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Localized electron to radical conversion in X-irradiated single crystals of 1,6-hexanediol and 1,8-octanediol
Radiat. Phys. Chem. Vol. 29, No. 6, pp. 439-445, 1987 Int. J. Radiat. AppL lnstrum. Part C Printed in Great Britain. All rights reserved
0146-5724/87 $3.00 +0.00 Copyright © 1987 Pergamon Journals Ltd
LOCALIZED ELECTRON TO RADICAL CONVERSION IN X-IRRADIATED SINGLE CRYSTALS OF 1,6-HEXANEDIOL AND 1,8-OCTANEDIOL MIKAEL LINDGREN, ANDERS LUND, MASAAKIOGASAWARAl and PER-OLOF SAMSKOG • Institute of Physics and Measurement Technology, Link6ping University, S-581 83 Link6ping, Sweden and 'Faculty of Engineering, Hokkaido University, Sapporo 060, Japan (Received 20 November 1986)
Abstract--The formation and reactions of radicals in X-irradiated single crystals of 1,6-hexanediol and 1,8-octanediol have been examined. ESR experiments have been performed between 4.2 K and room temperature in both the X- and Q-bands and correlated with kinetic data from pulse radiolysis. The primary oxidized and reduced species, localized cations and electrons, decompose to neutral hydroxyalkyl and alkyl radicals. Based on purely mechanistic aspects it is proposed that the localized electron, detected in this class of hydroxy-compounds, consists of a molecular anion stabilized by a neighbouring OH group. This anion is formed by electron attachment to a molecule within the hydrogen-bonded network. The activation energy for the decay of this species is determined by pulse radiolysis and correlated with the ESR experiments. Conversions between radicals occur thermally and by illumination, in agreement with results reported previously.
I. INTRODUCI'ION
Localized electrons are believed to be the primary reducing species formed by irradiation of organic compounds containing hydroxyl groups.~1-3)Recently, anisotropic hyperfine interactions were observed for electrons localized in single crystals of X-irradiated 1,6-hexane and 1,8-octanediol at 4.2 K. ~4) One fundamental character of the localized electrons is their sensitivity to visible light. Illumination with an ordinary slide projector lamp for only a few minutes is sufficient to remove any optical or ESR evidence of their existence, o,~) Based on the properties obtained from optical and ESR studies theoretical models have been proposed. In liquid media the electron is believed to be solvated by molecules containing polar groups. Detailed geometrical structures of such cavities in aqueous glasses have been determined from electron spin-echo modulation analysis. ¢7) This model requires several polar groups (I> 6). However, in crystals of simple diols, the hydroxyl groups form a two-dimensional layer of dipoles, cs) and it is improbable that the hydroxyl groups could rearrange to construct such a cavity. In the small-polaron model, c*)the excess electron is coupled to an intramolecular vibrational mode related to the OH-bond. This theoretical model applies to a simple chain of hydrogen-bonded alcohol molecules and a complicated three-dimensional cavity does not need to be built up. The second (cationic) product of the reaction: X
M .... , e - + M
+
(1)
has, so far, not been detected in these systems. It is evident that cations formed by such a reaction could be paramagnetic but probably these are very unstable and decompose to neutral radicals. The secondary reaction products, i.e. free radicals, produced by decomposition of cations or by reaction with the localized electron, have been extensively studied in numerous papers. The alkoxy radical which exhibits a strong G-value anisotropy, has been identified in many crystals of polyhydroxy compounds/~°-m2) However, alkoxy radicals are not stabilized in diols either at 4.2 or 77 K. ~6"n~ In a study of polycrystalline samples of Xirradiated n-alkanediols at temperatures above 77 K it has been reported "3J that mutal conversion occurs thermally and optically between two different radicals: --CH:--~H--CH2-, alkylradical
an
" -CH2--~H-OH
u.v. light
hydroxyalkylradical
(2)
The activation energy of the thermal decay from the alkyl to the hydroxyalkyl radical is, from kinetic data, calculated to about 7 kcal/mol. "3~ In our measurements on single crystals of irradiated 1,8-octanediol and 1,6-hexanediol at 4.2 K we could detect the alkyl radicals together with the radical species previously attributed to the localized electron: 4~ When the samples are bleached at 4.2 K the signal due to the hydroxyalkyl radical increases as the electron signal decreases. No change of the intensity is observed for the alkyl radical. In 1,6-hexanediol the relative yield of the alkyl radical was low compared to that in 1,8-octanediol.
439
440
MIKAELLINDGKENet al.
We have repeated the experiments performed by Hama e t al. ¢13) at 77 K and room temperature but this time in single crystals. For both diols we obtained similar results together with a new phenomenon. It was possible to produce alkyl radicals by irradiation with u.v. light only, without previous irradiation with photons of higher energy, e.g. X- or y-rays.
