High-resolution discrete absorption spectrum of α-methallyl free radical in the vapor phase

Article abstract of DOI:10.1016/j.saa.2005.09.038

The α-methallyl free radical is formed in the flash photolysis of 3-methylbut-1-ene, and cis-pent-2-ene in the vapor phase, and then subsequent reactions have been investigated by kinetic spectroscopy and gas-liquid chromatography. The photolysis flash was of short duration and it was possible to follow the kinetics of the radicals' decay, which occurred predominantly by bimolecular recombination. The measured rate constant for the α-methallyl recombination was (3.5 ± 0.3) × 10¹⁰ mol^-1 l s^-1 at 295 ± 2 K. The absolute extinction coefficients of the α-methallyl radical are calculated from the optical densities of the absorption bands. Detailed analysis of related absorption bands and lifetime measurements in the original α-methallyl high-resolution discrete absorption spectrum image were also carried out by image processing techniques.

Full text of DOI:10.1016/j.saa.2005.09.038

Spectrochimica Acta Part A 65 (2006) 143–146
High-resolution discrete absorption spectrum of
␣
-methallyl free radical in the vapor phase
a,∗
b
b
c
Fuat Bayrak c¸ eken , Ziya Telatar , Fikret Arı , Lale Tun c¸ y u¨ rek ,
d
e
˙
Ipek Karaaslan , Ali Yaman
a
˙
Department of Electronics Engineering, Yeditepe University, Istanbul 34755, Turkey
Department of Electronics Engineering, Ankara University, Ankara 06100, Turkey
Department of Systems Engineering, Yeditepe University, Istanbul 34755, Turkey
b
c
˙
d
˙
Department of Physics, Yeditepe University, Istanbul 34755, Turkey
Department of Physics, Ankara University, Ankara 06100, Turkey
e
Received 12 August 2005; accepted 15 September 2005
Abstract
The ␣-methallyl free radical is formed in the flash photolysis of 3-methylbut-1-ene, and cis-pent-2-ene in the vapor phase, and then subsequent
reactions have been investigated by kinetic spectroscopy and gas–liquid chromatography. The photolysis flash was of short duration and it was
possible to follow the kinetics of the radicals’ decay, which occurred predominantly by bimolecular recombination. The measured rate constant
1
0
−1
−1
for the ␣-methallyl recombination was (3.5 ± 0.3) × 10 mol l s at 295 ± 2 K. The absolute extinction coefficients of the ␣-methallyl radical
are calculated from the optical densities of the absorption bands. Detailed analysis of related absorption bands and lifetime measurements in the
original ␣-methallyl high-resolution discrete absorption spectrum image were also carried out by image processing techniques.
©
2005 Elsevier B.V. All rights reserved.
Keywords: Free radical; Flash spectroscopy; Lifetimes; ␣-Methallyl radical; High-resolution absorption spectra
1
. Introduction
this approximation, and in general the spectroscopic proof of
the production of free radicals is in accord with the mechanism
previously postulated from end product analysis. The photo-
sensitization or direct photolysis reveals that the excited states
formed in the primary step subsequently undergo a variety of
processes, predominantly, isomerisation, formation of radicals
through fission of some bands or fission to molecular products.
In photosensitized systems, most attention has been focussed on
isomerisation. With direct photolysis, quantitative information
on the decomposition mode has been derived from end product
analysis. A simple way of differentiating between free radical
and triplet-state absorption occurring in flash photolysis is to
compare the spectra observed in the presence and absence of air
[1–13].
The exact band analysis of digitally recorded multi-resolution
␣-methallyl free radical spectra requires image processing tech-
niques because of insufficiencies in the human visual system and
some degradation effects on the spectral image like noise, sat-
urated contrast and illumination values. Moreover, the human
visual system is sometimes insufficient to the little change in
intensity levels (two levels between neighborhood pixels) for
Electronic spectra of the ␣-methallyl free radical have been
observed in the far ultraviolet and its assignment discussed. The
intense region of absorption of the ␣- and ␤-methallyl radicals
lie 13 nm to the long wavelength of that of the allyl radical, and
are easily distinguished from it. Although the two methallyl rad-
icals have their intensity maxima in the same wavelength region,
the individual vibrational structures are quite characteristic, and
furthermore the ␤-methallyl radical absorbs in two sharp bands
at 225.1 and 223.1 nm, which are easily identified. In the flash
photolysis of all olefins including ethylene and 1,3-butadiene,
the methyl free radical absorption at 216 nm was detected. It
has been assumed that “methylation” of the allyl radical causes
only a minor perturbation of the conjugated ␲-system, and that
the three transitions have the same f-value. A general survey of
the opportunities offered by flash spectroscopy is reasonable in
∗
Corresponding author. Tel.: +90 216 578 0433; fax: +90 216 578 0400.
E-mail address: fubay@yeditepe.edu.tr (F. Bayrak c¸ eken).
1
386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.09.038

