Position-and conformation-dependent effects of fluorination on the structure and methyl internal rotation of methyl benzoates

The effects of fluorination on molecular structure and internal dynamics were investigated in monofluorinated methyl benzoates (methyl 2-fluorobenzoate, methyl 3-fluorobenzoate, and methyl 4-fluorobenzoate) using high-resolution rotational spectroscopy in combination with quantum-chemical calculations. Two conformers were identified for methyl 2-fluorobenzoate, whereas only one conformer each was observed for the 3- and 4-fluorinated species. All observed conformers exhibited A/E torsional splittings arising from internal rotation of the ester methyl rotor and were analyzed using the XIAM code. The experimentally derived barriers V3 lie in the range of 425–460 cm−1, exhibiting both position- and conformation-dependent behavior. The syn conformer of methyl 2-fluorobenzoate exhibits the largest barrier, whereas the barriers for anti-methyl 2-fluorobenzoate, anti-methyl 3-fluorobenzoate, and methyl 4-fluorobenzoate are close to that of methyl benzoate. Quantum-chemically predicted equilibrium structures indicate that the structural perturbations induced by fluorination are primarily localized near the substitution site. In the syn conformer of methyl 2-fluorobenzoate, the shorter F⋯HCH3 distance places the negative electrostatic region around fluorine closer to the positively polarized methyl hydrogen atoms. This proximity is consistent with enhanced local interactions that may contribute to its larger torsional barrier. These results demonstrate that the effect of fluorination on methyl internal rotation depends not only on the substitution position but also on the conformer-specific spatial arrangement of fluorine relative to the ester methyl rotor.