Proponium Ions and Analogs

Proponium Ions and Analogs The higher alkonium ions, such as protonated propane (C3H9+) [and also protonated butane (C4H1T), etc.], have been observed in the gas phase by In solution, the higher hydrocar- 

Hiraoka and Kebarle studied the C3H/ protonated propane cation in the gas phase 55 and found two isomers, the C-protonated (43) and the 2-H-protonated (44) cations. Ion 43 was shown to be of lower energy and also much more stable toward dissociation to C2H5+ -1- CH4 or.vrc-I J 1 + H2.Tlie energy barrier of the interconversion of the ions was calculated to be 9.1kcalmoH. 

Collins and O Malley identified 43 and 45 as the two isomers of the proponium ion (DFT and MP2 calculations). They also suggested that structure 44 is rather the transition state for the interconversion of the ions, and the 2//-protonated cation is better described as the van der Waals complex of sec-CsHv and H2. This suggestion, in fact, was already made by Hiraoka and 

Kebarle. Recently, Mota et al. calculated the potential energy surface of the C3H9+ cation [MP4(SDTQ)/6-311++G /MP2(full)/6-31G level]).The C-proponium cation (43) was again found to be of lowest energy but the complex xec-C ilh + H2 lies only 0.3 kcal mol above structure 43. The C-H bond lengths in the 3c-2e interactions are 1.272 and 1.188 A, whereas the C-C bond distance is 2.099 A.They also found that the interconversion between the -H-protonated cation (45) and the C-proponium cation (43) has no energy barrier. This may indicate that on zeolites where steric effects are important the primary, that is more accessible hydrogens are protonated initially to yield the 

1-//-proponium cation (45) (kinetic control), which then easily rearranges to give the thermodynamically more stable C-proponium ion. 

On the potential energy surface of diprotonated propane (C3H102+), three structures were located as minima (MP2/6-31G level).788 The 1 -//,2-//-diprotonated and 1-/7,3-//-diprotonated forms (458 and 459) are of Cj and C2v symmetry, respectively, whereas the l-//,C-diprotonated structure (460) has C symmetry. Structure 459, a distonic dication with the two charge-bearing centers separated by one carbon, is significantly more stable (by 22.6 kcal mol-1) than structure 458 (a gitonic dication with adjacent charges) and also more stable than structure 460 (by 6.9 kcal mol ). 

The tot-butyl cation (1) reforms isobutane via hydride abstraction from isobutane according to Bartlett et al.,834 Nenitzescu et al.,835 and Schmerling836 involving the tertiary C—H bond only through intermediate structure 463, and thus not exchanging the methine hydrogen. 

Evidence for a C—C protonated C4Hn+ ion (466) resembling cation 463 was obtained by Siskin.841 When studying the HF-TaF5-catalyzed ethylation of a large 

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