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 proton affinities (PA) for some simple polytetrafluoroethylene (PTFE) models, and also for propane as a polyethylene (PE) model, have been estimated. Using AM1, a systematic (more than 20-30 kcal mol(-1)) overestimation of the proton affinities was found. In contrast, a much better agreement with ab initio data was obtained by using MNDO and PM3. While the former values are too high, an underestimation by PM3 was established. Reliable results are obtained, however, when the mean values from MNDO and PM3 for PAs are used. Further investigations were carried out, applying MNDO and PM3 to larger perfluorinated alkanes and all three semiempirical methods to alkanes. Using the mean PAs from MNDO and PM3, convergence values of 116 kcal mol(-1) were obtained for PTFE and 154 kcal mol(-1) for PE. According to the theoretical results, protonation of alkanes leads to a C-H bond cleavage in agreement with experiment. Ab initio MPn/6-31G*//3-21G (n = 2,3) geometries of protonated propane, where all bonds are kept after protonation, correspond to a transition state. The calculated PA of approximately 135 kcal mol(-1) falls between the respective values for perfluorinated alkanes and alkanes. Neither by ab initio nor by semiempirical procedures was it possible to locate an alkane/proton complex corresponding to a minimum. Consequently, 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 processes is high (38 kcal mol(-1)).