Spectral image analysis for the absorption bands of the β-methallyl free radical in the vapor phase

Article abstract of DOI:10.1016/j.saa.2006.09.037

The radicals formed in the flash photolysis of 2-methylbut-1-ene and subsequent reactions have been investigated by kinetic spectroscopy and gas liquid chromatography. Less than 10% of photo products are formed by a molecular made of fission of the excited olefin, and of the radical modes the relative probabilities of band fission, β(C{single bond}H):β(C{single bond}H):α(C{single bond}C) are 13:1.37:1. The extinction coefficients of β-methallyl radical measured experimentally for all the absorption bands. The decay of the β-methallyl radical was second order. The rate constant for the β-methallyl radical recombination experimentally measured was 2.6 ± 0.3 × 10¹⁰ l mol^-1 s^-1 at 295 ± 2 K. The spectrum image showing the absorption bands was examined by image processing techniques in order to improve the visual experience of each band by localizing to a specific region of interest. Experimental results illustrate how the exact location of absorption bands was clearly extracted from the spectral image and further improvements in the visual detection of absorption bands.

Full text of DOI:10.1016/j.saa.2006.09.037

Spectrochimica Acta Part A 67 (2007) 1276–1280
Spectral image analysis for the absorption bands of the ␤-methallyl
free radical in the vapor phase
a,∗
b
b
a,1
Fuat Bayrak c¸ eken , Ziya Telatar , Fikret Arı , Aykut B. Bayrak
a
˙
Department of Biomedical Engineering, Yeditepe University, Istanbul 34755, Turkey
Department of Electronics Engineering, Ankara University, Ankara 06100, Turkey
b
Received 9 July 2006; accepted 15 September 2006
Abstract
The radicals formed in the flash photolysis of 2-methylbut-1-ene and subsequent reactions have been investigated by kinetic spectroscopy and gas
liquid chromatography. Less than 10% of photo products are formed by a molecular made of fission of the excited olefin, and of the radical modes
the relative probabilities of band fission, ␤(C H):␤(C H):␣(C C) are 13:1.37:1. The extinction coefficients of ␤-methallyl radical measured
experimentally for all the absorption bands. The decay of the ␤-methallyl radical was second order. The rate constant for the ␤-methallyl radical
1
0
−1 −1
recombination experimentally measured was 2.6 ± 0.3 × 10 l mol
s
at 295 ± 2 K. The spectrum image showing the absorption bands was
examined by image processing techniques in order to improve the visual experience of each band by localizing to a specific region of interest.
Experimental results illustrate how the exact location of absorption bands was clearly extracted from the spectral image and further improvements
in the visual detection of absorption bands.
©
2006 Elsevier B.V. All rights reserved.
Keywords: High resolution electronic absorption; ␤-Methallyl radical; Digital image analysis; Image adaptation; Second order decay
1
. Introduction
We have combined the techniques of flash, laser flash
photolysis and gas–liquid-chromatography end product anal-
ysis to elucidate the mechanism of photolytic decomposition
and radical recombination for an olefin of particular interest,
2-methylbut-1-ene. This molecule contains three band types sus-
ceptible to primary dissociation: ␣(C C), ␤(C C) and (C H).
It has not been investigated by direct photolysis except for the
qualitative identification of ␤(C C) fission as the major decom-
position made [1–27].
Photophysical and photochemical studies of simple olefins by
mercury photosensitization or direct photolysis reveal that the
excited olefins formed in the primary step subsequently undergo
a variety of processes, predominantly: isomerization; the forma-
tion of free radicals through fission of C H or C C bands (␣ or
␤
to the double band); or fission to molecular products.
In photosensitized systems most attention has been con-
centrated on isomerization. With direct photolysis quantitative
information on the decomposition made has been derived from
end product analysis. Bayrak c¸ eken using flash photolysis, iden-
tified the spectra of several free radicals in the vapor phase (i.e.
The visible and near ultraviolet absorption spectrum of a free
radical generally is shifted to longer wavelengths in the pres-
ence of an electric field because the polarizabilities of the ground
and excited states are different. In general, this shifted spectral
emissions are recorded to a suitable media (photographic paper,
digital imaging devices like CCD, etc.) sensitive to related wave-
lengths. If an image with large dynamic range is recorded on a
medium with a small dynamic range, such as film or paper, the
contrast and therefore the details of the image are reduced, par-
ticularly in the very bright and dark regions. In fact, the details
of small variations on the original analog spectrum image can
be re-emphasized by using digital image processing techniques
in which an image is digitally formed after the actual scan. The
measurement results extracted from experimental data might
␤
-methallyl, methyl, benzyl, ␣-chloro-benzyl, ␣-methallyl and
allyl free radicals), from the primary dissociation and made
approximate estimates of the relative probability of C C and
C H fission based on the assumption of equal “f” values for
allylic radicals.
∗
Corresponding author. Tel.: +90 216 578 0433; fax: +90 216 578 0400.
E-mail address: fubay@yeditepe.edu.tr (F. Bayrak c¸ eken).
Present address: SHER Institute, New York, NY, USA
1
1
386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.09.037

