The Structure of the 2-Norbornyl Cation:  The π-Complex and Beyond^,

Abstract

Calculations at Becke3LYP, MP2, and QCISD levels of theory are presented for different molecular structures of the 2-norbornyl cation (1). With the aid of the quantum theory of atoms in molecules (AIM) we show that 1 has a T-structure at the C6−C−C face (we view it as a π-complex with a tetracoordinate carbon atom C6) and that the (“nonclassical”) bridged structure with a pentacoordinate C6, which is not a minimum on the potential energy surface, is only reached when the C6−C1 and C6−C2 internuclear distances are shortened to 165 pm. At the Becke3LYP/6-31G(d,p) level in the gas phase this bridged species is 7.25 kcal mol-1 higher in energy than the equilibrium optimized geometry. NMR calculations show that 13C chemical shifts of C1, C2, and C6 of the bridged 165 pm species differ significantly from the experimental values.

Identification of carbonium and carbenium ions by QTAIM

“…28 The quantum theory of atoms in molecules (QTAIM) was used to study many carbonium ions, such as proponium and 2-norbornyl cations. [29][30][31][32][33][34][35][36][37][38][39] In this work we show that the QTAIM can be used to evaluate whether a carbocation is classical or not. It is also possible to determine the order of σ or π delocalization within the similar group of cations.…”

Topological study of the first step of nucleophilically unassisted solvolysis of protonated 2-endo/exo-norbornanol and protonated 2-endo/exo-oxabicycloheptanol

“…Table 2 shows some geometrical information along with r b , ∇ 2 r b , E at (W), q(W), SE at (C1-C3), e, |l 1 |/l 3 of the bond critical points 1-3, 5 and 6, and DI of their corresponding atomic pairs of the species 3-GS, 4-GS, 3-TS and 4-TS. Figure 5 shows the molecular graphs of the 2-norbornyl cation 36,38,39 and oxabicycloheptanyl cation. Both follow the Poincaré-Hopf relationship.…”

“…The quantum theory of atoms in molecules 34,35 (QTAIM) has been used to study the electronic nature of norbornyl cation and derivatives. [36][37][38][39] In this work, we applied QTAIM to reevaluate the first step of two unassisted nucleophilic solvolysis: [R-OH 2 ] + → R + + H 2 O, where R is 2-endo/exonorbornyl and 2-endo/exo-oxabicycloheptanyl moieties, that were previously studied by Schleyer and co-workers 33 by using molecular orbital theory. Certainly, different leaving groups would lead to different carbocation character to the transition state structure of the studied molecules in gas phase, but our work is restricted to water as leaving group for comparison reasons with the work of Schleyer and collaborators.…”

QTAIM-DI-VISAB computational study on the so-called nonclassical bicyclobutonium cation

“…Our recent computational studies on a number of cations, including 2-norbornyl, established that coordination based on the number of bond paths -as defined in a QTAIM 6 molecular graph -terminating at a nucleus in any species -cation, carbanion, radical, or carbene -should be used as the criterion of hyper-coordination and hyper-valency. 7,8,9,10 A QTAIM treatment precludes interpretational bias of the nature of bonding based on qualitative evidence such as interatomic distances. We argued that this approach should be used regardless of the nature of the intermediate to obviate the confusion and inaccuracies associated with arbitrarily using indicators such as dashed lines, dotted lines, cross-hatched lines, hollow tubes, and solid tubes in structural formulas.…”