Radical pairs and trapped electrons in single crystals of pentaerythritol

Chem. , 1984, 88 (15), pp 3292–3295. DOI: 10.1021/j150659a030. Publication Date: July 1984. ACS Legacy Archive. Cite this:J. Phys. Chem. 88, 15, 329...
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J. Phys. Chem. 1984,88. 3292-3295

matrices at 730 (sh) and 743 cm-'. Both bands are assigned to the 1:l complex; their relative intensity is more or less constant in the spectra studied. But the shift of the 743-cm-I band to 748 cm-I in the spectra of HC1-enriched matrices may indicate that the 1:2 complexes are likely to have also some component at 748 cm-I. The 743-cm-I band could be assigned to the hydrogen stretching mode in the 1:l complex in agreement with the studies in argon matrices. The second component at 730 cm-' may be due to one of the two hydrogen bending modes, very likely to the vb(H) in-plane mode. There is probably strong coupling between these two modes. The weak absorption at 704 cm-I is presumably an overtone of one of the other three hydrogen-bond vibrations which are expected to appear in the region below 400 cm-'. The other two absorptions are assigned similarly as in argon matrices. The different sensitivities of PyO-HCI and recently studied NHrHF' and amine-HX5s6 complexes to a nitrogen environment deserve notice. Both in argon and nitrogen matrices the v,(H) vibration of 4.0-HC1 complex is placed at 743 cm-l. In NH3-HF and in the amine-HX complexes the corresponding mode is very strongly perturbed by a nitrogen environment. The largest shift has been observed in the NH,-HCl complex where v, shifts from 1371 cm-' in argon to 720 cm-I in nitrogen.5 On the other hand, the v, mode in the (CH3)20-HCl complex shows relatively small frequency perturbation in a nitrogen en~ironment.~' The lower sensitivity of the PyO-HCl and (CH3)20-HCl complexes to a

nitrogen surrounding is probably related to the differences in the nature of the N.-H-X and O-.H--X bonds.

Conclusion Two kinds of pyridine N-oxide-hydrogen chloride complexes are identified in argon matrices; both are characterized by a very strong hydrogen bond. The v,(H) and vb(H) vibrations of the 1:l complex are observed at 743 and 863 cm-' with isotopic shift ratios of 1.53 and 1.41, respectively. For the 1:2 complexes the linear type structure is postulated. Only one hydrogen bond of this complex is identified at 703 cm-I with isotopic shift ratio of 1.25, and this vibration is assigned to the v, mode. In nitrogen matrices the strongly mixed vs(H) and vb(H) vibrations of the 1:l complex appear at 730 and 743 cm-', v,(H) being relatively weakly perturbed by the nitrogen environment. The position of the hydrogen-bond bands of PyO-HBr evidences partial proton transfer in these complexes. Acknowledgment. I am very grateful to Professor Henryk Ratajczak and Dr. Austin Barnes for helpful discussions and comments. This work was supported by the Polish Academy of Sciences MR.I.9 Research Project. Registry No. HC1, 7647-01-0; HBr, 10035-10-6; N2,7727-37-9; Ar, 7440-37-1; PyO, 694-59-7.

Radical Pairs and Trapped Electrons in Single Crystals of Pentaerythrltol. An Electron Spin Resonance and Pulse Radiolysis Kinetic Study Gosta Nilsson* and Anders Lund The Studsvik Science Research Laboratory, S-611 82 Nykoping, Sweden (Received: November 8, 1983)

The rate of transformation of radical pairs to monoradicals in y-irradiated single crystals of pentaerythritol C(CH,OH),, protonated or deuterated in the hydroxyl groups, was obtained by measuring the decay rate of the ESR signal of the radical pairs from 106 to 129 K. An isotope effect in the transformation rate was found and hydrogen atom tunneling seems to be involved. A mechanism is proposed. Pulse radiolysis data have shown that there is an isotope effect also in the decay rate of the optical absorption of electrons trapped in the crystal. The activation energies are 6.7 and 9.0 kcal/mol and the absorption maxima are at 430 and 510 nm for C(CH20H), and C(CH,OD),, respectively. A second moment analysis of the ESR line of the electrons shows that they are most likely trapped in interstitial positions.

