Chemistry Reference
In-Depth Information
nitrenium kind of ions (such as the 4-biphenylyl nitrenium and
N
-(4-biphenylyl)-
N
-
methyl nitrenium ions discussed here) may also induce a lower amount of the
iminocyclohexadienyl character of the aryl nitrenium ion.
Ortho
substitution relative
to the nitrenium ion attached to the phenyl ring may induce an appreciable effect on
the amount of charge delocalization and iminocyclohexadienyl character of the aryl
nitrenium ion and it is interesting to note that recent work for
ortho
-fluorine
substituents on singlet phenyl nitrenes demonstrated that a single fluorine substituent
has a moderate bystander effect on remote cyclization that is apparently not steric in
origin and two
ortho
fluorine substituent atoms have a greater effect on the
cyclization reaction.
62
Presently, it is not clear to what extent the
ortho
-substitution
relative to the nitrenium moiety attached to a
para
-substituted phenyl ring will affect
its iminocyclohexadienyl character, and this may be an interesting question to study
in the future.
The diphenyl nitrenium ion has the longest C
N bond length and smallest
amount of iminocyclohexadienyl character of the
para
-substituted aryl nitrenium
ions studied so far by transient vibrational spectroscopy in solution at room
temperature and the charge delocalization is approximately the same in both phenyl
rings since the structure has a greater amount of symmetry than the 2-fluorenyl
nitrenium and 4-biphenylyl nitrenium ions. This behavior also seems to correlate
with the observation of a higher intensity in the vibrational modes associated with the
C
Cbonds of the phenyl ring for the diphenyl nitrenium
ion and a greater amount of mutual exclusive character between the infrared and
Raman bands seen experimentally in the 1300-1700 cm
1
part of the spectra. This
demonstrates the usefulness of acquiring both infrared and Raman spectra (if possible)
for species with greater symmetry so that more vibrational bands can be seen to
improve the characterization of the structure and bonding of the species of interest.
Nbonds compared to theC
7.3.3 TR
3
Spectroscopy of Alkoxy-Substituted Phenyl Nitrenium Ions
Ultraviolet excitation of alkoxyphenyl azides in aqueous solutions results in intense
transient absorption bands with maxima around 290-305 nm on the nanoseconds to
milliseconds timescale
63,64
that have been assigned to alkoxyphenyl nitrenium ions
although these transient absorption bands are appreciably different from those of other
aryl nitrenium ions such as the 4-biphenylyl nitrenium and 2-fluorenyl nitrenium ions
that have transient absorption bands with maxima in the 430-460 nm region.
14,23-26,31
The 320 nm probe wavelength used in the transient resonance Raman experiments
waspickedtoberesonantwiththetransient absorption bands assigned to the
4-methoxyphenyl nitrenium and 4-ethoxyphenyl nitrenium ions. Figure 7.12
shows expanded views of the transient resonance Raman spectrum due to the
4-methoxyphenyl nitrenium ion after 266 nm photolysis of 4-methoxyphenyl azide
and 4-ethoxyphenyl azide in a water-acetonitrile (60:40) solvent. The lifetimes of
aryl nitrenium ions observed previously by transient absorption or TRIR experi-
ments displayed no appreciable difference when done in O
2
-andN
2
-saturated
solutions
36-40,51
, and it is not expected that removal of O
2
will make any noticeable
difference in the transient Raman spectra displayed in Figure 7.12.
