ON THE STRUCTURE OF BIS(9-FLUORENYL)DIMETHYLSILANE - A STERICALLY CROWDED MOLECULE WITH RELATIVELY LOW BARRIERS OF ROTATIONS AROUND THE SI-C(FLUORENYL) BONDS - X-RAY-DIFFRACTION ANALYSIS AND AM1 MOLECULAR-ORBITAL CALCULATIONS

The X-ray structure of bis(9-fluorenyl)dimethylsilane, the first silan e compound containing two voluminous fluorenyl groups, has been determ ined. The crystal system is orthorhombic with Aba2 space group. The un it cell (a = 18.672, b = 36.089, c = 12.802 Angstrom) contains two set s of four molecules, each one of one set being symmetrically related w ith the others of the same set. The conformations around the HC(fluore nyl)SiC(fluorenyl) bonds may be grossly described as in-in, in-out and out-out. The AM1 calculations have shown that the barrier between the in-out and in-in conformations is relatively low while that between t he in-out and out-out is higher; thus, probably, the first two are the most populated in solution. These results provide a better understand ing of some unusual chemical shifts previously observed in H-1-NMR for Me2Si(CHR2)(2) and Me2Si(CHR2)(CR'R-2). (C) 1997 Elsevier Science B.V .