High resolution mass spectrometry in molecular studies. Part XVII. Evidence for ring contractions in molecular ions: The fragmentation of N‐acetylmorpholine

In comparing N‐acetylmorpholine to closely related systems, two major differences in fragmentation are observed. M‐15 and M‐43 fragments, although quite common to such systems, are unusual with respect to their genesis. Formation of the M‐15 fragment by loss of C‐3 instead of the anticipitated loss of C‐2 or C‐8 is evident from shifts in the spectra of deuterated analogs. The M‐43 species arises from total loss of the N‐acyl substituent rather than the usual two‐step loss of ketene plus a ring hydrogen. These data are presented in support of mechanistic rationale involving radical induced ring contraction to a common intermediate molecular ion. Techniques of high resolution mass spectrometry and isotope labeling have been employed to substantiate certain mechanistic details.