Intramolecular Halogen Atom Coordinated H Transfer via Ion-Neutral Complex in the Gas Phase Dissociation of Protonated Dichlorvos Derivatives

Hydroxyl transfer versus cyclization reaction in the gas phase: Sequential loss of NH3 and CH2CO from protonated phenylalanine derivatives

Collisional activation of protonated phenylalanine derivatives deamination products leads to hydroxyl skeletal rearrangement versus cyclization reaction, and to form hydroxylbenzyl cation via elimination of CH₂CO. To better clarify this unusual fragmentation reaction, accurate mass measurements experiments, native isotope experiments, multiple-stage mass spectrometry experiments, different substituents experiments, and density functional theory (DFT) calculations were carried out to investigate the dissociation mechanistic pathways of protonated phenylalanine derivatives deamination products. In route 1, a three-membered ring-opening reaction and a 1,3-hydroxyl transfer (from the carbonyl carbon atom to the interposition carbon atom of carbonyl) occurs to form 3-hydroxy-1-oxo-3-phenylpropan-1-ylium, followed by dissociation to lose CH₂CO to give hydroxy (phenyl)methylium. In route 2, a successive cyclization rearrangement reaction and proton transfer occur to form a 2-hydroxylphenylpropionyl cation or protonated 2-hydroxy-4H-benzopyran, followed by dissociation to lose CH₂CO or CH≡COH to give 2-hydroxylbenzyl cation. In route 3, a successive hydroxyl transfer (from the carbonyl carbon atom to the ortho carbon atom on benzene) and two stepwise proton transfer (1,2-proton transfer to the ipso-carbon atom of the phenyl ring followed by 1,3-proton transfer to the ortho carbon atom of carbonyl) occurs to form a 2-hydroxylphenylpropionyl cation, which subsequently dissociates to form 2-hydroxylbenzyl cation by elimination of CH₂CO. DFT calculations suggested that route 1 was more favorable than route 2 and route 3 from a thermodynamic point of view.

Intramolecular benzyl cation transfer in the gas-phase fragmentation of protonated benzyl phenyl sulfones

In this study, the gas-phase fragmentations of protonated benzyl phenyl sulfones were investigated by electrospray ionization tandem mass spectrometry (ESI-MSⁿ ). Upon collisional activation, several characteristic fragment ions were observed, and the similar results occurred with different substituted benzyl phenyl sulfones. A mechanism involving an intramolecular benzyl cation transfer and the formation of intermediate ion was proposed and further identified by density functional theory (DFT) calculations. In addition, a reference compound, benzenesulfinic acid benzyl ester, has been synthesized, and its protonated ion has the same gas-phase behavior as compared to the protonated benzyl phenyl sulfone. This work provides access to some insight into the intramolecular benzyl-transfer reactions of benzyl phenyl sulfones in the gas phase and orients the characteristic peaks in collision-induced dissociation spectrometry (CID-MS).

An intriguing "reversible reaction" in the fragmentation of deprotonated dicamba and benzoic acid in a Q-orbitrap mass spectrometer: Loss and addition of carbon dioxide

RATIONALE

Loss of carbon dioxide is an important characteristic fragmentation reaction of deprotonated benzoic acid and its derivatives in electrospray ionization mass spectrometry. However, researchers have rarely noticed or believed that the loss of carbon dioxide in multistage mass spectrometry is a "reversible reaction," that is, the fragment anion generated by carbon dioxide loss can capture another carbon dioxide to regenerate its precursor ion.

METHODS

The fragmentation of the [M - H]^- ions of dicamba (3,6-dichloro-2-methoxybenzoic acid) and benzoic acid was performed with an electrospray ionization hybrid quadrupole-orbitrap mass spectrometer. The structural confirmation of the precursor ions and their product ions was supported by accurate mass (elemental composition) analysis. Pseudo-MS³ experiments (in-source collision-induced dissociation as MS² ) and isotope labelling experiments were used to confirm the addition of carbon dioxide to the product ions in MS² .

RESULTS

In the fragmentation of deprotonated dicamba (m/z 219), the relative abundance of the precursor ion does not decrease significantly or even increases as the collision energy increases. When the m/z 145 and 175 product ions were isolated in the mass analyzer, the ions 44 m/z units larger (m/z 189 and 219) were generated spontaneously, indicating the formation of carbon dioxide adduct ions. In the fragmentation of deprotonated [carboxyl-¹³ C]-benzoic acid (m/z 122), a deprotonated [carboxyl-¹² C]-benzoic acid ion (m/z 121) was generated which was derived from ¹³ CO₂ loss and ¹² CO₂ addition. The isotope labelling experiment further supports the formation of CO₂ -attached ions in the fragmentation of deprotonated benzoic acids.

CONCLUSIONS

Under collisional activation, deprotonated dicamba and benzoic acids easily undergo carbon dioxide loss, but the decarboxylated product anions have an appropriate nucleophilicity to carbon dioxide and they can capture a background carbon dioxide molecule remaining in the vacuum system to regenerate the precursor ions. This study provides a new and deeper understanding of the gas-phase chemistry of deprotonated benzoic acid derivatives in mass spectrometry.

Sulfur Transfer Versus Phenyl Ring Transfer in the Gas Phase: Sequential Loss of CH3OH and CH3O-P=O from Protonated Phosphorothioates

Collisional activation fragmentation of protonated phosphorothioates leads to skeletal rearrangement and formation of aryl sulfenylium cation (R-PhS⁺) via successive eliminations of CH₃OH and CH₃O-P=O. To better understand this unusual fragmentation reaction, isotope-labeling experiments and density functional theory (DFT) calculations were carried out to investigate two mechanistic pathways. In route 1, a direct intramolecular transfer of the R-phenyl group occurs from the oxygen atom to the sulfur atom on thiophosphoryl to form methoxyl S-(3-methyl-4-methylsulfanyl-phenyl) phosphonium thiolate (a4), which subsequently dissociates to form the m/z 169 cation. In route 2, the sulfur atom of the thiophosphoryl group undergoes two stepwise transfer (1,4-migration to the ortho-carbon atom of the phenyl ring followed by 1,2-migration to the ipso-carbon atom) to form an intermediate isomer, which undergoes the subsequent dissociation to form the m/z 169 cation. DFT calculations suggested that route 2 was more favorable than route 1 from the point view of kinetics. Graphical Abstract.