Pyrazine ionization potential

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... 

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)... 

The photoelectron spectrum of pyridazine is similar to those of pyrazine, pyrimidine, and triazine i.e., the lowest ionization potential corresponds to ionization of a lone-pair electron. The ionization potential is in agreement with the calculated value. These spectra were recorded also of pyridazine 1-oxide and 1,2-dioxide, and it was found that the perturbation of the t-system by the N—O group results in the separation of the lower excited states of the AT-oxide ions. ... 

Alternative approaches to improve polymer ambient stability by increasing the ionization potential have utilized electron poor comonomers in the polymer backbone. Electron deficient six-membered rings, such as 1,2-pyrazine [37] or perfluorobenzene [38], have also been introduced into the backbone. These are able to planarize the backbone through the formahon of intrachain S-N or S-F bonding interactions. Increases in ionizahon potenhal can therefore be attributed solely to an electron-withdrawing effect. Ambient stabilities were not reported, but charge carrier mobilities were on the order of 10 cm s. 

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... 

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. 

Ionization constants of pyrazine compounds are summarized by Barlin (B-82MI 603-0l>. The basicity constants of pyrazine and its methyl derivatives in water have also been measured, and show a linear inverse relationship with polarographic half-wave potentials <85JHC1143>. 

Fig. 4.6 A cut through the potential-energy surfaces of pyrazine along the normal coordinate Qsa from Ref. [27], The vertical energy differences and shifts are drawn on scale. The shaded areas symbolize the ionization continua. The arrows on the right-hand side indicate a possible two photon transition (Reprinted with permission from Ref. [27] Copyright (1991), American Institute of Physics)...

Although in the case of pyrazine the interpretation is complicated by cross-term signals and auto-ionizing Rydberg states, the experimental and theoretical investigations nevertheless demonstrate the considerable potential of femtosecond time-resolved photoelectron spectroscopy. As a beautiful example of a more favorable case, Stolow and coworkers have recently presented time-resolved ionization studies on decatetraene. In direct analogy to the discussion presented above, they managed to monitor the S2 Si internal conversion associated with a conical intersection in real time. 

In this section we discuss very briefly a few other examples of vibronic coupling in polyatomic molecules. The pyrazine cation has been treated along similar lines as the 5i -S2 excited manifold of the neutral species. Its X Ag ground state and the A Big first excited state contain a vacancy in the n and n molecular orbitals, respectively. Therefore, they differ from the S and S2 states basically in that the electron is ionized out of these two orbitals rather than promoted to the n virtual orbital. The analysis shows that the conical intersection occurring between the X-A potential-energy surfaces is essentially a replica of that in the S1-S2 electronic manifold of neutral pyrazine. This interesting structural relationship shows that the shape of the surfaces is governed by the occupied rather than the virtual molecular orbitals in this system. 

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