Molecular structures of neopentane and di-tert-butylmethane by vapor-phase electron diffraction

Abstract

Although the present molecules are much less strained than the tri-t-butyl member of the series CHn(t-Bu)4-n studied previously, di-t-butylmethane nevertheless exhibits striking steric deformations due to its pair of inescapable GG' conformations. The two adjacent t-butyl groups respond to the steric stress by undergoing torsional displacements of 15 +/- 6[deg] (3[sigma]), by tilting away from each other by 3-5[deg], and by opening up the central CCC bond angle to 125-128[deg] (parameter value sensitive to assumptions in analysis). Carbon-carbon bonds, with mean lengths of 1.545 +/- 0.005 A, are stretched on the average by about 0.008 A from the neopentane reference value. Derived molecular parameters are in substantial agreement with values calculated by molecular mechanics using model fields MUB-1 and MUB-2. The methylene 13C-H nmr coupling constant was found to be 125 Hz, a value indistinguishable from those reported for unstrained alkanes but not in accord with predictions from the formulas of Foote or Mislow for severely distorted methylene groups.Molecular parameters for neopentane included rg(C-C) = 1.534 +/- 0.003 A, rg(C-H) = 1.114 +/- 0.008 A and [angle]CCH = 112 +/- 3[deg]. The new value for the C-C bond satisfactorily resolves a discrepancy between previously reported bond lengths. These had disagreed significantly with each other but, to within their uncertainties, they are consistent with the new, intermediate value. Amplitudes of vibration were determined for both neopentane and di-t-butylmethane.