R2PI and MATI spectroscopy of 3-fluoro-5-methylanisole: Combined effect of meta-substituents on molecular conformation.

Abstract

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-CH3) and methoxy group (OCH3), namely the staggered (s)/eclipsed (e)-cis/trans 3F5MA. The theoretical calculations predicted that the stable conformer of cis 3F5MA is staggered in the S0 and S1 states, but eclipsed in the D0 state. While for trans 3F5MA, the staggered conformer is stable only in the S1 state and the eclipsed one is stable in the S0 and D0 states. The first electronic excitation energies (E1s) of cis and trans 3F5MA were determined to be 36,709 ± 3 and 36,615 ± 3 cm-1 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-1. Compared with the cis 3F5MA, more low-frequency vibronic bands assigned to m-CH3 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-CH3 torsion modes, revealing the s-cis → e-cis and s-trans → e-trans isomerizations upon the D0 ← S1 ionization. By comparing with several analogues, it is found that the s/e conformational preference in each electronic state is mainly influenced by OCH3 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 S0 and S1 states but false for the D0 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.