CHARGE-REMOTE FRAGMENTATION OF GAS-PHASE IONS - MECHANISTIC AND ENERGETIC CONSIDERATIONS IN THE DISSOCIATION OF LONG-CHAIN FUNCTIONALIZED ALKANES AND ALKENES

The collision-activated dissociation pathways of a number of long-chain functionalized alkane ions, CH3(CH2)nX+, and alkene ions have been examined by tandem mass spectrometry. Major dissociation pathways observed following collisional activation include the loss of C(n)H2n+2 units, the loss of C(n)H2n+1 units, the formation of C(n)H2n+1+, and the formation of X+ or XH+. Isotopic labelling data for [(CD3)3N(CD2)13CH3]+ show that no hydrogen/deuterium scrambling occurs between the charge site and the terminal methyl group for the losses of C(n)H2n+2 or C(n)H2n+1 from [(CH3)3N(CH2)13CH3]+. The collision energy required to detect charge-remote fragmentation in low-energy (electronvolt) collisional activation experiments was found to vary with compound type, ion formation method, mass of collision target, and target gas pressure. It is proposed that the dependence of the product distribution on the type of compound is caused by competition between charge-remote fragmentation and charge-directed processes such as the formation of C(n)H2n+1+ or the formation of X+ or XH+. A radical mechanism is proposed to account for several charge-remote dissociation pathways that cannot be accounted for by a previously proposed cyclic elimination mechanism. These processes include the loss of methane, the formation of odd- and even-electron product ions separated by 1 u, and the enhancement of certain dissociation products (e.g., allylic cleavage product ions) when unsaturation or substituent sites are introduced into the molecule.