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Ionisation and appearance potentials of alkylketones
Author(s) -
Mouvier G.,
Hernández R.
Publication year - 1975
Publication title -
organic mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.475
H-Index - 121
eISSN - 1096-9888
pISSN - 0030-493X
DOI - 10.1002/oms.1210101106
Subject(s) - kinetic energy , chemistry , ion , fragmentation (computing) , electron ionization , adiabatic process , ionization , mass spectrometry , polyatomic ion , atomic physics , ionization energy , analytical chemistry (journal) , physics , thermodynamics , organic chemistry , chromatography , quantum mechanics , computer science , operating system
The ionisation potentials of a series of alkylketones have been measured by the electron‐impact technique using the second derivative method. The ionisation efficiency curves were obtained with a conventional mass spectrometer and the data treated by means of a minicomputer. These results are compared with those given by photoelectron spectroscopy, which has enabled us to test our mass spectrometric method and to distinguish between the adiabatic and the vertical ionisation potentials. It has been observed that the electron‐impact technique can lead to the adiabatic ionisation potential in favourable cases. We have determined the appearance potentials of fragmentations [A] + = [RCO] + and [B] + = [CH 3 CO] + , arising from the rupture of bonds in the α position with respect to CO, for a large series of methylalkylketones. The standard heats of formation of these ions have been calculated. The excess energy of the fragmentation reaction, corresponding to the kinetic shift, is found to be small or negligible for the decomposition giving ion [A] + , requiring a smaller activation energy and high in the case of ions [B] + . This kinetic shift becomes larger for an increase in the activation energy and the vibrational degrees of freedom of the molecular ion, as postulated by the quasi‐equilibrium theory. An important fraction of the kinetic shift is due to the ‘competitive shift,’ i.e. the excess energy that must be supplied to the molecular ion for it to yield [B] + ions in sufficient amounts to be detected by the mass spectrometer. This is confirmed by the claculation of the fragmentation rate constants for the 2‐butanone molecular ion.