2. EXPERIMENTAL
Crystal preparation and the ESR cryogenic equipment for 4.2 K measurements in the X band together with the apparatus used for X-irradiation have been described elsewhere. ~4) The crystal structure of 1,6-hexanediol is given in a recent paper38) The ESR spectrometer operating at 35 GHz (Q-band) and the method of irradiation at 26 K with 50 kV X-rays have been described elsewhere3 ~4) In the pulse-radiolysis experiments the single crystals were placed in a cryostat and irradiated from opposite sides by single 3 ns pulses of 800 keV electrons from a double beam Febetron 708. Pulsed light from a 450 W Xenon lamp was used to detect the transient optical absorption. The light passed the sample perpendicular to the electron beams. A Tektronix digitizer, connected to a PDP 11-03 minicomputer, was used to collect and analyse the transient signal from the photomultiplier. The dose was measured using calorimetry. A more detailed description of the pulse radiolysis system is given in reference, o5) The photobleaching experiments at 4.2 K were carried out with a slide projector lamp together with an Oriel filter set allowing for depletion of selected wavelengths. The u.v.-irradiation at 77 K was performed using
a Bruker ER 200 ESR spectrometer and a Varian Xenon lamp operating at 300 W. In the ESR-experiments, an a*, b, c crystal axis system was chosen to coincide with that used previously. ~8~ 3. RESULTS 3.1. L o w temperature, 4 . 2 K
Immediately after X-irradiation of 1,8-octanediol a signal due to the localized electron (e~) and signals due to the hydroxyalkyl and alkyl radicals were observed. The spectra from the alkyl radical were broad compared to the others. The hyperfine interaction with four t - and one ~-proton gave a total width of about 140 G c6) which made this signal easy to identify since the width of the spectra due to the hydroxyalkyl radical and the e~: never exceeded 90 G. The e~ signal disappeared when the sample was illuminated with light of wavelength longer than 700 nm, and the signal from the hydroxyalkyl radical increased with about the same intensity. In Figs la and b, spectra are shown before and after photobleaching respectively, with the magnetic field projected in the a'c-plane 45 ° from the a*-axis (compare with Fig. 2 for hexanedioi). A stick plot diagram due to the hydroxyalkyl radical is shown below Fig. lb. In the case of 1,6-hexanediol the signal due to the alkyl radical was identified by small wiggles in the outer wings of the spectra. In Figs 2a and b spectra are shown before and after photobleaching respectively, at some different directions in the a'c-plane. With the magnetic field parallel to the a*-axis it became clear that the e& is completely converted to the hydroxyalkyl radical, and this only after 2 rain of illumination with light of wavelength longer than 700 nm. On comparing the spectra in Figs 2a and b
50G
(a)
{b}
Fig. !. X-bond ESR spectra of 1,8-octanediol, irradiated and measured at 4.2 K. The magnetic field is projected in the a'c-plane 45° from the a*-axis, (a) in the dark; Co) after photobleaching, 2 ~ 700 nm.
441
Electron to radical conversion
(b) B#o
"
30"
60"
B#c
120"
160"
G-2.0023
G=2.~23
Fig. 2. X-band ESR spectra of 1,6-hexanediol irradiated and measured at 4.2 K. The magnetic field is projected in the a'c-plane, (a) in the dark; (b) after photobleaching, g >I 700 nm.
when the magnetic field is projected along the a*-axis and 160° from the a*-axis respectively, i.e. after almost a full rotation, one can conclude that no localized electrons have been converted to hydroxyalkyl radicals during the measurement in the dark. Thus, it is tempting to assign all stronger lines in Fig. 2a to the e~. However, when the magnetic field is projected near the b or the e axis the spectrum due to the hydroxyalkyl radical gives rise to rather strong narrow lines. We cannot exclude that this species is also formed in the primary radiation process and hence, contributes to the signal in certain orientations. This is supported by the u.v.-illumination experiments performed at higher temperatures and the Q-band data described in following sections. In addition, an asymmetry of the spectra when the magnetic field is projected in the a*c plane supports the assumption that two species are present with slightly different g-factors. On rotating the crystal in the a ' b - and be-plane the spectra are identical at equal positive or negative angles from either of the axes as predicted from crystal symmetry. (s) This also causes site splitting which complicates the analysis. The overlapping of lines from different sites together with the contribution from the hydroxyalkyl radical, made it impossible to follow the e~ lines through R.P.C. 29/6--C
these rotations. In Figs 3a and b spectra are shown with the magnetic field parallel to the b-axis before and after illumination, respectively. In some orientations, the signal was recorded with a digitizer and a computer before and after illumination (2 > 700nm), respectively. By double integration, the total intensity could be evaluated. For hexanediol 93% of the total intensity remained, while for octanediol 90% was left after illumination. This implies a very efficient conversion from the ef~ to the hydroxyalkyl radical. 3.2. From 26 to 100K
In an attempt to obtain more information about the e~ signal some experiments were performed in the Q-band between 26 and 100 K for a single crystal of 1,6-hexanediol. The aim was to suppress forbidden transitions, which contribute considerably for couplings in the intermediate region (8-13G in the X-band). Also, different g-values allow a larger separation of different radicals, when the magnetic field is higher. Higher resolution gave no further information of the e~ but the signal due to the hydroxyalkyi radical was present at 26 K. This contributed considerably in spectra with the magnetic field projected
442
MIKA£L LINI~3REN e t
al,
50G
(b)
6 - 2.0023 Fig. 3. X-band ESR spectra of 1,6-hexanediol irradiated and measured at 4.2 K. The magnetic field is projected along the b-axis, (a) in the dark; (b) after photobleaching, ). >/700 nm.
near the b and c axes, as in the X-band measurements at 4.2 K. When the sample was slowly warmed up, the e& signal gradually disappeared. The disappearance between 75 and 85 K is in perfect agreement with the pulse-radiolysis experiments described below. In contrast to the photo-induced decomposition, which leads to the hydroxyalkyl radical, the thermally-induced decomposition gave no other paramagnetic species. This assured us that the hydroxyalkyl radical which was observed at certain orientations (e.g. along the b-axis) was present also immediately after X-irradiation at 26 K.
