Pyrazines 1,4-Diazines

The addition of electron-withdrawing substituents to the 1,4-diazine nucleus increases its participation in inverse electron demand [4 + 2] cycloadditions with electron-rich dienophiles. Both the rate and regiose-lectivity of the reaction are dependent on the number and position of the electron-withdrawing substituents present on the 1,4-diazine nucleus [Eq. (22) and Table 10-VII]. 

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Pyrazine (1,4-diazine) will bridge two OsNCLt fragments in [Cl4NOs(pyrazine)OsNCl4]2- where the chlorines are bent slightly away from the terminal N (IR l-(Os-N) 1105cm-1, Os-N 1.63 A) [190],... 

Pyrimidine (1,3-diazine) and pyrazine (1,4-diazine) exhibit weak fluorescences73,74 in solutions or as vapors at room temperature, and strong phosphorescences 76-79 in dilute solid solutions at low temperatures (77 or 90°K). The phosphorescent quantum yields have never been accurately measured in these solid solutions. In the vapor phase or in ordinary solutions, at room temperature, these two compounds do not phosphoresce. Radiationless deactivation processes must be considered again and a deactivation through an isomer cannot be excluded. 

This section deals with molecules derived from the diazines C4H4N2 pyrazine (1,4-diazine 7), pyrimidine (1,3-diazine 8) and pyridazine (1,2-diazine 9). 

In this section we shall present and compare different computational strategies one can adopt to simulate the effect of the environment on spectroscopic properties of solvated systems. In particular, as a representative example, we shall summarize the results of two studies [30,31] we have published in the last years on the environment effects on the nitrogen nuclear shieldings of a specific class of molecular systems containing sp2-type nitrogens three diazines, also known as pyridazine (1,2-diazine), pyrimidine (1,3-diazine) and pyrazine (1,4-diazine). 

Question 6.8 Predict the structure of the complex formed between pyrazine (1,4-diazine) and [Yb(CsH5)3]. 

Diazines are aromatic six-membered heterocycles that contain two sp -hybrid-ized nitrogen atoms in the ring. The three diazine isomers are pyridazine (1,2-dia-zine), pyrimidine (1,3-diazine) and pyrazine (1,4-diazine). The most important naturally occurring diazines are the pyrimidine bases uracil, thymine, and cytosine, which comprise the fundamental nucleoside building blocks in deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Pyrazines occur frequently as... 

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