Investigation of collision‐induced dissociations involving odd‐electron ion formation under positive electrospray ionization conditions using accurate mass

Abstract

Collision induced dissociation (CID) has been extensively used for structure elucidation. CID in the electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) modes has been found to generate mostly even-electron fragment ions while it has been occasionally reported to form odd-electron free radical ions. However, the structural requirements and the fragmentation mechanisms for free-radical CIDs have not been well characterized in the literature. For this purpose, we studied a series of aromatic and non-aromatic compounds such as sulfonamides, N-aryl amides, tert-butyl-substituted aromatic compounds, aryl alkyl ethers, and O-alkyl aryl oximes using the LTQ and LTQ Orbitrap linear ion trap mass spectrometers. The accurate measurement of the fragment ion masses established the unambiguous assignment of the fragment structures resulting from the test compounds. Our results showed that free radical fragmentation is structure dependent and is to a large extent correlated with the neighboring groups in the structures that stabilize the newly formed free radical ions.

From the mobile proton to wandering hydride ion: mechanistic aspects of gas‐phase ion chemistry

“…This situation occurs when the radical site is strongly stabilized in either (or both) the neutral or the charged fragments, for example by delocalization of the unpaired electron. Losses of • OH, • NO, • NO 2 from nitroaromatic MH + during ESI–CID experiments are for example frequently observed as recently reported …”

“…Dissociation of closed‐shell, even‐electron ions (such as ions of the type MH + or [M–H] − ) generally leads to two even‐electron species (an even‐electron ion plus a molecule) rather than two odd‐electron products . This observation is known as the ‘even‐electron rule’.…”

Derivatization of bisphenol A and its analogues with pyridine‐3‐sulfonyl chloride: multivariate optimization and fragmentation patterns by liquid chromatography/Orbitrap mass spectrometry

“…The formation of this radical product ion is presumably attributed to its stabilization through conjugation by the adjacent aromatic ring. Odd‐electron dissociations, which involve homolytic bond cleavages to yield radical product ions, are commonly observed in electron ionization (EI)‐MS, but they are less common in collision‐induced dissociation under ESI and atmospheric pressure chemical ionization (APCI) conditions 33, 34. Several exceptions to the 'even‐electron rule' have, however, been reported to involve radical eliminations leading to odd‐electron product ions of high stability 35, 36.…”

LC/MS study of the UV filter hexyl 2‐[4‐(diethylamino)‐2‐hydroxybenzoyl]‐benzoate (DHHB) aquatic chlorination with sodium hypochlorite

“…Because this reaction contradicts the rule of even electron ions and produces radial cation species, the abundance of ion 1d is rather low. Such homolytic dissociation has been shown to occur in several constituent amines . The alternative elimination of the whole aromatic part of the molecule (N–Ar bond cleavage) does not take place for some reason as its corresponding ion is absent in the spectra at any collision energy.…”