Dodecahedryl anion formation and an experimental determination of the acidity and C-H bond dissociation energy of dodecahedrane

Dodecahedryl anion (la) was generated in a Fourier transform mass spectrome ter by deprotonation of dodecahedrane (I). Examination of the acid and base behavior of 1 and 1a, respectively, enabled the acidity of 1 to be determi ned (DeltaH degrees (acid) = 402 +/- 2 kcal/mol). Good agreement is found w ith the 298 K computed MP2/6-31+G(d)//HF/6-31+G(d) value of 405.1 kcal/mol. In a similar manner, the electron affinity of dodecahedryl radical was mea sured (EA = 4 +/- 2 kcal/mol). These results were combined in a thermodynam ic cycle to afford the C-H bond dissociation energy of dodecahedrane (BDE = 92 +/- 3 kcal/mol), which is reasonably well reproduced (96.7 kcal/mol) at the MP2/HF level but leads to the suggestion that the reported heat of hyd rogenation of dodecahedrene is in error. The DePuy kinetic method for measu ring the acidity of 1 also was explored. It was found that this approach wo rks well with triphenylsilyldodecahedrane but gives poor results with triet hylsilyldodecahedrane. This latter failure is attributed to steric effects, and provides a rationale for several problem cases and a means to overcome these difficulties.