Table I. Kinetic data from the pulse radiolysis of 1,8-octanedioland 1,6-hexanediol E, (kml tool- ' ) 1,8-octancdiol* 1,6-hexanediol
~,
3.9 4.3
* Taken from Samskog et
(nm) 625 525
al. (t)
3.4. Pulse radiolysis
The pulse-radiolysis data of single crystals of 1,8-octanediol have been reported previously/6) In Table 1 we summarize the results together with those obtained with 1,6-hexanediol. The activation energies for the e ~ in the diois give a half life of about 3 rain
3.3. From 7 7 K to room temperature
To check the validity of the interpretation of Hama
0.25
et al.,(l~) who observed an interconversion between
the alkyl and the hydroxyalkyl radical in polycrystalline diols, their experiments were repeated in single crystals of 1,6-hexanediol and 1,8-octanediol. The results were essentially the same as reported previously, oe Interestingly, the relative intensities between the alkyl and the hydroxyalkyl radicals were the same at 77 K as when the sample was irradiated, illuminated and measured at 4.2 K. It was also observed that the alkyl radical could be generated by illumination with u.v.-light at 77 K without previous X-irradiation. The yield was very low with a resulting poor signal but this phenomenon was supported by annealing the crystals to room temperature where the more easily detected hydroxyalkyl radical appeared. The extent of alkyl radical formation produced by u.v.-light, was estimated to be ten times more efficient if the sample had been previously irradiated with X-rays.
/f'\ |
0.20
K 0 0.15
//
l
I o.I
I 4oo
I 5oo
I 6oo
I 7oo
I coo
Wave~n~ (rim) Fig. 4. Transient absorption spectrum of e ~ in a single crystal of 1,6-hexanediol recorded at 268 K.
Electron to radical conversion
16
C
~e 3.4
3.6
I
L
L
J
I
3.8
4.0
4,2
4.4
4,6
7"-~( K-~x 10-3)
Fig. 5. Arrhenius plot of the transient species in 1,6-hexanediol.
at 77 K and at temperatures below 70 K the localized electron can be stabilized for hours. This is in excellent agreement with the ESR data which showed that the e ~ signal "disappeared" between 75 and 85 K. In Fig. 4 the transient absorption is shown, recorded at 268 K. An Arrhenius plot of the kinetics is shown in Fig. 5. 4. DISCUSSION 4.1. Radical structures In single crystals containing hydroxyl groups, no detailed structure of the electron trap has been proposed and verified experimentally. The overlapping ESR-signal from the hydroxyalkyl radical also made it impossible in this study. However, a model based on purely mechanistic aspects will be given below. The radicals discussed here have been identified previously.(4.6,~3)In a recent paper ~s)a detailed description of the radical structure for the hydroxyalkyl
443
radical at room temperature was given. It is a n-type radical characterized by a g-value anisotropy in the range of 2.0023-2.0044 and hyperfine interaction with one "- and three /~-protons. The smallest /~-proton interaction is almost purely anisotropic and it was suggested to be due to a hydroxyl proton. The alkyl radical has two pairs of/~-protons with isotropic hyperfine interactions of ~28-30 G and 36--40 G. The ,,-proton has principal values of the hyperfine interaction tensor estimated to 9, 20 and 32 G. (6) It was not possible to make a more detailed analysis, either by us or previously,¢6'13) since the formation of the radical can probably occur by H abstraction from different carbon atoms within the chain, giving rise to overlapping spectra due to similar but not identical radicals. However, the identification as alkyl radicals can be safely made from the width of the spectra, as discussed in a previous section. The ESR parameters of the alkyl radicals are summarized in Table 2.
4.2. Primary reaction mechanisms Correlating the kinetic data from the pulseradiolysis measurements, with ESR experiments where the temperature can be varied, it is possible to identify and follow the primary radical reactions. The primary step in the ionizing process is the formation of an electron and a cation (reaction 1). In single crystals of saturated organic compounds, the cation has never been observed by means of ESR. It is believed to decompose rather quickly, even at 4.2 K, to form neutral radicals by abstraction of a proton. This is probably the mechanism for forming the radicals, observed already at 4.2 K, in our system of simple diols. The mechanisms leading to the generation of localized electrons are rather obscure. In liquids or glasses
Table2. ESR parametersfor the alkylradicalsin 1,6-hexane-and 1,8-octanediol.Hyperfinesplittings are givenin puss Direction cosines Principal values a* b c Tensor Hydroxyalkylradical(s) g 2.0025 0.9907 -0.0272 -0.1333 2.0033 - 0 . 1 3 2 8 -0.4066 -0.9039 2.004! 0.0296 - 0.9132 0.4065 --CH2(~HOH A, -26.2 0.0039 -0.3914 0.9202 - I 1.6 0.9999 0.0044 -0.0024 -4.1 0.0031 -0.9202 -0.3914 Apt 27.3 -0.0373 -0.9636 0.2647 24.0 -0.7367 0.2055 0.6442 23.1 0.6752 0.1710 0.7175 Ar~ 35.9 -0.0497 0.9429 -0.3292 32.8 0.9313 0.1628 0.3258 31.7 -0.3608 0.2904 0.8863 A~ -5.3 -0.0418 0.7548 -0.6546 - 3.4 0.9991 0.0390 - 0.0188 4.4 0.0113 -0.6548 -0.7557 Alkyl radicalt~'~3) g 2.0022.004 --CH2(~HCH2A, - 32 -20 -9 2A#~ 28-30 2A~ 36-40
444
MIKAELL1NDGRENetal.
