Rotamers of <em>m</em>-Methylanisole Studied by Mass-Analyzed Threshold Ionization Spectroscopy

Abstract

The S1←S0 electronic transition and threshold ionization of the cis and trans rotamers of mmethylanisole were investigated by using one-color resonant two-photon ionization and mass-analyzed threshold ionization techniques. The first electronic excitation energies (E1) of the cis and trans rotamers were determined to be (36049 ± 2) and (36117 ± 2) cm-1 , while the adiabatic ionization energies (Ip) were (64859 ± 5) and (65110 ± 5) cm-1 , respectively. The results of ab initio and density functional theory calculations provide a satisfactory interpretation for our experimental findings concerning the difference in the transitional energies of the cis and trans rotamers and assist in assigning the vibronic and cation spectra obtained in the present study. The observed active vibrations of both rotamers in the S1 and D0 states primarily consist of methyl torsion, in-plane ring deformation, and substituent-sensitive bending modes. Both experimental and theoretical results show that, for both cis and trans isomers, the geometry of the cation in the D0 state is somewhat different from that of the neutral species in the S1 state. In addition, the strengths of both the through-space substituent-substituent and substituent-ring interactions were found to follow the order: S0<S1<D0.

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

“…This indicates that the molecular geometry and the corresponding vibrational coordinates of the 6a 0 1 , ν s 1 , and 6a 0 1 τ 0 1 modes do not change much upon photoionization from the S 1 state to the D 0 state, as verified by the RMSD analysis (Figure S2). Similar phenomena via the benzene-ring-related vibrational levels were previously observed for anisole and its derivatives. ,,,, However, when the methyl torsion mode τ 0 1 was used as the intermediate level, the MATI spectra showed dramatically different characteristics as those via the benzene-ring-related vibrational levels. For the cis- rotamer, the most prominent peak was due to the 0 + band, and many other vibrational modes were also significantly excited (Figure a).…”

“…The current study along with several previous studies on other anisole derivatives ,,, and TA derivatives , show that the frequencies of the benzene-ring-related modes are not very sensitive to the orientations of the substituents. In this study, the frequencies of the substituent-sensitive modes, e.g., the methyl torsion mode τ 0 1 , are measured for both the cis- and trans- 3ClTA.…”

“…Similar phenomena via the benzene-ring-related vibrational levels were previously observed for anisole and its derivatives. 1,17,21,23,30 However, when the methyl torsion mode τ 0 1 was used as the intermediate level, the MATI spectra showed dramatically different characteristics as those via the benzenering-related vibrational levels. For the cis-rotamer, the most prominent peak was due to the 0 + band, and many other vibrational modes were also significantly excited (Figure 7a).…”

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

“…This is in accordance with the calculated results that the C2–C1–S–CH 3 dihedral angle for trans -2FTA in the S 1 state was reported to be only 15°, while it was close to 90° for gauche -3FTA as found in this study. For anisole derivatives, ,,,,, the torsion vibration was only observed previously to be active for 2-fluoroanisole . Compared with the substituent groups of −OCH 3 , −NH 2 , and −NHCH 3 , the S atom in the −SCH 3 group has a larger size, making the −SCH 3 group more flexible.…”