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Mass spectrometry of pyridine compounds–IV: The formation and decomposition of the [M – methyl] + ion [C 8 H 10 N] + from 4‐ t ‐butylpyridine in comparison with that of the [M – methyl] + ion [C 9 H 11 ] + from t ‐butylbenzene, as studied by D‐ and 13 C‐labelling
Author(s) -
Neeter R.,
Nibbering N. M. M.
Publication year - 1973
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.1210070910
Subject(s) - chemistry , hydrogen , ion , ethylene , cyclopropane , hydrogen atom , methyl group , population , pyridine , mass spectrometry , kinetic isotope effect , ring (chemistry) , medicinal chemistry , analytical chemistry (journal) , deuterium , atomic physics , organic chemistry , physics , group (periodic table) , catalysis , demography , chromatography , sociology
A strong secondary isotope effect is observed in the preferred loss of methyl vs. trideutero‐methyl from the molecular ions of appropriately labelled 4‐ t ‐butylpyridine and t ‐butylbenzene decomposing in the first and second field free regions of a double focusing mass spectrometer. This has been rationalised by invoking the theory of radiationless transitions 2 , which can account for the higher population of activated states responsible for loss of methyl vs. that for trideuteromethyl. 13 C‐Labelling at the central carbon atom of the t ‐butyl group indicates that the [M – methyl] + ions, decomposing further by elimination of ethylene, cannot be represented exclusively by a pyridylated (or phenylated) cyclopropane ion if present at all. It is concluded that ions with structures generated by 1,2‐hydrogen‐, 1,2‐pyridyl‐ (or 1,2‐phenyl‐) and 1,2‐methyl shifts must also play a role. D‐labelling further shows an extensive randomisation of side‐chain hydrogen atoms in the [M‐methyl] + ions of 4‐ t ‐butylbenzene; in this case, however, the expelled ethylene also contains ring hydrogen atoms (≤2). Presumably this is caused by exchange between the side‐chain and ortho‐hydrogen atoms in the initially generated phenyldimethylcarbinyl carbenium ion.