excess electrons are often considered to range through the medium until they are self-trapped. However, as mentioned above, as the hydroxyl groups form a fixed two-dimensional layer of dipoles in crystals of alkanediols, groups of dipoles arranged at favourable orientations for three-dimensional traps are less probable. An alternative mechanism is that the electron is localized at a defect of hydrogen bonding in the form of an anion which is stabilized by hydrogen bonding and by a neighbouring methylene group, probably of the lower layer (see insert below). However, since no detailed model has been verified experimentally, we regard this interpretation as tentative. This model is very similar either to the "small polaron" model proposed by Bush and Funabashi ~9) or the "diffuse anion" model by Webster. ¢m6) In the former, the excess electron is coupled to an intramolecular vibrational mode related to the OH bond and it will explain the structure observed in the absorption spectra of the localized electron at 4.2 K. ¢5) Decomposition following illumination can be incorporated into this model. Formation of the hydroxyalkyl radical is caused by loss of molecular hydrogen from the hydrogen-bonded anion defined above, .)4/ H
,HI H
(3) preserving the number of unpaired spins. This is consistent with the fact that the total intensity, due to the neutral radicals after illumination, is comparable to that of the intensity directly after X-irradiation at 4.2 K, containing the em~ signal. The disappearance of the ek~ upon heating can be explained by capture of an electron by the secondary cation. e- + ~CH2-OH ~ --* ~CH2-OH + H"
(4)
We emphasize that the conversion of localized electrons to neutral radicals is known not only in irradiated crystals of polyhydroxy compounds but also in irradiated alcohol glasses in general: localized electrons in methanol, ethanol or n-propanol disappear thermally in the dark at, for example, 100 K without forming detectable additional free radicals and they are photobleached to form hydroxyalkyl radicals. ~7) So far no explanation has been given for these facts on the basis of the cavity model, in which the excess electron attains a localized state occupying a void region, with a solvation shell around the localization site. The above considerations, however, do not preclude the possibility that electrons are captured interstitially in a cavity in the crystals. Previously, we
studied optical absorption spectra of the localized electron at 4.2 K and concluded that two distinctly different traps existed--shallow and deep. ~5)The absorption spectra of the shallowly trapped electron (with the maximum at ~ 1 3 0 0 n m in the case of octanediol and at ~ 1000 nm in the case of hexanediol) were extremely sensitive to light and their yields were relatively low. It is probable that the electrons of this type reside in interstitial cavities in the crystals. 4.3. Summary
The nature of the primary products induced by X-irradiation and the radicals formed from them, have been investigated in 1,6-hexanediol and 1,8octanediol by ESR and by pulse radiolysis. The diols have a simpler chemical and crystallographic structure than carbohydrates. Thus, 1,6hexanediol has a layer structure with the molecules connected by hydrogen bonds into infinite chains. ~s~ In carbohydrates, the hydrogen bonds extend in three dimensions. It is thought that in the latter case the primary reduction product is an excess electron localized in a cavity with the O - H dipoles of the surrounding molecules producing an energetically favourable trap. The ESR data are not sufficient precise to construct a definitive model for the reduction product in the diol case although the small polaron or diffuse anion are possible candidates. In other respects, i.e. the optical absorption spectra, the easy optical bleaching and the conversion to hydroxyalkyl radicals are analogous to those found for localized electrons in organic glasses and crystals. The alkoxy radical, which is usually assigned to an oxidation product, was not detected. Alkyl and hydroxyalkyl radicals were detected at 4.2 K even before photobleaching. It is possible that these radicals are formed in a secondary reaction of the primary (cationic) oxidation product. However, the different yields of hydroxyalkyl and alkyl radicals in 1,6hexanediol and 1,8-octanediol are difficult to account for and would seem to imply that the mode of fragmentation is sensitive to the chain length. Acknowledgements--One of the authors (M.O.) is supported by a Grant-in-Aid No. 61540310 for Scientific Research from the ministry of Education of Japan. We are indebted to Dr J. Byber8, Department of Physical Chemistry University of Aarhus, Denmark for the Q band ESR measurements at 26 K.
REFERENCES
1. H. C. Box, E. E. Budzinski and H. G. Freund, J. Chem. Phys. 1978, 6g, 1309. 2. H. C. Box, E. E. Budzinski, H. G. Freund and R. Potter, J. Chem. Phys. 1979, 70, 1320. 3. E. E. Budzinski, W. R. Potter, G. Potienko and H. C. Box, J. Chem. Phys. 1979, 70, 5040. 4. M. Ogasawara, M. Lindgren, A. Lund and G. Niisson, Chem. Phys. Lett. 1985, 117, 254. 5. M. Ogautwara, O. Claeson, H. Yoshida and A. Lund, J. Phys. Chem. 1984, 88, 5004.
Electron to radical conversion 6. P.-O. Samskog, A. Lund and G. Nilsson, Chem. Phys. Lett. 1981, 79, 447. 7. L. Koran, Acc. Chem. Res. 1981, 14, 438. 8. M. Lindgrcn, T. Gustafsson, J. Westerling and A. Lund, Chem. Phys. 1986, 106, 441. 9. R. L. Bush and K. Funabashi, J. Chem. Soc. Faraday. Trans. 2. 1977, 73, 274. 10. P.-O. Samskog, L. D. Kispcrt and A. Lund, J. Chem. Phys. 1982, 77, 2330. ! 1. H. C. Box and E. E. Budzinski, J. Chem. Phys. 1983, 79, 4142.
