Secondary and Tertiary 2-Methylbutyl Cations. 3. Theoretical Study of Structures and Interconversions of C5H11+ Ions in the Gas Phase1a

and, Dan Fǎrcaşiu; b, Steven H. Norton

doi:10.1021/jo970706k

Cited by 23 publications

(23 citation statements)

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“…We found, however, that the results of high-level ab initio calculations on cations with an isopentane carbon skeleton and the results of studies conducted in our laboratory on these cations generated solvolytically in trifluoroacetic acid (TFA) disagreed in some key conclusions. An attempt at taking into account the solvent by introducing a dielectric medium in the calculations was able to correct only part of the discrepancy .…”

Section: Introductionmentioning

confidence: 78%

“…We concluded, therefore, that the existing ab initio calculations on isolated carbocations cannot be considered satisfactory and we initiated a study of the structure and energy of carbocations in ion pairs. Altogether, our calculations took several years, commensurate with our resources, and required a few developmental steps; , some partial results were reported, but not in significant detail . A particular concern was that the size of the system forced some simplifying choices, particularly what anion could be employed.…”

Section: Introductionmentioning

confidence: 99%

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Theoretical Studies of Carbocations in Ion Pairs. 1. The 2-Propyl Cation

Fǎrcaşiu

Hâncu

1997

J. Phys. Chem. A

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Ab initio calculations with large basis sets and electron correlation were applied to the study of geometry of the 2-propyl cation in ion pairs with trihydro fluoroborate (A) or dihydrolithiate (B) as anion. The goal was to model the ion pair formed by ionization in a solvent with good anion-stabilizing properties, but of low dielectric constant, like trifluoroacetic acid (TFA). The effect of the anion, seen already at long distances, was that the preferred cation conformation changed from C1,C3 staggered as in the isolated carbocation (chiral 2-propyl cation, 1) to C1,C3 eclipsed (C s symmetry, 2). The optimized cation geometry was essentially the same at the MP2(FC)/6-31G*, MP2(FC)/6-31++G*, and MP2(FU)/6-311G** levels, but the position of the anion above the cation was somewhat more sensitive to the basis set. The preferred anion position was in the plane bisecting the C1−C2−C3 angle of 2 and in the region “inside” that angle. This “inside” displacement became more pronounced as the interionic distance, d, was made shorter; at the same time, the anion moved slightly off the bisecting plane. Elimination within the ion pair to form propene occurred at d < 2.5 Å. When the anion was allowed to “fall” freely, the reaction pathway was determined by the initial position of the anion: elimination for a position “inside” the C1−C2−C3 angle and recombination to 2-fluoropropane (occurring at d = 1.5−1.7 Å) for a position “outside” the C1−C2−C3 angle. The equilibrium cation geometry did not change significantly in 2.B relative to 2.A, but the distortions toward elimination occurred at longer distances for the more basic anion B and elimination itself took place at d = 3.5 Å. The energy difference beween 1.A (optimized with the methyl groups held staggered) and 2.A at d = 3.4 Å was 2.39 and 3.25 kcal/mol at the MP2(FC)/6-31++G* and MP2(FU)/6-311G** levels, respectively. The equilibrium position of the anion paired with 1 was above the C1−C2 bond, close to the syn hydrogen at C1. Thus, methyl rotation along the lowest-energy pathway involves also a movement of the anion relative to the cation.

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Section: Introductionmentioning

confidence: 78%

Section: Introductionmentioning

confidence: 99%

Theoretical Studies of Carbocations in Ion Pairs. 1. The 2-Propyl Cation

Fǎrcaşiu

Hâncu

1997

J. Phys. Chem. A

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“…Note also that ten of the 11 low-energy longarm-clinal forms are also closed forms, thus demonstrating a general prevalence for closed forms on the PES in agreement with past studies. [9][10][11][12][13][14][15][16][17][18][19][20] Curiously, the lowest-energy form, with two clinal longarms, is not fully closed but open-clinal on both sides.…”

Section: A Optimized Pw91/ 6-31g"d P… Structuresmentioning

confidence: 99%

“…PCP + structures have some CCC bond angles of Ͻ90°and are commonly seen as the lowest-energy PES minima in computational studies. [9][10][11][12][13][14][15][16][17][18][19][20] Fărcaşiu and co-workers [12][13][14][15] referred to them as "bridged" structures, but we prefer calling them "closed" structures, first to avoid confusion with H-bridged structures and second to recognize their dynamical relationship with classical "open" structures. We appear to be the first to point out that closed structures are rather rare in DFT simulations at catalytic ͑ϳ800 K͒ temperatures.…”

