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High-resolution discrete absorption spectrum of α-methallyl free radical in the vapor phase
Spectrochimica Acta Part A 65 (2006) 143–146
High-resolution discrete absorption spectrum of ␣-methallyl free radical in the vapor phase Fuat Bayrakc¸eken a,∗ , Ziya Telatar b , Fikret Arı b , Lale Tunc¸y¨urek c , ˙Ipek Karaaslan d , Ali Yaman e a
˙ Department of Electronics Engineering, Yeditepe University, Istanbul 34755, Turkey Department of Electronics Engineering, Ankara University, Ankara 06100, Turkey c Department of Systems Engineering, Yeditepe University, Istanbul ˙ 34755, Turkey d Department of Physics, Yeditepe University, Istanbul ˙ 34755, Turkey e Department of Physics, Ankara University, Ankara 06100, Turkey
b
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 for the ␣-methallyl recombination was (3.5 ± 0.3) × 1010 mol−1 l s−1 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 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 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 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: [email protected] (F. Bayrakc¸eken).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.09.038
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 photosensitization 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 techniques because of insufficiencies in the human visual system and some degradation effects on the spectral image like noise, saturated 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
144
F. Bayrak¸ceken 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 justnoticeable difference or short intensity discrimination. If the observer is assumed to spend some time before making a decision, 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 resolution 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 therefore 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 contrast 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 digitization process. In image enhancement, the main objective is to make the processed image better in some sense than the unprocessed image. In this case, the ideal image depends on the context 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 measurement 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 applying digital filtering techniques like smoothing and averaging [7].
Fig. 1. Flash photolysis of 3-methylbut-1-ene (3.4 Torr) + N2 (700 Torr) showing the spectrum of ␣-methallyl radical and its decay, flash energy was 1125 J (original spectrum is taken by the permission of F. Bayrakc¸eken).
2. Experimental
yn (i, j) =
The flash photolysis apparatus consisted of two parallel phototubes 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 microdensitometer 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 measured with a McLeod gauge (up to 0.1 Torr), a spiral gauge (0.1–50 Torr), and above 50 Torr, with a mercury manometer. Products and residual parent molecules were dissolved in several alcohols and the resulting solution concentrated to 20 l. A sample of 4 l was used for analysis with a Griffin and GeorgeD6 gas chromatograph with a 12% polyethylene glycol column at 200 ◦ C.
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 (ROI) being analysed as follows:
The image of the original multi-resolution ␣-methallyl spectra 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: y(i, j) {y(i, j)}max
yb (i, k) = y(i, j),
k ∈ ROIb , b = 1, 2, . . . , 10
(2)
(3)
where yb ’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 of the matrix yb (for each vertical axes) as, M
x(n) =
1 yb (i, n) M
(4)
i=1
Each element of the vector x(n) defines a distinct wavelength and the amplitude value of that element gives its optical density 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 density information and plotted again as shown in Fig. 2, which is an example of the separated band for 20 s measurement. The
F. Bayrak¸ceken et al. / Spectrochimica Acta Part A 65 (2006) 143–146
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: LT = distance{tD(t) − tD(t) }
Table 1 The rate constant and extinction coefficients for 3-methylbut-1-ene and cis-pent2-ene Compound
k (104 A−1 s−1 )
λ (nm)
ε (104 mol−1 1 cm−1 )
3-Methylbut-1-ene
2.07 1.46 1.04 1.18 0.82
237.8 236.0 235.0 233.3 232.4
5.00 3.52 2.51 2.85 1.98
cis-Pent-2-ene
2.07 1.44 1.00 1.10 0.95
237.8 236.0 235.0 233.3 232.4
5.00 3.48 2.42 2.66 2.29
(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 ␣-Methallyl radical was observed in flash photolysis of 3-methylbut-1-ene, cis-pent-2-ene, trans-but-2-ene, and but-1ene. 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 231 nm, because the sensitivity of the photographic plate was very low. Figs. 1–4 show flash photolysis of 3-methylbut-1ene, absorption spectrum of the ␣-methallyl free radical, lifetime measurements for 237.729 nm absorption band, and 3D representation for computed five absorption bands, 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 process 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 of two ␣-methallyl radical was 3.5 × 1010 mol−1 l s−1 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-state absorption occurring in the flash
Fig. 2. Absorption spectrum of the ␣-methallyl free radical for 20 s delay measurement.
