H. Bock et al., STRUCTURES AND MOLECULAR-PROPERTIES OF ST ERICALLY OVERCROWDED MOLECULES .37. HEXAKIS(TRIMETHYLSILYLMETHYL)BENZENE C6[CH2SI(CH3)3]6, Chemische Berichte, 126(10), 1993, pp. 2237-2241
Both the low first vertical ionization energy of only 740 eV, determin
ed by PE spectroscopy for C6[CH2Si(CH3)3]6, and the blocked substituen
t rotation in its radical cation as proven by ESR/ENDOR spectroscopy s
uggest considerable steric over-crowdedness. To further substantiate t
his interpretation of the molecular properties, the crystal structure
(hexagonal R3BAR, Z = 2) has been determined at 100 K. Accordingly the
bulky trimethylsilyl groups are arranged alternatingly above or below
the C6-ring plane out of which the methylene C centers care twisted b
y +/-9-degrees. Due to the angles CCSi widened to 115-degrees, however
, the intramolecular distances C(H)...(H)C between non-neighboured met
hyl groups of 443 and 452 pm even slightly exceed their van der Waals
radii sum of 400 pm and thus avoid severe spatial congestion.