Section: Introductionmentioning

confidence: 99%

On the structure and dynamics of secondary n-alkyl cations

East

Häfner

2009

The Journal of Chemical Physics

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Articles you may be interested inAdapting SAFT-γ perturbation theory to site-based molecular dynamics simulation. III. Molecules with partial charges at bulk phases, confined geometries and interfaces J. Chem. Phys. 141, 094708 (2014); 10.1063/1.4893966Study on the conformational equilibrium of the alanine dipeptide in water solution by using the averaged solvent electrostatic potential from molecular dynamics methodology J. Chem. Phys. 135, 194502 (2011); 10.1063/1.3658857 Combining ab initio quantum mechanics with a dipole-field model to describe acid dissociation reactions in water: First-principles free energy and entropy calculations J. Chem. Phys. 132, 074112 (2010); 10.1063/1.3317398 OD vibrations and hydration structure in an Al 3 + ( aq ) solution from a Car-Parrinello molecular-dynamics simulationStructure and dynamics of the silacyclobutane radical cation, studied by ab initio and density functional theory and electron spin resonance spectroscopy A variety of computational studies was undertaken to examine and establish the relative importance of open versus closed structures for unbranched secondary n-alkyl cations. First, the PW91 level of density functional theory was used to optimize over 20 minimum-energy structures of sec-pentyl, sec-hexyl, and sec-heptyl ions, demonstrating that closed structures are more stable than open ones on the potential energy surface ͑PES͒. Second, PW91 was used with a theoretical Andersen thermostat to perform a molecular dynamics simulation ͑150 ps͒ of C 9 H 19 + at a typical catalytic temperature of 800 K, demonstrating that the structure preference is inverted on the free-energy surface. Third, both quantum ͑rigid-rotor/harmonic oscillator͒ and classical partition functions were used to demonstrate that the simulated structure-opening at catalytic temperatures is due to the floppiness of the open forms, which improves its free energy by both lowering its zero-point vibrational energy and increasing its molecular entropy. The particular conformer of the preferred open form ͑at 800 K͒ is dependent on length of alkyl ion, with pentyl ions preferring syn/anti structures but longer ions preferring open-clinal ones. These results, plus an additional set of PES optimized structures from an alternative level of theory ͑MP2 / 6-31G͑d , p͒͒, are used to discuss the likely nature of secondary n-alkyl ions.

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“…Fǎrcaşiu et al have theoretically investigated the conformations of carbocations and the reactions of their precursors with borane at high-level ab initio calculations [4,5]. In 2007, Ricks and coworkers revealed that extraordinary stability of C 4 H 9 + species is brought about the charge distribution and hyperconjugation by means of IR spectral analysis [6].…”

Section: Introductionmentioning

confidence: 99%

Theoretical Studies on Structures and Electronic State of Alkyl‐Substituted Ethyl Cations

Shimasaki

Nakayasu

Shibata

et al. 2013

Journal of Chemistry

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This paper describes the gas phase structural details for a series of alkyl-substituted ethyl cations 1-11 by mainly natural bond orbital (NBO) analysis. For the simplest ethyl cation, two kinds of optimized structures (nonclassical 1 and classical 2) were found to have minima on the potential surface, and the nonclassical 1 was more stable than the corresponding classical 2 by 1.19 kcal/moL at Møller-Plesset (MP) level calculations in the gas phase. The calculated values of Wiberg's bond index revealed that the central ethylic C 1-C 2 bonds of the nonclassical cations 1, 6, 7, and 10 have 1.10-1.46 bond multiplicities. The compliance constant () values indicated that the cation center C 1 of nonclassical 2-butyl cation 6 (0.209Å/mdyn) acquires stronger hyperconjugative stabilization from C 2-R 3 (C-H) bond than that of the nonclassical ethyl cation 1 (0.388Å/mdyn). Moreover, the gas phase stabilities for the alkyl-substituted ethyl cations 1-11 were also estimated by the hydride affinities based on isodesmic equations.

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Secondary and Tertiary 2-Methylbutyl Cations. 3. Theoretical Study of Structures and Interconversions of C₅H₁₁⁺ Ions in the Gas Phase¹^a

Cited by 23 publications

References 30 publications

Theoretical Studies of Carbocations in Ion Pairs. 1. The 2-Propyl Cation

Theoretical Studies of Carbocations in Ion Pairs. 1. The 2-Propyl Cation

On the structure and dynamics of secondary n-alkyl cations

Theoretical Studies on Structures and Electronic State of Alkyl‐Substituted Ethyl Cations

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