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structure of the nitric oxide, its dimer, and its van der Waals complexes. In all, 16 valence electronic states
need to be considered for the dimer.
41
On the other hand, nitrogen dioxide's bent structure is readily accounted for by a Walsh bending
model from a linear
D
∞
h
point group to
C
2
v
.
79
The resulting unpaired electron is largely localized on the
nitrogen but there are many low lying excited states and configuration interaction with these is required for
a complete description of the electronic structure. Nevertheless,
S
/
2
remains a good quantum number
and the magnetic susceptibility for gaseous nitrogen dioxide at 293 K is 1.72 B.M. and its temperature
dependence is well accounted for by its equilibrium with the diamagnetic monomer.
80,81
The X-band EPR
spectrum of gaseous nitrogen dioxide at atmospheric pressure is a broad unresolved band which becomes
a triplet with
A
N
=
=
1
9 gauss at pressures between 14 and 1 mm Hg
P
NO
2
.
82
50
.
At lower pressures, 0.02 mm
Hg, the spectrum is described as having more than 200 lines.
83
At 4.2 K, and doped into solid argon,
nitrogen dioxide gives an axial EPR spectrum with g
=
0033(5) with an observed
coupling of 53.4 gauss.
84
At these temperatures the photolytic generation of nitrogen dioxide in a sodium
nitrite crystal gives a rhombic signal with
g
1
=
1
.
9920(5) and g
⊥
=
2
.
9910.
85
More recent
studies have demonstrated that nitrogen dioxide forms adducts/reacts with many solvents.
86,87
2
.
0057,
g
2
=
2
.
0015, and
g
3
=
1
.
There is a
rich literature for the photoelectron spectroscopy of nitrogen dioxide
88-90
and dinitrogen tetroxide.
91-97
A remarkable high resolution He-I study (h
v
=
21
.
22 eV) of gas phase nitrogen dioxide has exceptional
vibronic coupling in all of the observed bands.
98
Early theoretical attempts to understand the electronic structure of the nitric oxide dimer were unsuc-
cessful in accounting for the geometry, the vibrational data or its low
D
e
.
99 - 102
The subsequent efforts
to determine the electronic structure of the dimer have been well described in East and Bondybey's
analyses,
40,41
but surprisingly many problems remain to be resolved for all of the dimers, neutral and
charged ([N
2
O
2
]
+
,[N
2
O
2
]
−
)
alike. Even one of the most exhaustive post-Hartree-Fock studies by Roos
et al
. considered only a
cis
(NO)
2
geometry.
103
As noted by Schaefer
et al
. theory
104
has had a “muddled
relationship to experiment.” in this domain. Certainly part of the problem is due to the differing geometric
stabilities of the neutral versus the anionic and cationic dimers. Thus, although the neutral
cis
dimer is circa
45 kJ mol
−
1
more stable than the
trans
in an inert gas matrix, the
trans
is more stable by circa 6.0 kJ mol
−
1
for the cation and the
cis
and
trans
are almost equi-energetic for the anion. As a consequence, interpret-
ing the results of ionizing experiments such as photoelectron spectroscopy are complicated by geometry
considerations for the product states.
At first glance the nitrogen oxides are small molecules and should be ideally suited for modern
ab initio
calculations. But for a wide array of theoretical methods this class of compounds remains a challenge
to accurately model and predict the structure, spectroscopy, and reactivity of their dimers. New codes
and algorithms are often checked against the nitrogen oxides to assess improvements or advances in
methodology. Clearly the combination of weak bonds, numerous lone pairs, and low lying spin states
make this area as challenging for theoreticians as it is for experimentalists.
4.6 Reactivity of nitric oxide and nitrogen dioxide
and their van der Waals complexes
Central to any discussion of the reactivity of nitric oxide and nitrogen dioxide is consideration of the dimer-
ization equilibria in Equations 4.1 and 4.2. Both reactions are readily reversible and rapidly established;
their kinetic and thermodynamic data are contrasted in Table 4.5. The rate of nitric oxide dimerization and
its reverse must be rapid reactions, since there is no spectroscopic evidence for the presence of dimers in
reactions such as nitric oxide's oxygenation. Earlier reports suggested
54
that, based on determinations of
the second virial coefficients and Berthelot's equations, there was no evidence for associative dimerization
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