S. Li, W.Y. Fan / Chemical Physics Letters 427 (2006) 276–280
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from Sigma–Aldrich. Distillation and free-pump–thaw
cycles were carried out to remove impurities before these
compounds were used in the kinetic runs. A static Pyrex
cell (15 cm pathlength) with a temperature controller was
used as the reaction vessel initially evacuated by a rotary
pump to a background pressure of 2 · 10ꢀ3 Torr. The pres-
sures used for the t-butyl nitrite and the substrates were 1–
2 Torr and 10–20 Torr, respectively. About 100–760 Torr
SF6 buffer gas was flowed into the cell from its gas cylinder
(Linde gas, 99.9%).
sures and hence the concentration of the two products
formed in the abstraction reaction can then be determined.
A few control experiments were carried out initially.
Firstly, upon uv photolysis of a sample containing t-butyl
nitrite, the production of only acetone was observed, indi-
cating that unimolecular decomposition of the t-butoxy
radical is dominant in the absence of any substrates. Sec-
ondly, it was only upon uv lamp photolysis in the presence
of both t-butyl nitrite and the substrate that IR signals of
acetone and t-butanol began to appear and increased in
intensity with respect to the irradiation time.
The t-butoxy radical was generated by 360–380 nm
broadband lamp photolysis of t-butyl nitrite. The infrared
spectrum (Nicolet Nexus 870) was scanned every 3 min
from 1000 to 4000 cmꢀ1 with a resolution of 1 cmꢀ1. The
relative rate of the IR signal changes of acetone
(1738 cmꢀ1) and t-butanol (1140 cmꢀ1) were monitored
with respect to the irradiation time. Initial rate measure-
ments were used to minimize contributions from secondary
reactions caused by product buildup. The experiment was
repeated five times for each reaction pair. During the first
few minutes of irradiation, there were little formation of
acetone and t-butanol (absorbances <20%) so the Beer–
Lambert’s Law could be used to form a linear relationship
between the absorbance and concentration. For experi-
ments using di-t-butyl peroxide as the precursor, a
254 nm mercury lamp was used as the photodissociation
source in a quartz cell of similar size (15 cm long, 2.5 cm
diameter) while maintaining all other conditions. For the
Arrhenius parameter measurements, a heating tape was
used to increase uniformly the temperature of the reaction
cell with a thermocouple installed inside the reaction cham-
ber for recording the gas temperature. Throughout the irra-
diation, only minimal temperature rise was observed and
hence the temperature range of the experiment was
recorded as 299 1 ꢁC.
Based on the simple reaction scheme below and assum-
ing a constant photolysis rate of t-butyl nitrite, the relation-
ship between t-butanol and acetone formation over the
irradiation period (Dt) is given by Eqs. (1a)–(1f) [19,20].
hm
ðCH3Þ3CONO !ðCH3Þ3CO þ NO
ð3Þ
ð4Þ
ð5Þ
kd
ðCH3Þ3CO þ M ! CH3COCH3 þ CH3
ka
ðCH3Þ3CO þ RH !ðCH3Þ3COH þ R
d½t ꢀ BuOHꢁ
¼ ka½ðCH3Þ3COꢁ½donorꢁ
ð1aÞ
dt
d½acetoneꢁ
¼ kd½ðCH3Þ3COꢁ
dt
ð1bÞ
½t ꢀ BuOHꢁ ¼ ka½ðCH3Þ3COꢁ½donorꢁDt
½acetoneꢁ ¼ kd½ðCH3Þ3COꢁDt
ð1cÞ
ð1dÞ
½t ꢀ BuOHꢁ ka½donorꢁ
¼
ð1eÞ
½acetoneꢁ
kd
kd½t ꢀ BuOHꢁ
Ka ¼
½acetoneꢁ
ð1fÞ
½donorꢁ
Under conditions where the t-butoxy radical and sub-
strate concentrations are constant which would occur dur-
ing the initial stages of irradiation, Eqs. (1a) and (1b) may
be integrated to give (1c) and (1d) which upon rearrange-
ment lead to an expression for the rate coefficient shown
in (1f). Eqs. (1a) and (1b) predict linear productions of t-
butanol and acetone and indeed this is shown by the results
obtained for the reactions as shown in Fig. 1. The linear
plot demonstrates constant rate of production of t-butoxy
radical concentration at low substrate consumption. From
the ratio of acetone to t-butanol, the absolute rate coeffi-
cients of the H-atom abstraction reactions can be deter-
mined since kd has already been well-documented [7–11].
As nitric oxide is the other product from nitrite photolysis,
it may have some effect on the abstraction reactions. Hence
another precursor (di-t-butylperoxide) was used to produce
t-butoxy radical but essentially the same H-atom abstrac-
tion reaction rate coefficients were obtained. From these
results, the presence of NO has essentially no effect or at
least negligible within the experimental error of the rate
coefficients.
The structures of the reactants, products and the transi-
tion states were optimized using density functional theory
at UB3LYP/6-31+G* level in GAUSSIAN 03 that the enthal-
pies, free energies and activation energies of the abstraction
processes could be determined. Harmonic frequencies were
calculated at the optimized geometries to characterize sta-
tionary points as equilibrium structures, with all real fre-
quencies, or transition states, with one imaginary
frequency, and to evaluate zero-point energy (ZPE)
correction.
3. Results and discussion
The Beer–Lambert’s Law, Abs = e c l where Abs = mea-
sured absorbance; l = path length (cm), c is the partial con-
centration of the absorbing species (mol lꢀ1) and e is the
molar absorptivity (cm2 molꢀ1), has been used towards
determining the ratio of the products. The absorptivity of
the vibrational bands of the products was estimated by cal-
ibrating known pressures of samples of acetone and t-buta-
nol added to the same total pressure (buffer gas and
reactants) used in the abstraction experiment. The pres-
The rate coefficients for the abstraction rate of various
substrates are shown in Table 1. The relative rates depend
on the number and type of hydrogen atoms of the sub-