engleski

Proton transfer phenomena in crystals of alkali hydrogenchloranilates: semiquinone-like structures

Quinones and hydroquinones are good proton donors and acceptors. Chloranilic acid, being both a quinone and a hydroquinone, is capable of forming extensive hydrogen-bonded networks. Crystals of anhydrous chloranilic acid contain quinhydrone-like infinite hydrogen-bonded chains (I) of alternating quinone and hydroquinone moieties. Since the strongest hydrogen bonds are those between the charged moieties, hydrogencloranilate anion, a much stronger proton acceptor than the neutral acid, may be utilised in design of solid-state systems with low-barrier hydrogen bonds (LBHB's). Anhydrous alkali hydrogenchloranilates are very simple structures with semiquinone-like hydrogen-bonded chains (II) which are known to have LBHB's. Since they are not radicals, they are very stable and, therefore, very convenient for detailed study of LBHB's in the solid state. Our research focuses on sodium, potassium and rubidium hydrogenchloranilates, whose crystal structures involve the fragment II. X-ray structure of the potassium salt reveals O...O distance of 2.63 Å and a disordered proton. This bond is significantly shorter than in the neutral acid (2.77 Å ) which has no proton disorder. It is also shorter than the LBHB in crystals of p-benzosemiquinone radical (2.74 Å ). IR spectra display a Zundel-type continuum in the range 1500-400 cm-1 that is indicative of large amplitude motion. Therefore, we conclude that the hydrogen bond in crystals of potassium hydrogenchloranilate is indeed a low-barrier one and that the disorder is a result of the proton delocalisation.

proton transfer; semiquinone radical; low-barrier hydrogen bond

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