engleski

Strong π • • • π interactions in crystals of chloranilic acid and its alkali salts compete with hydrogen bonds

Strong π • • • π interactions have been observed in crystal structures of alkali hidrogen chloranilates and o-chloranilic acid, a novel isomer of p-chloranilic acid. The parallel conjugated rings are stacked directly one atop another without any offset. Centroid• • • centroid distances of 3.29 – 3.43 Å are the shortest ever observed. Such stacks are present in o-chloranilic acid trihydrate and ammonium and potassium hydrogen chloranilate dihydrate, and apparently dominate the crystal packing, while the medium-strong hydrogen bonds link the stacks together. The strong π • • • π interactions are due to the attractive forces exerted by local dipoles of adjacent molecules. To explain formation of such unusual stacks, a whole series of alkali hydrogen chloranilates has been prepared. Structures with larger cations, Rb and Cs reveal π -stacked benzenoid rings with a significant offset whereas their analogues with K+ and NH4+ show no offset and separation distances of conjugated rings are very short. However, those with small cations, Na and Li, are unstable and do not crystallise as hydrogen chloranilates. The cation size and their polarisability define the crystal packing patterns.

quinones ; chloranilic acid ; π • • • π interactions

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