C-H group acidity and the nature of C-H•••N interactions:: crystal structural analysis of pyrazine and methyl substituted pyrazines

Crystal structures of methyl-, 2,3-, 2,5- and 2,6-dimethyl-, and trimethylpyrazine (2-6) have been determined by X-ray diffraction analysis. Four of these compounds (2-4 and 6) are liquids and their single crystals have been grown in situ on the diffractometer using a miniature zone melting procedure. These structures are analysed together with those of pyrazine (1) and tetramethylpyrazine (7) in the context of C-H ... N interactions. Compounds 1-7 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. Compounds 1 and 2 are dominated by sp(2) C-H groups in their molecules and their supramolecular structures are generated by sp(2) C-H ... N. On the other hand the domination of sp(3) C-H groups at the molecular level in 6 and 7 results in crystal structures that are governed by sp(3) C-H ... N. A balance of sp(2) and sp(3) C-H groups in the dimethyl derivatives 3-5 leads to a situation where both kinds of C-H groups play a structure directing role. While the sp(2) C-H groups form C-H ... N, the sp(3) C-H groups are involved in C-H ...pi interactions. Thus the C-H group interactions follow a property unique to hydrogen bonds-stronger donors interact with stronger accepters while weaker donors approaching weaker accepters. 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.