Ring expansion and hydrogen scrambling in the molecular ion of 3‐methylthiophene

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Abstract
The ratio [M  D]/{[M‐D] + [M  H]} in the 70 eV mass spectra of six deuterated 3‐methylthiophenes has been determined. From these values the mole fractions of the molecular ions that lose hydrogen atoms specifically from the various positions of the molecule were calculated, as well as the mole fraction in which the hydrogen atoms are fully scrambled before hydrogen elimination. It appears that hydrogen atoms are mainly lost from a fully scrambled [C5H6S]+· ion and from the α‐position of the original molecular ion. A deuterium isotope effect of 1·60 to 1·72 was calculated for the hydrogen elimination. The reaction was also studied at low electron energies. In order to determine the degree of scrambling in the [C5H5S]+ ions, some decomposition reactions of this ion were investigated.