Jet-Cooled Fluorescence Spectroscopy of a Natural Product: Anethole

Abstract

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.

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

“…The ∠( i , a ) angles (see Table ) have small values for both conformers, in agreement with what is expected for the propenyl CH 3 groups (whose internal rotation axes are nearly parallel to the a principle axes). The experimental values of the V 3 barriers to internal rotation of the propenyl CH 3 top ( V 3 anti = 7.080(5) kJ mol –1 and V 3 syn = 6.978(4) kJ mol –1 ) are in agreement with the theoretical values (see Table ) and have a considerably smaller uncertainty with respect to the value (7.45 kJ mol –1 ), common to both conformers, reported in the fluorescence spectroscopy study . The obtained V 3 values are ca.…”

“…The experimental values of the V 3 barriers to internal rotation of the propenyl CH 3 top (V 3 anti = 7.080(5) kJ mol −1 and V 3 syn = 6.978(4) kJ mol −1 ) are in agreement with the theoretical values (see Table 1) and have a considerably smaller uncertainty with respect to the value (7.45 kJ mol −1 ), common to both conformers, reported in the fluorescence spectroscopy study. 26 The obtained V 3 values are ca. 10 and 20% lower, respectively, than the corresponding parameters of the isopropenyl methyl groups in propene 37 and isopropropenyl acetate.…”

“…25 The biological nature of E-anethole (E-An hereinafter) has motivated its investigation using different techniques: jet-cooled fluorescence spectroscopy, 26 matrix-isolation IR, 27 FTIR and FT-Raman spectroscopy. 28 The authors discussed structures and energies of the two most stable conformers mainly basing on theoretical calculations.…”

“…In nature, plants synthesize the E isomer almost exclusively, which holds the desired odor and taste of anethole, while the Z form is toxic and possesses an unpleasant scent and flavor . The biological nature of E -anethole ( E -An hereinafter) has motivated its investigation using different techniques: jet-cooled fluorescence spectroscopy, matrix-isolation IR, FTIR, and FT-Raman spectroscopy . The authors discussed the structures and energies of the two most stable conformers mainly based on theoretical calculations.…”

Experimental and theoretical study on the excited-state dynamics of ortho-, meta-, and para-methoxy methylcinnamate

“…and 32 958 cm −1 , respectively, giving the energy separation of 69 cm −1 . 31 Therefore, the observed species are ascribed to three distinct conformers out of four conformers and we assigned them by comparing (i) the calculated relative S 0 energies with the R2PI intensities and (ii) the calculated S 1 -S 0 electronic transition energies with the UV frequencies of the observed 0-0 bands. For instance, band C was assigned to the s-cis/anti conformer as it shows the strongest band intensity and appears at the highest frequency among three species in the R2PI spectrum.…”