Ambulation of Incipient Proton during Gas-Phase Dissociation of Protonated Alkyl Dihydrocinnamates

Xu, Sihang; Zhang, Yong; Errabelli, Ramu; Attygalle, Athula B.

doi:10.1021/acs.joc.5b01390

Cited by 9 publications

(7 citation statements)

References 21 publications

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“…As shown in Scheme , the corresponding proton is mobilized and assumed to undergo a 1,2‐shift along the aromatic ring and thus to weaken the otherwise rather strong C ar OMe bond. This mechanistic scenario is in line with nowadays accepted isomerization and fragmentation chemistry of gaseous aromatic ions . However, it may be noted that the minor loss of water, giving rise to ions at m / z 457.2050, from the protonated ester species indicates more complex, hidden rearrangements.…”

Section: Resultssupporting

confidence: 83%

The unusual fragmentation of long‐chain feruloyl esters under negative ion electrospray conditions

et al. 2019

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Long‐chain ferulic acid esters, such as eicosyl ferulate (1), show a complex and analytically valuable fragmentation behavior under negative ion electrospay collision‐induced dissociation ((−)‐ESI‐CID) mass spectrometry, as studied by use of a high‐resolution (Orbitrap) mass spectrometer. In a strong contrast to the very simple fragmentation of the [M + H]+ ion, which is discussed briefly, the deprotonated molecule, [M – H]−, exhibits a rich secondary fragmentation chemistry. It first loses a methyl radical (MS2) and the ortho‐quinoid [M – H – Me]‐• radical anion thus formed then dissociates by loss of an extended series of neutral radicals, CnH2n + 1• (n = 0–16) from the long alkyl chain, in competition with the expulsion of CO and CO2 (MS3). The further fragmentation (MS4) of the [M – H – Me – C3H7]− ion, discussed as an example, and the highly specific losses of alkyl radicals from the [M – H – Me – CO]‐• and [M – H – Me – CO2]‐• ions provide some mechanistic and structural insights.

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Section: Resultssupporting

confidence: 83%

The unusual fragmentation of long‐chain feruloyl esters under negative ion electrospray conditions

et al. 2019

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“…However, the protonated species produced under ESI conditions from compounds such as alcohols, esters and carboxylic acids are often too short‐lived to be detected as intact entities. On the other hand, Helium‐Plasma Ionization (HePI) is a potent soft‐ionization technique that generates detectable ions even from compounds with low proton affinity, such as acids and esters . For example, the HePI mass spectra recorded from benzoic acid, phenylacetic acid, dihydrocinnamic acid (Fig.…”

Section: Resultsmentioning

confidence: 99%

Collision‐induced dissociation processes of protonated benzoic acid and related compounds: competitive generation of protonated carbon dioxide or protonated benzene

Pavlov

Attygalle

2017

J. Mass Spectrom.

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Upon activation in the gas phase, protonated benzoic acid (m/z 123) undergoes fragmentation by several mechanisms. In addition to the predictable water loss followed by a CO loss, the m/z 123 ion more intriguingly eliminates a molecule of benzene to generate protonated carbon dioxide (H - O ═ C ≡ O, m/z 45), or a molecule of carbon dioxide to yield protonated benzene (m/z 79). Experimental evidence shows that the incipient proton ambulates during the fragmentation processes. For the CO or benzene loss, protonated benzoic acid transfers the charge-imparting proton initially to the ortho position and then to the ipso position to generate a transient species which dissociates to form an ion-neutral complex between benzene and protonated CO . The formation of the m/z 45 ion is not a phenomenon unique to benzoic acid: spectra from protonated isophthalic acid, terephthalic acid, trans-cinnamic acid and some aliphatic acids also displayed a peak for m/z 45. However, the m/z 45 peak is structurally diagnostic only for certain benzene polycarboxylic acids because the spectra of compounds with two carboxyl groups on adjacent ring carbons do not produce a peak at m/z 45. For the m/z 79 ion to be formed, an intramolecular reaction should take place in which protonated CO within the ion-neutral complex acts as the attacking electrophile to transfer a proton to benzene. Copyright © 2017 John Wiley & Sons, Ltd.

