salt from aminopyrazine cannot be converted into bromo-pyrazine under the conditions of the Sandmeyer reaction. 

Ionization. Ionization constants for pyrazine and several C-methylated derivatives have been redetermined for possible correlation with the polarographic half-wave potentials of the same compounds and their 1-alkyl iodides.1373... 

Ionization constants. Known pKt values for pyrazine and six methylated derivatives showed good correlation with newly calculated electron densities on the nitrogen atoms 1052 such a correlation was also observed for 1,4-di-methylpiperazine and a series of (distantly) related m- and p-bis(dimethy-laminomethyl)benzene derivatives.1039... 

Proton magnetic resonance measurements in dimethyl sulfoxide indicate that 2-aminopyrazine and its 3-methyl derivative exist predominantly in the amino form (979, 1086), in agreement with theoretical and experimental electronic spectra (1083). Proton magnetic resonance analysis of the methylation of 2-aminopyrazine with methyl iodide in dimethyl sulfoxide at room temperature showed it to be methylated 8.8 times as fast as pyrazine to give both 3-amino-l-methyl- and 2-amino-l -methylpyrazinium iodides in the ratio 2.9 I (666). Proton magnetic resonance spectra of ionized 2-aminopyrazine in liquid ammonia have also been reported (665). 

Second-order rate constants have been determined for the alkaline hydrolysis of 2-methoxycarbonylpyrazine in methanol-water (80%w/w) at 10° as 5.2x10 l/mol sec (1263) and for 2-ethoxycarbonylpyrazine at 30° in 50 and 60% dimethyl sulfoxide-water and 60, 70, and 80% ethanol-water as 95.9, 199, 12.4, 10.2, and 8.86x10 1/mol sec (1359). The failure of 2,5-dimethoxycarbonyl-pyrazine to undergo the Schmidt reaction has been accounted for in terms of its normal ionization, and I values in 100% sulfuric acid gave an average of 2.55 (1176). 

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