Kinetic studies on the thermal cis-trans isomerization of 1,3-diphenyltriazene in aqueous solution. Effects of acids and bases

Article abstract of DOI:10.1021/jo000599l

The thermal cis-to-trans isomerization of 1,3-diphenyltriazene (DPT) has been investigated in buffered aqueous solutions by means of laser-flash photolysis techniques. The cis-to-trans isomerization process is found to be catalyzed by general acids and general bases as a result of acid/base-promoted 1,3-prototropic rearrangements. Acid catalysis is attributed to rate-limiting proton transfer to the nitrogen-nitrogen double bond of cis-DPT, whereas base catalysis is attributed to rate-limiting base-promoted ionization of the amino nitrogen of cis-DPT leading to the isomerization. In addition, a process ascribed to the interconversion of cis rotamers through hindered rotation around the nitrogen-nitrogen single bond is also observed; at high pH this process becomes rate-limiting.

Full text of DOI:10.1021/jo000599l

J . Org. Chem. 2000, 65, 5739-5744
5739
Kin etic Stu d ies on th e Th er m a l Cis-Tr a n s Isom er iza tion of
1,3-Dip h en yltr ia zen e in Aqu eou s Solu tion . Effects of Acid s a n d
Ba ses
Mo´nica Barra* and Nan Chen
Guelph-Waterloo Centre for Graduate Work in Chemistry and Biochemistry, Waterloo Campus,
University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
mbarra@sciborg.uwaterloo.ca
Received April 19, 2000 (Revised Manuscript Received J une 27, 2000)
The thermal cis-to-trans isomerization of 1,3-diphenyltriazene (DP T) has been investigated in
buffered aqueous solutions by means of laser-flash photolysis techniques. The cis-to-trans
isomerization process is found to be catalyzed by general acids and general bases as a result of
acid/base-promoted 1,3-prototropic rearrangements. Acid catalysis is attributed to rate-limiting
proton transfer to the nitrogen-nitrogen double bond of cis-DP T, whereas base catalysis is
attributed to rate-limiting base-promoted ionization of the amino nitrogen of cis-DP T leading to
the isomerization. In addition, a process ascribed to the interconversion of cis rotamers through
hindered rotation around the nitrogen-nitrogen single bond is also observed; at high pH this process
becomes rate-limiting.
Sch em e 1
In tr od u ction
The study of compounds able to undergo reversible
isomerization around double bonds has been actively
pursued in recent years, as a result of the potential
application of these materials in information storage
systems and (photochemical) switching devices.1
Triazenes, compounds characterized by having a dia-
zoamino group, appear to always adopt the trans config-
uration in the ground state (as shown by X-ray diffraction
and dipole moment measurements);2 nonetheless, tri-
azenes are also known to exhibit (photoinduced) revers-
ible cis-trans isomerism. In contrast to the case of azo
compounds, mechanistic investigations on the cis-trans
isomerization of triazenes are very limited.3,4 In this
regard, it has been shown that the rate of the thermal
cis-to-trans isomerization of 1,3-diphenyltriazene (DP T)
depends strongly on the polarity of the solvent, the rate
increasing as the polarity of the medium increases.3a,4
Two mechanisms (Scheme 1) have been proposed for the
cis-to-trans isomerization of DP T in organic solvents:3a,4
(a) an intramolecular 1,3-hydrogen shift (nonpolar sol-
vents) and (b) a solvent-assisted 1,3-hydrogen shift in
which protic solvent molecules act as proton donors and
proton acceptors leading to the isomerization.
niques have been applied to the analysis of the isomer-
ization mechanism of DP T in aqueous solutions at 6 <
pH < 14.
