Conformational Equilibrium and Internal Dynamics of E-Anethole: A Rotational Study

The rotational spectra of the two conformers of E-anethole have been investigated using the free jet broadband millimeter-wave spectroscopic technique combined with theoretical calculations. Anti and syn conformers differ for the relative orientation of the propenyl and methoxy chains, with all heavy atoms coplanar to the benzene ring. Relative intensity measurements prove that the anti form is the global minimum, about 2.0(5) kJ mol-1 lower in energy with respect to the syn conformer, solving the contrasting results supplied by different theoretical methods. For both conformers, the barriers to internal rotation of the propenyl -CH3 group are low enough to generate fully resolved A-E splittings of the rotational transitions. The corresponding V3 barriers have been determined to be 7.080(5) and 6.978(4) kJ mol-1, respectively.