He X M, Swaminathan S, Craven B M, McMullan R K
Department of Crystallography, University of Pittsburgh, PA 15260.
Acta Crystallogr B. 1988 Jun 1;44 ( Pt 3):271-81. doi: 10.1107/s0108768187011108.
The crystal structure of parabanic acid (1H,3H-imidazoletrione, C3H2N2O3, Mr = 114.1) at 123 K [monoclinic, P2(1)/n, Z = 4, a = 10.704 (2), b = 8.187 (2), c = 4.969 (1) A, beta = 92.32 (1)o] has been determined by neutron and X-ray diffraction. Neutron reflections [1658, (sin theta)/lambda less than or equal to 0.78 A-1, lambda = 1.0470 (1) A, mu = 0.75 cm-1] in full-matrix least-squares refinement (wR = 0.055) gave nuclear parameters with bond lengths having estimated standard deviations (e.s.d.'s) of 0.001 A. X-ray reflections [3150 with magnitude of F magnitude of greater than or equal to 3 sigma and (sin theta)/lambda less than or equal to 1.3 A-1, Mo Ka, lambda = 0.7093 (1) A, mu = 1.708 cm-1] were used in full-matrix least-squares refinement (R = 0.022) assuming Stewart's rigid pseudoatom model, so as to determine the charge-density distribution. Sharp features near the O nuclei in the residual electron density map noted by Craven & McMullan (CM) [Acta Cryst. (1979), B35, 934-945] in their 298 K study were absent at 123 K. New refinements using CM's 298 K data give significant third-order thermal parameters for O atoms due to anharmonic molecular stretching, with good agreement between probability density functions, (p.d.f.'s) from X-ray and neutron diffraction. When deconvoluted from the thermal vibrations in the crystal, deformation charge densities derived at 298 and 123 K are in satisfactory agreement and conform closely to the 2 mm symmetry of the isolated molecule. The molecular dipole moment is 2.3 (3) D [7.7 (10) x 10(-30) C m]. A map of molecular electrostatic potential indicates that the carbonyl O atoms in parabanic acid are weakly electronegative. This is confirmed by calculation of the electrostatic energy of interaction for the two distinct H-bonded pairs of molecules isolated from the crystal. Energy values are weakly attractive [-7 (3) and 5 (2) kJ mol-1]. Similar calculations are less reliable for pairs of molecules forming intermolecular C....O distances as short as 2.75 A. However, depending on the model, energy values are repulsive or negligibly small. Thus the high stability of parabanic acid crystals (m.p. 516 K with decomposition) cannot be explained.
已通过中子和X射线衍射确定了仲班酸（1H,3H - 咪唑三酮，C3H2N2O3，Mr = 114.1）在123 K时的晶体结构[单斜晶系，P2(1)/n，Z = 4，a = 10.704 (2)，b = 8.187 (2)，c = 4.969 (1) Å，β = 92.32 (1)°]。中子反射[1658个，(sin θ)/λ ≤ 0.78 Å-1，λ = 1.0470 (1) Å，μ = 0.75 cm-1]用于全矩阵最小二乘法精修（wR = 0.055），得到的核参数中键长的估计标准偏差（e.s.d.）为0.001 Å。X射线反射[3150个，|F| ≥ 3σ且(sin θ)/λ ≤ 1.3 Å-1，Mo Kα，λ = 0.7093 (1) Å，μ = 1.708 cm-1]在假设斯图尔特刚性伪原子模型的情况下用于全矩阵最小二乘法精修（R = 0.022），以确定电荷密度分布。克雷文和麦克马伦（CM）[《晶体学报》（1979年），B35，934 - 945]在其298 K研究中指出的残余电子密度图中O原子核附近的尖锐特征在123 K时并不存在。使用CM的298 K数据进行的新精修给出了由于非谐分子拉伸导致的O原子显著的三阶热参数，X射线和中子衍射得到的概率密度函数（p.d.f.）之间吻合良好。当从晶体中的热振动中解卷积时，在298 K和123 K得到的变形电荷密度吻合良好，并且紧密符合孤立分子的2mm对称性。分子偶极矩为2.3 (3) D [7.7 (10)×10-30 C m]。分子静电势图表明仲班酸中的羰基O原子电负性较弱。从晶体中分离出的两对不同的氢键分子间相互作用的静电能计算证实了这一点。能量值具有弱吸引力[-7 (3)和5 (2) kJ mol-1]。对于形成分子间C....O距离短至2.75 Å的分子对，类似的计算可靠性较低。然而，根据模型不同，能量值可能是排斥的或可忽略不计的小。因此，仲班酸晶体（熔点516 K且分解）的高稳定性无法得到解释。
