Collect. Czech. Chem. Commun. 2009, 74, 101-114
Published online 2009-01-28 10:22:02

First and second ionization energies of 1,3,5-trimethylbenzene and 2,4,6-trimethylpyridine

Petr Milkoa, Detlef Schrödera, Karel Lemrb, Ján Žabkac, Christian Alcarazd,e and Jana Roithováa,f,*

a Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, v.v.i., Flemingovo nám. 2, 166 10 Prague 6, Czech Republic
b Department of Analytical Chemistry, Palacký University, Svobody 8, 771 46 Olomouc, Czech Republic
c J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Dolejškova 3, 183 00 Prague 8, Czech Republic
d Laboratoire de Chimie Physique, Bat. 350, UMR 8000 Centre Universitaire Paris-Sud, 91405 Orsay Cedex, France
e Synchrotron SOLEIL, L’orme des Merisiers, Saint-Aubin – BP 48, 91192 Gif-sur-Yvette Cedex, France
f Department of Organic Chemistry, Faculty of Science, Charles University, Hlavova 8, 128 40 Prague 2, Czech Republic


The first and second ionization energies of trimethyl substituted analogs of benzene and pyridine are determined by means of mass spectrometry in conjunction with synchrotron radiation. The first ionization energy of 1,3,5-trimethylbenzene amounts to (8.38 ± 0.05) eV and the second ionization energy to (22.8 ± 0.1) eV. The first ionization energy of 2,4,6-trimethylpyridine is determined as (8.65 ± 0.05) eV and the second ionization energy as (23.0 ± 0.1) eV. The ionization energies are compared with those of unsubstituted benzene and pyridine and the effects of the methyl groups are evaluated by means of isodesmic reactions. As expected, it is found that the electron-donating effect of the methyl groups stabilizes neutral pyridine and doubly charged pyridine more than neutral benzene and doubly charged benzene, respectively. Surprisingly, the opposite effect is found for the radical cations, which is ascribed to the unfavorable degenerate electronic structure of benzene radical-cation, which disappears upon the methyl substitution.

Keywords: Ionization energy; Isodesmic reactions; Jahn–Teller effect; Mass spectrometry; Synchrotron; Substituent effect; Arenes.

References: 37 live references.