Assembling an isomer grid: the isomorphous 4-, 3- and 2-fluoro-N'-(4-pyridyl)benzamides.

The three title isomers, 4-, (I), 3-, (II), and 2-fluoro-N'-(4-pyridyl)benzamide, (III), all C12H9FN2O, crystallize in the P2(1)/c space group (No. 14) with similar unit-cell parameters and are isomorphous and isostructural at the primary hydrogen-bonding level. An intramolecular C-H...O=C interaction is present in all three isomers [C...O = 2.8681 (17)-2.884 (2) A and C-H...O117-118 degrees ], with an additional N-H...F [N...F = 2.7544 (15) A] interaction in (III). Intermolecular amide-pyridine N-H...N hydrogen bonds link molecules into one-dimensional zigzag chains [graph set C(6)] along the [010] direction as the primary hydrogen bond [N...N = 3.022 (2), 3.049 (2) and 3.0213 (17) A]. These are augmented in (I) by C-H...pi(arene) and cyclic C-F...pi(arene) contacts about inversion centres, in (II) by C-F...F-C interactions [C...F = 3.037 (2) A] and weaker C-H...pi(arene)/C-H...F contacts, and in (III) by C-H...pi(arene) and C=O...O=C interactions, linking the alternating chains into two-dimensional sheets. Typical amide N-H...O=C hydrogen bonds [as C(4) chains] are not present [N...O = 3.438 (2) A in (I), 3.562 (2) A in (II) and 3.7854 (16) A in (III)]; the C=O group is effectively shielded and only participates in weaker interactions/contacts. This series is unusual as the three isomers are isomorphous (having similar unit-cell parameters, packing and alignment), but they differ in their interactions and contacts at the secondary level.