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Date: 4-7-2017
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Date: 11-6-2020
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Date: 25-6-2019
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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 XFxOy, 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.
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إدارة الغذاء والدواء الأميركية تقرّ عقارا جديدا للألزهايمر
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شراء وقود الطائرات المستدام.. "الدفع" من جيب المسافر
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ضمن مؤتمر الإمام الحجة(عجل الله فرجه) العلمي باحث كويتي يناقش إثبات ولادة الإمام المهدي (عجّل الله فرجه) من التّوقيعات الشّريفة
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