Resu lts
Laser excitation at 355 nm of DP T in buffered aqueous
solutions leads to instantaneous bleaching followed by
complete recovery of the initial absorbance of the solu-
tions (Figure 1 is representative). The instantaneous
bleaching, ascribed to photoinduced trans-to-cis isomer-
ization, would indicate that the extinction coefficients of
the cis isomer in the visible region are lower than those
corresponding to the trans form. This observation is in
agreement with the UV-vis absorption spectrum calcu-
lated for cis-DP T in poly(methyl methacrylate) film,
which shows that at λ > 350 nm cis-DP T has lower
absorptivity than trans-DP T.3b On the other hand, the
complete recovery would indicate that the entire photo-
induced process is reversible, and it is ascribed to the
thermal cis-to-trans isomerization, in agreement with
data in the literature.3a,4
The present study was undertaken in an attempt to
improve our understanding of the thermal cis-to-trans
isomerization mechanism of triazenes, particularly with
regard to the role of acids and bases as potential
catalysts. Thus, time-resolved laser-flash photolysis tech-
(1) Martin, P. J . in An Introduction to Molecular Electronics; Petty,
M. C., Bryce, M. R., Bloor, D., Eds.; Oxford University Press: New
York, 1995; Chapter 6.
(2) Benson, F. R. The High Nitrogen Compounds; J ohn Wiley &
Sons: New York, 1984; Chapter 3.
(3) Baro, J .; Dudek, D.; Luther, K.; Troe, J . Ber. Bunsen-Ges. Phys.
Chem. 1983, 87, (a) 1155; (b) 1161.
(4) Scaiano, J . C.; Chen, C.; McGarry, P. F. J . Photochem. Photobiol.
A: Chem. 1991, 62, 75.
Recovery traces were collected at different pHs using
series of solutions at constant buffer ratio (and, hence,
constant pH) but varying total buffer concentration.
10.1021/jo000599l CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/01/2000

5740 J . Org. Chem., Vol. 65, No. 18, 2000
Barra and Chen
F igu r e 1. Transient absorption spectra for DP T (pH ) 13,
NaOH buffer) obtained 0.72 µs (O), 2.72 µs (b), 5.92 µs (2),
and 13.5 µs (4) after laser pulse. Inset: kinetic traces recorded
at 390 nm at (a) pH ) 6.85 (0.01 M phosphate buffer) and (b)
0.0983 M NaOH buffer.
F igu r e 2. Buffer concentration dependence of the observed
rate constant for the slower process in the cis-to-trans isomer-
ization of DP T in phosphate buffer.
Recovery traces (e.g., Figure 1 inset) were very well
reproduced by one or two exponential terms, depending
on pH (see below). Deaerating the solutions showed no
measurable effect in any case.
P h osp h a te Bu ffer (6.0 < p H < 7.6). Recovery traces
are adequately described by two exponential terms with
preexponential factors independent of pH and buffer
concentration. The observed rate constant corresponding
to the faster process (kfobs) is also found to be independent
of pH and buffer concentration (Table S1)5 and has an
average value of (3.1 ( 0.1) × 105 s-1. The observed rate
constant corresponding to the slower process (ksobs), on
the other hand, increases as proton concentration and
buffer concentration increase as well (Table S2),5 which
is indicative of acid catalysis. Plots of ks
vs buffer
obs
concentration are fairly linear (Figure 2 is representa-
tive); hence, the data obey the linear rate law of eq 1, in
which kC is the catalytic rate coefficient for the buffer
and kO refers to the reaction through solvent-related
species.
F igu r e 3. Plot of catalytic rate constant vs molar fraction of
acid for cis-to-trans isomerization of DP T in phosphate buffer.
Inset: plot of left-hand side of eq 3 (see text) vs [H+].
ks ) ko + kc[buffer]
(1)
obs
Under the experimental conditions of this work, the
term [H+]2 is negligible in the denominator of eq 2; thus,
the latter can be rearranged to eq 3.
When the corresponding kC values are plotted against
the molar fraction of free acid xHA a nonlinear plot results
(Figure 3), which might indicate that even at these (high)
pHs the catalysis by H3PO4 contributes significantly. In
this case, the observed second-order catalytic rate coef-
ficient for the buffer kC is given by eq 2, where K1 and K2
are the first and second ionization constants for H3PO4.