F. Bayrak c¸ eken et al. / Spectrochimica Acta Part A 67 (2007) 1276–1280
1277
include some measurement noises because of insufficient num-
ber of samples. This type of noise can be removed by applying
digital filtering techniques like smoothing, averaging [15,26,28].
uum systems in the set-up the absorption spectrum was recorded
between 190 and 750 nm band.
The image of the original absorption spectrum of ␤-methallyl
radical was scanned by HP-Scanjet 7400c scanner at 600 dpi res-
olutiontoobtainahighresolutiondigitalimage. Theimageofthe
2
. Experimental
␤
-methallyl spectra having 11 time intervals and 6 main absorp-
tion bands recorded from the measurements 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 spectrum image was transformed to gray level image
as:
The 2-methylbut-1-ene used was supplied by Koch Light,
pure grade. The flash photolysis apparatus consisted of two
parallel photo-tubes arranged in series, contained in a reflec-
tor which was flushed with nitrogen. The photoflash energies
used were 780–1125 J, and the flash duration was 2 ␮s. All
quartz components were spectrosil grade. Free radical absorp-
tion spectra were recorded on Hilger small and medium quartz
instruments using Ilford XK fast blue sensitive plates sensi-
tized with sodium salicylate. The plates were photometered on
a Joyce–Loebel recording microdensitometer with step wedge
calibration. Product analysis was carried out on a Perkin-Elmer
F11 chromatograph fitted with a gas sampling valve. Peak areas
were electronically integrated and retention times were checked
by direct calibration. The 2-methylbut-1-ene was further puri-
y(i, j) = Gray-level{I(i, j)}
(1)
where I and y are the original spectrum image and resulting
256 gray-level image, respectively. The intensity levels of the
spectrum image was normalized by using MATLAB functions
in order to reduce the computational costs as follows:
y(i, j)
yn(i, j) =
(2)
{
y(i, j)}max
−
7
fied by repeated vacuum (background pressure was 10 Torr),
distillation before use. The range of pressures studied was:
where yn is the normalized image. Each horizontal time interval
from “before” to “after” in Fig. 1 were marked and separated
from the original spectrum image to define the region of interest
(ROI) being analyzed as follows:
2
-methylbut-1-ene (0.5–20 Torr) + argon (100–700 Torr), thus
maintaining isothermal conditions. 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 free radical.
The electronic spectra of the ␤-methallyl radical has been
recorded in the ultraviolet region, and its assignments discussed.
The intense regions of absorption of the ␣-methallyl and ␤-
methallyl radicals (238 nm) lie 13 nm to long wavelength of that
of the allyl radical, and are easily distinguished from it. Although
the two methallyl radicals 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 eas-
ilyidentified. Inallruns, theknowntransitionsofthe ␤-methallyl
radical was observed at the shortest delay times, and a typical
result is shown in Fig. 1. The flash source emits continuous radi-
ation at wavelengths down to 170 nm, but paramagnetic oxygen
cuts the emissions below 190 nm, therefore without using vac-
yb(i, k) = y(i, j), k ∈ ROI , b = 1, 2, . . . , 11
(3)
b
where y s show the blocks to be tried. Each time intervals
b
cropped as in Eq. (3) was defined as a 1-dimensional density
row vector x(n) by computing the averages through the columns
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
smoothed 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
Fig. 1. Flash photolysis of iso-butene (2 Torr) and N2 (700 Torr) showing the spectrum of the ␤-methallyl radical and its decay. Flash energy = 1125 J.