445
12. E. E. Budzinski and H. C. Box, J. Chem. Phys. 1985, 82, 3487. 13. Y. Hama, K. Hamanaka and T. Horinchi, Radiat. Phys. Chem. 1979, 13, 13. 14. J. R. Byberg and S. J. K. Jensen, J. Phys. Chem. 1970, 52, 5902. 15. A. Lund, G. Nilsson and P.-O. Samskog, Radiat. Phys. Chem. 1986, 27, 111. 16. B. Webster, J. Phys. Chem. 1975, 79, 2809. 17. A. J. Swallow, Radiation Chemistry, p. 194. Longman, London, 1973.
0146-5724/87 $3.00 +0.00 Copyright © 1987 Pergamon Journals Ltd
LOCALIZED ELECTRON TO RADICAL CONVERSION IN X-IRRADIATED SINGLE CRYSTALS OF 1,6-HEXANEDIOL AND 1,8-OCTANEDIOL MIKAEL LINDGREN, ANDERS LUND, MASAAKIOGASAWARAl and PER-OLOF SAMSKOG • Institute of Physics and Measurement Technology, Link6ping University, S-581 83 Link6ping, Sweden and 'Faculty of Engineering, Hokkaido University, Sapporo 060, Japan (Received 20 November 1986)
Abstract--The formation and reactions of radicals in X-irradiated single crystals of 1,6-hexanediol and 1,8-octanediol have been examined. ESR experiments have been performed between 4.2 K and room temperature in both the X- and Q-bands and correlated with kinetic data from pulse radiolysis. The primary oxidized and reduced species, localized cations and electrons, decompose to neutral hydroxyalkyl and alkyl radicals. Based on purely mechanistic aspects it is proposed that the localized electron, detected in this class of hydroxy-compounds, consists of a molecular anion stabilized by a neighbouring OH group. This anion is formed by electron attachment to a molecule within the hydrogen-bonded network. The activation energy for the decay of this species is determined by pulse radiolysis and correlated with the ESR experiments. Conversions between radicals occur thermally and by illumination, in agreement with results reported previously.
I. INTRODUCI'ION
Localized electrons are believed to be the primary reducing species formed by irradiation of organic compounds containing hydroxyl groups.~1-3)Recently, anisotropic hyperfine interactions were observed for electrons localized in single crystals of X-irradiated 1,6-hexane and 1,8-octanediol at 4.2 K. ~4) One fundamental character of the localized electrons is their sensitivity to visible light. Illumination with an ordinary slide projector lamp for only a few minutes is sufficient to remove any optical or ESR evidence of their existence, o,~) Based on the properties obtained from optical and ESR studies theoretical models have been proposed. In liquid media the electron is believed to be solvated by molecules containing polar groups. Detailed geometrical structures of such cavities in aqueous glasses have been determined from electron spin-echo modulation analysis. ¢7) This model requires several polar groups (I> 6). However, in crystals of simple diols, the hydroxyl groups form a two-dimensional layer of dipoles, cs) and it is improbable that the hydroxyl groups could rearrange to construct such a cavity. In the small-polaron model, c*)the excess electron is coupled to an intramolecular vibrational mode related to the OH-bond. This theoretical model applies to a simple chain of hydrogen-bonded alcohol molecules and a complicated three-dimensional cavity does not need to be built up. The second (cationic) product of the reaction: X
M .... , e - + M
+
(1)
has, so far, not been detected in these systems. It is evident that cations formed by such a reaction could be paramagnetic but probably these are very unstable and decompose to neutral radicals. The secondary reaction products, i.e. free radicals, produced by decomposition of cations or by reaction with the localized electron, have been extensively studied in numerous papers. The alkoxy radical which exhibits a strong G-value anisotropy, has been identified in many crystals of polyhydroxy compounds/~°-m2) However, alkoxy radicals are not stabilized in diols either at 4.2 or 77 K. ~6"n~ In a study of polycrystalline samples of Xirradiated n-alkanediols at temperatures above 77 K it has been reported "3J that mutal conversion occurs thermally and optically between two different radicals: --CH:--~H--CH2-, alkylradical
an
" -CH2--~H-OH
u.v. light
hydroxyalkylradical
(2)
The activation energy of the thermal decay from the alkyl to the hydroxyalkyl radical is, from kinetic data, calculated to about 7 kcal/mol. "3~ In our measurements on single crystals of irradiated 1,8-octanediol and 1,6-hexanediol at 4.2 K we could detect the alkyl radicals together with the radical species previously attributed to the localized electron: 4~ When the samples are bleached at 4.2 K the signal due to the hydroxyalkyl radical increases as the electron signal decreases. No change of the intensity is observed for the alkyl radical. In 1,6-hexanediol the relative yield of the alkyl radical was low compared to that in 1,8-octanediol.
439
440
MIKAELLINDGKENet al.
We have repeated the experiments performed by Hama e t al. ¢13) at 77 K and room temperature but this time in single crystals. For both diols we obtained similar results together with a new phenomenon. It was possible to produce alkyl radicals by irradiation with u.v. light only, without previous irradiation with photons of higher energy, e.g. X- or y-rays.