K1[H+] + K1K2
kc ×
) kH PO [H+] + K1kH PO
-
4
(3)
[H+]
3
4
2
A plot of the left-hand side of eq 3 vs [H+] gives a
satisfactory straight line (Figure 3 inset). From the
-
intercept and slope values for this plot kH
and kH
3PO4
2PO4
[H+]2
H3PO4
are obtained (Table 1). We are fully aware that this k
kc ) kH PO
+
value arises from conditions where H3PO4 is indeed a
minor species, but unfortunately, acid-catalyzed hydro-
lytic decomposition of DP T precluded accurate measure-
ments at pH < 6.6
Bor a te Bu ffer (7.9 < p H < 9.9). As in the case of
phosphate buffer, recovery traces are adequately de-
scribed by two exponential terms with preexponential
factors independent of pH and buffer concentration. The
[H+]2 + K1[H+] + K1K2
3
4
K1[H+]
- [H+]2 + K1[H+] + K1K2
kH PO
(2)
2
4
(5) Supporting Information: see paragraph at the end of this paper
regarding availability.

Thermal Cis-Trans Isomerization of 1,3-Diphenyltriazene
J . Org. Chem., Vol. 65, No. 18, 2000 5741
Ta ble 1. Ca ta lytic Ra te Con sta n ts for Cis-to-Tr a n s
Isom er iza tion of DP T in Aqu eou s Solu tion a
acidb
kacid, M-1 s-1
basec
kbase, M-1 s-1
H3PO4 (1.9d)
(6.4 ( 0.7) × 109 H2BO3- (8.94e) (1.1 ( 0.1) × 105
H2PO4- (6.46e) (7.8 ( 0.3) × 105 CO32- (9.76e)
(5.9 ( 0.4) × 105
HBO32- (11.6e) (2.8 ( 0.3) × 107
a
b
At 21 °C, µ ) 0.5 M (NaCl). pKa given in parentheses. c pKa
d
of conjugate acid given in parentheses. Taken from ref 28.
e Determined potentiometrically.
F igu r e 5. Plot of catalytic rate constant vs molar fraction of
base for cis-to-trans isomerization of DP T in borate buffer.
Inset: plot of left-hand side of eq 5 (see text) vs [H+].
A plot of the left-hand side of eq 5 vs [H+] gives a
satisfactory straight line (Figure 5 inset). From the
-
2-
intercept and slope values for this plot kH BO and kHBO
2
3
3
are obtained (Table 1).
Ca r bon a te Bu ffer (8.8 < p H < 10.9). At pH < 10.0
recovery traces are adequately described by two expo-
nential terms with preexponential factors independent
of pH and buffer concentration. The observed rate
constant corresponding to the faster process (kfobs), as in
the case of the other two buffer systems described above,
is found to be independent of pH and buffer concentration
(Table S5)5 and has an average value of (3.28 ( 0.04) ×
105 s-1. The observed rate constant corresponding to the
slower process (ksobs), on the other hand, increases as pH
and buffer concentration increase as well (Table S6),5
indicative of base catalysis as observed with borate
F igu r e 4. Buffer concentration dependence of the observed
rate constant for the slower process in the cis-to-trans isomer-
ization of DP T in borate buffer.
observed rate constant corresponding to the faster pro-
cess (kfobs) is also found to be independent of pH and
buffer concentration (Table S3)5 and has an average value
of (3.32 ( 0.04) × 105 s-1. The observed rate constant
corresponding to the slower process (ksobs), on the other
hand, increases as pH and buffer concentration increase
as well (Table S4),5 which is indicative of base catalysis,
with the exception of data at pH ) 7.93, which are
buffer. Plots of ks vs buffer concentration are fairly
obs
linear (Figure 6 is representative), once again, in agree-
ment with eq 1. When the corresponding kC values are
plotted against the molar fraction of free base xA- a linear
plot (with negligible intercept) results (Figure 6 inset);
independent of buffer concentration. Plots of ks
vs
obs
buffer concentration are fairly linear (Figure 4 is repre-
sentative), in agreement with eq 1. When the correspond-
ing kC values are plotted against the molar fraction of
free base xA- a nonlinear plot results (Figure 5), which
might indicate that even at these (low) pHs the catalysis
2-
from the slope of this plot kCO is obtained (Table 1).
3
At pH > 10.0 recovery traces are adequately described
by a single-exponential function. The corresponding
observed rate constants (Table S7)5 are essentially inde-
pendent of buffer concentration, and despite a slight
increase with pH, they are in very good agreement with
the kfobs values corresponding to the buffer systems
described above.