1
44
F. Bayrak c¸ eken et al. / Spectrochimica Acta Part A 65 (2006) 143–146
high-resolution digital images. One way to quantify the ability
to resolve two level visual stimuli which are the same except
for their intensities or luminances is by measuring the just-
noticeable difference or short intensity discrimination. If the
observer is assumed to spend some time before making a deci-
sion, the result is obtained when the observer is adapted to
the intensity level. When the intensity level the observer is
adapted to is of a different form, the observer’s intensity res-
olution ability decreases. From this fact, if an image with a
large dynamic range is recorded on a medium with a small
dynamic range, such as film or paper, the contrast and there-
fore the details of the image are reduced, particularly in the very
bright and dark regions. In fact, the details of small variations
on the original analog spectral image can be re-emphasized by
using digital image processing techniques in which an image is
digitally formed. On the other hand, increasing the local con-
trast and reducing the overall dynamic range can significantly
enhance the quality of such an image. With this aim, such image
enhancement algorithms are required for images after digitiza-
tion process. In image enhancement, the main objective is to
make the processed image better in some sense than the unpro-
cessedimage. Inthiscase, theidealimagedependsonthecontext
of the problem and is often not well defined. In that sense, an
image is enhanced by modifying its contrast and/or dynamic
range. Improved image maps, more information and less noise
from the raw data onto the actual image further increase their
resolution. On the other side, in some cases, the measure-
ment results extracted from the experimental data might include
some measurement noises because of the insufficient number
of samples. This type of noise can be eliminated by apply-
ing digital filtering techniques like smoothing and averaging
Fig. 1. Flash photolysis of 3-methylbut-1-ene (3.4 Torr) + N2 (700 Torr) show-
ing the spectrum of ␣-methallyl radical and its decay, flash energy was 1125 J
(
original spectrum is taken by the permission of F. Bayrak c¸ eken).
The image of the original multi-resolution ␣-methallyl spec-
tra having 10 time intervals and five main absorption bands
recorded from the measurements (Fig. 1) was also examined
by using image analysis techniques in order to obtain the exact
absorption band places from the spectra. For that reason the
original spectral image was transformed to a gray level image
as,
y(i, j) = Gray-level{I(i, j)}
(1)
where I and y are the original spectral images and the resulting
256 gray-level image, respectively. The intensity levels of the
spectral image was normalized by using MATLAB functions in
order to reduce the computational costs as follows:
[
7].
y(i, j)
2
. Experimental
yn(i, j) =
(2)
{
y(i, j)}max
The flash photolysis apparatus consisted of two parallel pho-
where yn is the normalized image. Each horizontal time interval
from “before” to “after” in Fig. 1 were marked and separated
from the original spectral image to define the region of interest
totubes arranged in series, contained in a reflector, which was
flushed with oxygen-free nitrogen. The photoflash energies used
were 780–1125 J, and the flash duration was 4 ␮s. All quartz
components were of spectrosilica grade. Spectra were recorded
on Hilger medium quartz instrument using Ilford-XK fast blue
sensitive plates sensitized with sodium salicylate. The plates
were photometered on a Joyce-Loebel recording microdensito-
meter with step wedge calibration. All the chemicals used were
spectroscopically pure. All kinetic spectroscopy was carried out
for single flashes giving <1% decomposition, as multiple flashes
resulted in the appearance of a continuous absorption spectrum
in the wavelength region of the methyl radical. Low pressures
of parent molecules were flashed after mixing with excess of
nitrogen to maintain isothermal conditions, pressures were mea-
sured with a McLeod gauge (up to 0.1 Torr), a spiral gauge
(
ROI) being analysed as follows:
yb(i, k) = y(i, j), k ∈ ROIb, b = 1, 2, . . . , 10
where y ’s show the blocks to be tried. Each time interval
cropped, as in Eq. (3), was defined as a one-dimensional density
row vector x(n) by computing the averages through the columns
(3)
b
of the matrix y (for each vertical axes) as,
b
M
ꢀ
1
x(n) =
yb(i, n)
(4)
M
i=1
Each element of the vector x(n) defines a distinct wavelength
and the amplitude value of that element gives its optical den-
sity in the spectral image. The elements of the vector x(n) were
smoothened by curve fitting in order to avoid noise effects and
obtain a better visual graphical representation for all absorption
bands than the original. Each band was identified from den-
sity information and plotted again as shown in Fig. 2, which is
an example of the separated band for 20 ␮s measurement. The
(
0.1–50 Torr), and above 50 Torr, with a mercury manometer.
Products and residual parent molecules were dissolved in sev-
eral alcohols and the resulting solution concentrated to 20 ␮l. A
sample of 4 ␮l was used for analysis with a Griffin and George-
D6 gas chromatograph with a 12% polyethylene glycol column
◦
at 200 C.

