Chemistry Reference
In-Depth Information
1.85
1.80
1.75
1.70
1.65
1.60
µ
calc
µ
obs
1.55
1.50
100
150
200
250
300
Temperature (K)
Figure 4.3
Observed (open triangles) and calculated (filled squares) magnetic moments from the Van Vleck
distributionlawmodel.
Van Vleck, Equation 4.9.
56-58
1
−
e
−
x
+
xe
−
x
µ
eff
= 2
h
∆n
(4.9)
173
T
x=
=
x
+
xe
−
x
kT
π
∗
orbital results in a net combination of angular momentum of the electron and the
orbital where the spin either opposes the orbital momentum, with a result of 1
Occupation of the
−
1
/
2
or
1
/
2
or combines with
/
2
. These two doublet states are designated
2
2
it to give 1
+
1
/
2
or
3
1
/
2
and
3
/
2
respectively and differ in
energy by 1.47 kJ mol
−
1
or 124.2 cm
−
1
. At room temperature slightly more than a third of the molecules
2
are in the lower energy
/
2
state. The resultant magnetic susceptibility is then the combination of these
two states and the expectation values are
1
0B.M. for
2
2B.M for
2
/
2
. In addition to
affecting the magnetic susceptibility and magnetic moment for nitric oxide, the combined orbital momenta
broaden and shift the EPR resonance of the unpaired electron from
g
e
, so that with conventional cavity
instruments an EPR resonance is not observable for nitric oxide and cooling to low temperatures gives
the diamagnetic dimer. Adduct formation with either surfaces
59
µ
=
1
/
2
and
µ
=
3
or metal ions
60
π
∗
degeneracy
breaks the
and leads to observable EPR resonances.
61
Turner's early molecular photoelectron spectra
62
for nitric
oxide have since been followed by many studies,
63-73
including those of the dimer under a variety of
conditions.
40,74 - 76
Low lying excited states for nitric oxide include a Rydberg
2
lp
lp
4
π
∗
0
ryd
A
2
σ
σ
σ
π
σ
state from the
4
2
excitation of an electron to an s-type Rydberg orbital, and two,
and B
states, which arise from
π
→
π
∗
excitation and correspond to
3
π
∗
2
a
π
configurations. Collectively these three excited states lie
between 4.7 and 5.6 eV in higher energy,
77,78
and need to be considered for understanding the electronic
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