Two conformers observed and characterized in isobutylbenzene.

The microwave spectrum of isobutylbenzene (2-methyl-1-phenylpropane) reveals the presence of two conformers that are characterized by their microwave spectra and by quantum chemical calculations. The more stable conformer has a gauche configuration of the C(phenyl)-C(1)-C(2)-H chain coupled with a approximately 80 degrees dihedral angle between the phenyl group and the C(phenyl)-C(1)-C(2) plane with C(1) symmetry. The less stable conformer has a plane of symmetry, C(s), with an anti configuration of the C(phenyl)-C(1)-C(2)-H chain coupled with a 90 degrees dihedral angle between the phenyl group and the C(phenyl)-C(1)-C(2) plane. The rotational constant values are 3070.9273(4) MHz, 736.01980(6) MHz, and 680.92889(6) MHz for the C(1) species and 2500.780(8) MHz, 885.72743(10) MHz, and 770.42036(10) MHz for the C(s) species. Quantum chemical calculations are in agreement with these structures and predict a relative energy between those two conformers of 0.4 kcal/mol.