Conformations of β-Fluorophenetole and Their Reactivities Studied by Supersonic Jet/REMPI Spectroscopy

The structure and spectroscopy of β-fluorophenetole (2-phenoxy-1-fluoroethane, FCH<sub>2</sub>CH<sub>2</sub>OPh) have been studied by X-ray crystallography and Raman scattering of the solid and by resonance-enhanced multiphoton ionization (REMPI) excitation spectra of a supersonic-jet-cooled gaseous sample, as well as by ab initio calculations. Fluorine and oxygen are synclinal (with an FCCO torsion angle near 70°) in the dominant conformational isomer for both the crystalline and gas phases. The minor conformer observed in the gas phase has antiperiplanar substituents (FCCO torsion angle = 180°), with a relative abundance comparable to that previously inferred from NMR measurements in solution. Hartree−Fock-based computations, as well as second-order Møller−Plesset and density functional geometry optimizations, predict the structural features closely, and the computed (unscaled) normal modes ≤350 cm<sup>-1</sup> have frequencies not far from those measured by vibrational spectroscopy. CI singles calculations give reasonable estimates of the isomeric differences in the UV absorptions and fit the observed overtones well, though they err in predicting the absolute wavelengths. Ab initio calculations of the electronic ground states do not give a useful ordering of the relative energies of the conformational isomers, for they predict high stability for a highly nonplanar structure for which no experimental evidence is seen. Atoms-in-molecules analysis of theoretical electron densities correlates the preferences for synclinal versus antiperiplanar geometries (in 1-phenoxypropane as well as β-fluorophenetole) with double bowing of the bond paths between two methylene carbons, which (in-planar conformations with <i>C</i><i><sub>s</sub></i> symmetry) cut across the lines of centers. Time-of-flight mass spectrometry of isotopically substituted analogues ionized by REMPI shows that deuterium substitution does not decelerate the rate of decomposition of radical cations nor do different conformers manifest any differences in their fragmentation patterns.