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Conformations of β-Fluorophenetole and Their Reactivities Studied by Supersonic Jet/REMPI Spectroscopy
journal contribution
posted on 1999-01-29, 00:00 authored by Bryan E. Kohler, Thomas Hellman Morton, Viet Nguyen, Thomas A. ShalerThe structure and spectroscopy of β-fluorophenetole (2-phenoxy-1-fluoroethane, FCH2CH2OPh) 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-1 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
Cs 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.
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vibrational spectroscopyREMPIM øllerbond pathsdensitydeuterium substitutionCI singles calculationsgas phasesantiperiplanar geometriesconformerground statesFCCO torsion angleexcitation spectramethylene carbonselectron densities correlatesgas phasenonplanar structureantiperiplanar substituentsfluorophenetoleFCHAb initio calculationsCHgeometry optimizationsab initio calculationsisomeric differencessynclinalfragmentation patternsNMR measurementsReactivities StudiedUV absorptions
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