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. 2001 Jun;57(Pt 3):311-6. doi: 10.1107/s0108768101003779. Epub 2001 Jun 1.
The structure of the title compound has been studied by neutron diffraction at 30 and 295 K, with the emphasis on the location of the protons. There are two crystallographically independent H atoms in two very short hydrogen bonds, 2.437 (2) and 2.442 (2) A at 30 K. The structure could be refined successfully in the centrosymmetric space group P1;, with the H atoms located at the centres of symmetry. However, the form of the thermal ellipsoids of hydrogen indicated either asymmetric hydrogen bonds or overlap of two closely spaced, partially occupied positions around the centres of symmetry. Several different types of refinements have then been applied, including unconventional models; with all atoms except hydrogen constrained in P1;, but with hydrogen allowed to refine without any constraints in P1, anisotropic refinement of all atoms resulted in clearly off-centred hydrogen positions. Significance tests clearly showed that the results from this constrained refinement give the most satisfactory description of the structure. This structure may be described as 'pseudo-centrosymmetric with non-centred protons'. The results demonstrate that it is very important to also include refinement models with non-centrosymmetric hydrogen in a centrosymmetric environment when studying very short hydrogen bonds. The shifts of the two H atoms from the centres of symmetry are 0.15 (1) and 0.12 (1) A, respectively, at 30 K, and 0.15 (1) A for both H atoms at room temperature. At 30 K: R(F) = 0.036 for 1485 reflections; at 295 K: R(F) = 0.035 for 1349 reflections.
已通过在30 K和295 K下的中子衍射研究了标题化合物的结构，重点是质子的位置。在两个非常短的氢键中存在两个晶体学独立的H原子，在30 K时键长分别为2.437(2)和2.442(2)Å。该结构可以在中心对称空间群P1̅中成功精修，H原子位于对称中心。然而，氢的热椭球形式表明存在不对称氢键或对称中心周围两个紧密间隔、部分占据位置的重叠。随后应用了几种不同类型的精修，包括非常规模型；除氢原子外的所有原子在P1̅中受到约束，但氢原子在P1中无任何约束地进行精修，所有原子的各向异性精修导致氢原子位置明显偏离中心。显著性检验清楚地表明，这种约束精修的结果对结构给出了最令人满意的描述。这种结构可描述为“具有非中心质子的伪中心对称”。结果表明，在研究非常短的氢键时，在中心对称环境中纳入具有非中心对称氢的精修模型非常重要。在30 K时，两个H原子相对于对称中心的位移分别为0.15(1)和0.12(1)Å，在室温下两个H原子的位移均为0.15(1)Å。在30 K时：1485个反射的R(F)=0.036；在295 K时：1349个反射的R(F)=0.035。
