Isotopomer distributions of neutral products from a doubly labeled cation in the gas phase. Interconversion of 1-fluoro-1-propyl cation and 1-fluoroisopropyl cation on the C3H6F+ potential energy surface

Shaler, Thomas A.; Morton, Thomas Hellman

doi:10.1021/ja00018a009

Cited by 23 publications

(18 citation statements)

References 4 publications

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“…Recently, however, we pointed out that such a phenomenological interpretation can lead to contradictions and that a mechanistic analysis is to be preferred, even though it requires the study of additional isotopically substituted analogues . Given a set of mechanistic hypotheses, the relative product yields from a unimolecular decomposition can be dissected using a first-order kinetic scheme (either by solving a set of coupled differential equations or by means of the steady-state approximation 3 ) under the following circumstances: the concentration of the reactant must decay exponentially (as when it rapidly equilibrates with an infinite heat bath or consists of a set of isolated, monoenergetic molecules 2 ) or else the reaction must have gone to completion (in which case the result represents an ensemble average). While there are other circumstances where a steady-state analysis provides a satisfactory approximation, the photoionization results analyzed above correspond to collections of essentially monoenergetic reactants.…”

Section: Resultsmentioning

confidence: 99%

Regio- and Stereochemistry of Alkene Expulsion from Ionized sec-Alkyl Phenyl Ethers

et al. 1999

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Photoionization mass spectrometry of isotopically substituted 2-phenoxypropane (iPrOPh), 2-phenoxybutane (sBuOPh), and 3-phenoxypentane (3AmOPh) permits the analysis of branching ratios for competing pathways by which the radical cations expel neutral alkene to yield ionized phenol. Ionization energies (IEs) of 2-phenoxyalkanes do not differ significantly between 2-phenoxypropane and 2-phenoxyoctane and are unaffected by deuterium substitution. IEs for 3-phenoxyalkanes are 0.04 eV lower than for the 2-phenoxyalkanes. Measurements of PhOD•+:PhOH•+ ratios from deuterated analogues as a function of photon energy lead to a dissection of two mechanisms: direct syn elimination via four-member transition states (which differentiates between stereochemically distinct positions on an adjacent methylene group) and formation of ion−neutral complexes (which affords hydrogen transfer from all positions of the side chain). Syn elimination from ionized sBuOPh partitions among trans-2-butene, cis-2-butene, and 1-butene in a ratio of approximately 6:5:4, exhibiting no systematic variation with internal energy. The proportion of ion−neutral complex formation for sBuOPh increases with energy, from virtually nil at 9.6 eV to about 20% at 9.81 eV to slightly more than one-half at 11.92 eV. Ion−neutral complexes from sBuOPh yield nearly equal proportions of 1-butene and 2-butenes, with little variation as a function of internal energy, while those from 3AmOPh yield about 80−90% 2-pentenes. DFT calculations confirm the preference for syn elimination from ionized iPrOPh at low internal energies. The computed energy of that transition state agrees with published experimental determinations. Analysis of the electron density using the atoms-in-molecules approach shows that the transition state does not possess cyclic topology, unlike vicinal eliminations from neutral molecules (which pass through bona fide cyclic transition states). Cyclic topology is seen for a structure that precedes the potential energy maximum, but that ring disappears at the top of the barrier. Both syn elimination and ion−neutral complex formation from the radical cation proceed far along the pathway for bond heterolysis before arriving at a point at which the two types of mechanism diverge from one another.

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Section: Resultsmentioning

confidence: 99%

Regio- and Stereochemistry of Alkene Expulsion from Ionized sec-Alkyl Phenyl Ethers

et al. 1999

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“…[1][2][3][4][5] Even the α-monofluoroalkyl carbocations are too strongly destabilized to be prepared and observed in the solution phase. However, an α-monofluoromethyl carbocation, 1-fluoro-2-propylium cation (CH 3 CH + CH 2 F), was characterized as a transient intermediate in the gas phase, 6 and an extremely unstable α-trifluoromethylvinyl cation, 3,3,3-trifluoro-1-phenyl-2-propenium cation, was generated through a photochemical reaction. 7 We have earlier demonstrated that the π-delocalized, α-trifluoromethylallyl cations 1-3, formed in superacidic media at low temperatures, have relatively low C 1 -C 2 rotational barriers of about 9 kcal/mol, implying that the positive charge is substantially localized on the tertiary carbon (C 3 ).…”

Section: Introductionmentioning

confidence: 99%

Ab initio structural studies of cyclobutylmethyl cations: effect of fluoroalkyl groups on the relative stability of the carbocations

Reddy¹,

Rasul²,

Prakash³

2017

Arkivoc

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Ab initio calculations at MP2/cc-pVTZ level show that the trifluoromethyl group has a strong destabilizing effect on the nonclassical, σ-bridged cyclobutylmethyl cations. The GIAO-MP2 derived 13 C NMR chemical shifts indicate substantial charge delocalization from the neighboring cyclobutyl ring for carbocations with an α-fluorolkyl group as compared to the 1-cyclobutylethyl cation, and this enhanced charge delocalization in case of the α-(trifluoromethyl)cyclobutylmethyl cation would lead to the ring-opening rearrangement to form the relatively more stable nonclassical primary cyclobutylmethyl cation, in which the carbocation center is farthest from the strongly electron-withdrawing trifluoromethyl group.

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“…3 As an example, the molecular formula c 3 H 6 f + encompasses at least 10 stable structures. 4 Not surprisingly, EBflow experiments demonstrate that the fluoronium metathesis reaction shown in Equation (2) (transposition of f + and oxygen, which are isoelectronic) produces the 2-fluoroisopropyl cation, 2, from the reaction of acetone with perfluorinated carbocations. 5 purely mass spectrometric approaches can provide structural insights, but these aches work best when all isomeric options can be explored.…”

Section: Introductionmentioning

confidence: 99%

“…On the hand, 13 unexpectedly, neutral product collection in the EBflow reveals that fluoronium metathesis on propionaldehyde also gives 2 as the major product, mixed with cH 3 cH 2 cH=f + (which interconverts with cH 3 cHcH 2 f + ) as only a minor product. 4 How does this rearrangement occur? fluoronium metathesis is an extremely exothermic reaction.…”

Section: Introductionmentioning

confidence: 99%

Structures of Products from Positive Ion–Molecule Reactions by Means of Vibrational Spectroscopy

Morton

2010

Eur J Mass Spectrom (Chichester)

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Isotopomer distributions of neutral products from a doubly labeled cation in the gas phase. Interconversion of 1-fluoro-1-propyl cation and 1-fluoroisopropyl cation on the C3H6F+ potential energy surface

Cited by 23 publications

References 4 publications

Regio- and Stereochemistry of Alkene Expulsion from Ionized sec-Alkyl Phenyl Ethers

Regio- and Stereochemistry of Alkene Expulsion from Ionized sec-Alkyl Phenyl Ethers

Ab initio structural studies of cyclobutylmethyl cations: effect of fluoroalkyl groups on the relative stability of the carbocations

Structures of Products from Positive Ion–Molecule Reactions by Means of Vibrational Spectroscopy

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