EXPERIMENTAL AND THEORETICAL INVESTIGATION OF THE MOLECULAR-STRUCTURE OF CYCLOPROPYLGERMANE

In this work the gas-phase molecular structure of cyclopropylgermane (CPG) has been investigated by electron diffraction and ab initio molecular orbital calculations. The geometry optimization has been performed with use of the basis sets 3-21G*, 4-21G*, and STO-3G. The ring parameters (r(a) values with 3-sigma uncertainties) derived from the electron diffraction study are the following: r(C1-C2) = 1.521 (7) angstrom, r(C2-C3) = 1.502 (9) angstrom, r(C-H) = 1.091 (3) angstrom, and angle H-C-H = 118.2 (2.3)-degrees. Other parameters are r(Ge-C) = 1.924 (2) angstrom, angle C1-Ge-H6 = 108.8 (1.2)-degrees, angle C1-Ge-ring plane = 55.5 (1.6)-degrees, angle C1-H5-ring plane = 57.3 (1.9)-degrees. Furthermore, both the experimental and the theoretical studies have revealed that the GeH3 group is tilted toward the ring plane. The values for this tilt angle obtained from electron diffraction and from calculations are 3.4 (2.0)-degrees and 2.1-degrees (when the 4-21G* basis set is used), respectively. This tilt has been rationalized to be the result of hyperconjugative interaction. The geometric parameters of cyclopropylmonofluorogermane, cyclopropyldifluorogermane, and cyclopropyltri-fluorogermane also have been optimized. The progressive shortening of the Ge-F bond with increasing fluorination is interpreted as being the consequence of a fluorine negative hyperconjugation effect. The barrier heights for internal rotation for the GeH3 and SiH3 groups and their fluorinated counterparts in various compounds have been calculated. Furthermore, the structural results obtained assess the strong pi-donor character of the cyclopropyl system and demonstrate that pi-acceptor ability of the germyl group is less pronounced than that of the silyl group.