Crystal structure disorders: disorders involving F and O atoms

 Not all structure solutions are straightforward. Some involve disordering of atomic positions, a problem that, for example, made the elucidation of the structure of C₆₀ difficult. Examples of disordered structures occur commonly in oxofluorides because the O and F atoms are similar in size and possess similar electronic properties. Thus, in a crystal containing molecules of an oxofluoride XF_xO_y, a given atomic position might be occupied by O in one molecule and by F in another molecule. The overall result is modelled by fractional occupation of each site by O and F. Fractional occupancies can lead to difficulties in determining true X^_F and X^_O bond lengths and true bond angles. The compound [F₂NO]‏[AsF₆]^- represents a classic example of the problem. Although first prepared and characterized

in 1969, its structure was not reported until 2001. The [F₂NO]‏ ions in crystalline [F₂NO][AsF₆] are disordered such that the fluorine occupancy of each ‘F’ position is 78% and 77% respectively (rather than being 100%), and the fluorine occupancy of the ‘O’ position is 45% (rather than being 0%). The paper cited in the further reading below illustrates how the structural data can be treated so that meaningful N^_O and N^_F bond lengths and F^_N^_F and F^_N^_O bond angles are obtained. Crystalline [F₂NO][AsF₆] is composed of infinite chains of alternating cations and anions. There are close contacts between the N atom of each cation and the F atoms of adjacent [AsF₆]^- ions as shown in the figure.

مواضيع ذات صلة

كيمياء القصدير

تطبيقات الطبقات الثنائية الهيدروكسيد

تطبيقات عملية التبادل الأيوني

مواد التبادل الأيوني Ion exchanger

ظاهرة التبادل الايوني Ion Exchange

تاريخ التبادل الايوني Ion Exchange History

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كيمياء البلاديوم ومعقداته

طرائق تقدير النحاس

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طرائق تقدير النيكل

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