Conformations of beta-fluorophenetole and their reactivities studied by supersonic jet REMPI spectroscopy

The structure and spectroscopy of beta-fluorophenetole (2-phenoxy-1-fluoroe thane, FCH2CH2OPh) have been studied by X-ray crystallography and Raman sca ttering of the solid and by resonance-enhanced multiphoton ionization (REMP I) 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 degrees) in the dominant conformational isomer for b oth the crystalline and gas phases. The minor conformer observed in the gas phase has antiperiplanar substituents (FCCO torsion angle = 180 degrees), with a relative abundance comparable to that previously inferred from NMR m easurements in solution. Hartree-Fock-based computations, as well as second -order Moller-Plesset and density functional geometry optimizations, predic t the structural features closely, and the computed (unscaled) normal modes less than or equal to 350 cm(-1) have frequencies not far from those measu red by vibrational spectroscopy. CI singles calculations give reasonable es timates of the isomeric differences in the UV absorptions and fit the obser ved overtones well, though they err in predicting the absolute wavelengths. Ab initio calculations of the electronic ground states do not give a usefu l ordering of the relative energies of the conformational isomers, for they predict high stability for a highly nonplanar structure for which no exper imental evidence is seen. Atoms-in-molecules analysis of theoretical electr on densities correlates the preferences for synclinal versus antiperiplanar geometries (in 1-phenoxypropane as well as beta-fluorophenetole) with doub le bowing of the bond paths between two methylene carbons, which (in-planar conformations with C-g symmetry) cut across the lines of centers. Time-of- flight mass spectrometry of isotopically substituted analogues ionized by R EMPI shows that deuterium substitution does not decelerate the rate of deco mposition of radical cations nor do different conformers manifest any diffe rences in their fragmentation patterns.