Cheetham G M, Carr P D, Dodd I M, Kariuki B M, Harding M M
J Synchrotron Radiat. 1995 Nov 1;2(Pt 6):300-8. doi: 10.1107/S0909049595012799.
Experiments are described to show some of the potential of the synchrotron radiation Laue method for the study of structural change within single crystals. In the metastable tetragonal crystals of P(4)N(4)Cl(8) the eight-membered P(4)N(4) ring is in a boat conformation, with symmetry {\bar 4}. On heating to ca 340 K the crystals transform, slowly to a second tetragonal form in which the ring conformation is a chair, its symmetry {\bar 1}. Both structures are known [Hazekamp, Migchelsen & Vos (1962). Acta Cryst. 15, 539-543; Wagner & Vos (1968). Acta Cryst. B24, 707-713]. In the transformation the molecular packing, unit-cell dimensions and crystal quality remain almost unchanged. To study this transformation, series of Laue diffraction patterns were recorded at 2-3 min intervals over a period of 30-40 min, while the temperature was raised to 373 K. For two series, reflection intensities were measured and they allowed determination and refinement of the fraction of boat and chair molecules present in a mixed boat/chair model of the structure. No significant change in the crystal occurs below ca 340 K; at or above 340 K, 40-50% of the molecules are converted from boat to chair conformations within 5 min, but the remainder of the conversion is much slower, even when the temperature is raised towards 370 K.
本文描述了一些实验，以展示同步辐射劳埃法在研究单晶结构变化方面的潜力。在P(4)N(4)Cl(8)的亚稳四方晶体中，八元环P(4)N(4)呈船式构象，对称性为{\bar 4}。加热至约340 K时，晶体缓慢转变为第二种四方晶型，其中环构象为椅式，对称性为{\bar 1}。两种结构均已为人所知[哈泽坎普、米格切尔森和沃斯（1962年）。《晶体学报》，15卷，539 - 543页；瓦格纳和沃斯（1968年）。《晶体学报》，B24卷，707 - 713页]。在转变过程中，分子堆积、晶胞尺寸和晶体质量几乎保持不变。为了研究这种转变，在将温度升至373 K的过程中，每隔2 - 3分钟记录一系列劳埃衍射图样，持续30 - 40分钟。对于两个系列，测量了反射强度，并据此确定和精修了结构的船式/椅式混合模型中船式和椅式分子的比例。在约340 K以下，晶体没有明显变化；在340 K及以上，40 - 50%的分子在5分钟内从船式转变为椅式构象，但其余的转变要慢得多，即使温度升至370 K也是如此。
