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Molecular Motion in Solid [Sb(CH 3 ) 4 ]X, X = Cl, Br, I, PF 6 , and BF 4 . 1 H, 19 F NMR Second Moment and Crystal Structure

1989; Volume: 93; Issue: 11 Linguagem: Inglês

10.1002/bbpc.19890931132

0005-9021

G. Burbach, Shi-qi Dou, Alarich Weiß,

Crystal Structures and Properties

Abstract The tumbling motion of the tetramethylstibonium cation [Sb(CH 3 ) 4 ] + has been studied in the ionic compounds [Sb(CH 3 ) 4 ]X, X = Cl, Br, I, PF 6 , and BF 4 , by measurement of the second moment of the protons M 2 ( 1 H) in the temperature range 95 ≧ T/K ≧ 340. — At room temperature the iodide, hexafluorophosphate, and tetrafluoroborate crystallize with hexagonal unit cell. The crystal structure of [Sb(CH 3 ) 4 ]I was determined by single crystal X‐ray diffraction: a = 728.4(3) pm, c = 1037.7(5) pm, space group C 4 6v — P6 3 mc, Z = 2. — M 2 ( 1 H ) of [Sb(CH 3 ) 4 ]I shows cation tumbling at T > 320 K. The methyl groups are rotating about their threefold axes in the whole temperature range studied. — In [Sb(CH 3 ) 4 ]PF 6 and [Sb(CH 3 ) 4 ]PF 4 both, cation and anion are tumbling at T ≧ 259 K and T ≧ 248 K, respectively, as seen by M 2 ( 1 H) and M 2 ( 19 F). A jump in the second moments and differential thermal analysis (DTA) of [Sb(CH 3 ) 4 ]BF 4 reveals a solid‐solid phase transition with hysteresis (width 28 K). Activation energies for the tumbling motion of the tetramethylstibonium cation have been determined.