The peculiar structure of dioxygen difluoride, FOOF anomeric delocalization

6 The peculiar structure of dioxygen difluoride, FOOF anomeric delocalization 

Replacement of the two H atoms in HOOH by F atoms, does not change the shape or symmetry of the molecule. See Fig. 17.4. Note, however, that the 0-0 bond distance is 26 pm shorter than in HOOH and indistinguishable for the 00 double bond distance in the O2 molecule, while the O-F bond distance is 18 pm longer than in OF2 Note, also, that the ZOOF valence angle is 14° smaller than the ZOOH valence angle in HOOH, and that the dihedral angle t(XOOX) has been reduced by 30° to a value close to 90°. 

Quantum chemical calculations indicate that the barriers to internal rotation in FOOF are much higher than in HOOH the energies of the planar syn and anti forms are both calculated to be about 100 kJ moP above that of the equilibrium structure[6]. Interestingly the 0-0 bond distances in the planar forms are similar to the 0-0 bond distance in HOOH while the O-F bond distances are similar to the O-F bond distance in OF2. 

The results for FOOF may be generalized in the following manner Consider a molecular fragment AOEX where the O atom forms a single, covalent bond to an atom E from Group 

Similar anomeric effects are also observed in molecules containing A2NEX fragments. The available evidence shows that in such molecules the anomeric stabilization is at its maximum when the electron lone pair orbital on the N atom is anti the E-X bond, i.e. when the NEX plane bisects the ANA angle. See Fig. 17.5. 

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