Electron-Impact Ionization of Mandelic Acid and Mandelic Acid Methyl Ester as Prototypes for the C 6 H 5 CH(OH)-X System: Ionization and Appearance Energies,Activation Energies and Enthalpies of Formation

Electron-impact ionization mass spectra, the decay of metastable ions, ionization and appearance energies and bond energies, as dissociation energies, are reported for mandelic acid (MA) and mandelic acid methyl ester (MAME) . An ionization energy of 8.55 eV was obtained for both compounds. For the formation of C₆H₅⁺ ions, appearance energies of 14.25 eV or 14.40 eV were determined for MA or MAME, respectively. On the main fragmentation route from molecular ions to these C₆H₅⁺ ions, an activation energy of (1.1±0.1)eV in excess of the calculated total reaction enthalpy was found for the elimination of CO from the prominent C₇H₇O⁺ ions to C₆H₇⁺ ions, involving the transfer of two H-atoms. The second (minor) fragmentation route via C₇H₆O⁺ions resembles the benzaldehyde decomposition pathway and the individual steps were found to be near to their calculated thermochemical reaction enthalpies. Gas-phase enthalpies of formation of (–4.652±0.3 eV) [≡ (–448.8±30) kJ mol⁻¹] for MA and (–4.243±0.3) eV [≡ (–409.4±30) kJ mol⁻¹] for MAME are derived, based on the appearance energy of their C₇H₇O⁺ ions at m/z 107 and the formation enthalpy adopted from protonated benzaldehyde. The results are compared with data for benzylalcohol.