Ion-Molecule Complexes in Unimolecular Fragmentations of Gaseous Cations. Alkyl Phenyl Ether Molecular Ions

Article abstract of DOI:10.1021/ja00525a022

A decomposition pathway that bears a formal resemblance to first-order elimination in solution is demonstrated for parent ion fragmentations of alkyl phenyl ethers under electron impact (EI) and chemical ionization (CI).The sequence of steps in the gas phase, parent ion -> ion-molecule complex -> fragments, is analogous to first-order elimination in solution (which goes through ion pairs).Such a mechanism for expulsion of PhOH^+. from molecular ions has been tested by examing neutral products from 70-eV electron bombardment of neopentyl phenyl ether in a specially constructed electron bombardment flow (EBFlow) reactor.The C5H10 isomers 2-methyl-1-butene (2) and 2-methyl-2-butene (3) are recovered in the same ratio (2/3=1.14) as is produced by gas-phase deprotonation of tert-amyl cation.This result is validated by a mass spectrometric study of deuterated analogues, for which the ratio of γ-transfer (corresponding to product 2) to α-transfer (corresponding to product 3) is calculated to be 1.46.Intermediacy of an ion-molecule complex, PhOCH2C(CH3)3^+. -> .CH3CH2(CH3)2C⁺> -> PhOH^+. + 2 or 3, predicts this outcome, where the species in brackets represents an electrostatically bound comlex of the neutral phenoxyl radical and an alkyl cation.This mechanism explains the published mass spectrometric (EI an CI) and EBFlow results for n-propyl and n-butyl phenyl ethers.Several confirmatory experiments support the intermediacy of an ion-molecule complex and rule out other interpretations of the experiment data.