2-
by HBO3 contributes significantly. In this case, the
observed second-order catalytic rate coefficient for the
buffer kC is given by eq 4, where K1 and K2 are the first
and second ionization constants for H3BO3.
K1[H+]
- [H+]2 + K1[H+] + K1K2
Na OH Bu ffer (0.005 M e [HO-] e 0.20 M). Recovery
traces are very well reproduced by a single-exponential
function. Observed rate constants are independent of
hydroxide ion concentration (Table S8),5 and the average
value, namely (3.39 ( 0.05) × 105 s-1, is in excellent
agreement with the values corresponding to kfobs in the
buffer systems described above.
kc ) kH BO
+
2
3
K1K2
k
(4)
2- [H+]2 + K1[H+] + K1K2
HBO3
Under the experimental conditions of this work, the
term K1K2 is negligible in the denominator of eq 4; thus,
the latter can be rearranged to eq 5.
p H P r ofile. The intercepts of all buffer dependence
plots (i.e., kO in eq 1) are well defined, and they provide
(6) Triazenes are very sensitive to acids, decomposing in aqueous
media to nitrogen, amines and alcohols/phenols. The rate constant
determined for proton-catalyzed decomposition of DP T under the
experimental conditions of this work is (1.37 ( 0.03) × 103 M-1 s-1
(ref 8).
kc([H+]2 + K1[H+]) )
kH BO -K [H+] + K K k
2-
2 HBO3
(5)
1
1
2
3

5742 J . Org. Chem., Vol. 65, No. 18, 2000
Barra and Chen
(i) the rate constant for the horizontal segment at pH >
10 (dashed line in Figure 7) is, within experimental
errors, identical to the values determined for kfobs at pH
< 10, (ii) the preexponential factors for the two first-order
processes observed at pH < 10 are independent of pH,
(iii) a change in the ionization state of the substrate
requires the point of intersection of the two arms of the
profile to occur 0.301 units above the pH-rate curve (not
the case of Figure 7), and (iv) cis-DP T is indeed expected
to be less acidic than trans-DP T (see below), the down-
ward bend shown in Figure 7 is ascribed to a change in
rate-controlling step.
The empirical rate law for the rate profile correspond-
ing to kO at pH < 10 consists of three terms as given in
eq 6, each term representing a different mechanism.7
ko ) c1[H+] + c2 + c3[HO-]
(6)
The first term of eq 6 represents the left-hand side of
the V-shaped profile which gives way to a diagonal
portion of slope ) -1, signifying acid catalysis. The
second term is attributable to an “uncatalyzed” reaction.
Finally, the third term represents the right-hand side of
the V-shaped profile, which gives way to a diagonal
portion of slope ) 1, signifying base catalysis. Analysis
by least-squares fitting according to eq 6 (solid line in
F igu r e 6. Buffer concentration dependence of the observed
rate constant for the slower process in the cis-to-trans isomer-
ization of DP T in carbonate buffer. Inset: catalytic rate
constant vs molar fraction of base.
Figure 7) leads to values of (1.3 ( 0.1) × 1010 M-1 s-1
,
(2.2 ( 0.2) × 103 s-1, and (1.5 ( 0.1) × 109 M-1 s-1 for c1
to c3, respectively.
Discu ssion
The pKa for trans-DP T determined under the experi-
mental conditions of this work is (13.0 ( 0.1),8 in good
agreement with the reported value in 20% ethanol
aqueous solution, namely (13.26 ( 0.04).9 Not surpris-
ingly, DP T is a significantly stronger acid than aniline
(pKa ∼ 27 at 25 °C),10 owing to the electron-withdrawing
character (i.e. inductive and resonance effects) of the
PhNdN- group. In the case of cis-DP T, one would
anticipate resonance delocalization to be more restricted
as a result of steric hindrance, and consequently, the
-NH- group of cis-DP T is expected to be less acidic than
that in the trans isomer. For comparative purposes, it
should be pointed out here that the cis isomers of
2-hydroxy-5-methylazobenzene (pKa ) 10.7)11 and 4-[4′-
(dimethylamino)phenylazo]benzenesulfonate (pKa ) 5.0)12
have indeed been shown to be significantly weaker acids
than the corresponding trans forms (pKa ) 9.411 and
2.7013-2.87,14 respectively). Thus, laser excitation of
trans-DP T under the experimental conditions of this
work would result in instantaneous formation of neutral
cis-DP T.
