Chemistry Reference
In-Depth Information
1.8
1.7
1.6
0
50
100
150
200
250
300
Temperature (K)
Figure 4.2
N-N bond lengths (
A
)inN
2
O
4
as a function of temperature for (
) X-ray diffraction, (
•
)neutron
diffraction,(
)electrondiffractionand(
∗
)forIRdata.ThebestfitlinefortheX-raydiffractiondatadescribedby
Cartwright
45
isalsoshown.
first to investigate this and employed variable temperature X-ray diffraction to determine these dimensions
in the solid state. Although other weak intermolecular interactions such element hydrogen
51
and halogen
52
bond lengths are known to vary with temperature, for covalent species or van der Waals complexes such
as dinitrogen tetraoxide this temperature dependence has not been well studied. If Broadley's 1949 X-ray
diffraction result with a very short 1.64(3) A N-N bond length is removed, there is a small apparent increase
in N-N bond length with temperature in Figure 4.2. Clearly more data at intermediary temperatures are
required before this correlation is well established, but the overall affect has the magnitude found in other
systems. A conclusion is that close lying non-bonding or antibonding excited states do not appear to be
thermally populated, and this result finds support from recent femtosecond spectroscopy of the dimer.
53
(4.8)
N
O
N
O
4.5 Electronic structure of nitrogen oxides
2
2
2
lp
In the molecular orbital description for nitric oxide the valence ground state configuration is
σ
σ
lp
σ
π
∗
orbital with substan-
tial localization on the nitrogen. Although this is a good start to understanding nitric oxide's electronic
structure, and it is certainly better than valence bond descriptions
54
(Equation 4.8) (as a double bond and
a three electron bond between the atoms),
55
simple MO theory does not explain the magnetic suscepti-
bility or the EPR data in a satisfactory way. A plot of the variable temperature magnetic susceptibility
of gaseous nitric oxide is shown in Figure 4.3, along with a theoretical fit from a model developed by
4
π
∗
1
X
2
π
, which suggests a bond order of 2.5 with the unpaired electron in a
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