Attila Kovács, István Macsári and István Hargittai:

Intramolecular hydrogen bonding in fluorophenol derivatives:

2-fluorophenol, 2,6-difluorophenol and 2,3,5,6-tetrafluorohydroquinone.

The Journal of Physical Chemistry A, 103 (1999) 3110-3114.

Intramolecular F...H hydrogen bonding in 2-fluorophenol, 2,6-difluorophenol, and 2,3,5,6-tetrafluorohydroquinone has been studied by ab initio molecular orbital calculations. Geometry optimizations at the MP2/6-31G** level resulted in two planar local minima on the potential energy surface, characterised by different orientations of the hydroxy hydrogen. In the conformers where the hydroxy hydrogen points towards a fluorine, the computations suggest weak intramolecular F...H hydrogen bonding interactions. Characteristic changes in the geometrical parameters upon hydrogen bonding are manifested in the lengthening of the C–F bond involved in the interaction by 0.01 Å, in the lengthening of the O–H bond by 0.003 Å, in the decrease of the C–O–H bond angle by 1º, and in a tilt of the C–F and C–O bonds towards each other, as compared with the geometries of the parent molecules. The hydrogen bonds shorten in the order 2-fluorophenol > 2,6-difluorophenol > 2,3,5,6-tetrafluorohydroquinone (C2h) > 2,3,5,6-tetrafluorohydroquinone (C2v) implying strengthening hydrogen bonds in the opposite direction. 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 system.