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

Abstract

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...

Differentiation of bisphenol F diglycidyl ether isomers and their derivatives by HPLC-MS and GC-MS—comment on the published data

“…It is well known that electron ionization mass spectra of 1,2-disubstituted aromatic compounds are substantially different from those of their positional isomers. There are a number of examples that in ESI conditions the ortho effect also takes place [25][26][27][28][29][30][31].…”