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Extinction coefficients for the β-methallyl radical
Volume 43, number 1
EXTINCTION COEFFICIENTS
CHEMICAL
PHYSICS
LETIERS
FOR THEfl-METHALLYL
I October IF76
RADICAL
Fuat B_AYRAK$“KEN 0.D. T. U. Teorik Rimya B~liinrii. Ankara. Turkey Rcccived 10 hlay 1976
The radicals formed in the flash photolysis of 2.mcthylbot-I-cne and iso-butcnc and subsequent re;rctions have been invcstigatcd by kinetic spec:roscopy and gas liquid chromatography. The cxtincfion cocfficicnts of the pmetlidlyl radical wcrc measured from the absorption bands. The decay of the radical was second order. The measured rate constant for ~methallyl combination was(2.6 2 0.3) X 10” mol-’ I2 s-l at 295 f 2 K.
I. Introduction The electronic
2. Experimental spectra of the P-methallyl
radical
been discovered in the far ultra-violet and its assignments discussed. The intense regions of absorption of the a- and P-methallyl radicals (238 nm) lie 13 nm lo long wavelengtll of that of the ally1 radical, and are easily distinguished from it [ I] . Although the two methallyl radicals have their intensity maxima in the same wavelength region, the individual vibrational structures arc quite characteristic, and furthermore the&methallyl radical absorbs in two sharp bands at 225.1 and 223.1 nm, which arc easily identiiied. Photochemical studies of simple olefins by mercury photosensitisation or direct photolysis reveal that the excited olefins formed in the primary step subsequently undergo a variety of processes, predominantly: isomcrisation; the formation of radicals through fission of C-H or C-C bonds (a or p to the double bond); or fission to molecular products. In photosensitized systems [2-71, most attention has been concentrated on isomerisation. With direct photolysis (8- 12 ] quantitative information on the decomposition mode has been derived from end product analysis. The most detailed work has been carried out for 2-methylbut-J-ene, and isobutene. The rate constant for fl-methallyl combination measured was (2.6 -+0.3) X lOlo mol-l P s-* at 295 t 2 K [ 131. has
The flasJ1 photolysis apparatus consisted of two parallel photo-tubes arranged in series, contained in a reflector which was flushed with nitrogen. The photoflash energies used were 780- J 125 J, and t!c flash duration was 2 ps. All quartz components were Spcctrosil grade. Spectra were recorded on a HiJger medium quartz spectrograph, slit-width 0.025 mm. The Ilford HP3 plates used were developed in llford PQ Universal developer. The spectra were photometered on a JoyceLoeblc double beam recording microdensitometer mode1 MKlJB. Calibrated optical densities on the photographic plate were obtained with a seven step filter (HiJgcr 1273).
3. Results and discussion In all runs, the known transitions [ 11 of the Pmethallyl radical was observed at the shortest delay times, and a typical result is shown in fig. 1. The flash source emits continuous radiation at wavelengths down to J $0 run. The absorption spectrum of 2methylbut-lene indicates that absorption of a quantum of radiation up to 200 nm results in the fonnation of excited molecules with a range of energies. The major transitions are: V + N (n* + n) the upper state being twisted 90” from the planar configuration at its energy minimum, and R + N where the upper
183
Volume 43, number 1
ire...:.
_
1 October 1976
CHEMICAL PHYSICS LETTERS
>:‘-
‘< ..’
230
.
,.
.
. ..
,.
2.0
nni.
.. . -
..;
&rnethallyl f:ig. I. Flash photolysis of iso-butcnc (2 torr) and N2 (700 corr) showins the spectrum of the P_mcthallyl radial and its decay. i.lash energy = I 125 J.
levels are Rydbcrg states. Dissociation may occur by one of several primary paths, where between 200 and 380 kJ mol-I, energy could appear as excitation of the radical products. Chcsick ilO] suggested the formation, by intersystem crossing, of ground state molecules with high vibrational excitation followed by dissociation, whereas Borrell and Cashmore [ 12 J favored the Rydbcrg state as the precursor of radical formation. The observed spectra clearly demonstrate that the predominant fate of the excited olefin is fission of tht? /3(C-C) bond to give methyl and &methallyl radicals,
photolysis flash is virtually over. Table 1 shows the rate constants and extinction coefticients for 2methylbut-lene and isobutene.
CH,CH,C(CH,) = CH; + i’H, + CH,C(CH,)=CH,. The decay of all bands could be fitted well to second order plots with slopes consistent with their relative optical densities at the respective wakelengrhs. A typical example is shown in fig. 2. No significant effect on the slope of such plots was produced by varying the pressure of the parent molecule by factors of up to 4, and nitrogen or argon pressure by factors of up to 3. Measurements commenced at 5 L(Sdelay, when the 184
Time
(ps)
Fig. 2. Second order plot for the decay of the 8-mcthallyl radical mwsurcd at 234.7 (curve A). 239.9 (B), 236.1 (C), 238.0 (D). 225.1 (E). 223.1 (F) nm.