F igu r e 7. Rate profile for cis-to-trans isomerization of DP T
in aqueous solution: O, rate constants for reaction through
solvent-related species (obtained from the intercept of buffer
dependence plots for phosphate, borate and carbonate buffers);
3, observed rate constants for faster process detected at pH <
10 in phosphate, borate and carbonate buffers; 4, observed
rate constants for single process detected in NaOH buffer.
rate coefficients for cis-to-trans isomerization of DP T
through solvent-related species. The resulting kO values
and the observed rate constants measured in NaOH
buffer are displayed in the rate profile shown in Figure
7; the latter also displays the observed rate constants for
the faster process observed at pH < 10. The pH-rate
curve corresponding to isomerization through solvent-
related species is characterized by a V-shaped profile
(with a hardly discernible horizontal section) and a
downward bend at pH ca. 10.5. Downward bends of this
type are generally interpreted as a result of either a
change in the state of ionization of the substrate or a
change in rate-controlling step.7 Taking into account that
The kinetic data presented in the previous section can
be interpreted in terms of the general mechanism shown
in eq 7, where A and B represent neutral cis-DP T forms,
(8) Chen, N. M. Sc., University of Waterloo, 2000.
(9) Benes, J .; Bera´nek, V.; Zimprich, J .; Vetesnik, P. Collect. Czech.
Chem. Commun. 1977, 42, 702.
(10) Albert, A.; Serjeant, E. P. The Determination of Ionization
Constants, 3rd ed.; Chapman & Hall: New York, 1984; p 149.
(11) Wettermark, G.; Langmuir, M. E.; Anderson, D. G. J . Am.
Chem. Soc. 1965, 87, 476.
(12) Sanchez, A. M.; Barra, M.; de Rossi, R. H. J . Org. Chem. 1999,
64, 1604.
(13) Tawarah, K. M.; Abu-Shamleh, H. M. Dyes Pigments 1991, 16,
241.
(7) Loudon, G. M. J . Chem. Educ. 1991, 68, 973.
(14) Reeves, R. L. J . Am. Chem. Soc. 1966, 88, 2240.

Thermal Cis-Trans Isomerization of 1,3-Diphenyltriazene
J . Org. Chem., Vol. 65, No. 18, 2000 5743
and the second step corresponds to the thermal cis-to-
trans isomerization catalyzed by general acids or general
bases.
analogous to the acid-catalyzed enol-ketonization mech-
anism22 and to the acid-catalyzed double-bond rearrange-
ment of alkenes.23
A yk1z B acid or8 trans-DP T
(7)
(12)
k-1
base
It is well-known that rotation around the nitrogen-
nitrogen single bond of triazenes has a relatively high
free energy barrier, which is ascribed to the partial
double bond character that results from 1,3-dipolar
contributions (eq 8).15-21 Hence, cis-DP T is proposed to
exist as a pair of rotamers (eq 9) which are expected to
undergo cis-to-trans isomerization at different rates as
a result of stereoelectronic factors. In fact, based on the
1,3-hydrogen shift mechanisms shown in Scheme 1
rotamer B is predicted to isomerize more rapidly than
rotamer A.
The general base-catalyzed mechanism, on the other
hand, is interpreted as represented by eq 13 and involves
rate-determining base-promoted ionization of the amino
nitrogen of rotamer B to give a resonance-stabilized
anion, which then combines with a proton at the position
that gives the thermodynamically more stable trans
triazene. This mechanism resembles the base-catalyzed
enolization process22 as well as the base-catalyzed double-
bond rearrangement of alkenes.23
(8)
(9)
(13)
As already mentioned, two well-distinguishable pro-
cesses are observed at pH < 10. The faster process,
independent of pH and buffer concentration, is ascribed
to the interconversion of cis rotamers (i.e., eq 9), and the
corresponding observed rate constant kfobs is given by eq
10. The second process detected, catalyzed by general
acids or general bases, is ascribed to the thermal cis-to-
trans isomerization and the corresponding observed rate
The “uncatalyzed” process (i.e., c2 in eq 6) would
correspond to solvent water molecules acting as proton
donors (eq 12) or as basic proton acceptors (eq 13).
