Competitive homolytic and heterolytic decomposition pathways of gas‐phase negative ions generated from aminobenzoate esters

Abstract

An alkyl-radical loss and an alkene loss are two competitive fragmentation pathways that deprotonated aminobenzoate esters undergo upon activation under mass spectrometric conditions. For the meta and para isomers, the alkyl-radical loss by a homolytic cleavage of the alkyl-oxygen bond of the ester moiety is the predominant fragmentation pathway, while the contribution from the alkene elimination by a heterolytic pathway is less significant. In contrast, owing to a pronounced charge-mediated ortho effect, the alkene loss becomes the predominant pathway for the ortho isomers of ethyl and higher esters. Results from isotope-labeled compounds confirmed that the alkene loss proceeds by a specific γ-hydrogen transfer mechanism that resembles the McLafferty rearrangement for radical cations. Even for the para compounds, if the alkoxide moiety bears structural motifs required for the elimination of a more stable alkene molecule, the heterolytic pathway becomes the predominant pathway. For example, in the spectrum of deprotonated 2-phenylethyl 4-aminobenzoate, m/z 136 peak is the base peak because the alkene eliminated is styrene. Owing to the fact that all deprotonated aminobenzoate esters, irrespective of the size of the alkoxy group, upon activation fragment to form an m/z 135 ion, aminobenzoate esters in mixtures can be quantified by precursor ion discovery mass spectrometric experiments.

Forensic analysis of water‐based lubricants using liquid extraction surface analysis high‐resolution tandem mass spectrometry

“…Subsequent loss of carbon dioxide is demonstrated by the observation of a dehydrophenoxide radical anion (C 6 H 4 O •− ) at m/z 92.03. This pathway is consistent with the unimolecular dissociation of structurally related p ‐aminobenzoate ester anions . Similarly, collisional activation of ethyl paraben ( EP ) ions (C 9 H 9 O 3 − , m/z 165.06) yields product ions at m/z 136.02 and 92.03 via an analogous loss of an ethyl radical and subsequent decarboxylation.…”

Formation of Carbamate Anions by the Gas-phase Reaction of Anilide Ions with CO2

“…Although it is not customary to record negative-ion mass spectra of aromatic amines, gaseous anions can be generated under mass spectrometric conditions even from aniline (Supplementary Figure 2a) albeit its gas-phase acidity is relatively low. On the other hand, more acidic amines such as procaine (N,N-diethylaminoethyl 4-aminobenzoate), and benzocaine (ethyl 4-aminobenzoate) undergo more facile deprotonation and generate abundant gaseous anions under negativeion ESI conditions [21]. Although molecular and electronic structures and vibrational frequencies of anilide and substituted-anilide ions had been theoretically evaluated [22], and anilide-ions have been employed as nucleophiles to form aromatic substitution products [23], to our knowledge no mass spectrometry-based structural work has been carried out previously to determine the efficacy of anilide ions as reagents for nucleophilic addition reactions.…”

“…Furthermore, the product ion spectrum recorded using N 2 as the collision gas from deprotonated ethyl 4-aminobenzoate showed two distinct fragment-ion peaks at m/z 135 and 136, attributed to ethyl radical and ethylene losses, respectively [21] (Supplementary Figure 5a). When the collision gas was changed to CO 2 , two new peaks appeared in the spectrum (Supplementary Figure 5b).…”

Helium-Plasma-Ionization Mass Spectrometry of Metallocenes and Their Derivatives

“…Although plasma-ionization approaches, such as direct analysis in real time (DART) and flowing atmospheric-pressure afterglow (FAPA), appear to be appealing because samples need not be dissolved in a solvent prior to analysis, these ambient methods have been applied thus far only infrequently for the analysis of organometallics. ,,− The recent review by Vikse and McIndoe does not cite any plasma-based ambient ionization methods for MS analysis of organometallic compounds . Among the ambient methods available, helium-plasma ionization (HePI) , is a simple ionization method for the MS analysis of organic − and even rather involatile chemicals such as inorganic nitrates and mercury salts. , Compared to common APCI techniques, which require high-voltage corona discharges, the glow discharge of HePI is generated at low voltages. Moreover, HePI does not employ a corona needle; it uses a discharge capillary to generate the microplasma.…”