Ge₆Me₁₂ and Ge₆H₁₂ Cyclohexagermanes: Conformer Equilibria Studied by Raman Spectroscopy and Quantum Chemical Calculations

Abstract

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 (Ge6Me12), 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 Ge6Me12 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 Ge6Me12 and of the parent Ge6H12 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 Ge6H12, 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 Ge6Me12. They also give reasonable geometry parameters, relative energies and barriers for the conformer interconversion.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Chemical Properties of Tetragermaplatinacyclopentane. Insertion of an Alkyne into a Pt–Ge Bond and Silylation Caused by H2SiPh2

“…The PtÀGe bond of 7 (2.497(1) Å) is slightly longer than those of bis(germyl)platinum complex 4 (2.4454(5), 2.4380(5) Å) and germaplatinacycles 2 (2.451(1), 2.4542(9) Å) and 3 (2.4643(4), 2.4769(4) Å). 10 Three GeÀGe bonds of 7 (2.481(2), 2.455(2), 2.435(2) Å) are longer than those of cyclo-(GeMe 2 ) 6 (2.409(2)À2.430(2) Å), 12 but shorter than that of t Bu 3 GeGe t Bu 3 (2.705(1), 2.714(1) Å, the longest GeÀGe bond). 13 The PtÀGe1ÀGe2 and Ge1ÀGe2ÀGe3 angles (119.80(5)°, 113.81(5)°) are larger than Ge2ÀGe3ÀGe4 and Ge3ÀGe4ÀC2 angles (103.90(5)°, 105.6(3)°) to cause more severe deviation of Ge3 and Ge4 from the coordination plane than Ge1 and Ge2.…”

Singly Bonded Catenated Germanes: Eighty Years of Progress

“…The structure of 116 was shown to adopt three different crystalline forms depending on the solvent of crystallization, and the structure of 117 was determined in 1975 and has also recently been reinvestigated . The structures of the heptabenzene and ditoluene solvates of 116 have been obtained, and the two structures differ only slightly.…”

Molecular Structures of the Digermanes Me2XGeGeXMe2 (X = Me, Cl, and H): An Ab Initio Study Combined with Experimental Investigation by Raman Spectroscopy and Gas Electron Diffraction

“…However, the conformational equilibrium of permethylcyclohexagermane, where the barrier for the ring inversion is significantly larger because of concerted rotation about six Ge-Ge bonds, has been recently resolved with Raman spectroscopy. 15 Because the conformers could not be resolved by Raman spectroscopy, the conformational properties of 1-3 have also been probed by gas electron diffraction. Because GED has a time scale ∼10 7 times faster than Raman spectroscopy, any additional conformers should be observable.…”

“…In these cases, the low barriers to rotation prevent the observation of separate conformers. However, the conformational equilibrium of permethylcyclohexagermane, where the barrier for the ring inversion is significantly larger because of concerted rotation about six Ge−Ge bonds, has been recently resolved with Raman spectroscopy . Because the conformers could not be resolved by Raman spectroscopy, the conformational properties of 1 – 3 have also been probed by gas electron diffraction.…”

“…13 From a recent theoretical investigation of cyclohexagermane by Hölbling et al, it is known that the barriers for ring inversion calculated with B3LYP are very reasonable when relativistic effects for Ge are included. 15 Without these, the barriers are overestimated by a factor of 4.…”