Chemistry Reference
In-Depth Information
1
2-NpN, unlike the case with singlet 1-naphthyl nitrene,
26
suggesting a picoseconds
lifetime or shorter for
1
2-NpN, even at 77 K.
53 1
2-NpN must cyclize with a rate
10
7
s
1
at 77 K. Assuming a normal Arrhenius pre-exponential A
factor of 10
13
s
1
for cyclization, one concludes that the barrier to isomerization of
1
2-NpN must be less than 2.1 kcal/mol.
53
constant
1.1
1.2.1.3 Phenyl Azide. The chemistry, kinetics, and spectroscopy of phenyl nitrene
have been extensively reviewed previously
9
and in this section, we will focus mainly
on the excited state of phenyl azide. Phenyl azide (PhN
3
) is a convenient light-
activated precursor of singlet phenyl nitrene (
1
PhN).
9
Unfortunately, it is well-
documented that on photolysis of PhN
3
in acetonitrile or cyclohexane, a polymeric
tar is formed. In an ultrafast time-resolved experiments, tar can form on the surface of
the flow cell and prevent spectroscopic analysis under certain conditions.
56
The tar is
mainly formed by polymerization of the cyclic ketenimine (Scheme 1.1, K), which is
produced by cyclization of
1
PhN and subsequent rearrangement of benzazirine (BA).
Wirz's group managed to perform ultrafast studies on phenyl azide in dichloro-
methane (DCM), a solvent which may help to dissolve the polymeric tar to an extent,
and reported that the excited state lifetime of phenyl azide is 100 ps.
56
Due to solvent
absorption, we could not use DCM as solvent and could not attempt to repeat Wirz's
experiment with an excitation wavelength of 270 nm. However, we were able to
repeat this experiment in acetonitrile (ACN) containing 1M diethylamine to
scavenge ketenimine K and prevent tar formation.
57
Although this experiment
suffers from two-photon absorption by the solvent mixture (with related solvent
artifacts for the first few picoseconds after the laser pulse), it is clear that no transient
absorption forms several picoseconds after electronic excitation of phenyl azide in
ACN containing diethylamine. We also find that the lifetime of the excited state of
phenyl azide is
1 ps, which is significantly shorter than that reported by Wirz et al.
In Section 1.3, we will note that aryl nitrenes protonate efficiently in 88% formic
acid and that two very short-lived nitrenes,
o
-biphenylyl nitrene and 1-naphthyl
nitrene, can be protonated in this solvent as well to form their corresponding
nitrenium ions (see Section 1.2). Taking advantage of formic acid, an excellent
nitrene trap, we avoided tar formation and were able to study the photochemistry
of phenyl azide using ultrafast spectroscopic methods.
57
In 100% formic acid, the
excited state of phenyl azide decays within our instrument response function
(300 fs) and a growth below 400 nm is observed, which is assigned to singlet
phenyl nitrene (
1
PhN). This assignment is consistent with the singlet nitrene
spectrumreportedpreviously.Basedonour ultrafast time-resolved studies of
phenyl azide in acetonitrile with diethylamine and in formic acid, we conclude the
lifetime of phenyl azide singlet excited state is no longer than 1 ps, similar to its
biphenylyl and naphthyl counterparts (Table 1.1).
1.2.2 Ultrafast IR Studies
Ultrafast time-resolved vibrational spectroscopy has the potential for monitoring the
dynamics of intermediates involved in photochemistry and providing structural