145
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 radicals. 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. 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 analytical 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
Fig. 3. Decay time measurements for 237.729 nm absorption band.
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F. Bayrak¸ceken 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)
232.189 233.249 234.959 236.053 237.729
121 122 114 113 103
Fig. 6. The plot of the optical density vs. time (decay of the ␣-methallyl radical) measured at 237.8 nm (curve A), 236 nm (B), 235 nm (C), 233.3 nm (D), 232.4 nm (E).
Fig. 4. 3D representation for computed five absorption bands.
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 section, computed decay times for five absorption bands are given in Table 2. In the experimental studies of a radical lifetime measurement, 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 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
Fig. 5. Second order plot for the decay of the ␣-methallyl radical (from Fig. 1), measured at 237.8 nm (curve A), 236 nm (B), 235 nm (C), 233.3 nm (D), 232.4 nm (E).
related regions more efficiently. Fig. 3 shows the detailed plot of 237.729 nm absorption band obtained from all time measurements 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”
[1] A.B. Callear, H.K. Lee, Trans. Faraday Soc. 64 (1968) 308. [2] A.B. Callear, H.K. Lee, Trans. Faraday Soc. 64 (1968) 2017. [3] F. Bayrakc¸eken, J.H. Brophy, R.D. Fink, J.E. Nicholas, J. Chem. Soc., Faraday Trans. I 69 (1973) 228. [4] F. Bayrakc¸eken, Chem. Phys. Lett. 43 (1976) 183. [5] F. Bayrakc¸eken, Chem. Phys. Lett. 74 (2) (1980) 183. [6] F. Bayrakc¸eken, L. Sagat, Chem. Phys. Lett. 159 (1) (1989) 109. [7] F. Bayrakc¸eken, F. Arı, Z. Telatar, Spectrochimica Acta, Part A (2005), in press. ¨ ugedik, ˙I. Karaaslan, Spectrochim. [8] F. Bayrakc¸eken, S. Aktas¸, A. Unl¨ Acta, Part A 59 (2003) 1663. [9] G.R. DeMar´e, J. Photochem. 30 (4) (1985) 407. [10] F. Bayrakc¸eken, J.E. Nicholas, J. Chem. Soc., Sect. B (1970) 691. [11] G. Porter, M.I. Savadatti, Spectrochim. Acta 22 (1966) 803. [12] K. Watanabe, J. Chem. Phys. 26 (1957) 542. [13] S.W. Benson, The Foundation of Chemical Kinetics, McGraw- Hill, New York, 1960, pp. 299–305.
High-resolution discrete absorption spectrum of ␣-methallyl free radical in the vapor phase Fuat Bayrakc¸eken a,∗ , Ziya Telatar b , Fikret Arı b , Lale Tunc¸y¨urek c , ˙Ipek Karaaslan d , Ali Yaman e a
˙ Department of Electronics Engineering, Yeditepe University, Istanbul 34755, Turkey Department of Electronics Engineering, Ankara University, Ankara 06100, Turkey c Department of Systems Engineering, Yeditepe University, Istanbul ˙ 34755, Turkey d Department of Physics, Yeditepe University, Istanbul ˙ 34755, Turkey e Department of Physics, Ankara University, Ankara 06100, Turkey
b
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 for the ␣-methallyl recombination was (3.5 ± 0.3) × 1010 mol−1 l s−1 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 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 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 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: [email protected] (F. Bayrakc¸eken).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.09.038
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 photosensitization 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 techniques because of insufficiencies in the human visual system and some degradation effects on the spectral image like noise, saturated 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
144
F. Bayrak¸ceken 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 justnoticeable difference or short intensity discrimination. If the observer is assumed to spend some time before making a decision, 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 resolution 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 therefore 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 contrast 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 digitization process. In image enhancement, the main objective is to make the processed image better in some sense than the unprocessed image. In this case, the ideal image depends on the context 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 measurement 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 applying digital filtering techniques like smoothing and averaging [7].
