Chemistry Reference
In-Depth Information
R
′
R
R
R
′
R
R
′
R
R
′
N
N
N
N
+
+
+
+
1
σ
2
1
p
2
3
σp
1
σp
FIGURE 6.1.
Electronic configurations for a bent nitrenium ion.
180
). In that case,
discussion, nitrenium ions generally exist in a bent geometry (
u
<
the
orbital, possessing more
s
character, would be lower in energy than the
p
orbital. Whether the singlet or the triplet is the ground state will depend if the energy
difference between these orbitals is sufficient to compensate for the electronic
exchange energy available to the triplet state. Specifically, any effects that increase
the orbital energy splitting, such as more acute bond angle or electron donation
specifically into the
p
orbital will stabilize the singlet relative to the triplet. Likewise
effects that diminish the splitting, such as a more obtuse bond angle, or substituents
that remove electron density from the
p
orbital, will favor the triplet state relative to
the singlet.
In the absence of a more explicitly quantitative analysis, there is no way to know
which state is the ground state or how drastic the structural variations in R and R'
would need to be to perturb this energy difference. Nonetheless, the qualitative
considerations do suggest that, like the carbenes
3
and nitrenes,
4
nitrenium ions ought
to have low-energy singlet and triplet states. Understanding these energy differences
is interesting from a fundamental point of view. In particular, accurate modeling of
open-shell molecular systems remains an interesting problem in computational
chemistry.
A more practical motivation for these studies is the longstanding interest in
novel polymeric materials having interesting magnetic properties.
5
Va r i ou s
schemes for incorporating organic radicals, diradicals, carbenes, and nitrenes
into high-spin polymers and even materials showing bulk magnetism have been
explored, although commercial products based on these ideas remain elusive.
6
One
obstacle has been the chemical instability of many neutral open-shell organic
species. Carbenes, nitrenes, and most organic radicals tend to rapidly dimerize
and/or combine with oxygen. Nitrenium ions show none of these behaviors,
although the data set is admittedly more limited. Presumably, the positive charge
inhibits dimerization, and their high ionization potentials attenuate their reactivity
toward oxygen. In any case, if stable triplet nitrenium ions could be identified,
there is some promise that they could be used to create high-spin or even
ferromagnetic polymers or solids. However, it should be stressed that such
applications remain a distant prospect at best. There are very few nitrenium
ions that are known to have triplet ground states, and those that do are unstable.
The more immediate challenge is to identify the types of nitrenium ions likely to
possess triplet ground states, and then to identify the means of stabilizing them.
s
