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adduct of nitric oxide and bisulfite, sulfur-based diazeniumdiolates are not isolated from the reactions of
mercaptans and nitric oxide.
4.10 The colored species problem in nitric oxide chemistry
Sporadic and diverse reports over the last century have described colored species forming during the
manipulation and/or reaction of nitric oxide with various substrates. It is clear that the deep blue solutions
and solids often condensed at low temperature are due to dinitrogen trioxide and result from the reaction of
nitric oxide and nitrogen dioxide. The later is often a contaminant of commercial samples of nitric oxide,
but of course it can also arise from the adventitious reaction of dioxygen with nitric oxide. However,
there are related, but harder to rationalize, reports of red and red-blue adducts at low temperature. These
often appear in the presence of Lewis acids. Characteristic features of these adducts are their ease of
dissociation and reactivity. There is thus only limited characteristic data relating to these adducts with
Seel's determination of a
40 M
−
1
cm
−
1
,
186
and Laane's vibrational spectroscopy
characterization being particularly good examples.
187
In the later case, the vibrational data from the four
nitric oxide isotopomers was interpreted in terms of the formation of an asymmetric nitric oxide dimer
(ONON). While it is not clear how the Lewis acid would promote the formation of this isomer over the
more frequently observed symmetric dimer, the Lewis acids may stabilize the asymmetric isomer and
allow for its observation.
λ
max
of 580 nm and an
ε
=
4.11 Conclusions
Although the spectroscopic, physical, and theoretical basis for nitric oxide dimerization is now well under-
stood and characterized, the practical consequences of this propensity in terms of nitric oxide's chemistry
and biology remain important problems. Ultimately, these are mechanistic problems, and it should be no
surprise that this chemistry remains so poised. The inorganic chemistry of nitrogen is filled with spec-
tacular examples of kinetic control and it is in its oxides that mechanistic issues are critical. In the late
1880s the mechanism of nitric oxide's oxidation was a source of considerable controversy between two
rival camps
188,189
using classical product analysis to advocate different mechanisms. A century later, in the
1990s, similar difficulties were encountered in attempts to understand the chemistry of peroxynitrite.
190 - 192
While the community waits for a resolution to this vexing discussion, which may very well require new and
orthogonal methods to untangle, large areas of this ostensibly simple chemistry remain to be explored.
And there are many lessons in this chemistry for all chemists interested in transient and persistent radicals.
Given these difficulties in understanding simple diatomic and triatomic radicals, the chemistry of larger
homologues may well pose even more nuance and problems.
References
1.
N. V. Sidgwick,
The Organic Chemistry of Nitrogen
, Clarendon Press, Oxford, 1966.
2.
M. Zhou, L. Zhang and Q. Qin,
J. Am. Chem. Soc.
,
122
, 4483 - 4488 (2000).
3.
N. Shafizadeh, P. Brechignac, M. Dyndgaard,
et al
.,
J. Chem. Phys.
,
108
, 9313 - 9326 (1998).
4.
R. E. Miller,
Science
,
240
, 447 - 53 (1988).
5.
C. R. Dennis, C. J. Whitham, R. J. Low and B. J. Howard,
Chem. Phys. Lett.
,
282
, 421 -428 (1998).
6.
Y. Kim and H. Meyer,
Int. Rev. Phys. Chem.
,
20
, 219 -282 (2001).
7.
D. S. King and J. C. Stephenson,
J. Chem. Phys.
,
82
, 5286 -8 (1985).
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