DIFLUORODIOXIRANE - AN UNUSUAL CYCLIC PEROXIDE

The surprisingly high stability of the dioxirane CF2O2 (2), its unusua l geometry, its infrared spectrum, and NMR chemical shifts are determi ned and analyzed on the basis of extended ab initio calculations inclu ding seven different methods and nine different basis sets. At the hig hest level of theory, the CCSD(T) approach has been used together with a cc-VTZ2P+f basis set, which leads to an accurate description of geo metry and vibrational frequencies. Stabilizing CF,CF bond interactions add 19.5 kcal/mol and CF,CO bond interactions 12 kcal/mol to the stab ility of the dioxirane ring. The latter effect reduces ring strain fro m 32.8 (dioxirane) to 20.5 kcal/mol (2) where 17 kcal/mol are due to C O bond strengthening and 4.6 kcal/mol to 00 bond weakening. Changes in CO and OO bond strength are caused by a transfer of negative charge f rom the CF2 group to the antisymmetric Walsh MO of the ring. The calcu lated Delta H-f degrees(298) value of 2 is -102 +/- 1.5 kcal/mol, whic h indicates that 2 is thermodynamically rather stable. Calculated C-13 (133 ppm) and O-17 NMR chemical shifts (403 ppm) are unusually positi ve for an organic cyclic peroxide, but should facilitate the identific ation of 2 in the presence of its isomers F2COO and FC(=O)OF, which po ssess Delta H-f degrees(298) values of -60 and -104 kcal/mol, respecti vely.