Introduction Radical pairs and trapped electrons have recently been observed by ESR specroscopy in y-irradiated single crystals of deuterated pentaerythritol, C(CH20D), (PET-D), at 77 K.' We found that the pairs are formed by two (CH20D)3CCHODradicals. The sites of the pairs were identified and the separation of the radicals was found to be 4.37 A. Radical pairs are a main primary product and the selective formation of mainly one type of radical pair was discussed in the paper.' The central part of the ESR spectrum of irradiated PET-D is a single line with a peak-peak line width of 5 G and with an almost isotropic g factor close to the free-electron value. The line was only observed in the dark at low microwave power and could be bleached by filtered light with X > 390 nm. For these reasons we have assigned the line to electrons which are trapped in the 1attice.l We have continued our investigation of single crystals of PET-H and PET-D. In the present paper we report on an isotope effect (1) G. Nilsson, A. Lund, and P.-0.Samskog, J. Phys. Chem., 86, 4144 (1982).

0022-3654/84/2088-3292$01.50/0

in the decay kinetics of the radical pairs which has led us to propose a decay mechanism for the pairs. Pulse radiolysis was used to study the optical absorption of the trapped electron and the kinetics of its decay and from ESR line width measurements we have drawn the conclusion that the electron is trapped in an interstitial position.

Experimental Section The purification and deuteration of PET and the preparation of crystals have been described in the previous paper.' The crystal has a body-centered tetragonal lattice, space group 14, having axes of a and b = 6.087 A and c = 8.757 A with two molecules in the unit cell.2 The four C H 2 0 H groups of each molecule occupy equivalent positions in the unit cell. The molecules are linked together by hydrogen bonds. Each OH group takes part in two such bonds, both close to the (002) plane, making an angle of approximately 90° with each other. Thus, four oxygven atoms are situated ner the (002) plane at the corners of a distorted square. (2) D. Eilerman and R. Rudman, Acta Crystallogr., Sect. B, 35, 2458 (1979).

0 1984 American Chemical Society

The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3293

Radical Pairs and Trapped Electrons in PET

-m .-P

v)

a Figure 1. Crystal structure of PET projected on the ab plane. Black

2

circles are carbon atoms, Large open circles are oxygen atoms and small open circles are hydrogen atoms. Only four OH groups are shown. Hydrogen atoms bound to carbon atoms are omitted. The carbon atom at the center of the figure is at z = l/*. TABLE I

101 h x ,

species radical pairs radical pairs electrons electrons

matrix PET-H PET-D PET-H PET-D

nm 430 510

TI/Z:

ps

0.4 1.0

EA,

kcal/mol 4.5 7.4 6.7 9.0

10-'OA, s-l

6X 1.8 18

' ' ' ' ' ' '

' ' '

'

Time

' ' '

'

1

Figure 2. Decay of radical pairs ( 0 )in PET-H at 119 K (time scale: 0.198 min/div) and (0)in PET-D at 120 K (time scale: 0.746 min/div).