1
278
F. Bayrak c¸ eken et al. / Spectrochimica Acta Part A 67 (2007) 1276–1280
and N → R where the upper levels are Rydberg states. Dissocia-
tion may occur by one of several primary paths, where between
−
1
2
00 and 380 kJ mol , energy could appear as excitation of
the radical products. Bayrak c¸ eken suggested the formation, by
intersystem crossing, of the ground state molecules with high
vibrational, rotational and translational excitations followed by
dissociation, whereasfavoredtheRydbergstatesastheprecursor
of the radical formation. The observed absorption spectra clearly
demonstrate that the predominant fate of the excited olefin is
fission of the ␤(C C) band to give methyl and ␤-methallyl
radicals:
∗
•
•
•
CH3CH2C(CH3) = CH → C H3 + C H2C(CH3) = CH2
2
(
6)
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. A typical example is shown in
Fig. 4. No significant effect on the slope of such plots was pro-
duced by varying the pressures of the parent molecule by factors
of up to four, and nitrogen or argon pressure by factors of up to
three. Measurements commenced at 5 ␮s delay, when the photol-
ysis flash is virtually over. Table 1 shows the rate constants and
extinction coefficients for 2-mehthylbut-1-ene, and isobutene
Fig. 2. Absorption spectrum of the ␤-methallyl free radical for 20 ␮s delay
measurement.
[Figs. 1 and 4 and Table 1, are reproduced by the permission of
Fuat Bayrak c¸ eken, Ref. [11]].
Decomposition by a “molecular made” may follow the for-
mation of molecules with high vibrational excitation in the
electronic ground state. These species may be formed by inter-
nal conversion from the excited electronic state. These species
may be formed by internal conversion from the excited elec-
tronic state, to overlapping high vibrational levels of the ground
state, or by electronic-vibrational energy transfer on collision
with other olefin molecules. Such highly excited ground state
molecules could undergo 1-3 hydrogen atom shifts and ␣(C C)
cleavage to molecular rather than radical products. Concentra-
Fig. 3. Band positions for 20 ␮s mesurement.
an example of the separated band for 20 ␮s measurement. The
maximum points of the curve marked by arrows in Fig. 3 defines
the places of the related absorption bands as 223.095, 225.130,
2
33.947, 236.155, 237.919 and 239.887 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:
LT = distance{tD(t)max − tD(t)1/e
}
(5)
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.
3
. Results and discussion
The absorption spectrum of 2-methylbut-2-ene, indicates that
the absorption of a quantum of radiation up to 200 nm results in
the formation of excited molecules with a range of energies. The
*
major transitions are: N → V (␲ → ␲ ), the upper state being
Fig. 4. Second order plot for the decay of the ␤-methallyl radical measured at
234.7 (curve A), 239.9 (B), 236.1 (C), 238.0 (D), 225.1 (E) and 223.1 nm (F).
◦
twisted 90 from the planar configuration at its energy minimum,

