Intramolecular motion in the triptycenegermanyl radical: single crystal EPR study at variable temperature and DFT calculations

X-irradiation of single crystals of Tp-GeH3 (Tp: triptycene) led to the trapping of the radical Tp-GeH2. The angular variations of the resulting EPR spectra were recorded at 300 and 77 K. The drastic temperature dependence of the spectra was caused by both a strong anisotropy of the g-tensor and a rotation of the GeH2 moiety around the C-Ge bond. The determination of the EPR tensors as well as the analysis of this motion required to take the presence of disorder in the crystal into account. In accordance with DFT calculations, Tp-GeH2 is shown to be pyramidal and to adopt, in its lowest energy structure, a staggered conformation. Rotation around the C-GeH2 bond is blocked at 90 K and is almost free above 110 K. The experimental barrier, obtained after simulation of the EPR spectra as a function of the rotational correlation time, is equal to 1.3 kcal mol(-1), which is slightly inferior to the barrier calculated by DFT (3.6 kcal mol(-1)). Calculations performed on Tp-CH3, Tp-GeH3 and Tp-GeH2 show that the rotation barrier DeltaE(rot) around the C-Ge bond drastically decreases by passing from the germane precursor to the germanyl radical and that DeltaE(rot) increases by passing from the germane to its carbon analogous. Structural parameters involved in these barrier differences are examined. (C) 2003 Elsevier Science Ltd. All rights reserved.