2. EXPERIMENTAL
Crystal preparation and the ESR cryogenic equipment for 4.2 K measurements in the X band together with the apparatus used for X-irradiation have been described elsewhere. ~4) The crystal structure of 1,6-hexanediol is given in a recent paper38) The ESR spectrometer operating at 35 GHz (Q-band) and the method of irradiation at 26 K with 50 kV X-rays have been described elsewhere3 ~4) In the pulse-radiolysis experiments the single crystals were placed in a cryostat and irradiated from opposite sides by single 3 ns pulses of 800 keV electrons from a double beam Febetron 708. Pulsed light from a 450 W Xenon lamp was used to detect the transient optical absorption. The light passed the sample perpendicular to the electron beams. A Tektronix digitizer, connected to a PDP 11-03 minicomputer, was used to collect and analyse the transient signal from the photomultiplier. The dose was measured using calorimetry. A more detailed description of the pulse radiolysis system is given in reference, o5) The photobleaching experiments at 4.2 K were carried out with a slide projector lamp together with an Oriel filter set allowing for depletion of selected wavelengths. The u.v.-irradiation at 77 K was performed using
a Bruker ER 200 ESR spectrometer and a Varian Xenon lamp operating at 300 W. In the ESR-experiments, an a*, b, c crystal axis system was chosen to coincide with that used previously. ~8~ 3. RESULTS 3.1. L o w temperature, 4 . 2 K
Immediately after X-irradiation of 1,8-octanediol a signal due to the localized electron (e~) and signals due to the hydroxyalkyl and alkyl radicals were observed. The spectra from the alkyl radical were broad compared to the others. The hyperfine interaction with four t - and one ~-proton gave a total width of about 140 G c6) which made this signal easy to identify since the width of the spectra due to the hydroxyalkyl radical and the e~: never exceeded 90 G. The e~ signal disappeared when the sample was illuminated with light of wavelength longer than 700 nm, and the signal from the hydroxyalkyl radical increased with about the same intensity. In Figs la and b, spectra are shown before and after photobleaching respectively, with the magnetic field projected in the a'c-plane 45 ° from the a*-axis (compare with Fig. 2 for hexanedioi). A stick plot diagram due to the hydroxyalkyl radical is shown below Fig. lb. In the case of 1,6-hexanediol the signal due to the alkyl radical was identified by small wiggles in the outer wings of the spectra. In Figs 2a and b spectra are shown before and after photobleaching respectively, at some different directions in the a'c-plane. With the magnetic field parallel to the a*-axis it became clear that the e& is completely converted to the hydroxyalkyl radical, and this only after 2 rain of illumination with light of wavelength longer than 700 nm. On comparing the spectra in Figs 2a and b
50G
(a)
{b}
Fig. !. X-bond ESR spectra of 1,8-octanediol, irradiated and measured at 4.2 K. The magnetic field is projected in the a'c-plane 45° from the a*-axis, (a) in the dark; Co) after photobleaching, 2 ~ 700 nm.
441
Electron to radical conversion
(b) B#o
"
30"
60"
B#c
120"
160"
G-2.0023
G=2.~23
Fig. 2. X-band ESR spectra of 1,6-hexanediol irradiated and measured at 4.2 K. The magnetic field is projected in the a'c-plane, (a) in the dark; (b) after photobleaching, g >I 700 nm.
when the magnetic field is projected along the a*-axis and 160° from the a*-axis respectively, i.e. after almost a full rotation, one can conclude that no localized electrons have been converted to hydroxyalkyl radicals during the measurement in the dark. Thus, it is tempting to assign all stronger lines in Fig. 2a to the e~. However, when the magnetic field is projected near the b or the e axis the spectrum due to the hydroxyalkyl radical gives rise to rather strong narrow lines. We cannot exclude that this species is also formed in the primary radiation process and hence, contributes to the signal in certain orientations. This is supported by the u.v.-illumination experiments performed at higher temperatures and the Q-band data described in following sections. In addition, an asymmetry of the spectra when the magnetic field is projected in the a*c plane supports the assumption that two species are present with slightly different g-factors. On rotating the crystal in the a ' b - and be-plane the spectra are identical at equal positive or negative angles from either of the axes as predicted from crystal symmetry. (s) This also causes site splitting which complicates the analysis. The overlapping of lines from different sites together with the contribution from the hydroxyalkyl radical, made it impossible to follow the e~ lines through R.P.C. 29/6--C
these rotations. In Figs 3a and b spectra are shown with the magnetic field parallel to the b-axis before and after illumination, respectively. In some orientations, the signal was recorded with a digitizer and a computer before and after illumination (2 > 700nm), respectively. By double integration, the total intensity could be evaluated. For hexanediol 93% of the total intensity remained, while for octanediol 90% was left after illumination. This implies a very efficient conversion from the ef~ to the hydroxyalkyl radical. 3.2. From 26 to 100K
In an attempt to obtain more information about the e~ signal some experiments were performed in the Q-band between 26 and 100 K for a single crystal of 1,6-hexanediol. The aim was to suppress forbidden transitions, which contribute considerably for couplings in the intermediate region (8-13G in the X-band). Also, different g-values allow a larger separation of different radicals, when the magnetic field is higher. Higher resolution gave no further information of the e~ but the signal due to the hydroxyalkyi radical was present at 26 K. This contributed considerably in spectra with the magnetic field projected
442
MIKA£L LINI~3REN e t
al,
50G
(b)
6 - 2.0023 Fig. 3. X-band ESR spectra of 1,6-hexanediol irradiated and measured at 4.2 K. The magnetic field is projected along the b-axis, (a) in the dark; (b) after photobleaching, ). >/700 nm.
near the b and c axes, as in the X-band measurements at 4.2 K. When the sample was slowly warmed up, the e& signal gradually disappeared. The disappearance between 75 and 85 K is in perfect agreement with the pulse-radiolysis experiments described below. In contrast to the photo-induced decomposition, which leads to the hydroxyalkyl radical, the thermally-induced decomposition gave no other paramagnetic species. This assured us that the hydroxyalkyl radical which was observed at certain orientations (e.g. along the b-axis) was present also immediately after X-irradiation at 26 K.
