Proponium ions

Proponium ions can be formed via protonation of propane, when CH3 attacks C2H6 or C2H5+ attacks CH4 (34). The protonation outcome could occur at the primary C-H, secondary C-H or the C-C, affording respectively the 1-H-proponium ion, 2-H-proponium ion, or the C-proponium ion (Figure 11). The latter can also be formed when C2H5+ attacks the C-H bond of methane. Propane is an interesting molecule to study, since it is the smallest alkane containing all representative types of bonds present in a linear alkane. Therefore, it is the simplest prototype of linear alkanes. 

Figure 11. Different ways for the formation of the isomeric proponium ions.

Figure 12 shows the calculated main geometrical parameters for the proponium ions, their corresponding van der Waals complexes and also their relative energies. 

Figure 12. The isomers of the proponium ion. Values on the bottom of the structures correspond to relative energies (AH) in kcal/mol obtained at MP4(SDTQ)/6-311++G //MP2 (full)/6-31C level.

BOMD calculations on several forms of the proponium ion have been performed at several temperatures (84). 

BOMD study of the l-H-proponium ion shows that this species quickly interconverts to the C- proponium ion, after the H2 moiety involved in the 3c2e bond rotates up toward the C-C bond (Figure 13). 

Two isomeric proponium ions, cation 455 and 457 were computed by Collins and O Malley831 (DFTand MP2 calculations). The 2-//-protonated cation 456, in turn, was suggested to be rather the transition state for the interconversion of the ions. Structure 456 is better described as the van der Waals complex of, vec-C3H7+ and H2, which was already suggested by Hiraoka and Kebarle.748 Mota and co-workers832 have recently... 

The formation of C-ethanemethonium can occur by protoiytic attack of C-H or C-C bonds, leading to H-proponium or C-proponium pentacoordinated ions, respectively. Proponium ions can further evolve towards isomeric C-ethanemethonium ion via bond-bond rearrangement. The nature of the initial interaction between proton and propane molecule is of little importance because intramolecular reatrangements of energetically close carbonium ion forms take place easily [26]. 

A bimolecular reaction of C-ethanemethonium or C-proponium ions with propane molecule may result in formation of ethane and buthonium ion. The latter may further evolve to n-butane or i-butane via deprotonation and recovering the Brpnsted acid site. This reaction step may occur either via consecutive mechanism [15, 16] involving formation of CH3 carbenium ion bound to the zeolite framework, followed by its addition to propane molecule, or via concerted mechanism involving CHs transfer from carbonium ion to the propane molecule. This mechanistic pathway explains butanes primary formation. Ethane was also found to be a primary product, however selectivity to ethane formation was much lower comparetively to butanes. It may be due to the fact that ethane... 

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