The conformational behavior of isobutylbenzene in CS2/C6D12 solution at 300 K

The 1H nuclear magnetic resonance spectrum of isobutylbenzene as a dilute solution in CS2/C6D12 at 300 K is analyzed. The gauche conformer (statistical weight 2) predominates, as was previously shown for a jet-cooled stream of this compound. The apparent twofold barrier to rotation about the Csp2—Csp3 bond is 10.5 ± 0.5 kJ/mol, as must be the case for the analogous gauche conformer of n-propylbenzene. The internal barriers are compared for ethylbenzene, gauche isobutylbenzene, and neopentylbenzene. AM1 and STO-3G MO computations of the conformer stabilities and barriers are reported, the latter being in better agreement with experiment.