Geometries and rotation barriers of 2-methyl-1,3-butadiene and of 2,3-dimethyl-1,3-butadiene by ab initio gradient calculation

Abstract

The structures of 2-methyl-1,3-butadiene (isoprene) and 2,3-dimethyl-1,3-butadiene have been determined by ab initio gradient computation at the double-zeta Hartree-Fock level as a function of torsional angle around the central CC bond. Both compounds are most stable in the anti conformation and have subsidiary minima at a gauche conformation approximately 40° from syn. The dependence on torsional angle of the structural parameters and of the total energy is discussed together with similar results previously obtained for the parent molecule, butadiene. In terms of classical effects, the CC and CC bond lengths can be attributed mainly to hybridization of the carbon atoms. Resonance makes a minor contribution toward shortening the single bond and lengthening the double bonds in the planar forms and is mainly responsible for the rotational barrier around the central CC bond, since the absence of coplanarity at 90° hinders electron delocalization in the π system. Stability of the syn conformations is strongly decreased by CH 2⋯CH 2, CH 2⋯CH 3 and CH 3⋯CH 3 steric interactions, which are of comparable strength.