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“…However, this process may be useful for identification of compounds (eg, isomer differentiation) and can be very interesting from the gas-phase ion chemistry point of view, as described in details in a number of papers. [1][2][3][4][5][6][7][8][9][10][11][12] There is also an interesting example of the loss of very long alcohols from protonated tetrahydropyranyl ethers. 13 Loss of methanol molecule has been also observed for protonated methyl esters of aminoacids.…”

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confidence: 99%

“…It is clearly seen at Figure 1, for methyl meta-anisate, the loss of methanol molecule is accompanied by efficient intramolecular H/D exchange (in contrast to the ortho and para isomers).It can be taken for granted that the loss of methanol molecule occurs from the structure protonated at methoxy oxygen atom of the ester group (obviously the most stable is the structure protonated at carbonyl oxygen atom) as already proposed for other kinds of protonated esters. 6,9,12,14 If this process is accompanied by intramolecular hydrogen exchange, a structure protonated at aromatic ring (most probably at ortho position) should be also formed. Thus, the process can be shown as it is drawn in Scheme 1.Evidently, the intramolecular hydrogen exchange rate is faster for the meta isomer, compared to those for the ortho and para isomers.…”

mentioning

confidence: 99%

“…It can be taken for granted that the loss of methanol molecule occurs from the structure protonated at methoxy oxygen atom of the ester group (obviously the most stable is the structure protonated at carbonyl oxygen atom) as already proposed for other kinds of protonated esters. 6,9,12,14 If this process is accompanied by intramolecular hydrogen exchange, a structure protonated at aromatic ring (most probably at ortho position) should be also formed. Thus, the process can be shown as it is drawn in Scheme 1.…”

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confidence: 99%

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Intramolecular hydrogen exchange prior to methanol loss from protonated methyl benzoates bearing different ring substituents under CID conditions

et al. 2018

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The loss of methanol from protonated methoxy-group-containing organic compounds (ethers or esters) under collision-induced dissociation (CID) conditions seems to be a trivial process. However, this process may be useful for identification of compounds (eg, isomer differentiation) and can be very interesting from the gas-phase ion chemistry point of view, as described in details in a number of papers. [1][2][3][4][5][6][7][8][9][10][11][12] There is also an interesting example of the loss of very long alcohols from protonated tetrahydropyranyl ethers. 13 Loss of methanol molecule has been also observed for protonated methyl esters of aminoacids. 14,15 It has been briefly shown in the supporting information of the previous paper that the loss of methanol from protonated methyl benzoate is also not trivial process. Namely, the product ion spectra of [C 6 H 5 COOCH 3 +D] + and [C 6 D 5 COOCH 3 +H] + have shown that part of the eliminated methanol molecules contain hydrogen atoms originating from the aromatic ring. 16 In other words, the loss of methanol from protonated methyl benzoate is accompanied by intramolecular (intraionic) hydrogen exchange (for the ions [C 6 H 5 COOCH 3 +D] + and [C 6 D 5 COOCH 3 +H] + , we deal with intramolecular H/D exchange). It is worth adding that the product ion spectrum of [C 6 H 5 COOCD 3 +H] + has shown that the hydrogen atoms from methyl group are not directly involved in the hydrogen exchange. 16 It is also worth adding that the loss of water molecule from protonated benzoic acid is also accompanied by intramolecular hydrogen exchange as shown by the product ion spectra of respective deuterium-containing ions. 17In this work, we decided to study the influence of substituents on the hydrogen exchange for methyl benzoates bearing different ring substituents. Figure 1 shows the product ion spectra of [M + D] + ions of methyl anisates. The product ion spectra of [M + D] + ions obtained at different collision energies as well as other product ion spectra of [M + H] + ions are shown in the supporting information (Figure 1s-4s). It is clearly seen at Figure 1, for methyl meta-anisate, the loss of methanol molecule is accompanied by efficient intramolecular H/D exchange (in contrast to the ortho and para isomers).It can be taken for granted that the loss of methanol molecule occurs from the structure protonated at methoxy oxygen atom of the ester group (obviously the most stable is the structure protonated at carbonyl oxygen atom) as already proposed for other kinds of protonated esters. 6,9,12,14 If this process is accompanied by intramolecular hydrogen exchange, a structure protonated at aromatic ring (most probably at ortho position) should be also formed. Thus, the process can be shown as it is drawn in Scheme 1.Evidently, the intramolecular hydrogen exchange rate is faster for the meta isomer, compared to those for the ortho and para isomers. To answer this question, we have performed respective calculations. The results of the calculations have clearly shown that for meta isomers, t...

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Ambulation of Incipient Proton during Gas-Phase Dissociation of Protonated Alkyl Dihydrocinnamates

Cited by 9 publications

References 21 publications

The unusual fragmentation of long‐chain feruloyl esters under negative ion electrospray conditions

The unusual fragmentation of long‐chain feruloyl esters under negative ion electrospray conditions

Collision‐induced dissociation processes of protonated benzoic acid and related compounds: competitive generation of protonated carbon dioxide or protonated benzene

Intramolecular hydrogen exchange prior to methanol loss from protonated methyl benzoates bearing different ring substituents under CID conditions

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