Identification of carbonium and carbenium ions by QTAIM

Abstract

A ferramenta de Gassman-Fentiman da demanda crescente de elétrons foi usada para identificar íons carbênios e íons carbônios. Contudo, devido ao seu entendimento ambíguo, ela foi pivô de uma disputa histórica. Nós aplicamos a metodologia da Teoria Quântica de Átomos em Moléculas -QTAIM -para caracterizar íons carbênios e íons carbônios de uma forma mais eficaz e mais fácil. Essa metodologia pode ser usada para avaliar se um carbocátion é clássico ou não. Além disso, é possível classificar um conjunto de íon carbônio na ordem de magnitude da deslocalização σ, ou π. Há poucas diferenças entre os resultados de nosso modelo baseado na QTAIM e da ferramenta de Gassman-Fentiman. Contudo, diferentemente da ferramenta de Gassman-Fentiman, os cátions 7-anisol-7-norbornenila e 2-anisol-2-norbornila são não-clássicos, embora sejam os menos íons não-clássicos em seus conjuntos correspondentes de cátions estudados.The Gassman-Fentiman tool of increasing electron demand was used to identify carbonium and carbenium ions. Nonetheless, due to its ambiguous understanding, it was pivot of a historical dispute. We applied the Quantum Theory of Atoms in Molecules (QTAIM) metodology to characterize the carbonium and carbenium ions on an easier and more effective way. By comparing selected topological information of reference carbenium ions the QTAIM metodology can be used to evaluate whether a carbocation is classical or not. In addition, it is possible to rank a set of carbonium ions in order of their corresponding σ or π delocalization. There are few differences between our QTAIM-based model and Gassman-Fentiman tool. Unlike Gassman-Fentiman tool results, 7-anisyl-7-norbornenyl and 2-anisyl-2-norbornyl cations are non-classical, although they are the least nonclassical ions in their corresponding set of studied cations.Keywords: norbornyl cation, norbornenyl cation, QTAIM, carbonium ion, carbenium ion IntroductionIn 1949 Winstein and Trifan 1,2 postulated the assistance of σ electron delocalization to account for the large rate differences in acetolysis of exo-and endo-2-norbornyl brosilates. On the other hand, Brown 3,4 attributed these differences to steric effects. Thereafter, heated debates on nature of norbornyl cation, named the nonclassical ion controversy, took place.3-5 Nonetheless, experimental evidences 6-9 indicated that norbornyl cation had no trivalent carbenium center characteristic of a classical ion. Eventually, the nonclassical nature 10 of the hypercoordinate norbornyl cation, characterized by delocalized σ electrons in a threecenter two-electron (3c-2e) bonding, was proved. 11Carbenium and carbonium ions, formerly named classical and nonclassical ions, respectively, can be identified by an index developed by Schleyer and collaborators 12 based on the total 13 C NMR chemical shift difference between a carbocation and the corresponding neutral hydrocarbon. In the classical trivalent carbocations the chemical shift difference, ∆δ, is more than 350 ppm while in nonclassical carbocations ∆δ < 100 ppm.Gassman and Fent...

Structure, energy, vibrational spectrum, and Bader's analysis of π⋯H hydrogen bonds and H−δ⋯H+δdihydrogen bonds

“…507 Based on the kinetic and potential energy operators given above, 508,509 the QTAIM theory parameters identify maxima and minima of the electron density on the surface of the molecule and classify the chemical bonds as closed shells guided by r 2 r 4 0, or additionally as shared interactions if r 2 r o 0. 510,511 This theoretical argument has been used successfully to establish the intermolecular topology of several systems, [512][513][514][515][516][517][518][519][520][521][522][523][524][525][526][527][528][529][530][531] e.g., compounds with p bonds 532 and binary complexes, 533,534 including those formed by acetylene and hydrofluoric acid. 535 It is not common to study ternary hydrogen complexes from the theoretical viewpoint.…”

Latin American contributions to quantum chemical topology

“…• Some cations with nonclassical classification exhibit similar deviations as the classical ones. The QTAIM is useful to unambiguously identify carbonium and carbenium ions in an easier way than by using the Gassman-Fentiman tool, as shown by Firme and coworkers, [28] who found systems with small delocalization indexes between atomic basins C1 and C2, δ(C1, C2), of about 10 −1 , similar to those found between water molecules in small H 2 O clusters. [10] This condition is an indicative of molecules with weak ring delocalization and therefore of carbenium ions.…”