335 0-

"At 296 K. A projection of the crystal structure on the ab plane is shown in Figure 1. Crystals intended for ESR measurements were irradiated with X-rays (70 kV, Au anode) at 77 K to a dose of about 2 Mrd. To measure the decay kinetics of the radical pairs the samples were cooled to a fixed temperature in a stream of nitrogen gas. The temperature of the gas was controlled by a thermocouple. The intensity of one of the ESR peaks of the radical pairs was then followed as a function of time. The measurements on trapped electrons were made in complete darkness and at low microwave power. All measurements were performed with a Varian E9spectrometer operating at X band. The pulse radiolysis experiments were carried out with nanosecond time resolution by using a doublebeam electron accelerator (Febetron 708) as the radiation source. The equipment has been described in a previous paper.3 The dose per pulse was 248 krd. Results and Discussion Decay of Radical Pairs. The radical pairs decay by first-order kinetics in PET-H and PET-D as shown by Figure 2. The decay was measured over the temperature interval 106-129 K. At higher temperatures the decay was too fast, and at lower temperatures too slow to be conveniently followed by ESR. Over the temperature interval studied the Arrhenius plots are linear for both the protonated and the deuterated samples (Figure 3). The parameters obtained from the plots are collected in Table I. The data show that the activation energy difference for the decay of the deuterated pair and the protonated pair, EAD- EAH, is about 3 kcal/mol(l2.6 kJ/mol). The ratio of the preexponential factors of the protonated and deuterated pairs, AH/AD, is 3.3 X 10-5. The activation energy difference is an indication of an isotope effect in the decay of the radical pairs. The energy difference is, however, larger than the difference between the zero-point energies for the stretching vibrations of the 0-H and 0-D bonds. This difference is about 1.4 kcal/mol.4,5 The ratio AH/AD is also (3) P.-0. Samskog, G. Nilsson, and A. Lund, J . Chem. Phys., 68, 4986 (1978). (4) R. P. Bell, 'The Proton in Chemistry", 2nd ed., Chapman and Hall, London, and Cornell University Press, Ithaca, NY, 1973. ( 5 ) E. F. Caldin and V. Gold, "Proton-Transfer Reactions", Wiley, New York, 1975.

-1

s

-

-2-

--3C

-4-

-T 7

I

I

a

I

I

9

I

I

10

T-'(K-'x lo-? Figure 3. Arrhenius plots for the decay of radical pairs in PET-H (0) and in PET-D (X). The unit of k is m i d .

much less than 0.5, which is the lower limit of this ratio predicted by semiclassical t h e ~ r y . The ~ , ~ two observations imply that hydrogen atom tunneling may contribute to the decay of the radical pairs. Tunneling will show up as a positive deviation from the Arrhenius plot a t low temperatures, causing low values of In A and AH/AD since In A is extrapolated from a finite temperature interval to T' = 0. We tried to make a rough check of the curvature of the Arrhenius plot by measuring the decay rate at 77 K. The decay was, however, too slow to give a reliable value. The decay rate of radical pairs and its temperature dependence have also been measured by Haven et al. in hydroxyurea6 and by Yahimenko et al.' and by Toriyama et al. in dimethylglyoxime.* In both matrices a hydrogen-deuterium kinetic isotope effect was found in the hydrogen atom transfer reaction by which the radicals in a pair diffuse apart. The difference EAD- EAHwas about 6-7 kcal/mol (25-29 kJ/mol) and AH/AD was of the same order of magnitude as we have found. Toriyama et a1.8 could show that there was a positive of the Arrhenius plot at low temperatures ( 6 ) Y . Haven, R. C. Williams, P.J. Hamrick, Jr., and H. Shields, J . Phys. Chem., 60, 127 (1974). (7) 0.E. Yahimenko and S.C. Lebedev, Int. J . Radiat. Phys. Chem., 3, 17 (1971). (8) K. Toriyarna, K. Nunome, and M. Iwasaki, J . Am. Chem. SOC.,99, 5823 (1977).

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The Journal of Physical Chemistry, Vol. 88, No. 15, 1984

Nilsson and Lund 0.1

.

I

I

I

I

I

I

\

Figure 4. Crystal structure of PET projected on the ab plane. The plane for the body-centered molecule has been extended in the lower right corner of the figure. The dotted line between C , and C2connects the two radicals of a pair. which is strong evidence that tunneling must be involved in the hydrogen-transfer reaction. The kinetic isotope effect in the decay of the radical pairs in PET is proof that the hydroxyl hydrogen atoms take part in the decay. We therefore propose that a hydroxyl hydrogen atom is transferred to the radical site from an adjacent undamaged molecule ( C H 2 0 H ) 3 C C H 2 0 H+ HOCHC(CH,OH), ( C H 2 0 H ) 3 C C H 2 0 HOCH2C(CH20H)3(1)

+

-

0.0021

I

I

1

2

I

I

I

4

5

6

Figure 5. Decay of the optical absorption in PET-H at 480 nm and 295 K shown as an example of the resolution of the transient into two firstorder decays. The optical densities at t = 0 were used to construct the spectra in Figure 6 . ~