Evidently, prototropic rearrangements of the type shown
in eqs 12 and 13 do not lead to a change in the nitrogen-
nitrogen double bond configuration when applied to
rotamer A.
constant ks is given by eq 11, where K1 ) k1/k-1, and
obs
kH O, kacid, and kbase represent the uncatalyzed and acid/
2
base catalytic rate coefficients, respectively.
At high pH, the interconversion of rotamers becomes
rate-controlling, and only one process is detected. The fact
that the rate constants observed with NaOH buffer are
identical to the kfobs values determined at pH < 10 would
indicate that k-1 is indeed significantly smaller than k1
(i.e., eq 10 simplifies to kfobs ) k1) and K1 ) k1/k-1 . 1.
This observation is consistent with the relative stability
predicted for this pair of rotamers (i.e., A less stable than
B). Thus, the factor K1/(1 + K1) in eq 11 is for practical
purposes unity, and the coefficients c1 to c3 in eq 6 would
kfobs ) k1 + k-1
(10)
K1
ks
)
(kH O [acid] +
+
k
kbase[base])
∑
∑
obs
acid
2
1 + K1
(11)
No evidence of concerted acid/base-catalysis, as might be
expected from mechanism (b) in Scheme 1, was obtained
under the experimental conditions of this work. The
general acid-catalyzed mechanism is interpreted as
shown in eq 12, and involves rate-determining proton
transfer to the nitrogen-nitrogen double bond of rotamer
B to give a resonance-stabilized cation, which then loses
a proton from the position that gives the thermodynami-
cally more stable trans-triazene. This mechanism is
correspond, respectively, to kH , kH O, and kHO- in eq 11.
+
2
In fact, the catalytic rate coefficients thus ascribed to H+
and HO- are consistent with diffusion-controlled proto-
nation rate constants.24
To the best of our knowledge, no energy barrier for
hindered rotation around the nitrogen-nitrogen single
bond in disubstituted triazenes has been determined so
far. Nonetheless, one would anticipate a higher free
energy of activation for the cis isomer relative to the trans
form due to steric interactions. Based on the application
of the Eyring equation25 to k1, a value of 3.3 × 105 s-1 (at
21 °C) for this rate constant would correspond to a free
(15) Marullo, N. P.; Mayfield, C. B.; Wagener, E. H. J . Am. Chem.
Soc. 1968, 90, 510.
(16) Akhtar, M. H.; McDaniel, R. S.; Feser, M.; Oehlschlager, A. C.
Tetrahedron 1968, 24, 3899.
(17) Lunazzi, L.; Cerioni, G.; Foresti, E.; Macciantelli, D. J . Chem.
Soc., Perkin Trans. 2 1978, 686.
(18) Sieh, D.; Wilbur, D. J .; Michejda, C. J . J . Am. Chem. Soc. 1980,
102, 3883.
(19) Golding, B. T.; Kemp, T. J .; Narayanaswamy, R.; Waters, B.
W. J . Chem. Res., Synop. 1984, 130.
(20) Lippert, Th.; Wokaun, A.; Dauth, J .; Nuyken, O. Magn. Reson.
Chem. 1992, 30, 1178.
(21) Panitz, J .-C.; Lippert, Th.; Stebani, J .; Nuyken, O.; Wokaun,
A. J . Phys. Chem. 1993, 97, 5246.
(22) Toullec, J . Adv. Phys. Org. Chem. 1982, 18, 1.
(23) Mackenzie, K. in The Chemistry of Alkenes; Patai, S., Ed.; J ohn
Wiley & Sons: London, 1964; Vol. 1, Chapter 7.
(24) Eigen, M. Angew. Chem., Int. Ed. Engl. 1964, 3, 1.
(25) Lowry, T. H.; Richardson, K. S. Mechanisms and Theory in
Organic Chemistry; Harper & Row: New York, 1987; p 209.

5744 J . Org. Chem., Vol. 65, No. 18, 2000
Barra and Chen
Laser experiments were carried out using a Q-switched Nd:
YAG laser (Continuum, Surelite I) operated at 355 nm (4-6
ns pulses, < 30 mJ /pulse) for excitation. Further details on
this laser system have already been reported.12,27 Solutions
were contained in quartz cells constructed of 7 × 7 mm2
Suprasil tubing. Transient absorption spectra were collected
under flow conditions in order to ensure the irradiation of fresh
portions of sample by each laser pulse. All measurements were
carried out at 21 °C.