Fig. 1. Flash photolysis of 3-methylbut-1-ene (3.4 Torr) + N2 (700 Torr) showing the spectrum of ␣-methallyl radical and its decay, flash energy was 1125 J (original spectrum is taken by the permission of F. Bayrakc¸eken).
2. Experimental
yn (i, j) =
The flash photolysis apparatus consisted of two parallel phototubes 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 microdensitometer 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 measured with a McLeod gauge (up to 0.1 Torr), a spiral gauge (0.1–50 Torr), and above 50 Torr, with a mercury manometer. Products and residual parent molecules were dissolved in several alcohols and the resulting solution concentrated to 20 l. A sample of 4 l was used for analysis with a Griffin and GeorgeD6 gas chromatograph with a 12% polyethylene glycol column at 200 ◦ C.
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 (ROI) being analysed as follows:
The image of the original multi-resolution ␣-methallyl spectra 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: y(i, j) {y(i, j)}max
yb (i, k) = y(i, j),
k ∈ ROIb , b = 1, 2, . . . , 10
(2)
(3)
where yb ’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 of the matrix yb (for each vertical axes) as, M
x(n) =
1 yb (i, n) M
(4)
i=1
Each element of the vector x(n) defines a distinct wavelength and the amplitude value of that element gives its optical density 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 density information and plotted again as shown in Fig. 2, which is an example of the separated band for 20 s measurement. The
F. Bayrak¸ceken et al. / Spectrochimica Acta Part A 65 (2006) 143–146
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: LT = distance{tD(t) − tD(t) }
Table 1 The rate constant and extinction coefficients for 3-methylbut-1-ene and cis-pent2-ene Compound
k (104 A−1 s−1 )
λ (nm)
ε (104 mol−1 1 cm−1 )
3-Methylbut-1-ene
2.07 1.46 1.04 1.18 0.82
237.8 236.0 235.0 233.3 232.4
5.00 3.52 2.51 2.85 1.98
cis-Pent-2-ene
2.07 1.44 1.00 1.10 0.95
237.8 236.0 235.0 233.3 232.4
5.00 3.48 2.42 2.66 2.29
(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 ␣-Methallyl radical was observed in flash photolysis of 3-methylbut-1-ene, cis-pent-2-ene, trans-but-2-ene, and but-1ene. 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 231 nm, because the sensitivity of the photographic plate was very low. Figs. 1–4 show flash photolysis of 3-methylbut-1ene, absorption spectrum of the ␣-methallyl free radical, lifetime measurements for 237.729 nm absorption band, and 3D representation for computed five absorption bands, 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 process 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 of two ␣-methallyl radical was 3.5 × 1010 mol−1 l s−1 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-state absorption occurring in the flash
Fig. 2. Absorption spectrum of the ␣-methallyl free radical for 20 s delay measurement.
145
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 radicals. 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. 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 analytical 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
Fig. 3. Decay time measurements for 237.729 nm absorption band.
146
F. Bayrak¸ceken 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)
232.189 233.249 234.959 236.053 237.729
121 122 114 113 103
Fig. 6. The plot of the optical density vs. time (decay of the ␣-methallyl radical) measured at 237.8 nm (curve A), 236 nm (B), 235 nm (C), 233.3 nm (D), 232.4 nm (E).
Fig. 4. 3D representation for computed five absorption bands.
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 section, computed decay times for five absorption bands are given in Table 2. In the experimental studies of a radical lifetime measurement, 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 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
Fig. 5. Second order plot for the decay of the ␣-methallyl radical (from Fig. 1), measured at 237.8 nm (curve A), 236 nm (B), 235 nm (C), 233.3 nm (D), 232.4 nm (E).
related regions more efficiently. Fig. 3 shows the detailed plot of 237.729 nm absorption band obtained from all time measurements 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”
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