Structure and vibrations of 2-fluoro-N-methylaniline in the S0, S1 and D0 states: REMPI and MATI spectroscopy and theoretical calculations

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.

Conformational preferences and isomerization upon excitation/ionization of 2-methoxypyridine and 2-N-methylaminopyridine

Conformers from the rotations of the methyl group and the methoxy or methylamino group, namely staggered (s)/eclipsed (e)-cis/trans 2-methoxypyridine (2MOP) and 2-N-methylaminopyridine (2NMP), are studied using theoretical calculations in combination with one-color resonance-enhanced two-photon ionization (1C-R2PI) and mass-analyzed threshold ionization (MATI) spectroscopies. The calculations predict that, for cis 2MOP, trans 2MOP and trans 2NMP, only the s conformers are stable in the S₀, S₁ and D₀ states. However, for cis 2NMP, the stable conformer is staggered in the S₀ state but eclipsed in the S₁ and D₀ states, indicating an isomerization upon the excitation or ionization from the S₀ state. This is experimentally supported by the 1C-R2PI and MATI spectra of 2NMP. Due to the relative instability, the number density of trans 2MOP is too low in the sample to be detected. All the bands in the 1C-R2PI and MATI spectra of 2MOP are assigned to s-cis 2MOP. The energy differences between cis and trans conformers are derived from excitation and ionization energies, indicating another conformational isomerization: stable trans 2NMP in the S₀ and S₁ states but stable cis 2NMP in the D₀ state. For 2MOP, the so-called syn preference previously found for the S₀ state is also observed in the S₁ and D₀ states. The conformational preference and isomerization are discussed with natural bond orbital calculations and reduced density gradient analysis. For 2MOP, the syn preferences are mainly caused by the exchange repulsion among several σ-orbitals of the OCH₃ group and the pyridine ring. While the relative stabilities of the s and e conformers of cis 2MOP and cis 2NMP are simultaneously influenced by steric repulsion and orbital interactions.