Bayreuth 1998 – scientific programme

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CP: Chemische Physik

CP 10: Dynamik molekularer Systeme

CP 10.2: Poster

Tuesday, March 10, 1998, 18:00–20:00, P1

Quantitative study of the solid-state dynamic properties of
E(SiMe₃)₄ (E=C,Si) by variable-temperature one- and
two-dimensional ¹³C and ²⁹Si CP/MAS NMR — •Xavier Helluy, Jörg Kümmerlen, and Angelika Sebald — Bayerisches Geoinstitut, Universität Bayreuth

Compounds E(SiMe₃)₄ with E=C (1) and with E=Si (2) are known to form plastically crystalline phases at ambient temperatures. We will consider the molecular dynamic properties of solid 1 and 2 at temperatures below the phase transition temperature to the plastically crystalline state. While 1 and 2, from a chemical point of view, are closely related homologues, the low temperature dynamic properties of these two crystalline compounds differ considerably. Quantitative analysis of variable-temperature ¹³C and ²⁹Si CP/MAS NMR spectra of 1 and 2 by means of spectral lineshape simulations in the exchange broadened regime, in conjunction with quantitative analysis of two-dimensional ¹³C and ²⁹Si exchange (2D EXSY) experiments on 1 and 2, provides detailed insight. Both solid 1 and 2 display C₃ molecular symmetry in their respective low-temperature phases. For 1, only thermally activated reorientation of the entire molecule is observed but no internal SiMe₃ reorientation. For 2 both internal SiMe₃ reorientation and reorientation of the entire molecule occur.