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

Abstract

Resonance-enhanced two-photon ionization (R2PI) and massanalyzed 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 S 0 , S 1 , and D 0 states. Both rotamers adopt planar structures with cis-being slightly more stable than trans-in the respective S 0 , S 1 , and D 0 states. The conformation, substitution, and isotope effects on the molecular structure, active vibrations, and electronic transition and ionization energies of 3ClTA are analyzed.

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

“…They are consistent with the 1C-R2PI spectroscopic measurement (Figure ) that the trans -0 0 0 band is slightly stronger than that of the trans -torsion mode τ 0 1 at 44 cm –1 . This is similar to the case of 3ClTA, for which both theoretical calculations and spectroscopic measurements confirmed its planarity in the S 1 state . The 0 + band is the most prominent peak in the MATI spectrum of trans -3FTA via the 0 0 0 level, as shown in Figure a.…”

“…In addition to the −SCH 3 group, the substitution effect of the F atom may also contribute. Our previous study showed that the torsion vibrational peaks of both cis - and trans -3ClTA in the R2PI spectra were much weaker than that of the 0 0 0 bands . The electron-withdrawing effect of the F atom predominates over the electron-donating effect, which increases instead of neutralizing the positive hole on the benzene ring .…”

“…This is opposite to the substitution effect caused by the Cl atom in 3ClTA, for which the electron-donating effect predominates. Such effects could be reflected by the calculated atomic charges, e.g., the charge on the F atom in 3FTA is in the range of −0.27 to −0.34 e (Table ), while it is −0.04 to 0.12 e on the Cl atom in 3ClTA . The ESP maps in Figure S3 clearly show that the ESP around the F atom is more negative than that around Cl.…”

“…However, several previous studies using ultraviolet photoelectron and Raman spectroscopy also show that both planar and perpendicular (nonplanar) rotamers could coexist in the S 0 state. − For TA derivatives, both planar and nonplanar structures have been observed. For example, the trans- 2-fluorothioanisole molecule (2FTA) was found to be planar in the S 0 and D 0 states, while it is slightly distorted in the S 1 state; stable planar cis- and trans- rotamers were observed in each of the S 0 , S 1 , and D 0 states for 3-chlorothioanisole (3ClTA) . The stable structures for most anisole − derivatives have been found to be planar, while stable nonplanar structures were determined for C 6 H 5 SCF 3 − and N-methylaniline (NMA) derivatives. − …”

“…For example, the trans-2fluorothioanisole molecule (2FTA) was found to be planar in the S 0 and D 0 states, while it is slightly distorted in the S 1 state; 11 stable planar cis-and trans-rotamers were observed in each of the S 0 , S 1 , and D 0 states for 3-chlorothioanisole (3ClTA). 12 The stable structures for most anisole 13−18 derivatives have been found to be planar, while stable nonplanar structures were determined for C 6 H 5 SCF 3 19−21 and N-methylaniline (NMA) derivatives. 22−25 The intrinsic nature, orientation, and relative position of the substituted functional groups are key factors in determining the stable structures of benzene derivatives.…”