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Data S1. Supporting Information Figure 1s. Product ion spectra of [M + D]+ ion of methyl ortho-anisate. Figure 2s. Product ion spectra of [M + D]+ ion of methyl meta-anisate. Figure 3s. Product ion spectra of [M + D]+ ion of methyl para-anisate. Figure 4s. Product ion spectra of [M + H]+ ions of methyl anisates. Figure 5s. Product ion spectra of [M + D]+ ions of methyl fluorobenzoates. It is clearly seen that for meta isomers loss methanol molecule is accompanied by intramolecular H/D exchange. Figure 6s. Product ion spectra of [M + D]+ ions of methyl hydroxybenzoates. It is clearly seen that for meta isomers loss methanol molecule is accompanied by intramolecular H/D exchange. Figure 7s. Product ion spectra of [M + D]+ ions of methyl naphthoate. It is clearly seen that for both isomers loss methanol molecule is accompanied by intramolecular H/D exchange. Figure 8s. Product ion spectra of [M + H]+ and [M + D]+ ions of ethyl picolinate. It is clearly seen that the loss of ethanol molecule proceed, however, it is not accompanied by intramolecular hydrogen exchange. Figure 9s. Product ion spectra of protonated ethyl nicotinate and ethyl isonicotinate. It is clearly seen that the loss of ethanol molecule does not proceed. Figure 10s. Product ion spectra of [M + H]+ ions of methyl (D3) anisates. Figure 11s. Exemplary full scan mass spectra of methyl para-anisate. Scheme 1s. Relative energies of protonated methyl ortho-anisate. Scheme 2s. Relative energies of protonated methyl meta-anisate. Scheme 3s. Relative energies of protonated methyl para-anisate. Scheme 4s. Resonance structures of methyl anisates protonated at ortho position to the ester group. Scheme 5s. Relative energies of some structures of the protonated methyl fluorobenzoates (all structures are shown in the supporting information of J. Mass. Spectrom. 2018, 53, 379). Scheme 6s. Relative energies of some structures of the protonated methyl hydroxybenzoates (all structures are shown in the supporting information of J. Mass. Spectrom. 2018, 53, 379). Scheme 7s. Relative energies of some structures of the protonated methyl naphthoates (all structures are shown in the supporting information of J. Mass. Spectrom. 2018, 53, 379). Scheme 8s. Relative energies of protonated ethyl picolinate. Protonation at ethoxy oxygen atom is very likely, thus loss of ethanol molecule occurs. Protonation at carbon atom of aromatic ring is very unlikely, thus intramolecular hydrogen exchange does not occurs during ethanol loss (Figure 5s). Scheme 9s. Relative energies of protonated ethyl nicotinate. Protonation at ethoxy oxygen atom is very unlikely, thus the loss of ethanol molecule does not occur (Figure 9s). Scheme 10s. Relative energies of protonated ethyl isonicotinate. Protonation at ethoxy oxygen atom is very unlikely, thus the loss of ethanol molecule does not occur (Figure 9s). Scheme 11s. Formation of product ion at m/z 111 from protonated molecule of methyl (D3) para-anisate. It is worth adding that loss of methyl radical has been observed for methoxylated flavonoids (P.J.A. Madeira, C.M. Borges, M.H. Florêncio. Rapid Commun. Mass Spectrom. 2010; 24: 3432-3440). Table 1s. Natural Bonding Analysis of methyl otho-anisate protonated at ortho position to the ester group. Table 2s. Natural Bonding Analysis of methyl meta-anisate protonated at ortho position to the ester group. Table 3s. Natural Bonding Analysis of methyl para-anisate protonated at ortho position to the ester group. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.