Resonance-enhanced two-photon ionization spectroscopy and theoretical calculations of 3,5-difluoroanisole and its Ar-containing complex

Substitution-induced Nonplanarity of 3-Fluorothioanisole in the First Electronically Excited State

Vibronic spectra of 3-fluorothioanisole (3FTA) in the first electronic excited state (S₁) and the cationic ground state (D₀) have been obtained by one-color resonant two-photon ionization (1C-R2PI) and mass-analyzed threshold ionization (MATI) spectroscopy. Spectroscopic measurements and theoretical calculations indicate that both cis- and trans-rotamers of the 3FTA molecule are stable and coexist in the S₀ (the electronic ground state) and D₀ states, and the cis-rotamer is shown to be slightly more stable than the trans-rotamer. In the S₁ state, theoretical calculations predict a stable gauche-structure of 3FTA, manifested by the observation of strong activation of the vibrational modes involving the motion of the -SCH₃ group in the low-frequency regions of the 1C-R2PI and MATI spectra. The electronic excitation energy from the S₀ state to the S₁ state (E₁) and the adiabatic ionization energy (IE) are respectively determined to be 34 820 ± 3 and 65 468 ± 5 cm^-1 for cis-3FTA, and those of the trans-rotamer are respectively determined to be 35 047 ± 3 and 65 644 ± 5 cm^-1. The structural properties of the stable rotamers of 3FTA and their comparison with other F- and Cl-substituted thioanisole derivatives are discussed as well.

Photoionization Spectroscopic and Theoretical Study on the Molecular Structures of cis- and trans-3-Chlorothioanisole

Resonance-enhanced two-photon ionization (R2PI) and mass-analyzed threshold ionization (MATI) spectra are measured for the cis- and trans-3-chlorothioanisole (3ClTA). The first electronic excitation energy (E ₁) and the adiabatic ionization energy (IE) of the cis-rotamer are determined to be 33 959±3 and 65 326±5 cm^-1, respectively, and those of the trans-rotamer are determined to be 34102±3 and 65 471±5 cm^-1, respectively. Density functional theory (DFT) calculations confirm that both the cis- and trans-rotamers of 3ClTA are stable and coexist in their respective S₀, S₁, and D₀ states. Both rotamers adopt planar structures with cis- being slightly more stable than trans- in the respective S₀, S₁, and D₀ states. The conformation, substitution, and isotope effects on the molecular structure, active vibrations, and electronic transition and ionization energies of 3ClTA are analyzed.

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

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.

Rotamers and isotopomers of 3-chloro-5-fluoroanisole studied by resonant two-photon ionization spectroscopy and theoretical calculations

The ab initio and density functional theory (DFT) calculations reveal that two rotamers, denoted by cis and trans 3-chloro-5-fluoroanisole (3C5FA), are stable for each of the S(0), S(1), and D(0) states. In the one-color resonant two-photon ionization (R2PI) spectra, the band origins of the S(1)←S(0) electronic transition (0(0) bands) of cis(35)Cl-3C5FA and cis(37)Cl-3C5FA are both located at 36,468 ± 3 cm(-1), while the 0(0) bands of trans(35)Cl-3C5FA and trans(37)Cl-3C5FA are found to be 36,351 ± 3 and 36,354 ± 3 cm(-1). The two rotamers display very similar vibrational frequencies in the S(1) state, and the observed active modes mainly involve the in-plane ring deformation vibrations. By the two-color R2PI spectroscopy, the adiabatic ionization energies (IEs) of both isotopomers of 3C5FA are determined to be 69,720 ± 15 cm(-1) for the cis rotamer and 69,636 ± 15 cm(-1) for the trans rotamer. The substitution, conformation, and isotope effects on the properties of 3C5FA, including the molecular structures, vibrational frequencies, and electronic transition and ionization energies, were also discussed in detail.