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Study of the rate of methyl loss and the structure of the product ion as a function of the lifetime of the molecular ion of pent‐3‐en‐2‐ol
Author(s) -
Zappey Herman W.,
Ingemann Steen,
Nibbering Nico M. M.
Publication year - 1991
Publication title -
organic mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.475
H-Index - 121
eISSN - 1096-9888
pISSN - 0030-493X
DOI - 10.1002/oms.1210260411
Subject(s) - ion , function (biology) , product (mathematics) , materials science , chemistry , organic chemistry , biology , mathematics , genetics , geometry
The normalized unimolecular rate constant for loss of a methyl radical from pent‐3‐en‐2‐ol molecular ions with lifetimes ranging from 10 −11 to 10 −9 s was studied by field ionization kinetics (FIK). The normalized rate curve shows maxima at molecular ion lifetimes of 10 −10.5 and 10 −10.1 s. Based on results for deuterium and 13 C‐labelled pent‐3‐en‐2‐ol, the maximum at 10 −10.5 s is ascribed to loss of the methyl group at the 1‐position by a direct cleavage reaction. The maximum at 10 −10.1 s is attributed to a 1,2‐H shift‐initiated rearrangement of the molecular ion, which leads to loss of the methyl group at the 5‐ and 1‐positions. The time dependence of the processes in the form of the maxima on the normalized rate curve is discussed qualitatively in terms of a lower critical energy and a tighter transition state of the 1,2‐H shift than those of the direct cleavage reaction. Collision‐induced dissociation of the [C 4 H 7 O] + product ions in combination with FIK provides evidence that at molecular ion lifetimes corresponding to the first maximum on the rate curve protonated crotonaldehyde is formed, whereas protonated methyl vinyl ketone and the butyryl cation are formed at times corresponding to the second maximum.