Structures of the strained molecules hexamethylethane and 1,1,2,2-tetramethylethane by gas-phase electron diffraction

Abstract

Hexamethylethane has bond lengths of rg(C-C central) = 1.582 +/- 0.01 A, rg (C-C terminal) = 1.542 +/- 0.002 A, rg (C-H) = 1.113 +/- 0.004 A, and bond angles of [angle]CcCcCt = 111.0 +/- 0.3[deg] and [angle]CCH = 111.5 +/- 1.4[deg] (uncertainties 2[sigma]). It suffers a mean twist from Dsd symmetry of 5 +/- 4[deg]. Tetramethylethane is approximately 60 % gauche, 40 % trans, in composition in the gas-phase with bond lengths rg (C-C central, trans) = 1.544 +/- 0.006 A, rg (C-C central, gauche)-rg (C-C central, trans) = 0.002 A (assumed), rg (C-C terminal) = 1.539 +/- 0.002 A, rg (C-H ave) = 1.115 +/- 0.004 A, and angles are distributed around the average angle of 111.35 +/- 0.4[deg] in accord with a picture of steric interactions. The gauche conformer is twisted 65 +/- 5[deg] from the eclipsed configuration. Amplitudes of vibration were determined for both molecules. The structure of (CH3)3BN( CH3)3 is considered in the light of (CH3)3CC( CH3)3 results and it is concluded that the B-N length is intermediate between the values proposed by Lide and by Geller.