A systematic study of the types of fragmentation undergone by intemdly excited protonated molecules is presented. Two representative functional groups, ethers and ketones, are selected, and both aliphatic and aromatic substituents are examined. The ions are formed by protonation or alkylation in a chemical ionization source, and caused to fragment by collision at high relative kinetic energy. The prevalence of 1,3-reaurangements and the breakdown of the even eleetron fragmentation rule highlight the ion chemistry uncovered here. Specific findings include: (i) electron unpairing reactions associated with -1 radical loss are common. These are probably energetically less favorable simple bond cleavages and are observed in competition with entropically less favorable elimination reactions. (ii) Alkane elimination is a characteristic reaction, which often occurs by a four-centered mechanism. Several variations on this reaction type are encountered. (%) AlLene elimination is another ubiquitous reaction which occurs by a four-centered mechanism in the systems studied here. Competition between atkene and alkane loss is extremely sensitive to the particular system in hand.

The fragmentation of protonated amines and esters is examined by collision induced dissociation of mass selected ions generated by chemical ionization. Four fragmentation types are observed: (i) loss of an alkane moiety, explicable in terms of a four-centered reaction, (i) loss of alkene, also explicable as a four-centered process, (iii) loss of an alkyl radical, requiring electron unpairing, (iv) losses of other neutral molecules, viz. acids and alcohols from protonated esters, or amines and ammonia from protonated amines. Proposed mechanisms were checked by characterizing selected product ions through their collision induced dissociation spectra. The usefulness of these generalizations is substantiated by the successful prediction of the fragmentation patterns and the order of relative abundances of daughter ions for eight unknowns including some containing functional groups not studied previously.