A comparative study of the structure of 2,3-dimethyl-2-butene

Abstract

The molecular structure of 2,3-dimethyl-2-butene (DMB) is calculated by means of molecular mechanics, using two different force fields and by ab initio calculations, using a 4-31G/4-21G basis set. Both calculational methods show good mutual correspondence, except for the orientation of the methyl groups. Comparison with three experimental structure models indicates that the best experimental, static description of the molecule is given by Tokue et al. [5]. The r g parameters of this model, a planar C skeleton with four methyl hydrogens eclipsing the double bond, agree very well with the theoretical models, after conversion to r g geometry. The calculations point to a shallow potential around this eclipsed D 2h form and to complicated large amplitude motions connected with the methyl groups. The latter aspect makes it impossible to prove the existence of deviations from eclipsed D 2h by electron diffraction alone. In fact, simulated experiments show that the present precision of an electron diffraction analysis is insufficient to reveal reliably structural details beyond those already given [5], nor to discriminate between the results of ab initio and molecular mechanics calculations. The calculations provide arguments to resolve a conflict in the literature regarding the value of the barrier to methyl rotation. A value around 1 kcal mol −1 is favoured.