MOLECULAR-STRUCTURE OF 2-FLUOROPHENOL AND 2,6-DIFLUOROPHENOL FROM GAS-PHASE ELECTRON-DIFFRACTION

The molecular structure of 2-fluorophenol and 2,6-difluorophenol have been determined by electron diffraction in the gas phase. The molecular geometries indicate possible formation of a weak intramolecular hydrogen bond between the OH and F substituents in ortho positions. The hydrogen bonds in 2-fluorophenol and 2,6-difluorophenol may be somewhat weaker than in tetrafluorohydroquinone but the relatively large experimental errors hinder a detailed comparison. The respective nearest H...F interactions, at 2.13 +/- 0.05 and 2.05 +/- 0.08 angstrom, are closer than the sums of van der Waals radii, 2.55 angstrom; the C-O-H angles, 101.9 +/- 3.9-degrees and 96.7 +/- 4.3-degrees, are smaller than in phenol. The ring parameters are as follows: 2-fluorophenol r(g)(C-C)mean 1.395 +/- 0.004 angstrom, angle C-C1-C = 119.8 +/- 2.0-degrees, angle C-C2-C = 121.5 +/- 2.1-degrees, angle C-C6-C = 118.2 +/- 2.0-degrees, angle C-C3-C = 119.8 +/- 2.5-degrees; 2,6-difluorophenol r(g)(C-C)mean 1.394 +/- 0.004 angstrom, angle C-C1-C = 117.9 +/- 3.5-degrees, angle C-C2-C = 122.4 +/- 2.5; angle C-C4-C = 119.0 +/- 2.4-degrees. The large uncertainties do not allow the determination of angular ring deformation; however, the ring bond angles are consistent with additivity of the individual substituent effects with no appreciable consequence of substituent interaction within experimental error. The C-F bond lengths (r(g)) are 1.353 +/- 0.012 and 1.352 +/- 0.017 angstrom (mean value), the C-O bond lengths are 1.378 +/- 0.010 and 1.362 +/- 0.0.036 angstrom in 2-fluorophenol and 2,6-difluorophenol, respectively. A second form of 2-fluorophenol, not allowing O-H...F interaction, seems also to be present in the vapor of this substance.