Mass-spectrometric Study on Ionization and Dissociation of Formaldehyde, Acetaldehyde, Acetone and Ethyl Methyl Ketone by Electron Impact

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A study of the ionization and dissociation of formaldehyde, acetaldehyde, acetone and ethyl methyl ketone, all of which have an oxygen atom, was carried out through the appearance potential method. As appearance potentials were obtained by the retarding potential difference method, break points, which correspond to successively occurring ionization, appeared in the ionization efficiency curves of molecules and fragments. By considering the relation of energy, it is concluded that the ionization of the molecule takes place at first by the removal of one of the lone pair electrons of oxygen atom in the molecule, and that, in the formation of a fragment ion including an oxygen atom, dissociation takes place to create a triple bond, satisfying the trivalency of O+ in the fragment ions. The total bond energy of O+ in CH3C≡O+ and C2H5C≡O+ are calculated as about 200 kcal./mol., while in HC≡O+ it is 170∼180 kcal./mol. as in the case of diethyl ether reported previously. This fact may be explained by the number of resonance structures related. The second and the third ionization potentials are thought to correspond to the removal of the π-electron and σ-electron in C–O bond respectively. The heat of formation of CHO+, CH3CO+ and C2H5CO+ are obtained from various reactions.
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