Table I. Kinetic data from the pulse radiolysis of 1,8-octanedioland 1,6-hexanediol E, (kml tool- ' ) 1,8-octancdiol* 1,6-hexanediol
~,
3.9 4.3
* Taken from Samskog et
(nm) 625 525
al. (t)
3.4. Pulse radiolysis
The pulse-radiolysis data of single crystals of 1,8-octanediol have been reported previously/6) In Table 1 we summarize the results together with those obtained with 1,6-hexanediol. The activation energies for the e ~ in the diois give a half life of about 3 rain
3.3. From 7 7 K to room temperature
To check the validity of the interpretation of Hama
0.25
et al.,(l~) who observed an interconversion between
the alkyl and the hydroxyalkyl radical in polycrystalline diols, their experiments were repeated in single crystals of 1,6-hexanediol and 1,8-octanediol. The results were essentially the same as reported previously, oe Interestingly, the relative intensities between the alkyl and the hydroxyalkyl radicals were the same at 77 K as when the sample was irradiated, illuminated and measured at 4.2 K. It was also observed that the alkyl radical could be generated by illumination with u.v.-light at 77 K without previous X-irradiation. The yield was very low with a resulting poor signal but this phenomenon was supported by annealing the crystals to room temperature where the more easily detected hydroxyalkyl radical appeared. The extent of alkyl radical formation produced by u.v.-light, was estimated to be ten times more efficient if the sample had been previously irradiated with X-rays.
/f'\ |
0.20
K 0 0.15
//
l
I o.I
I 4oo
I 5oo
I 6oo
I 7oo
I coo
Wave~n~ (rim) Fig. 4. Transient absorption spectrum of e ~ in a single crystal of 1,6-hexanediol recorded at 268 K.
Electron to radical conversion
16
C
~e 3.4
3.6
I
L
L
J
I
3.8
4.0
4,2
4.4
4,6
7"-~( K-~x 10-3)
Fig. 5. Arrhenius plot of the transient species in 1,6-hexanediol.
at 77 K and at temperatures below 70 K the localized electron can be stabilized for hours. This is in excellent agreement with the ESR data which showed that the e ~ signal "disappeared" between 75 and 85 K. In Fig. 4 the transient absorption is shown, recorded at 268 K. An Arrhenius plot of the kinetics is shown in Fig. 5. 4. DISCUSSION 4.1. Radical structures In single crystals containing hydroxyl groups, no detailed structure of the electron trap has been proposed and verified experimentally. The overlapping ESR-signal from the hydroxyalkyl radical also made it impossible in this study. However, a model based on purely mechanistic aspects will be given below. The radicals discussed here have been identified previously.(4.6,~3)In a recent paper ~s)a detailed description of the radical structure for the hydroxyalkyl
443
radical at room temperature was given. It is a n-type radical characterized by a g-value anisotropy in the range of 2.0023-2.0044 and hyperfine interaction with one "- and three /~-protons. The smallest /~-proton interaction is almost purely anisotropic and it was suggested to be due to a hydroxyl proton. The alkyl radical has two pairs of/~-protons with isotropic hyperfine interactions of ~28-30 G and 36--40 G. The ,,-proton has principal values of the hyperfine interaction tensor estimated to 9, 20 and 32 G. (6) It was not possible to make a more detailed analysis, either by us or previously,¢6'13) since the formation of the radical can probably occur by H abstraction from different carbon atoms within the chain, giving rise to overlapping spectra due to similar but not identical radicals. However, the identification as alkyl radicals can be safely made from the width of the spectra, as discussed in a previous section. The ESR parameters of the alkyl radicals are summarized in Table 2.
4.2. Primary reaction mechanisms Correlating the kinetic data from the pulseradiolysis measurements, with ESR experiments where the temperature can be varied, it is possible to identify and follow the primary radical reactions. The primary step in the ionizing process is the formation of an electron and a cation (reaction 1). In single crystals of saturated organic compounds, the cation has never been observed by means of ESR. It is believed to decompose rather quickly, even at 4.2 K, to form neutral radicals by abstraction of a proton. This is probably the mechanism for forming the radicals, observed already at 4.2 K, in our system of simple diols. The mechanisms leading to the generation of localized electrons are rather obscure. In liquids or glasses
Table2. ESR parametersfor the alkylradicalsin 1,6-hexane-and 1,8-octanediol.Hyperfinesplittings are givenin puss Direction cosines Principal values a* b c Tensor Hydroxyalkylradical(s) g 2.0025 0.9907 -0.0272 -0.1333 2.0033 - 0 . 1 3 2 8 -0.4066 -0.9039 2.004! 0.0296 - 0.9132 0.4065 --CH2(~HOH A, -26.2 0.0039 -0.3914 0.9202 - I 1.6 0.9999 0.0044 -0.0024 -4.1 0.0031 -0.9202 -0.3914 Apt 27.3 -0.0373 -0.9636 0.2647 24.0 -0.7367 0.2055 0.6442 23.1 0.6752 0.1710 0.7175 Ar~ 35.9 -0.0497 0.9429 -0.3292 32.8 0.9313 0.1628 0.3258 31.7 -0.3608 0.2904 0.8863 A~ -5.3 -0.0418 0.7548 -0.6546 - 3.4 0.9991 0.0390 - 0.0188 4.4 0.0113 -0.6548 -0.7557 Alkyl radicalt~'~3) g 2.0022.004 --CH2(~HCH2A, - 32 -20 -9 2A#~ 28-30 2A~ 36-40
444
MIKAELL1NDGRENetal.
