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Orientational Disorder in Solids Composed of the Ellipsoid-Shaped Molecules Trichloromethyltrimethylsilane, Cl3csi(Ch3)3 T-Butyltrichlorosilane, (Ch3)3csicl3 and T-Butyldimethylchlorosilane, (Ch3)3csi(Ch3)2cl

Berichte Der Bunsen-Gesellschaft-Physical Chemistry Chemical Physics 98(8): 1096-1100

Autoren/Herausgeber: Brauninger S
Dou SQ
Fueß H
Schmahl WW
Strauss R
Weiss A
Erschienen: 1994

The ethane structure-related molecules Cl3CSi(CH3)3 (1), (H3C)3CSiCl3 (2), and (CH3)3CSi(CH3)2Cl (3) crystallize from the melt to an orientational disordered body centered cubic (plastic) phase I, Im3m, Z = 2. By DTA/DSC the following thermodynamic data have been determined (temperatures in K, DELTAH in kJ mol-1). (1): T(m) = 405.3; DELTAH(m) = 7.36; DELTAS(m)/R = 2.17; T(I,II) = 285.5; DELTAH(I,II) = 11.16; DELTAS(I,II)/R = 4.68. (2): T(m) = 373.4; DELTAH(m) = 6.69; DELTAS(m)/R = 2.13; T(I,II) = 206.4; DELTAH(I,II) = 6.01; DELTAS(I,II)/R = 3.45. (3): T(m) = 358.1; DELTAH(m) = 5.66; DELTAS(m)/R = 1.87; T(I,II) = 203.6; DELTAH(I,II) = 7.75; DELTAS(I,II)/R = 4.51. At 293 K the lattice constants of the bcc unit cells are (1): 796.6 (2) pm (303 K), (2): 794.5 (7) pm, (3): 803.6(9) pm.|(1) Shows a two line Cl-35 NQR spectrum (nu(i) in MHz, T = 77 K) with nu1 = 37.213, nu2 = 37.033, fade out temperature T(f) = 253 K; (2): Doublet, nu1 = 19.060, nu2 = 18.260, T(f) = 177 K; (3): nu = 16.786, T(f) = 200 K.|The H-1-NMR line width and the second moment M2 (H-1) of (2) show the orientationally disordered state above T(I,II), hysteresis in T(I,II), the freezing-in of the t-butyl group rotation below T(I,II), and a freezing-in of the individual CH3 group rotation below 100 K.