Intramolecular hydrogen bonding in fluorophenol derivatives: 2-fluorophenol, 2,6-difluorophenol, and 2,3,5,6-tetrafluorohydroquinone

Intramolecular F ... H hydrogen bonding in 2-fluorophenol, 2,6-difluorophen ol, and 2,3,5,6-tetrafluorohydroquinone has been studied by ab initio molec ular orbital calculations. Geometry optimizations at the MP2/6-31G** level resulted in two planar local minima on the potential energy surface, charac terized by different orientations of the hydroxy hydrogen. In the conformer s where the hydroxy hydrogen points toward a fluorine, the computations sug gest weak intramolecular F ... H hydrogen bonding interactions. Characteris tic changes in the geometrical parameters upon hydrogen bonding are manifes ted in the lengthening of the C-F bond involved in the interaction by 0.01 Angstrom, in the lengthening of the O-H bond by 0.003 Angstrom, in the decr ease of the C-O-H bond angle by 1 degrees, and in a tilt of the C-F and C-O bonds toward each other, as compared with the geometries of the parent mol ecules. The hydrogen bonds shorten in the order 2-fluorophenol > 2,6-difluo rophenol > 2,3,5,6-tetrafluorohydroquinone (C-2h) > 2,3,5,6-tetrafluorohydr oquinone (C-2v), implying strengthening of hydrogen bonds in the opposite d irection. The strength of hydrogen bonding and its overall consequences in the rest of the molecule are less pronounced in systems where the hydrogen bond occurs in five-membered rings than when it is part of a six-membered s ystem.