Electron-impact ionization of benzoic acid, nicotinic acid and their n-butyl esters: An approach to regioselective proton affinities derived from ionization and appearance energy data

Abstract

Electron-impact ionization mass spectra, the decay of metastable ions, ionization and appearance energies and bond energies, as dissociation energies, are reported for the title compounds. An ionization energy of 9.47 eV was obtained for benzoic acid, 9.43 eV for benzoic acid n-butyl ester, 9.61 eV for nicotinic acid and 9.97 eV for nicotinic acid n-butyl ester. Molecular ions of both butyl esters show two common main fragmentation pathways: the first process is a McLafferty rearrangement, characterized by the transfer of one H-atom from the aliphatic ester chain, which leads to the ions of either the organic acid or 1-butene. From their appearance energies and known thermodynamic data, gas-phase formation enthalpies ( Δ H f 0 ) of the parent n-butyl esters are calculated. Values of Δ H f 0 = ( − 4.29 ± 0.3 ) eV for benzoic acid n-butyl ester and Δ H f 0 = ( − 3.66 ± 0.3 ) eV for nicotinic acid n-butyl ester were obtained. The second process is characterized by the transfer of two H-atoms from the ester chain leading to a protonated form of the corresponding organic acids and C 4H 7 radicals. Good evidence is provided for the formation of methylallyl radicals. Appearance energies are used to calculate a proton affinity (PA) for benzoic acid. The obtained value of PA = (8.73 ± 0.3) eV, corresponding to a protonation of the carbonyl group, is in close corroboration with published data (PA = 8.51 eV). Activation energies for the intermediate H-transfers were found to be insignificant. This methodic gateway is applied to the system of nicotinic acid and its butyl ester. Adopting the formation of a methylallyl radical, the obtained proton affinity of nicotinic acid, PA = 8.58 eV, is very near to the published data of benzoic acid. An alternative fragmentation mechanism leading to a value of PA ≈ 9.5 eV (typical for a protonation of the pyridine-nitrogen) is very unlikely. It is concluded that this transfer of two H-atoms from the ester chain is controlled by a charge switching between the carboxylic oxygen atoms which leads to a regiospecific protonation site, in this case to the protonated carbonyl group. This is conform with a B3LYP DFT calculation with a corresponding proton affinity of PA = 8.29 eV.