Jet-cooled fluorescence spectroscopy of a natural product: anethole

The jet-cooled fluorescence spectroscopy of the natural product molecule anethole ((E)-1-methoxy-4-(1-propenyl)benzene) has been studied. Single vibronic level fluorescence spectroscopy was used to verify the existence of two rotamers, syn and anti, with electronic origins at 32,889 and 32,958 cm(-1), respectively. The excitation and emission spectra show characteristics similar to those of styrene and styrene derivatives, including Cs symmetry and low amplitude motions of the propenyl (vinyl) group. As in styrene, the low amplitude modes show substantial Duschinsky mixing. Interestingly, the methoxy group shows very little activity in the spectroscopy of anethole but is found to influence the activity of the propenyl group. This activity is easily observed in the Franck-Condon activity of the propenyl-bending mode. Differences are explained using simple molecular orbital arguments. The observed torsional structure was modeled and compared to ab initio calculations, allowing us to determine a barrier to internal rotation of 623 cm(-1) for the propenyl group, in agreement with similar molecules. Calculated potential energy surfaces (using density functional theory) were used to construct a more complete representation of the torsion surface of anethole, incorporating the torsions of both substituents. Several anomalous features of the excitation spectra were assigned to van der Waals clusters of anethole with water. The assignments and analyses presented here are also consistent with density function calculations.