Bridged and Open Carbocation Structures as a Function of the Correlation Level in ab Initio Calculations:  The 4-Methyl-2-pentyl Cation

An investigation of the alkylation reaction of propene with the 2-propyl cation by ab initio density functional methods at the B3LYP/6-31G** level found a distorted trimethyl-1-protonated cyclopropane as an energy minimum along the reaction coordinate (intermediate) and an open ion, the 4-methyl-2-pentyl cation (1), as another energy minimum (product). In contrast, the open ion 1 was not an energy minimum in MP2/6-31G** calculations. Attempts at geometry optimizations of 1 at that level led invariably to the protonated cyclopropane structure (more symmetrical than in the previous case). The ion 1 was in fact a transition structure in the MP2/6-31G** optimizations. A coupled cluster (CCSD/6-31G**) geometry optimization showed, however, the open ion 1 as a true energy minimum. This result brings a note of caution concerning MP2 geometry optimizations of carbocations. In particular, when these calculations find small energy differences between bridged and open structures, but find only the bridged structures as energy minima, the results might be in error. What the level of calculation is at which the predicted carbocation structures can be considered definitive remains an open question.