4
212 Yuan et al.
Asian J. Chem.
photolysis of the sample by the analyzing light, suitable filters
were employed.
−
aq
k
2
e + BrB → Product
(2)
(3)
Gas chromatography (7890A,Agilent, USA) coupled with
a mass spectrometry (5975C, Agilent, USA) (GC-MS) was
performed to identify the degradation products. GC-MS was
equipped with a DB-5 MS column (30 m × 0.25 mm × 0.25
µm). Helium was used as the carrier gas at a constant flow
−
aq
k3
•-
e +PyTS → pyTS
In summary, the concentration change of hydrated electrons
in the solution was in agreement with eqn. (1):
−
aq
d[e ]
−
aq
2
−
−
aq
-1
−
= k [e ] + k [e ][BrB] + k [e ][PyTS]
(
1)
rate of 1 mL min . The GC temperature program was 60 °C
1
2
aq
3
dt
-1
(
hold for 5 min) to 280 °C (hold for 10 min) at 5 °C min .
−
Both injection and ion source temperature were 250 °C. Before
GC-MS analysis, 50 mL irradiated samples were extracted by
d[e ]
aq
−
2
−
−
aq
−
= k [e ] + k ′[e ]+ k ′[e ]
(2)
1
aq
2
aq
3
dt
5
mL dichloromethane.
After integrating eqn. (2), the concentration change of
hydrated electrons was obtained, as shown in eqn. (3):
The concentration of Br- generated from bromobenzene
degradation was determined by IC, which consisted of a hydro-
philic anion exchange column and an autosampler. A Dionex
IonPac AS11 column (2 × 250/4 × 250 mm) and the eluent of
′
′
′
2
′
3
−
aq
−(k2 + k3)
(
k + k )[e ] e
−
aq
0
(e ) =
′
′
(3)
−
aq
−(k +k )t
2
3
′
2
′
3
-1
k [e ] (1− e
) + (k + k )
1
0 mM KOH at a flow rate of 0.33 mL min were employed.
1
0
Injection volume was 10 mL.
−
aq
Take A
t
(e ) = εcl into eqn. (3) to obtain the absorbance
Calculation method: Density functional theory (DFT)-
UB3LYP method in combination with the 6-31 + G (d,p) basis
sets were used to obtain the structure and the optical properties
change of hydrated electrons in the solution, as shown in eqn.
4).
(
(
absorption peak and oscillator strength) of anion radical. All
′
′
′
2
′
3
−
aq
−(k2 + k3)t
calculations were performed by a Gaussian 09 software
package.
εl(k + k )A [e ] e
−
aq
0
0
A(e ) =
′
′
(4)
−
−(k +k )t
2
3
′
2
′
3
2
k A [e ](1− e
) + εl(k + k )
1
0
aq
RESULTS AND DISCUSSION
14
The study conducted by Huang et al. showed that k =
1
9
-1 -1
Production of hydrated electrons: The laser flash photo-
(6.5 ± 0.2) × 10 L mol s and k was substituted into eqn. (4)
and k = (9.7 ± 0.3) × 10 s was obtained. The rate constant of
the eqn (3) was (2.7 ± 0.1) × 10 s , which was studied in our
previous research .Therefore the first-order reaction constant
1
-5
5
-1
lysis of 9.83 × 10 M PyTS in N
2
saturated aqueous solution
2
5
-1
was performed and two-photon ionization process of PyTS
3
12
occurred, which led to the formation of triplet PyTS ( PyTS),
+
·
12,13
5
-1
cation radicals (PyTS ) and hydrated electrons . The
of eqn (2) was (7.0 ± 0.3) × 10 s (Fig. 1).
3
+·
absorption bands of PyTS and PyTS were at < 500 nm (the
absorption peak of the former was at 430 nm, the latter 510
nm). The broad and irregular absorption at 550-750 nm was
attributed to the characteristic absorption of hydrated electrons
Based on the rate constant of first-order reaction, it can
be seen that at the initial time, the ratio of hydrated electrons
consumption rate in reaction (1), (2) and (3) was:
−
−
−
k [e ]: k [e ][PyTS]: k [e ][BrB] = 1:11:25.5
1
aq
2
aq
3
aq
(
peak at around 700 nm). That is, the absorption at 690 nm
was mainly caused by hydrated electrons and less interfered
by other transient absorptions. Therefore, the kinetics of the
reaction between hydrated electrons and bromobenzene in
solution was analyzed based on the absorption changes at this
particular wavelength.
Thus, at the initial time, the self-quenching reaction of
hydrated electrons took up only a small proportion, while the
reactions with bromobenzene and pyrenetetrasulfonate had
contributed more greatly to the decay of hydrated electrons,
where the rate of reaction with bromobenzene was about two
times of that with pyrenetetrasulfonate.
Kinetics analysis of the reaction
Rate constant of first-order reaction: Based on the
hydrated electrons absorbance at this wavelength (λ = 20560
mol L cm ) , the concentration of hydrated electrons at the
0
.10
-1
-1 14
k = 9.7 ± 0.3E5
-
6
initial time of the reaction could be calculated as 3.8 × 10
-1
0.05
0.00
mol L . There were mainly three reactions where hydrated
electrons were involved, as shown in eqns. (1), (2). In the
solution, the concentrations of bromobenzene (4.2 × 10 mol
-5
-1
L ) and pyrenetetrasulfonate (PyTS) in rate constant of first-
-5
-1
order reaction (9.8 × 10 mol L ) were much greater than the
concentration of hydrated electrons. Therefore, the eqns. (2)
and (3) could be approximated to first-order reaction. There-
fore, it could be seen that the decay of hydrated electrons was
consistent with one second-order reaction and two quasi first-
order reactions.
-0.05
0.000000
0.000005
Time (s)
0.000010
2
H O
−
aq
−
aq
2
-
e
+ e
H
2
+ 2OH
(1)
Fig. 1. Decay kinetic of hydrated electrons at 690 nm
k1