Mixtures of Cl2, toluene, and the bath gas were compressed in
an oil-free diaphragm compressor (Nova Swiss), and then
flowed through the high-pressure cell (path length of 10 cm
and optical diameter of 0.9 cm). For experiments o1 bar, a
flow cell made of glass was used (path length of 52 cm and
optical diameter of 3 cm). Flow rates were controlled by flow
meters (Tylan, model FM361 and FM362) at rates such that
reagents and products were removed from the observation
volume between the laser pulses. Total pressures were
measured with high-pressure meters (Burster, model 8201).
Two platinum resistance thermometers were directly attached
to the front and back of the cell to measure the temperature.
The progress of reaction (1) was monitored by recording the
absorption signal of benzyl radicals at 253 nm on time scales of
ms to ms. The light source for the absorption measurements
was a high-intensity Hg–Xe lamp (Ushio, model UXM-200 H,
200W). At 253 nm, the absorption of benzyl radicals dominates
over contributions from other species involved (sbenzyl ¼ 1.3 ꢁ
10ꢂ16 cm2 moleculeꢂ1).16 The absorption signal was detected
by a standard prism-monochromator (Zeiss, model MM3) and
photomultiplier (RCA, 1P28A) arrangement with a bandwidth
of 2 nm, and recorded using a digital storage oscilloscope
(LeCroy, band width 200 MHz). Typically several hundred
shots were averaged. The bath gases of helium, argon, xenon,
N2, and CO2 were of a purity higher than 99.998%. Impurities
in the bath gases, especially oxygen, were carefully removed by
a series of gas cleaning adsorbers (Messer-Griesheim, model
Oxisorb, and Alltech, model Oxytrap) and dust filters. All
chemicals were purified in a pump–thaw–freeze cycle prior
to use.
measured without the laser beam at the same experimental
conditions. Several hundred spectra were usually averaged.
3. Results
Fig. 1 shows typical absorption signals at 253 nm, recorded
after the 308 nm-laser photolysis of Cl2 in the presence of
toluene and the bath gas CO2 at 300 K. All absorption–time
profiles showed clean second-order decays of the benzyl
concentrations and agreed entirely with an assignment to
reaction (1).
Reaction (3) is known to solely proceed through a fast
H-atom abstraction channel (k3 ¼ 6.1 ꢁ 10ꢂ11 cm3 moleculeꢂ1
sꢂ1
) such that chlorine atoms rapidly, completely, and
stoichiometrically lead to benzyl radicals.17 The quantitative
analysis of our experimental observation was carried out
considering the major channel (1) and also accounting for
other possible side reactions such as the recombination of
chlorine atoms and the reaction between chlorine atoms and
benzyl radicals:
Cl þ Cl - Cl2
C6H5CH2 þ Cl - C6H5CH2Cl
(4)
(5)
These two side reactions, however, turned out to be insigni-
ficant because the concentration of toluene was always present
in significant excess. Residual absorptions from products like
dibenzyl and benzyl chloride are also negligible due to their
small absorption coefficients (sdibenzyl E sbenzyl chloride E 1 ꢁ
10ꢂ19 cm2 moleculeꢂ1 18
compared to the strong absorption
)
from benzyl radicals (sbenzyl ¼ 1.3 ꢁ 10ꢂ16 cm2 moleculeꢂ1).16
The residuals of the fits did not show any systematic devia-
tions, giving additional support to the reported values of k1.
Typical concentrations used in our experiments were: [Cl]0 (¼
[benzyl]0) ¼ (1 ꢂ 5) ꢁ 1013 molecule cmꢂ3 and [toluene] ¼
2.2. Transient absorption spectra
Prior to the analysis of the kinetics, it was necessary to check
the pressure dependence of the absorption of benzyl radicals.
In addition, possible contributions to the absorption signals
from other reactants, intermediates, and products (for
example, dibenzyl or benzyl chloride) over the observation
wavelength range were investigated. It turned out that benzyl
absortpion was predominant over the wavelength range
240–270 nm and times up to a few ms.
(0.7 ꢂ 7) ꢁ 1016 molecule cmꢂ3
.
Several other precursor molecules for benzyl radicals were
tested but found to be inadequate for our purpose. For
example, benzyl iodide, ethylbenzene, and benzyl chloride also
yield benzyl radicals in a direct photolytic step. However,
subsequent side reactions of other photolytic products (such
as halogen atoms or ethyl radicals) caused large difficulties of
the analysis.
Pressure-dependent transient spectra of benzyl radicals were
recorded at room temperature in a high-pressure flow cell
closed by two quartz windows (path length of 29.5 cm and
optical diameter of 0.9 cm). A Jobin-Yvon-Spex type of
spectrograph (model SPEX 270M) with a holographic grating
(1200 or 2400 g mmꢂ1, blazed at 500 nm) and a 384 ꢁ 576 pixel
ICCD camera (LaVison, model FlameStar IIF) served as the
detection system (wavelength range of 190–850 nm). The
highest resolution of the spectrograph (0.096 nm) was deter-
mined by an entrance slit width of 12 mm. The minimum gating
time was 1 ns. A high-pressure xenon arc lamp (Osram, model
XBO 150W/1) was used as the light source. A set of lenses
focused the light beam as it emerged from the highcell on to the
entrance slit of the spectrograph. The 308 nm-excimer laser
beam (Lambda Physik, model EMG 101MSC) and the light of
the Xe lamp were directed collinearly through the highcell, via
a set of laser mirrors (Laseroptik, high reflectance at 308 nm
and high transmittance at 253 nm, 451) in a counter-propagat-
ing arrangement. A laser power meter recorded the laser energy
behind the high-pressure cell. A low-pressure mercury lamp
(ORIEL, model 65130, 22–44 W) was used to calibrate the
wavelength of the spectrum before the measurement. A pulse/
delay generator (Stanford Research Systems, model DG535)
was interfaced with the laser controlling system and the data
acquisition system of the ICCD camera. Spectral analysis
software (LaVision, DaVis version 6.0) was used for post-
processing the measured spectra. Prior to obtaining the tran-
sient spectrum of benzyl radicals, a reference spectrum was
Fig. 1 Absorption signals at 253 nm, recorded after the photolysis at
308 nm of mixtures of Cl2, toluene and CO2 at T ¼ 300 K. Main figure:
time-profiles at different CO2 pressures, from the top: 35, 20, 10, and
6 bar. The ratio of [Cl2]/[CO2] was kept constant (1.3 ꢁ 10ꢂ5). Inserted
figure: (K) maximum absorption values as a function of the CO2 bath
gas pressure, (--) ¼ linear fit.
4134
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 4 1 3 3 – 4 1 4 1
T h i s j o u r n a l i s & T h e O w n e r S o c i e t i e s 2 0 0 4