Conformational stability, r0 structural parameters, ab initio calculations, and vibrational assignment for germacyclopentane

The infrared spectra (3500-50 cm(-1)) of the gas and solid and the Raman spectrum (3200-30 cm(-1)) of liquid germacyclopentane, c-C4H8GeH2, have been recorded. Additionally, the infrared spectra (3200-400 cm(-1)) of liquid xenon solutions have been recorded at -65 and -95 degrees C. In all of the physical states only the twisted C-2 conformer was detected. The conformational energetics have been calculated with the Moller-Plesset perturbation method to the second order, (MP2(full)) as well as with density functional theory by the B3LYP method utilizing a variety of basis sets up to 6-311G(2d,2p). All of these calculations predict only the twisted conformer as the stable form with an average barrier to planarity of 2756 cm-1 (32.97 kJ/mol) from the MP2 calculations and a significantly lower value of 2128 cm-1 (25.45 kJ/mol) from the DFT calculations with neither calculations being significantly effect by inclusion of diffuse functions. The C-s conformer (envelope) has a lower energy of 700 cm(-1) (MP2) and 400 cm(-1) (B3LYP) than the planar form. Thus, the path between the two identical C-2 conformers is by a pseudorotational motion rather than through the planar form. From the isolated GeH frequency from the GeHD isotopomer the GeH distance was calculated to be 1.532(2) A. By utilizing the previously reported microwave rotational constants for four isotopomers (Ge-70, Ge-72, Ge-73, Ge-74) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r(0) structural parameters were obtained. The determined heavy atom distances are: r(0)(GeC) = 1.972(5); r(0)(C2C4) and (C3C5) = 1.541(5); and r(0)(C4C5) = 1.533(5) A and the angles in degrees: angle CGeC = 93.6(5)degrees; angle GeCC = 102.6(5)degrees; angle CCC = 109.4(5)degrees with the two dihedral angles angle GeCCC = -38.4(3)degrees and angle CCCC = 53.7(3)degrees. A complete vibrational assignment is given for the twisted (C2) conformer for the normal species and the GeD2 isotopomer which are supported by normal coordinate calculations with scaled force constants from MP2(full)/6-31G(d) calculations. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules. (C) 2006 Elsevier B.V. All rights reserved.