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J. Chem. Phys., Vol. 111, No. 2, 8 July 1999
Mark L. Campbell
importance of electronic effects in the kinetics of these reac-
tions.
V. SUMMARY
We have measured the second-order rate constants as a
function of temperature for the reactions of ground state lan-
thanide atoms with nitrous oxide. Large variations in the
reactivity are observed. Energy barriers vary from 0.8 for
1
1
Ce( G ) to 20.2 kJ/mole for Yb( S ). The reaction barriers
4
0
are found to correlate to the energy required to promote an
electron out of the filled 6s subshell.
FIG. 5. Plot of the experimental activation energies vs the sums of the
ionization energy and s-p promotion energy.
ACKNOWLEDGMENT
This research was supported by a Cottrell College Sci-
ence Award of the Research Corporation. MLC is a Henry
Dreyfus Teacher Scholar.
therefore, these two properties largely determine the activa-
tion barrier.
In the resonance model, the activated complex is de-
scribed by three resonance structures. The first structure has
a covalent bond between the oxygen and the metal in which
the metal uses the s orbital for bonding. The second structure
1
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also has a covalent bond except the metal has p character.
Ϫ
The last structure results from the interaction of the N O
2
ϩ
and M ions. The activation energy is calculated based on
the wave functions of these structures and their contribution
to the electronic structure. According to the resonance
theory, as the ionization potential and s-p promotion energy
of the metal increase, the energy differences between the
resonating structures should also increase. This energy dif-
ference increase causes the resonance energy to decrease and
the activation energy to increase. Table III lists the ionization
energies and s-p promotion energies of the lanthanides stud-
ied in these experiments. Figure 5 presents a plot of the
activation energies of the lanthanides against the sums of the
ionization energy and s-p promotion energy for each lan-
thanide. The energy barriers for the majority of the lan-
thanides follow the pattern suggested by Fontijn’s resonance
theory, although the correlation is not as good as for the
5
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1
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1
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4
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2
1
s –s electron promotion energy correlation presented ear-
lier. Unfortunately, theoretical calculations involving the lan-
thanides are extremely difficult due to complications arising
from electron correlation, relativistic and spin-orbit effects.
The SECI calculations of Fontijn and co-workers represent a
transitional solution to these difficulties. Hopefully further
advances in dynamics calculations will some day clarify the
1
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