Vofumc 43. number
1
CHEMICAL
PHYSICS LETTERS
I October 1976
Table 1 Compound -__------. isobutenc
2-methylbur-ltnc
k (IO4 A-’ s-‘) 6.70 6.20 5.50 5.10 3.60 2.90
236.1 238.0 225.1 223.1
1.73 1.60 1.42 1.32 0.93 0.75
6.70 6.25 5.50 5.05 3.50 2.90
234.7 239.9 236.1 238.0 225.1 223.1
1.73 1.61 1.42 1.31 0.9 I 0.75
References [ l] A.B. Callear and fl.K. Lee. Trans. Faraday Sot. 64 (1968) 2017. [2] F.f’. Lossine. D.G.M. hiarsdcn and J.B. Farmer. Can. J. Chem. 34 (1956) 701. [3] Xf. Avrahami and P. Kcbarle. J. Phys. Chem. 67 (1963) 354. 141 R.J. Cvctanovic and L.C. Doyle. J. Chcm. Phys. 37 (1962) 543. [S J J.R. Majer, B. hlile and J.C. Robb. Trans. Faraday Sot. 57 (1961) 1342. [6] RJ. Cvetanovic. fi.E. Gunning and E.W.R. Steacic, J. Chem. Phys. 5 1 (1969) 573.
km) 234.7
233.9
c (lo4 mot-r
P cm-r)
[ 7) M. Avrahami and P. Kcbarlc. J. Chcm. ISI 19! I101 1111
1121
I131
Phys.
38 (1963)
700. P. Borrell ani P. Cashmore. International Conference on Photochemistry, Munich (1967) Vol. I, p. 163. N. Hruumiya, S. Shida and S. Arai, Bull. Cbcm. Sot. Japan 38 (1965) 142. J.P. Chesick, J. Chcm. Phys. 45 (1966) 3934. P. Borrell and F.C. James. Trans. f’araday Sot. 62 (1966) 2452. B. Borrcll. P. Cashmorc. A. Cervenkcn and f:.C. James, Transitions Non Rad. Mol., 20th Reunion Sot. Cfrem. Phys. (1969) p. 229. F. Bayrakqeken. J.H. Brophy. R.D. Fink and J.E. Nicholas. J. Chcm. Sot. Faraday Trans. I69 (1973) 228.
185
EXTINCTION COEFFICIENTS
CHEMICAL
PHYSICS
LETIERS
FOR THEfl-METHALLYL
I October IF76
RADICAL
Fuat B_AYRAK$“KEN 0.D. T. U. Teorik Rimya B~liinrii. Ankara. Turkey Rcccived 10 hlay 1976
The radicals formed in the flash photolysis of 2.mcthylbot-I-cne and iso-butcnc and subsequent re;rctions have been invcstigatcd by kinetic spec:roscopy and gas liquid chromatography. The cxtincfion cocfficicnts of the pmetlidlyl radical wcrc measured from the absorption bands. The decay of the radical was second order. The measured rate constant for ~methallyl combination was(2.6 2 0.3) X 10” mol-’ I2 s-l at 295 f 2 K.
I. Introduction The electronic
2. Experimental spectra of the P-methallyl
radical
been discovered in the far ultra-violet and its assignments discussed. The intense regions of absorption of the a- and P-methallyl radicals (238 nm) lie 13 nm lo long wavelengtll of that of the ally1 radical, and are easily distinguished from it [ I] . Although the two methallyl radicals have their intensity maxima in the same wavelength region, the individual vibrational structures arc quite characteristic, and furthermore the&methallyl radical absorbs in two sharp bands at 225.1 and 223.1 nm, which arc easily identiiied. Photochemical studies of simple olefins by mercury photosensitisation or direct photolysis reveal that the excited olefins formed in the primary step subsequently undergo a variety of processes, predominantly: isomcrisation; the formation of radicals through fission of C-H or C-C bonds (a or p to the double bond); or fission to molecular products. In photosensitized systems [2-71, most attention has been concentrated on isomerisation. With direct photolysis (8- 12 ] quantitative information on the decomposition mode has been derived from end product analysis. The most detailed work has been carried out for 2-methylbut-J-ene, and isobutene. The rate constant for fl-methallyl combination measured was (2.6 -+0.3) X lOlo mol-l P s-* at 295 t 2 K [ 131. has
The flasJ1 photolysis apparatus consisted of two parallel photo-tubes arranged in series, contained in a reflector which was flushed with nitrogen. The photoflash energies used were 780- J 125 J, and t!c flash duration was 2 ps. All quartz components were Spcctrosil grade. Spectra were recorded on a HiJger medium quartz spectrograph, slit-width 0.025 mm. The Ilford HP3 plates used were developed in llford PQ Universal developer. The spectra were photometered on a JoyceLoeblc double beam recording microdensitometer mode1 MKlJB. Calibrated optical densities on the photographic plate were obtained with a seven step filter (HiJgcr 1273).
