Dissociative Protonation and Proton Transfers: Fragmentation of α, β-Unsaturated Aromatic Ketones in Mass Spectrometry

Abstract

In mass spectrometry of the alpha,beta-unsaturated aromatic ketones, Ph-CO-CH=CH-Ph', losses of a benzene from the two ends and elimination of a styrene are the three major fragmentation reactions of the protonated molecules. When the ketones are substituted on the right phenyl ring, the electron-donating groups are in favor of losing a styrene to form the benzoyl cation, PhCO(+), whereas the electron-withdrawing groups strongly favor loss of benzene of the left side to form a cinnamoyl cation, Ph'CH=CHCO(+). When the ketones are substituted on the left phenyl ring, the substituent effects on the reactions are reversed. In both cases, the ratios of the two competitive product ions are well-correlated with the sigma p(+) substituent constants. Theoretical calculations indicate that the carbonyl oxygen is the most favorable site for protonation, and the olefinic carbon adjacent to the carbonyl is also favorable especially when a strong electron-releasing group is present on the right phenyl ring. The energy barrier to the interconversion between the ions formed from protonation at these two sites regulates the overall reactions. Transfer of a proton from the carbonyl oxygen to the ipso position on either phenyl ring, which is dissociative, triggers loss of benzene.

Gas-Phase Intramolecular Cyclization of Argentinated N-Allylbenzamides

“…From then on, Hammett plots have been extensively employed in the study of gas-phase chemistry [47][48][49][50] owing to its intuitive presentation of the substituent effect on the direct cleavage as well as on competitive fragmentation pathways. To get a more direct understanding on the relationship between substituents and competitive pathways, the logarithm of the intensity ratios of these two competitive product ions are usually utilized to plot with the substituent constants [31,37,50,51].…”

Ion-neutral complexes resulting from dissociative protonation: Fragmentation of α-furanylmethyl benzyl ethers and 4-N,N-dimethylbenzyl benzyl ethers

“…The compounds selected in this study were expected to have the particular fragmentation loss of CH 2 O, which occurs when a proton migrates from the thermodynamically favored site to the key dissociative protonation site [19,23,24]. The compounds tested were selected to be representative of different kinds of molecules, including (1) ␣-aromatic heterocycle methyl benzyl ethers, and (2) substituted-benzyl benzyl ethers.…”

Gas Phase Chemistry of Li+with Amides: the Observation of LiOH Loss in Mass Spectrometry

“…In the past few decades, continuous efforts have been devoted to the investigation of the dissociation reactions of charged compounds derived from protonation [4][5][6][7][8] or deprotonation [9][10][11][12][13]. On the other hand, the mass spectrometric fragmentations of metal cationized, especially lithiated biomolecules, have attracted much attention because of the desire to measure intrinsic metal ion affinity and identify the binding sites [14,15], and because the interesting dissociation pathways exhibited by these compounds are usually different from the protonated and deprotonated analogues [16,17].…”