Three-Center−Two-Electron and Four-Center−Four-Electron Bonds. A Study by Electron Charge Density over the Structure of Methonium Cations

Okulik, Nora Beatriz; Peruchena, Nélida María; Jubert, Alicia H.

doi:10.1021/jp063709m

Cited by 23 publications

(34 citation statements)

References 40 publications

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“…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.…”

Section: Introductionmentioning

confidence: 78%

Identification of carbonium and carbenium ions by QTAIM

Firme¹,

Antunes²,

Esteves³

2009

J. Braz. Chem. Soc.

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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...

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Section: Introductionmentioning

confidence: 78%

Identification of carbonium and carbenium ions by QTAIM

Firme¹,

Antunes²,

Esteves³

2009

J. Braz. Chem. Soc.

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“…Computational studies examined various aspects of the CH 5 + cation -potential energy surface, including the pathways leading to the fragments CH 3 + and H 2 , 152 fluxionality including the effect of deuteration, 153 and electron density charge topology as studied by Bader's theory of atoms in molecules. 154 Solvolysis of the mesylate (79) results in ring-opened products consistent with the bridged cation (80) as the intermediate. 155 The exo-isomer of (79) undergoes solvolysis with direct displacement of the leaving group and complete retention of the bicyclic structure.…”

Section: Carbonium (Bridged) Ionsmentioning

confidence: 97%

Carbocations

McClelland¹

2010

Organic Reaction Mechanisms Series

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“…However, most of these studies agreed in considering CH 5 + ion as a very fluxional species [63][64][65][66][67], whose basic components are a tripod CH 3 and a H 2 moiety. There are however some specific cases in which the coordination of carbon was found to be larger than four.…”

Section: Pentacovalent Carbon Atomsmentioning

confidence: 99%

Unexpected Gas-Phase Ion Chemistry Results Unraveled by Computational Chemistry

Gámez¹,

Corral²,

Mó³

et al. 2010

COC

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An overview of recent studies, which show the important role played by computational chemistry in explaining and predicting the behavior of a great variety of systems is presented. The first example illustrates the useful interplay experiment-theory which allowed to unambiguously characterize the X2(CH3)7 + (X = Si, Ge, Sn, Pb) cations as paradigmatic examples of molecular systems involving pentacovalently bound carbon atoms. Also, the possibility of deeply analyzing the bonding in molecular systems, led to a rationalization of the quite different behavior of acetylene and iminoborane derivatives, in spite of being isoelectronic species. Similarly, the analysis of the bonding perturbations usually associated with ion-molecule interactions in the gas phase, or with protonation and deprotonation processes, has made possible the characterization of non-conventional complexes as the key structures in the interactions of unsaturated and aromatic Si and Ge containing compounds with transition metal ions or to explain the dramatic enhanced acidity of alkylboranes and ,unsaturated borane derivatives.

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Three-Center−Two-Electron and Four-Center−Four-Electron Bonds. A Study by Electron Charge Density over the Structure of Methonium Cations

Cited by 23 publications

References 40 publications

Identification of carbonium and carbenium ions by QTAIM

Identification of carbonium and carbenium ions by QTAIM

Carbocations

Unexpected Gas-Phase Ion Chemistry Results Unraveled by Computational Chemistry

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