Photochemistry of 1H-Benzotriazole
J. Am. Chem. Soc., Vol. 122, No. 24, 2000 5853
spectra indicate that 1 also predominates in various solvents,26
but that the 2H-tautomer is the most stable form in the gas
Table 3. Results of DFT Calculations of 1 and 2 and of the Free
Energies of Reaction and Activation for the Cyclization Reaction
2
f 1
2
7,28
phase.
Evidence for a thermal equilibrium between 1H-
HF (6-21G*) B3LYP (6-31G*)
∆G° a
kcal mol
benzotriazole (1) and the quinonoid valence isomer 2 has not
been obtained. However, replacement of the NH group of 1 by
an O atom in benzo[1,2,3]oxadiazole shifts the equilibrium to
-
1
structure
E
h
E
h
2
1
-391.141101
-391.197773
-395.817656
-395.867179
-395.812361
0
-28.0
3.7
the o-quinonoid structure 6-diazo-2,4-cyclohexadien-1-one
transition state 2 f 1
29
(1,2-benzoquinone diazide).
a
Including zero-point energies and thermal contributions to the free
energies at 298 K.
Irradiation of 1 in solution generates the quinonoid imine 2
within a few picoseconds by N-N bond fission. The triplet state
of 1 has been generated and identified as a long-lived intermedi-
ate by sensitization with benzophenone. It is not formed by
intersystem crossing following direct excitation of 1, and it is
not involved in the formation of 2. The observation of saturable,
detectable transient absorption at 380 nm. This observation
shows that the quantum yield of intersystem crossing of 1 is
very small (<1%) at room temperature.
Quantum Yields of Photodecomposition. Material balance
1
8
1-4
specific base catalysis in the decay of 2 (Figure 2) provides
conclusive evidence for a rapid and reversible proton-transfer
reaction that is established prior to the rate-determining cy-
clization step (Scheme 2). The maximum rate of cyclization, k′
is notoriously poor in preparative photolyses of benzotriazole.
Irradiation of 1 in aqueous acid is reported to give a single
2a
photoproduct, 2-aminophenol, in 16% yield. The same product
is formed in neutral solution but is photooxidized upon
7
-1
+
)
3.2 × 10 s , is reached when pH exceeds pKa(2 ) ) 12.7,
continued irradiation. Aniline is the major product formed in
% yield by irradiation of 1 in acetonitrile.3 Accurate deter-
a
where the equilibrium shifts to the neutral diazo compound 2.
6
At the same time, the absorption maximum shifts from 405 nm
mination of quantum yields is difficult under these circum-
stances. Order-of-magnitude estimates were obtained by 254-
+
(
2 ) to 430 nm (2). The spectral shift is similar to that observed
-
4
upon deprotonation of phenol-2-diazonium ion to 1,2-benzo-
nm irradiation of 1 (∼2 × 10 M) in aqueous and aprotic
solutions and spectrophotometric monitoring of the photoreac-
tion. The quantum yields depended on the pH of the solution
3
0
quinone diazide (353 to 395 nm). In less basic solutions, pH
+
<
pKa(2 ), the observed rate constant decreases linearly with
increasing proton concentration, until a constant value of k ) 3
(Figure 4), but were less than 1% throughout. The quantum yield
-
1
s
is reached around pH ) 5. This reaction is 7 orders of
of photodecomposition was below 0.1% upon triplet sensitiza-
tion of 1 by irradiation of benzophenone in acetonitrile at 365
nm. The UV-spectral changes induced by irradiation of the
N-methyl derivative 5 in 0.1 N aqueous HClO4 were similar to
those of 1, and the quantum yield of disappearance was about
the same for both compounds, 0.8%.
magnitude slower than that in aqueous base and corresponds to
+
cyclization of the diazonium ion 2 . The barrier for cyclization
of 2 determined by the DFT calculations (Table 3) is only 3.7
-
1
kcal mol . We could not locate a minimum for the primary
+
product that would be formed by cyclization of 2 , namely a
protonated form of 1 carrying two protons at N1. This suggests
Calculations. Density functional theory (DFT) was used to
calculate the energies of the valence isomers 1 and 2 and of the
transition state for the cyclization reaction, 2 f 1. Geometries
determined at the Hartree-Fock level with the 6-21G(d) basis
set were used as input for B3LYP(6-31G(d)) full geometry
optimization. B3LYP is a combination of Becke’s three-
+
that the observed, slow cyclization of 2 occurs in concert with
+
+
proton transfer to water, 2 f 1 + H .
Imine 2 is quite sensitive to light when generated in rigid
6
-8
glasses.
Due to its short lifetime in solution, secondary
irradiation of 2 does not occur with continuous light sources.
+
20
Although the lifetime of diazonium ion 2 is much longer, we
parameter exchange functional with a slightly modified Lee-
+
2
1,22
could not detect any photosensitivity of 2 in solution. The
Yang-Parr (LYP)
correlation functional as implemented in
+
2
3
decay rate of 2 in aqueous acid (0.1 N HClO4) was insensitive
GAUSSIAN 98. The transition structure was located on the
2
4
to 100-fold changes in the intensity of the monitoring light, and
the quantum yield of nitrogen elimination determined for 254-
nm irradiation was independent of the light intensity (varied
up to 10-fold using neutral density filters).
potential energy surface using the STQN method. Wave
function stability was tested, and harmonic vibrational frequen-
cies were calculated by using analytical second derivatives. The
results are summarized in Table 3.
Benzotriazole (1) is an amphoteric molecule. It absorbs at
Discussion
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The relative stability of the prototropic tautomers of benzo-
triazole (1H vs 2H) has been studied extensively. The solid
contains exclusively the 1H-tautomer 1.25 Electronic absorption
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