Structural and Conformational Properties of 2-Propenylgermane (Allylgermane) Studied by Microwave and Infrared Spectroscopy and Quantum Chemical Calculations

The structural and conformational properties of allylgermane have been investigated using Stark and Fourier transform microwave spectroscopies, infrared spectroscopy, and high-level quantum chemical calculations. The parent species H₂CCHCH₂GeH₃ was investigated by microwave spectroscopy and infrared spectroscopy, while three deuterated species, namely, H₂CCDCH₂GeH₃, H₂CCHCHDGeH₃, and H₂CCHCH₂GeD₃, were studied only by infrared spectroscopy. The microwave spectra of the ground vibrational state as well as of the first excited state of the torsion vibration around the sp²−sp³ carbon−carbon bond were assigned for the ⁷⁰Ge, ⁷²Ge, and ⁷⁴Ge isotopomers of one conformer. This rotamer has an anticlinal arrangement for the CCCGe chain of atoms. The infrared spectrum of the gas in the 500−4000 cm^-1 range has been assigned. No evidence of additional rotameric forms other than anticlinal was seen in the microwave and infrared spectra. Several different high-level ab initio and density functional theory calculations have been performed. These calculations indicate that a less stable form, having a synperiplanar conformation of the CCCGe link of atoms, may coexist with the anticlinal form. The energy differences between the synperiplanar and anticlinal forms were calculated to be 5.6−9.2 kJ/mol depending on the computational procedure. The best approximation of the equilibrium structure of the anticlinal rotamer was found in the MP2/aug-cc-pVTZ calculations. The barrier to internal rotation of the germyl group was found to be 6.561(17) kJ/mol, from measurements of the splitting of microwave transitions caused by tunneling of the germyl group through its threefold barrier.