Evidence for H⋅(β) and H⋅(δ) competitive migrations in 2‐methyl pentanoic acid radical cations

Audier, H. E.; Sozzi, G.; Maquestiau, A.

doi:10.1002/rcm.1290010205

Cited by 3 publications

(1 citation statement)

References 4 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…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

View full text Add to dashboard Cite

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.

show abstract

Section: Resultsmentioning

confidence: 99%