An Electron Localization Function (ELF) Study of the 2-Norbornyl Cation

Abstract

Wave functions obtained at Becke3LYP, Becke3PW91, MP2, and QCISD levels of theory for the 2-norbornyl cation (1) are used in a topological analysis of the Becke−Edgecombe electron localization function (ELF) with the Silvi suite of programs. Partitioning of the molecular space of the cation into basins of attractors yields information on the shared valence basin interactions at the C1−C2−C6 face of 1 and other Cs species in which the C1−C6 and C2−C6 distances are increased incrementally to 193.5 pm. At the Becke3LYP/cc-PVTZ//Becke3LYP/cc-PVTZ level, a single trisynaptic basin with a population of 0.83 electrons connects C1 and C2 to C6 in the optimized geometry where the C1/C2−C6 distances are 188.9 pm. When the C1/C2−C6 distances are increased by 4.5 pm to 193.5 pm, the shared trisynaptic basin splits into a monosynaptic basin at C6 with a population of 0.49 electrons and a disynaptic basin between C1 and C2 with a population of 2.75 electrons; and ΔET is only 0.12 kcal mol-1. Disynaptic basins with electron populations in the region of 0.5 electrons link C1−C6 and C2−C6 in the equilibrium optimized geometries of 1 obtained at the MP2, Becke3PW91, and QCISD levels (the C1/C2−C6 distances range from 182.5 to 184.6 pm). This is the case even when the large correlation-consistent basis set cc-PVTZ is used to obtain the wave functions. But these shared interactions are lost when the C1/C2−C6 distances are increased in the range of 8 pm. While the nature of the basins at the C1−C2−C6 face depends on the C1/C2−C6 distances and the basis set, on the whole, increases in energy only ranging up to 0.55 kcal mol-1 are found with large basis sets when the distances are increased by 4−8 pm. At the point where the shared valence interactions between C1/C2 and C6 are lost, there is no discontinuity in the calculated total energy. According to our ELF analysis the formation of 1 from the Cs primary “open” cation involves a transfer of electrons from the valence basin of the double bond at C1−C2 to a basin at C6 The stabilization of 1 relative to the open primary cation results from an electrostatic interaction between a carbocation and the electrons of a carbon−carbon double bond.