excess electrons are often considered to range through the medium until they are self-trapped. However, as mentioned above, as the hydroxyl groups form a fixed two-dimensional layer of dipoles in crystals of alkanediols, groups of dipoles arranged at favourable orientations for three-dimensional traps are less probable. An alternative mechanism is that the electron is localized at a defect of hydrogen bonding in the form of an anion which is stabilized by hydrogen bonding and by a neighbouring methylene group, probably of the lower layer (see insert below). However, since no detailed model has been verified experimentally, we regard this interpretation as tentative. This model is very similar either to the "small polaron" model proposed by Bush and Funabashi ~9) or the "diffuse anion" model by Webster. ¢m6) In the former, the excess electron is coupled to an intramolecular vibrational mode related to the OH bond and it will explain the structure observed in the absorption spectra of the localized electron at 4.2 K. ¢5) Decomposition following illumination can be incorporated into this model. Formation of the hydroxyalkyl radical is caused by loss of molecular hydrogen from the hydrogen-bonded anion defined above, .)4/ H
,HI H
(3) preserving the number of unpaired spins. This is consistent with the fact that the total intensity, due to the neutral radicals after illumination, is comparable to that of the intensity directly after X-irradiation at 4.2 K, containing the em~ signal. The disappearance of the ek~ upon heating can be explained by capture of an electron by the secondary cation. e- + ~CH2-OH ~ --* ~CH2-OH + H"
(4)
We emphasize that the conversion of localized electrons to neutral radicals is known not only in irradiated crystals of polyhydroxy compounds but also in irradiated alcohol glasses in general: localized electrons in methanol, ethanol or n-propanol disappear thermally in the dark at, for example, 100 K without forming detectable additional free radicals and they are photobleached to form hydroxyalkyl radicals. ~7) So far no explanation has been given for these facts on the basis of the cavity model, in which the excess electron attains a localized state occupying a void region, with a solvation shell around the localization site. The above considerations, however, do not preclude the possibility that electrons are captured interstitially in a cavity in the crystals. Previously, we
studied optical absorption spectra of the localized electron at 4.2 K and concluded that two distinctly different traps existed--shallow and deep. ~5)The absorption spectra of the shallowly trapped electron (with the maximum at ~ 1 3 0 0 n m in the case of octanediol and at ~ 1000 nm in the case of hexanediol) were extremely sensitive to light and their yields were relatively low. It is probable that the electrons of this type reside in interstitial cavities in the crystals. 4.3. Summary
The nature of the primary products induced by X-irradiation and the radicals formed from them, have been investigated in 1,6-hexanediol and 1,8octanediol by ESR and by pulse radiolysis. The diols have a simpler chemical and crystallographic structure than carbohydrates. Thus, 1,6hexanediol has a layer structure with the molecules connected by hydrogen bonds into infinite chains. ~s~ In carbohydrates, the hydrogen bonds extend in three dimensions. It is thought that in the latter case the primary reduction product is an excess electron localized in a cavity with the O - H dipoles of the surrounding molecules producing an energetically favourable trap. The ESR data are not sufficient precise to construct a definitive model for the reduction product in the diol case although the small polaron or diffuse anion are possible candidates. In other respects, i.e. the optical absorption spectra, the easy optical bleaching and the conversion to hydroxyalkyl radicals are analogous to those found for localized electrons in organic glasses and crystals. The alkoxy radical, which is usually assigned to an oxidation product, was not detected. Alkyl and hydroxyalkyl radicals were detected at 4.2 K even before photobleaching. It is possible that these radicals are formed in a secondary reaction of the primary (cationic) oxidation product. However, the different yields of hydroxyalkyl and alkyl radicals in 1,6hexanediol and 1,8-octanediol are difficult to account for and would seem to imply that the mode of fragmentation is sensitive to the chain length. Acknowledgements--One of the authors (M.O.) is supported by a Grant-in-Aid No. 61540310 for Scientific Research from the ministry of Education of Japan. We are indebted to Dr J. Byber8, Department of Physical Chemistry University of Aarhus, Denmark for the Q band ESR measurements at 26 K.
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12. E. E. Budzinski and H. C. Box, J. Chem. Phys. 1985, 82, 3487. 13. Y. Hama, K. Hamanaka and T. Horinchi, Radiat. Phys. Chem. 1979, 13, 13. 14. J. R. Byberg and S. J. K. Jensen, J. Phys. Chem. 1970, 52, 5902. 15. A. Lund, G. Nilsson and P.-O. Samskog, Radiat. Phys. Chem. 1986, 27, 111. 16. B. Webster, J. Phys. Chem. 1975, 79, 2809. 17. A. J. Swallow, Radiation Chemistry, p. 194. Longman, London, 1973.