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J. Chem. Phys. 72, 5656 (1980); http://dx.doi.org/10.1063/1.438982 (11 pages)
Polymorphism of crystalline poly(hydroxymethyl) compounds. III. The structures of crystalline and plastic tris(hydroxymethyl)aminomethane
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Add to FacebookBetween 5 °K and its m.p. (446.0 °K), tris, H2NC(CH2OH)3, exhibits one phase transition, at 407.3 °K, to form the orientationally disordered (plastic) phase I. The ordered form, phase II, is orthorhombic, Pn21a, a=8.844(1) Å, b=7.794(1) Å, c=8.795 (1) Å, Z=4, Dx=1.327 g/cm3, and Dm=1.320 g/cm3. The crystal structure can be described in terms of layers of molecules approximately normal to the c axis, with strong intralayer hydrogen bonding and weak interlayer hydrogen bonding. The crystal structure of phase II was studied using x‐ray diffraction at 144, 295, 297, 348, 382, and 395 °K. Full‐matrix least‐squares refinement of single‐crystal diffractometer data resulted in R values between 2–4% for all but the highest‐temperature data sets. Although the integrity of the structure is maintained throughout this temperature range, subtle effects are noted that indicate the increased thermal motion as phase I is approached. Phase I is body‐centered‐cubic, Im3m, with a=6.876(6) Å at 423 °K, Z=2, and Dx=1.237 g/cm3. A comparison of the two structures and the experimental investigation of a single sample in both phases indicate that the mechanism of the phase transition involves a realignment of the molecules within the the layers and a lessening of the interlayer distance. The formation of a bcc lattice in phase I, rather than a fcc lattice as was found in the other members of this series, has been tentatively interpreted in terms of the ellipsoidal shape of the tris molecule.
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