C-H group acidity and the nature of C-H...N interactions: crystal structural analysis of pyrazine and methyl substituted pyrazines.
Crystal structures of pyrazine, methyl-, 2,3-, 2,5- and 2,6-dimethyl-, and trimethylpyrazine have been detd. by X-ray diffraction anal. Four of these compds.(methyl-, 2,3-, 2,5-dimethyl- and trimethylpyrazine) are liqs. and their single crystals have been grown in situ on the diffractometer using a miniature zone melting procedure. These structures are analyzed together with those of pyrazine and tetramethylpyrazine in the context of C-H...N interactions. Compds. studied were chosen because they contain only C, H and N atoms, and also C-H groups of variable acidity. This facilitates the comparison of C-H...N geometries with respect to the C-H group acidity. Compds. pyrazine and methylpyrazine are dominated by s C-H groups in their mols. and their supramol. structures are generated by s C-H...N. On the other hand the domination of sp3 C-H groups at the mol. level in trimethylpyrazine and tetramethylpyrazine results in crystal structures that are governed by s C-H...N. A balance of sp2 and sp3 C-H groups in the di-Me derivs. leads to a situation where both kinds of C-H groups play a structure directing role. While the s C-H groups form C-H...N, the s C-H groups are involved in C-H...p interactions. Thus the C-H group interactions follow a property unique to hydrogen bonds-stronger donors interact with stronger acceptors while weaker donors approaching weaker acceptors. Inter-structural comparison revealed that H...N distances decrease with increasing C-H acidity and therefore C-H...N interactions could be considered as weak hydrogen bonds.
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