A new charge-associated mechanism to account for the production of fragment ions in the high-energy CID spectra of fatty acids

A new mechanism, termed a charge-assisted process, is proposed as an additional mechanism to the charge-remote process to account for ions of the [M - CnH2n+2] series found in the positive and negative high energy CID spectra of fatty acids and related compounds when ionized as closed-shell ([M - H]- or [M + X]+) species. The new mechanism is based on that commonly invoked to account for similar ions in the electron-impact spectra of derivatized fatty acids whereby the positive charge on the derivative abstracts a hydrogen atom from various positions of the alkyl chain to leave a radical that initiates a radical-induced cleavage of the chain. It is proposed that in the high energy CID spectra of closed-shell ions, similar hydrogen migrations occur but unpairing of electrons is avoided by charge transfer to the alkyl chain. This charge then initiates a concerted cleavage of the chain to give an allylic carbonium (positive ion spectrum) or carbanion (negative ion spectrum). The mechanism avoids the need to involve radicals or loss of hydrogen atoms from even-electron (closed shell) ions and provides a driving force for the reaction, namely, the formation of ions with a stabilized charge. An extension of the mechanism is also proposed to account for the formation of odd-electron ions from these compounds. The charge-assisted mechanism does not rule out the occurrence of other mechanisms that have been accepted for many years but provides an alternative process that can account for some spectral features which were difficult to explain earlier.