F. Bayrak c¸ eken et al. / Spectrochimica Acta Part A 65 (2006) 143–146
145
maximum level of the curve in Fig. 2 defines the places of the
related absorption bands as 232.189, 233.249, 234.959, 236.053
and 237.729 nm.
The lifetime (LT) measurement was realized by defining the
interval between the maximum value of the signal and the signal
value at 1/e level on time graph as follows:
Table 1
The rate constant and extinction coefficients for 3-methylbut-1-ene and cis-pent-
-ene
2
4
−1 −1
4
−1
−1
Compound
k (10 A
s
)
λ (nm)
ε (10 mol 1 cm )
2
.07
237.8
236.0
235.0
233.3
232.4
5.00
3.52
2.51
2.85
1.98
1.46
1.04
1.18
3-Methylbut-1-ene
LT = distance{tD(t) − tD(t)}
(5)
0.82
where t is the time scale position in the spectrum and D(t)’s are
the peak values and the 1/e value of the curve, respectively.
2
1
1.00
1.10
.07
.44
237.8
236.0
235.0
233.3
232.4
5.00
3.48
2.42
2.66
2.29
cis-Pent-2-ene
3
. Results and discussion
0
.95
␣
-Methallyl radical was observed in flash photolysis of
3
-methylbut-1-ene, cis-pent-2-ene, trans-but-2-ene, and but-1-
photolysis is to compare the spectra observed in the presence and
absence of air. Since the rate of quenching of triplet states by
paramagnetic oxygen is so fast, no triplet -state absorption is
seen in the presence of air, whereas both radical and triplet-state
absorptions are seen in the absence of air. This method depends
on the fact that, in general, free radicals react rather slowly with
oxygen but it is only suitable, when the presence of air does not
interfere with the photolytic formation of triplet states and radi-
cals. The ␣-methallyl radical absorption spectrum was observed
weakly for trans-but-2-ene and but-1-ene, therefore the data for
these two molecules were not included in Table 1. There is some
uncertainty in the range 250–254 nm, owing to interference from
silica lines.
ene. The high-resolution absorption spectrum of this radical is
shown in Fig. 1. The effect of the addition of an excess of inert
gas between 200 and 700 Torr did not change the intensity of
absorption bands of the free radical much in the vapor phase.
It was not possible to photometer the spectra accurately below
2
31 nm, because the sensitivity of the photographic plate was
very low. Figs. 1–4 show flash photolysis of 3-methylbut-1-
ene, absorption spectrum of the ␣-methallyl free radical, lifetime
measurements for 237.729 nm absorption band, and 3D repre-
sentationforcomputedfiveabsorptionbands, respectively. Fig. 5
shows the second order plot for the decay of the ␣-methallyl
radical at 237.8, 236, 235, 233.3 and 232.4 nm. The decay of
all bands could be fitted well to second order plots with slopes
consistent with their relative optical densities at the respective
wavelengths. No significant effect on the slope of such plots
were produced by varying the pressure of the parent molecule
by factors of up to 5 and the nitrogen pressure by factors of
up to 3. At the total pressures employed in this work, the pro-
cess is not thermolecular involving either the parent molecule or
the nitrogen present in excess. Table 1 shows the rate constants
and absolute extinction coefficients for 3-methylbut-1-ene and
cis-pent-2-ene. Table 2 shows the computed 1/e decay values
for the absorption bands. The rate constant of recombination
A more detailed investigation of these and similar systems
would be facilitated by an apparatus with which the ambient
temperature could be varied, and with a more sensitive analyt-
ical technique. This would make possible the extension of the
method to analogous compounds with lower vapor pressures,
␣
-methallyl and ␤-methallyl in particular, and probably result
in a more complete kinetic analysis.
Selected absorption regions from the original spectral image
of ␣-methallyl radical given in Fig. 1 have been examined in
the experimental studies in order to observe and to evaluate
10
−1 −1
of two ␣-methallyl radical was 3.5 × 10 mol l s at 295 K.
The flash duration time was short enough to follow the decay of
the radical at room temperature. A single way of differentiating
between radical and triplet-stateabsorption occurring in the flash
Fig. 2. Absorption spectrum of the ␣-methallyl free radical for 20 ␮s delay
measurement.
Fig. 3. Decay time measurements for 237.729 nm absorption band.