For each solution, kinetic traces were recorded at 390 nm
in different time domains (0.08-8 µs per point) and then
combined for analysis; if at all necessary, traces were previ-
ously normalized to account for slight differences in signal
intensity due to laser power fluctuations. Observed rate
constants were determined by using the general curve fitting
procedure of Kaleidagraph 3.0.5 software from Abelbeck
Software.
Molar fractions were calculated from the observed pH by
using the ionization equilibrium constants determined poten-
tiometrically under our experimental conditions, unless stated
otherwise. Thus, ionization constants were obtained by ex-
trapolating the pH values of solutions containing the corre-
sponding conjugate acid and base in 1:1 ratio to zero buffer
concentration; resulting values are summarized in Table 1 and
are in excellent agreement with reported values under similar
conditions.28
energy of activation of ca. 9.8 kcal/mol. This value is
comparable to the free energy barriers typically deter-
mined for hindered rotation in trisubsituted triazenes in
organic solvents, namely 10-20 kcal/mol.15-20 On the
other hand, an appreciable higher free energy barrier for
hindered rotation is expected upon protonation or depro-
tonation of the triazene moiety, owing to the increase of
the partial double bond character as a result of charge
delocalization. Thus, interconversion between the corre-
sponding conjugate acids and bases of rotamers A/B can
be ignored.
In summary, the thermal cis-to-trans isomerization of
DP T in aqueous solution is found to be catalyzed by
general acids and general bases. Catalysis is attributed
to acid/base-promoted 1,3-prototropic rearrangements
leading to isomerization, consistent with previous studies
in protic organic solvents.3a,4 In addition, a process
characterized by an estimated free energy barrier of 9.8
kcal/mol is also observed. This process is ascribed to the
interconversion of neutral cis rotamers through hindered
rotation around the nitrogen-nitrogen single bond.
Exp er im en ta l Section
Proton concentrations for the rate profile (Figure 7) were
calculated from the observed pH by using a value of 0.732 for
the proton activity coefficient,29a except for data from NaOH
buffer. In this case proton concentrations were calculated
1,3-Diphenyltriazene (Aldrich) was purified by treatment
with Cd(OH)2 generated in situ from Cd(NO3)2 (Allied Chemi-
cal) in basic methanol (BDH, ACS grade) aqueous solution,
as described in the literature.26 Aqueous buffer solutions were
prepared using analytical grade salts and water purified in a
Millipore apparatus.
To minimize any acid-catalyzed substrate decomposition,
samples were analyzed immediately after adding the substrate
dissolved in methanol (BDH, Omnisolv grade) to solutions
containing all the other constituents. The total methanol
concentration was 2% (by volume), and the ionic strength was
kept constant at 0.5 M using NaCl as compensating electrolyte.
according to eq 14 using (γH γHO /aH O) ) 0.51629b and Kw
)
+
-
2
29c
0.6806 × 10-14
.
KW
1
[H+] )
(14)
-
γH+γOH-/a
H2O [OH ]
Ack n ow led gm en t. This work was supported by
research and equipment grants from the Natural Sci-
ences and Engineering Research Council (NSERC) of
Canada.
(26) Dwyer, F. P. J . Soc. Chem. Ind. London 1937, 56, 70.
(27) Barra, M.; Agha, K. A. J . Photochem. Photobiol., A: Chem. 1997,
109, 293.
(28) Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum
Press: New York, 1974; Vol. 6.
(29) Harned, H. S.; Owen, B. B. The Physical Chemistry of Electro-
lytic Solutions; Reinhold Publishing Corp.: New York, 1958; (a) p 748;
(b) p 752; (c) p 638.
Su p p or tin g In for m a tion Ava ila ble: Observed rate con-
stants for thermal cis-to-trans isomerization of 1,3-diphenyl-
triazene in buffered aqueous solutions as a function of pH and
buffer concentration. This material is free of charge via the
Internet at http://pubs.acs.org.
J O000599L