Conformational studies by dynamic NMR. 7. Stereochemical processes in 2,3-dimethylbutane

The 90.5-MHz 13C NMR spectrum of 2,3-dimethylbutane at -180°C shows that the gauche and anti rotational isomers are present in the statistical ratio of 2:1. Line shape analysis of the methine signals gives a free energy of activation of 4.3 ± 0.2 kcal/mol for the anti-gauche interconversion. These results are supported by ab initio SCF-MO calculations which reproduce the anti-gauche rotational barrier and predict that the gauche-gauche barrier would be ca. 8 kcal/mol. Accordingly a direct gauche-gauche interconversion can be ruled out and the gauche rotamers must interchange via the anti rotamer.