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The resonance-enhanced two-photon ionization (R2PI) and mass-analyzed threshold ionization (MATI) spectra of 3-fluoro-5-methylanisole (3F5MA) were recorded to explore the conformers arising from the rotation of meta-methyl group (m-CH) and methoxy group (OCH), namely the staggered (s)/eclipsed (e)-cis/trans 3F5MA. The theoretical calculations predicted that the stable conformer of cis 3F5MA is staggered in the S and S states, but eclipsed in the D state. While for trans 3F5MA, the staggered conformer is stable only in the S state and the eclipsed one is stable in the S and D states. The first electronic excitation energies (Es) of cis and trans 3F5MA were determined to be 36,709 ± 3 and 36,615 ± 3 cm by the R2PI spectroscopy. Correspondingly, by the MATI spectroscopy, the adiabatic ionization energies (IEs) were measured to be 66,908 ± 5 and 66,692 ± 5 cm. Compared with the cis 3F5MA, more low-frequency vibronic bands assigned to m-CH torsions are observed for the trans conformer in the R2PI spectrum, supporting the theoretically predicated e-trans → s-trans isomerization upon excitation. The MATI spectra of the cis and trans conformers are similar. Most of the observed cationic bands are related to the m-CH torsion modes, revealing the s-cis → e-cis and s-trans → e-trans isomerizations upon the D ← S ionization. By comparing with several analogues, it is found that the s/e conformational preference in each electronic state is mainly influenced by OCH group instead of the F atom. The combined effects of meta-substituents on molecular conformation and transition energies are discussed in detail. A kind of additivity of meta-substituent effects on s/e preference is qualitatively true for the S and S states but false for the D state. This is different from that of ortho-substituent effect, which is previously reported that the additivity of ortho-substituent effects on methyl rotation barriers in 2-fluoro-6-chlorotoluene is applicable to all the three electronic states while the additivity of meta-substituent effect on cis/trans preference is found to be true in all three electronic states of 3F5MA.
This article was published in the following journal.
Name: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy
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