(
)
138
G.R. Dey et al.rChemical Physics Letters 310 1999 137–144
of N2-purged solutions of 5=10y3 mol dmy3 4-hy-
2. Experimental
droxythiophenol in n-butyl chloride. The spectrum
after 100 ns, which is elicited here immediately after
the electron pulse, exhibits two absorption bands
with lmax values at 340 and 510 nm and a shoulder
at 560 nm. These main bands are to a considerable
extent already formed during the electron pulse, and
subsequently grow in amplitude within ;2 ms. By
contrast, the shoulder disappears within ;200 ns.
Pulse radiolysis experiments were performed with
Ž
.
high-energy electron pulses 1 MeV, 15 ns duration
produced by a pulse transformer type electron accel-
Ž
erator ELIT Institute of Nuclear Physics, Novosi-
.
birsk, Russia . The dose per pulse determined with
an electron dosimeter was ;50 Gy. Irradiation of
the samples was carried out in an online flow system
with an optical path length of 10 mm of a quartz cell.
The optical detection system consists of a pulsed
The main absorption is attributed to the thiyl radical
Ø
w
x
HOArS , which tallies with the literature 3,7 .
Ž
Considering the time profiles at ls500 nm in-
set in Fig. 1 , the majority of the thiyl radical
Ž
xenon lamp, a SpectraPro-500 monochromator Ac-
.
.
ton Research ,
a
R4220 photomultiplier tube
Hamamatsu or a FND-100Q silicon photo diode
Laser Components , and a TDS 640 digitizing os-
cilloscope Tektronix . More details about the pulse
absorption is evidently formed in a delayed manner,
Ž
Ž
.
Ž .
as expected for the radical H abstraction reaction 5 .
.
A bimolecular rate constant k5 s1=108 dm3 moly1
sy1 was determined from the dependence on scav-
enger concentration. Similar rate constants were
Ž
.
w x
radiolysis apparatus are given elsewhere 4 . The
solutions were purged with ultrapure N2 and O2.
4-hydroxythiophenol from Aldrich was used as re-
w x
found by Schoneich et al.
8 for the analogous
¨
reaction of aliphatic thiols. As can be seen, the
growth of HOArSØ is not purely exponential, which
could be caused by superposition with the ionic
processes discussed subsequently.
Ž
.
ceived. Spectrograde n-butyl chloride Aldrich was
tested spectrometrically before use. All other chemi-
cals used were analytical reagent grade.
To remove the ionic contributions from the spec-
tra of Fig. 1, we added 0.1 mol dmy3 ethanol as a
scavenger for the parent ions of the solvent which
acts under deprotonation.
3. Results and discussion
The main reactive species generated by the radiol-
w x
ysis of pure n-butyl chloride 5 are given by reac-
Ž .
tion 3
BuClØqqC2 H5OH ™ BuClØqC2 H5OHq2
.
6
Ž .
BuCl–r_r_r_™BuClØq,BuØ;Cly
The radical cation BuClØq exhibits a broad absorp-
tion band with lmax f500 nm and has a half-life of
;100 ns. In the presence of solute molecules with a
lower ionization potential such as 4-hydroxy-
thiophenol, the solvent radical cations BuCl
undergo the rapid electron transfer reaction 4 6 .
Furthermore, it should be noted that because of its
soft S–H bond, HOArSH can also undergo a rela-
.
3
Ž .
Ž
This produced divergent spectral behavior Fig.
.
Ž
.
2 , which differs from the initial situation Fig. 1 by
the disappearance of the fast transient absorption
around 550 nm, a different relationship between the
520 and the 340 nm peak, and a slightly different
spectral structure in the range between 400 and 460
nm. Furthermore, the time profiles given as insets
exhibit more homogeneous time behavior without
the marked fast initial step in Fig. 1. We therefore
suppose that Fig. 2 gives the actual spectrum of the
Øq
Ž
.
can
Ž . w x
Ž .
tively fast reaction 5 with the radiolytically formed
Ž .
thiyl radical formed according to reaction 5 .
w
Ž .x
butyl radical cf. reaction 3 .
Ž
.
To analyze the ionic processes Gfi f0.2 , we
decided to remove the dominating radical reaction
Gf3–4 by rapid deactivation of the butyl radi-
cals. This was performed by purging the sample with
BuClØqqHOArSH ™ BuClqHOArSH Øq
BuØqHOArSH ™ BuHqHOArSØ .
,
4
Ž .
Ž
.
5
Ž .
Fig. 1 shows transient optical absorption spectra
obtained at different times during the pulse radiolysis
oxygen to transform the alkyl radicals in under 50 ns
Ž .
into alkylperoxyl radicals 7 , which were found to