Olovsson I, Ptasiewicz-Bak H, Gustafsson T, Majerz I
Materials Chemistry, Angström Laboratory, University of Uppsala, Box 538, SE-751 21 Uppsala, Sweden.
Acta Crystallogr B. 2002 Aug;58(Pt 4):627-31. doi: 10.1107/s0108768102004858. Epub 2002 Jul 30.
In our earlier neutron diffraction study of the title compound at 30 K and 295 K an unconventional strategy in the refinement of hydrogen was applied and the same procedure has now been followed in the present investigation at 170 K and 90 K. There are two short O...H...O hydrogen bonds [2.437 (2) A and 2.442 (2) A at 30 K] and the 'heavy-atom' structure is centrosymmetric (P1) with centres of symmetry in the middle of the O...O bonds. However, statistical significance tests clearly show that an asymmetric location of both H atoms gives the most satisfactory description of the structure at all temperatures. The shift of hydrogen from the centre of symmetry is 0.15, 0.14, 0.15 and 0.15 A for H2 at 30, 90, 170 and 295 K, respectively, and 0.15, 0.15, 0.15 and 0.12 A for H4 (sigma = 0.01 A). Furthermore, the behaviour of H2 is very interesting: at 295 K and 170 K it is located on one side of the symmetry centre but at 90 K and 30 K it is located on the other side. A detailed determination of the unit-cell parameters by X-ray diffraction in the whole temperature range from 30 K to 295 K has revealed that the data points of the cell parameters as a function of temperature fall on two different straight lines with a sudden change in the slope around 135 K. It appears likely that the change in the location of H2 as the temperature is lowered is related to this behaviour. At 170 K, R(F) = 0.029 for 1236 reflections; at 90 K, R(F) = 0.030 for 1457 reflections.
在我们早期对该标题化合物在30 K和295 K下的中子衍射研究中，采用了一种非常规的氢原子精修策略，并且在目前170 K和90 K的研究中遵循了相同的程序。存在两个短的O...H...O氢键[30 K时为2.437(2) Å和2.442(2) Å]，且“重原子”结构是中心对称的(P1)，对称中心位于O...O键的中间。然而，统计显著性检验清楚地表明，两个H原子的不对称位置在所有温度下都能给出对结构最令人满意的描述。对于H2，在30、90、170和295 K时，氢原子从对称中心的位移分别为0.15、0.14、0.15和0.15 Å，对于H4为0.15、0.15、0.15和0.12 Å(标准偏差σ = 0.01 Å)。此外，H2的行为非常有趣：在295 K和l70 K时它位于对称中心的一侧，但在90 K和30 K时它位于另一侧。通过在30 K至295 K的整个温度范围内进行X射线衍射对晶胞参数进行详细测定，结果表明，晶胞参数作为温度函数的数据点落在两条不同的直线上，在135 K左右斜率发生突然变化。随着温度降低，H2位置的变化似乎与这种行为有关。在170 K时，1236个反射的R(F) = 0.029；在90 K时，1457个反射的R(F) = 0.030。
