Vibrational Spectra and Theoretical Calculations of cis- and trans-3-Fluoro-N-methylaniline in the Neutral (S0) and Cationic (D0) Ground States

2019

ChemistryOpen

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Multiple approaches reveal the strong effects of a positive charge introduced by protonation or ionization on the conformation of o ‐heteroaromatic ethers and amides. The ethers and amides containing an ortho ‐N heteroatom are syn ‐preferring while those containing an ortho ‐O or ortho ‐S heteroatom are mostly anti ‐preferring. However, for all the monocyclic o ‐heteroaromatic ethers and amides, the protonated ones are all anti ‐preferring while the ionized ones are all syn ‐preferring. Interestingly, although both the protonation and ionization introduce a positive charge, they have such different effects on molecular conformation, very informative for understanding the origin of conformational preferences. Detailed analysis shows that the population of the introduced positive charge dictates the conformational preferences via electrostatic and orbital interactions. Compared to ortho ‐heteroatoms, meta ‐heteroatoms have weaker effect on conformational preference. Achieved by complete inductive method, the regularity of conformational preferences and switching provides easy ways to modulate conformers (by pH or redox), and makes this kind of ether or amide bond a conformational hinge applicable to design of functional molecules (drugs and materials) and modulation of molecular biological processes.

Section: Conformational Preferences Of O-heteroaromatic Ethers: Effecsupporting

confidence: 61%

Modulation of Conformational Preferences of Heteroaromatic Ethers and Amides through Protonation and Ionization: Charge Effect

Dai

2019

ChemistryOpen

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“…3, cis 2MOP is more stable than trans 2MOP in the S 0 , S 1 and D 0 states, suggesting the existence of the syn preference in all the three electronic states. It is interesting to know whether the syn conformational preference found for many compounds in the S 0 state 12,16,18,25 will remain in the S 1 and D 0 states. The question seems still open and some results of qualitative estimations are listed in Table 2.…”

Section: C-r2pi Spectra Of 2mop and 2nmpmentioning

confidence: 99%

“…Weersink et al 17 examined the excitation spectrum of N,N-dimethylaniline to analyze geometrical shifts and proposed that the structural change along torsional motions of the two CH 3 rotors upon excitation is responsible for the weak origin band. Recently, we found that the ionization could alter the relative stability of 3-fluoro-N-methylaniline 18 conformers related to the rotation of the methylamino group, giving a stable cis in the S 0 state but a stable trans in the D 0 state. While spectroscopic and theoretical investigations 19 on conformers related to the CH 3 rotation demonstrated that for diethyl ether (DEE), the neutral state has a stable conformer quite different from that of the cationic state, but for diethyl sulfide (DES), the two states have similar conformers.…”

Section: Introductionmentioning

confidence: 99%

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

Dai

Liu

Phys. Chem. Chem. Phys.

et al. 2018

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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 S, S and D states. However, for cis 2NMP, the stable conformer is staggered in the S state but eclipsed in the S and D states, indicating an isomerization upon the excitation or ionization from the S 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 S and S states but stable cis 2NMP in the D state. For 2MOP, the so-called syn preference previously found for the S state is also observed in the S and D 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 OCH 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.

“…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. − …”

Section: Introductionmentioning

confidence: 97%

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

Hou

et al. 2022

J. Phys. Chem. A

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.