Correlations between molecular structure and mass spectra I.: An anomalous type of rearrangement ions found in the mass spectra of carboxylic acids and nitriles

An important type of rearrangement ions is common to the mass spectra of several classes of organic compounds, e.g. ketones, aldehydes, acids, nitriles, etc. For its explanation McLafferty proposed a six‐membered ring transition state through which a hydrogen atom was thought to be transferred. In the spectra of certain branched acids and nitriles we have observed an additional type of rearrangement ions. In the case of the nitriles one must discriminate between the actual rearrangement ions CnH2n‐1N+ and the ions Cn+1H2n+1+, of nearly the same mass. With a mass spectrometer of high resolving power (A.E.I., MS 9) the presence of both types of ions was established; their relative abundances vary widely for different substances. Although at first sight related to the McLafferty‐rearrangements, the newly found rearrangements are not readily explained in exactly the same way. They can be accounted for by a simple modification of McLafferty's scheme, involving a seven‐ or even eight‐membered ring transition state instead of the usual six‐membered one.