-

followed by the isomerization reaction ( C H 2 0 H ) 3 C C H 2 0 (CH20H)3CCHOH

I

3 Time(ps)

ah

N

0

17(2)

Reactions 1 and 2 are repeated, leading to a separation of the radicals in the pair and to an increased number of monoradicals. This mechanism is of course also valid for the diffusion of the monoradicals in the lattice. In Figure 4 we have extended the plane for the body-centered molecule shown in Figure 1 and connected the radicals in one of the eight equivalent radical pair sites by a dotted line. The figure shows that the pair is formed by a spin at carbon atom C1 of the body-centered molecule and a spin at carbon atom C2of a corner molecule of the unit cell. The distance C1-C2 is 4.37 A.' There are two hydroxyl hydrogen atoms on undamaged molecules, HI and H2, which are the nearest neighbors to the radical centered a t C,; the distances are 3.18 and 3.56 A, respectively. If the transfer is restricted to H I , then the radical can diffuse back to its original position by similar transfer steps in the same plane. If, however, H2 is transferred, then the radical moves to the plane above and becomes centered at carbon atom C3. The radical separation C2-C3 has increased to 4.72 A and can continue to increase by further transfer steps from plane to plane. When the radicals in a pair diffuse apart by the hydrogen atom transfer mechanism, two new monoradicals are formed. No grow-in of monoradicals was, however, observed when the radical pairs decayed. Instead the monoradicals decayed in the temperature interval studied. The reason could be that after the first diffusion step which converts the radicals in a pair to monoradicals the two radicals are in the same crystal plane and immediately recombine. There is therefore no production of monoradicals when the radical pairs decay and bulk monoradicals disappear by radical-radical recombination. Optical and Kinetic Properties of Trapped Electrons and Radicals. By pulse radiolysis of PET-H and PET-D we observed transient optical absorptions which could be resolved into firstorder decays (Figure 5). The half-lives at 295 K are 1.7 and 0.4 p s for PET-H and 5.1 and 1.O p s for PET-D, respectively. The absorption spectra of the four transients are shown in Figure 6. The Arrhenius plots for the two short-lived species are shown in Figure 7 and their optical and kinetic parameters are collected in Table I. In both the protonated and deuterated samples there is a long-lived optical absorption which extends into the UV and a

-1

2 z6E

'i 5 -

E

c

I

z40

w

32-

-

1

300

400 500 600 Wavelength(rnrn)

10

Figure 6. Optical absorption of radicals in PET-H ( 0 )and PET-D (0) and of electrons in PET-H ( X ) and PET-D (0)at 295 K.

5

3.0

3.5 T-'(K-'

4.0

4.5

5.0

.io-9 Figure 7. Arrhenius plots for the decay of electrons in PET-H ( 0 )and PET-D (0). The unit of k is s-].

short-lived absorption with a maximum in the visible. The UV absorption is probably due to radicals. The kinetic data in Table I show that there is an isotope effect in the decay of the visible transient. This would be expected if the visible absorption is due

The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3295

Radical Pairs and Trapped Electrons in PET

I I Y

V

Figure 8. ESR spectra at 10 pW of polycrystalline PET-H (a) and PET-D (b) irradiated and measured at 77 K. The central peak is due to trapped electrons.

to trapped electrons which can decay by the following first-order reaction:

e,

-

(CH20H),CCH,0-

+H

(3)

The half-life of the visible absorption in the deuterated sample at 137 K calculated from the Arrhenius parameters in Table I is about 1 min. The half-life of the electron signal in the ESR spectrum a t the same temperature is 4 min. The agreement is reasonable and supports the assumption that trapped electrons are the cause of the visible absorption. The peaks of the optical absorption bands of the electron in the two matrices are at remarkably short wavelengths. This suggests that the electron is deeply trapped. The peak positions are, however, not unreasonable. We have observed A, = 500 nm for trapped electrons in rhamnose crystals at 278 K9and Li = 400 nm for electrons in the same matrix and Kevan found ,A, at 4 K.Io Finally, Buxton and Salmon found A, = 450-475 nm for electrons in sucrose crystals at 6 K." Figure 6 shows that the product of the extinction coefficient at ,A, and the C value is only 400 for PET-H and 270 for PET-D in units of M-' cm-' e< (100 eV)-'. The reason is probably that G is very small. Electron Trap. The ESR line widths for the protonated and deuterated samples (Figure 8) are 11 and 5 G , respectively. The (9) P.-0. Samskog, A. Lund, and G. Nilsson, J . Chem. Phys., 73,4862 (1980). (10) A. S. W. Li and L. Kevan, J . Chem. Phys., 76, 5647 (1982). (1 1) G . V. Buxton and G . A. Salmon, Chem. Phys. Lett., 73,304 (1980).

