A MODIFIED MCLAFFERTY REARRANGEMENT IN THE ELECTRON-IMPACT MASS-SPECTRA OF DANSYLATED AMINO-ACID METHYL-ESTERS

Electron impact mass spectra of dansylated amino-acids and their methy l esters show a very intense peak at m/z 171. A systematic study faile d to show the genesis and origin of this ion. The present work is devo ted to the electron impact mass spectrometric (EIMS) analysis of the a ppearance of this ion m/z 171 in the spectra of dansylated amino-acid methyl esters and its diagnostic importance in the analysis of dansyl derivatives, A structural model for this study is offered by a series of N-alkylated aromatic sulphonamides. The EIMS spectra of these compo unds indicate that the classic fragmentation is accompanied by hydroge n migration from aliphatic radicals to the aromatic ring in a manner a nalogous with dansylated amino-acids. A modified McLafferty rearrangem ent mechanism is proposed for this EI reaction. The metastable transit ions and the deuterium-labelled compounds confirm this mechanism and t he origin of the itinerant hydrogen.