1
46
F. Bayrak c¸ eken et al. / Spectrochimica Acta Part A 65 (2006) 143–146
Table 2
Computed 1/e decay times values for absorption bands
Band position (nm) Lifetime (␮s)
2
2
2
2
2
32.189
33.249
34.959
36.053
37.729
121
122
114
113
103
Fig. 6. The plot of the optical density vs. time (decay of the ␣-methallyl rad-
ical) measured at 237.8 nm (curve A), 236 nm (B), 235 nm (C), 233.3 nm (D),
232.4 nm (E).
measurements in the original spectral image. The lifetime for
this band was computed from Fig. 1, as 103 ␮s. After applying
the image analysis techniques explained in the experimental sec-
tion, computed decay times for five absorption bands are given
in Table 2. In the experimental studies of a radical lifetime mea-
surement, it is expected that computed decay time intervals for
all absorption bands must approximately be close to each other.
Computational results given in Table 2 corrected this hypothesis
with a considerable amount of deviation.
Fig. 4. 3D representation for computed five absorption bands.
Fig. 4 shows 3D representation of the five bands given in
Table 2. Figs. 5 and 6 show the second order plot for the decay
of the ␣-methallyl radical and the plot for the decay of the radical
absorbance versus time. The computed 1/e decay times are given
in Table 2. The approximate value of the lifetime of the radical
is 150 ± 5 ␮s.
References
[
[
[
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2
32.4 nm (E).
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related regions more efficiently. Fig. 3 shows the detailed plot
of 237.729 nm absorption band obtained from all time measure-
ments including before and after time intervals. This figure was
plotted to compute the lifetime measurement of the band. The
zero value is about “before” and the 180 value is about “after”
[
[