Chemistry Reference
In-Depth Information
3
CH
3
CH
3
CH
3
+
N
N
N
Triplet sensitizer
+
N
N
+
N
N
N
N
Triplet quencher
CH
3
CH
3
CH
3
81
FIGURE 6.17.
The
D
E
st
value for the stable nitrenium ion was determined by measuring rates
of triplet energy transfer using laser flash photolysis.
diaryl nitrenium ions show reasonable (
20 nm) agreement between observed and
calculated spectra. Second, the observed species decays rapidly (300 ns) in manner
that both the first-order and the unaffected reactions by changes in the solvent. The
stable products from this species all result from what would be second-order
reactions.
On this basis, it was suggested that the transient detected in the LFP experiments
was the excited state, open-shell (
n
,
. The 300 ns decay seen in the
absence of nucleophiles is attributed to relaxation to the (undetected) lower energy
n
2
singlet state.
Boche et al. characterized several 1,2,3-triazolium salts (e.g.,
p
) singlet of
80
, Fig. 6.17) and
argued that these species could be described as nitrenium ions on the basis of
resonance structure that assigns a positive charge to the central nitrogen atom.
49
As
might be expected, the two flanking nitrogen atoms impart significant stability to this
ion through
81
electron donation. In fact, triazolium ions are indefinitely stable at
room temperature. However, this stability also makes it possible to examine their
singlet triplet energy gaps using standard photophysical methods.
Thus, 1,3-dimethylbenzotriazolium
p
perchlorate was prepared and its triplet
state energy was determined using laser flash photolysis.
50
Specifically, rate
constants for energy transfer from a series of excited triplet state sensitizers
having different excited state energies to the triazolium ion combined with rate
constants for energy transfer from the excited triplet state of the triazolium ion to
triplet quenchers having different triplet energies were usedtoinferatripletenergy
of 66
81
64.1
kcal/mol shows excellent agreement with the experiment. This result demonstrates
that DFT calculations provide chemically accurate values of
3 kcal/mol for the triazolium ion. The DFT-derived value of
E
st
for nitrenium
D
ions.
Cramer and Sullivan
51
examined
E
st
for several
exo
-heteroaromatic nitrenium
D
ions (
82-88
, Table 6.10) based on pyridine (
82-84
), pyrimidine (
85-87
), and
triazene (
). The effect of adding nitrogen atoms to the aromatic rings is to lower
the energy of the filled
88
orbitals and thus cause less stabilization of the singlet
relative to the triplet. The three pyridine isomers were all ground state singlets,
with
p
E
st
shifting toward the triplet as the nitrogen is moved from the
ortho
to the
para
positions. The pyrimidines and the triazine
D
88
derivatives are predicted to be
ground state triplets.
