PROTON AFFINITIES OF SOME POLYFLUOROALKANES IN COMPARISON TO THE UNSUBSTITUTED ALKANES - THE ESTIMATION OF THE PROTON AFFINITIES OF POLYTETRAFLUOROETHYLENE AND POLYETHYLENE BY APPLYING THEORETICAL METHODS

Applying ab initio and semiempirical quantum chemical methods, the pro ton affinities (PA) for some simple polytetrafluoroethylene (PTFE) mod els, and also for propane as a polyethylene (PE) model, have been esti mated. Using AM1, a systematic (more than 20-30 kcal mol(-1)) overesti mation of the proton affinities was found. In contrast, a much better agreement with ab initio data was obtained by using MNDO and PM3. Whil e the former values are too high, an underestimation by PM3 was establ ished. Reliable results are obtained, however, when the mean values fr om MNDO and PM3 for PAs are used. Further investigations were carried out, applying MNDO and PM3 to larger perfluorinated alkanes and all th ree semiempirical methods to alkanes. Using the mean PAs from MNDO and PM3, convergence values of 116 kcal mol(-1) were obtained for PTFE an d 154 kcal mol(-1) for PE. According to the theoretical results, proto nation of alkanes leads to a C-H bond cleavage in agreement with exper iment. Ab initio MPn/6-31G//3-21G (n = 2,3) geometries of protonated propane, where all bonds are kept after protonation, correspond to a t ransition state. The calculated PA of approximately 135 kcal mol(-1) f alls between the respective values for perfluorinated alkanes and alka nes. Neither by ab initio nor by semiempirical procedures was it possi ble to locate an alkane/proton complex corresponding to a minimum. Con sequently, one may conclude that protonation of PTFE is accompanied by moderate PA without bond cleavage, while PE may be protonated only by destruction of the C-H bond. The energy difference of these two proce sses is high (38 kcal mol(-1)).