F. Bayrak c¸ eken et al. / Spectrochimica Acta Part A 67 (2007) 1276–1280
1279
Table 1
The rate constant and extinction coefficients for isobutene and 2-methylbut-1-
ene
4
−1 −1
4
−1
−1
Compound
Isobutene
k (10 A
s
)
λ (nm)
ε (10 l mol cm )
6.70
234.7
239.9
236.1
238.0
225.1
223.1
1.73
1.60
1.42
1.32
0.93
0.75
6
5
5
3
2
.20
.50
.10
.60
.90
2-Methylbut-1-ene
6.70
234.7
239.9
236.1
238.0
225.1
223.1
1.73
1.61
1.42
1.31
0.91
0.75
6
5
5
3
2
.25
.50
.05
.50
.90
tion of ␤-methallyl radical of up to 1.5 × 10−6 mol l , were
measured spectroscopically. Radical–radical reactions in such
cases are bimolecular, of very low activation energy, and con-
siderably faster than competing radical–molecule reactions.
Reactions between H atoms and allylic radicals are faster
than H addition to olefins at such concentrations. Hence the
major product yields should be explicable in terms of direct
combination:
−1
Fig. 5. Computed band curve for 233.947 nm absorption band.
Table 2
Bandwidth and band positions for absorption bands
Band index
Bandwidth
Band position
Band position
experimental (nm)
calculated (nm)
calculated (nm)
1
2
3
4
5
6
1.178
1.694
1.224
1.318
1.624
1.310
223.095
225.130
233.947
236.155
237.919
239.887
223.1
225.1
234.7
236.1
278.0
239.9
•
•
2
R → R R
(7)
or cross combination reactions:
•
•
•
R + R → R R
(8)
The cross combination reaction is significantly slower than
the direct combination processes. Minor products, including
those which are partly formed by “molecular” processes, partly
radical reactions, reveal the participation of a complex range of
reactions including a small contribution of unimolecular decom-
position of a ␤-methallyl radical, which has a high proportion of
the excess energy available as stated above. Other reactions must
involve a small fraction of ␤-methallyl radical, together with
ethyl and a C3 radical from direct ␣(C C) fission, and H atoms
in the experimental studies in order to observe and to evalu-
ate related regions more efficiently. Fig. 5 shows absorption
curve for 233.947 nm band. In this figure, horizontal dashed line
indicates1/elevelforbandwidthmeasurement. Calculatedband-
width and band position values with experimental band position
results for all six bands are given in Table 2. As shown, results
are confirmed by experimental measurements. Fig. 6 shows the
detailed time plot of 233.947 nm absorption band obtained from
and a C radical, from (C H) fission. The relative probability
5
of ␤(C C) to (C H) fission may be estimated from the yields
of products of either methyl or ␤-methallyl radicals relative to
those of C radicals giving:
5
␤
(C–C) : (C–H) = 9.5 ± 1.0 : 1
(9)
The analogous case of but-1-ene which also has ␤(C C) and
(
C H) bands were studied by direct photolysis. In the high tem-
perature experiments, in the absence of added inert gas, similar
initial radical concentrations were obtained. The kinetic anal-
ysis of the decay of ␤-methallyl radical is consistent with the
product analysis. A good second order plots for the bimolecular
combination were obtained. The 1/e decay time of the radical
was measured experimentally by using calibrated optical densi-
ties on the photographic plate were obtained with a seven-step
filter (Hilger F 1273) was 75 ± 5 ␮s.
Selected absorption regions from the original spectral image
of ␤-methallyl radical given in Fig. 1 have been examined
Fig. 6. Decay time measurements for 233.947 nm absorption band.

1
280
F. Bayrak c¸ eken et al. / Spectrochimica Acta Part A 67 (2007) 1276–1280
time measurements. This figure was plotted to compute the life-
time measurement of the band. The lifetime for this band was
calculated as 70 ␮s. In both cases measured and calculated decay
times were the same within the experimental error.
[2] M. Avrahami, P. Kebarle, J. Phys. Chem. 67 (1963) 354.
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Adaptation could correct for the spectrographic recording
errors by rebalancing the sensitivity across the motion mecha-
nism, removing the perceptual bias. In the same way, adaptation
can maintain perceptual constancy by recalibrating the visual
system whenever the observer changes. As in the case of adap-
tation, there are numerous other examples of these “spatial
contrast” effects in visual coding. A second implication of adap-
tation as far as refractive errors are concerned is that adaptation
might not only make the world look better. Adaptation also
makes another prediction about visual experience. As we adopt
to stimuli, we are often unaware of the profound sensitivity
changes that occurring, yet these become strikingly obvious
when we see the after-effect, and what we notice is how the
stimulus deviates from the images we are currently adapted to.
It is as is the visual system is building a predictive code for the
environment, discounting properties that fit with expectations
while highlighting the errors.
[
[
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[
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7