3. Results and discussion In all runs, the known transitions [ 11 of the Pmethallyl radical was observed at the shortest delay times, and a typical result is shown in fig. 1. The flash source emits continuous radiation at wavelengths down to J $0 run. The absorption spectrum of 2methylbut-lene indicates that absorption of a quantum of radiation up to 200 nm results in the fonnation of excited molecules with a range of energies. The major transitions are: V + N (n* + n) the upper state being twisted 90” from the planar configuration at its energy minimum, and R + N where the upper
183
Volume 43, number 1
ire...:.
_
1 October 1976
CHEMICAL PHYSICS LETTERS
>:‘-
‘< ..’
230
.
,.
.
. ..
,.
2.0
nni.
.. . -
..;
&rnethallyl f:ig. I. Flash photolysis of iso-butcnc (2 torr) and N2 (700 corr) showins the spectrum of the P_mcthallyl radial and its decay. i.lash energy = I 125 J.
levels are Rydbcrg states. Dissociation may occur by one of several primary paths, where between 200 and 380 kJ mol-I, energy could appear as excitation of the radical products. Chcsick ilO] suggested the formation, by intersystem crossing, of ground state molecules with high vibrational excitation followed by dissociation, whereas Borrell and Cashmore [ 12 J favored the Rydbcrg state as the precursor of radical formation. The observed spectra clearly demonstrate that the predominant fate of the excited olefin is fission of tht? /3(C-C) bond to give methyl and &methallyl radicals,
photolysis flash is virtually over. Table 1 shows the rate constants and extinction coefticients for 2methylbut-lene and isobutene.
CH,CH,C(CH,) = CH; + i’H, + CH,C(CH,)=CH,. The decay of all bands could be fitted well to second order plots with slopes consistent with their relative optical densities at the respective wakelengrhs. A typical example is shown in fig. 2. No significant effect on the slope of such plots was produced by varying the pressure of the parent molecule by factors of up to 4, and nitrogen or argon pressure by factors of up to 3. Measurements commenced at 5 L(Sdelay, when the 184
Time
(ps)
Fig. 2. Second order plot for the decay of the 8-mcthallyl radical mwsurcd at 234.7 (curve A). 239.9 (B), 236.1 (C), 238.0 (D). 225.1 (E). 223.1 (F) nm.
Vofumc 43. number
1
CHEMICAL
PHYSICS LETTERS
I October 1976
Table 1 Compound -__------. isobutenc
2-methylbur-ltnc
k (IO4 A-’ s-‘) 6.70 6.20 5.50 5.10 3.60 2.90
236.1 238.0 225.1 223.1
1.73 1.60 1.42 1.32 0.93 0.75
6.70 6.25 5.50 5.05 3.50 2.90
234.7 239.9 236.1 238.0 225.1 223.1
1.73 1.61 1.42 1.31 0.9 I 0.75
References [ l] A.B. Callear and fl.K. Lee. Trans. Faraday Sot. 64 (1968) 2017. [2] F.f’. Lossine. D.G.M. hiarsdcn and J.B. Farmer. Can. J. Chem. 34 (1956) 701. [3] Xf. Avrahami and P. Kcbarle. J. Phys. Chem. 67 (1963) 354. 141 R.J. Cvctanovic and L.C. Doyle. J. Chcm. Phys. 37 (1962) 543. [S J J.R. Majer, B. hlile and J.C. Robb. Trans. Faraday Sot. 57 (1961) 1342. [6] RJ. Cvetanovic. fi.E. Gunning and E.W.R. Steacic, J. Chem. Phys. 5 1 (1969) 573.
km) 234.7
233.9
c (lo4 mot-r
P cm-r)
[ 7) M. Avrahami and P. Kcbarlc. J. Chcm. ISI 19! I101 1111
1121
I131
Phys.
38 (1963)
700. P. Borrell ani P. Cashmore. International Conference on Photochemistry, Munich (1967) Vol. I, p. 163. N. Hruumiya, S. Shida and S. Arai, Bull. Cbcm. Sot. Japan 38 (1965) 142. J.P. Chesick, J. Chcm. Phys. 45 (1966) 3934. P. Borrell and F.C. James. Trans. f’araday Sot. 62 (1966) 2452. B. Borrcll. P. Cashmorc. A. Cervenkcn and f:.C. James, Transitions Non Rad. Mol., 20th Reunion Sot. Cfrem. Phys. (1969) p. 229. F. Bayrakqeken. J.H. Brophy. R.D. Fink and J.E. Nicholas. J. Chcm. Sot. Faraday Trans. I69 (1973) 228.
185