Ge 6 Me 12 and Ge 6 H 12 Cyclohexagermanes: Conformer Equilibria Studied by Raman Spectroscopy and Quantum Chemical Calculations

In the first experimental study of rotational isomerism of a compound possessing a fourth row element backbone, the totally symmetric Ge–Ge stretching mode of dodecamethylcyclohexagermane (Ge 6 Me 12 ), which is the most intense Raman band in the range 200–300 cm –1 , was analyzed with temperature‐dependent vibrational Raman spectroscopy in both the crystalline and solution states. Below 50 °C, the Raman spectrum of the solid shows a single band at 232 cm –1 originating from the chair conformer. Between 50 and 60 °C, a shoulder at 227 cm –1 appears, which is associated with a twisted conformer. In solution, the band attributed to the chair conformer displays a shoulder arising from an averaged twist/boat conformer, even at –80 °C. The intensity of the shoulder increases with temperature. By using the van't Hoff relationship, an enthalpy difference of 2.4 kJ mol –1 between chair and twist/boat conformers was obtained. An improved X‐ray crystal structure of Ge 6 Me 12 was also obtained, and an average Ge–Ge bond length of 242 pm was found, about 5 pm longer than previously reported. With the help of quantum chemical DFT and MP2 calculations, the geometries and energies of the conformers of Ge 6 Me 12 and of the parent Ge 6 H 12 compound were investigated thoroughly by employing 6‐31+G*, D95V and aug‐cc‐pVTZ basis sets, as well as SDD and aug‐cc‐pVTZ‐PP pseudopotentials on the Ge atoms. B3LYP/aug‐cc‐pVTZ calculations predict two conformers for Ge 6 H 12 , chair and twist, with zero‐point vibrational energy (ZPVE) corrected relative energies of 0.0 and 5.7 kJ mol –1 , respectively. They predict three conformers, chair (0.0), twist (5.2) and boat (9.2), for Ge 6 Me 12 . They also give reasonable geometry parameters, relative energies and barriers for the conformer interconversion.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)