increased line width in the protonated sample is caused by electron-proton hyperfine interactions. Vincow and Johnson have derived expressions for the second moments of ESR lines with proton hyperfine interactions.I2 The second moment due to deuteron hyperfine interaction is smaller by the factor * / , ( I . L ~ / ~ ~ ) ~ = 0.063. From the point dipole approximation one obtains the relation r = CN116.Here r is the distance between the electron and the hydrogen atom in angstroms and N the number of hydrogen atoms with which the electron interacts and C = 1.58 A is calculated from the measured second moments. There are two types of possible sites for the trapped electron. The electron can be trapped either in a molecular vacancy or in an interstitial position. We assume that in both cases there is no reorientation of the dipoles (OH groups) forming the trapping site. If the electron is trapped in a vacancy, it has four nearest-neighbor hydroxyl hydrogen atoms in the same plane at a distance of 4.23 8, and eight nearest-neighbor hydroxyl hydrogen atoms in the planes above and below at a distance of 4.77 A. The distance calculated from the formula with N = 12 yields r = 2.4 8,and with N = 4 one obtains r = 2.0 A. Thus, the vacancy model can be excluded. If the electron is trapped in an interstitial position for example in the center of the distorted square of oxygen atoms in the ab plane (Figure l), the distance to the four nearest-neighbor hydroxyl hydrogen atoms is r = 1.6 8,. The distance calculated from the formula with N = 4 is however 2.0 A. This indicates that the electron is localized above or below the plane. A displacement of 1.2 8, or 30% of the distance between the (002) planes is sufficient to give an electron-hydrogen atom distance of 2.0 A. For all other interstitial positions, except the one proposed, N becomes too small and r too large to fit the formula. The first observation of excess electrons trapped in interstitial positions in a hydrogen-bonded molecular crystal was probably the ESR experiment made by Bennett et ale', They found that trapped electrons in polycrystalline ice interact with six hydrogen atoms which could be the six nearest hydrogen atoms on the wall of the natural cavity in ice. The fact that electrons can be trapped in interstitial positions in polyhydroxy compounds and not necessarily in molecular vacancies, which has often been proposed, has become apparent during the last few year^.'^-'^ The traps are formed by two or three hydroxyl groups and the excess electron seems not to introduce rearrangements of the surrounding molecules or the polar g r o ~ p s . ' ~ * ~ ~ Registry No. C(CH,OD),, 18356-91-7; D2,7782-39-0; pentaerythritol, 115-77-5. (12) G. Vincow and P. M. Johnson, J . Chem. Phys., 39, 1143 (1963). (13) J. E. Bennett, B. Mile, and A. Thomas, J . Chem. SOC.A , 9, 1393 (1 967). (14) H. C. Box, E. E. Budzinski, and H. G. Freund, J . Chem. Phys., 69, 1309 (1978). (15) H. C. Box, E. E. Budzinski, H. G . Freund. and W. R. Potter. J . Chem. Ph$.,'70, 1320 (1979). (16) E. E. Budzinski, W. R. Potter, G. Potienko, and H. C. Box, J . Chem. Phys., 70, 5040 (1979). (17) S. E. Locker and H. C. Box, J . Chem. Phys., 72, 828 (1980). (18) E. E. Budzinski, W. R. Potter, and H. C. Box, J . Chem. Phys., 72, 972 (1980). (19) P.-0. Samskog, L. D. Kispert, and A. Lund, J. Chem. Phys., 78,5790 (1 983). (20) P.-0. Samskog, L. D. Kispert, and A. Lund, J . Chem. Phys.,79,635 (1983). '