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Radical reactions in malonic acid single crystal between 4 K and 300 K
Chemical physics 12 (1976) 169-176 0 North-Holland Publishing Company
RADICAL REACTIONS 1N MALONlC ACID SINGLE CRYSTAL BETWEEN 4 K AND 300 K
M. KIKUCHI*, N. LERAY, J. RONCIN Laboratoire
de Rhonance
Electronique
et Ioniqlde, Universite’de ParisSlrd,
Centre d’Orsay - 91405
Orsay, France
and
B. JOUKOFF CNETDepartment
PEC/CCM,
196 rue de Paris - 92 Bagneox. France
Received 21 July 1975
For three out of the four different radicals which are produced by y-irradiation in malonic a!id single crystals, two stereo. isomers have been observed. The structures ?nd orientations of these radicals were determined. CH2COOH proceeds from the cation and&already formed at 4 K; as to HC(COOH)z, it proceeds from the molecular form of the protonated anion COOHCHzC(OH)z.
1. Introduction
also concerned
Parent anions and cations are, with trapped electrons and excited molecules the primary species produced by ionizing radiations in organic solids. Usually the ionic species are very reactive and, even when they are trapped in the crystal, they can decompose or react with molecules of the matrix to give either other transient species, e.g., ions or free radicals, or stable final products. Since many of these primary or secondary transient species are paramagnetic, ESR spectroscopy is a very useful tool to study the reaction steps leading to stable products formation. ESR spectroscopy on single crystals associated to crystallographic data may also give useful inforrnations on the stereospecificity of solid state reactions. There has been much work done by ESR [l] or, more recently by ENDOR [2], on radiation damages in dicarboxylic acids and aminoacids. The main scope of these experiments was the identification and structural study of the free radicals produced in the solid by ionizing radiations. However some authors were _l
Resent address: hl. KiiuchiQawa, Radiation
Chemistry,
Department
Laboratory of Applied of Nuclear Engineering
-
University of Tokyo, 7-3-l Hongo Bunkyoku, Tokyo, Japan.,
with the determination
of reaction steps
[31. 1: the case of malonic acid, two radicals eH2COOH
and CH(COOH), [4] have already been dbserved to be stable at room temperature. Since there are crystallographical data available for this molecule, we shall discuss here the reaction steps leading to the formation of those tw6 relatively stable radicals. The crystallography of malonic acid was studied by Coedkoop [S] , who showed that the molecules are arranged in zigzag along the C-axis with the carboxylic groups linked through two hydrogen bonds. In the present work we use the same notation as Goedkoop for the atoms of the malonic acid molecule (fig. 1). In the crystal cell, this molecule has no element of symmetry and the two carboxylic groups are not equivalent: the C,0,,012H12 plane is nearly parallel to the C&C2 plane (13”), while the C,021022H,2 plane is perpendicular to it. This particularity leads us to expect they will play different roles in the various reactions.
2. Experimental The’ 13C hyperfime intera&&ris a sensitive probe ’ of the unpaired electron distiibution in the region of ..
170
hf. Kikuchi et al./Radical
reactiom
in ndonic
acid single crystal
Fig. 1. MaIonicacid molecule. the 13C nucleus. Unfortunately, with samples containing 13C in natural abundance, the 13C features are usually hidden in the Gngs of the spectrum of unlabelled spdcies. However, in perdeuterated molecules, hyperfme and superhyperfme couplings of protons are reduced and, comequently, in many cases, determination of 13C coupling tensors becomes possible. For -this reason, most of our experiments were performed
I
&
Ha Fig. 2. ESR spectrum oi malonic acid single crystal irradiated
at 77 K. Spectrum recorded at 90 K.
on perdeuterated malonic acid single crystals. MaIonic &id from Flub (99% purity) was used .without further purification. Perdeuterated malonic acid was obtdined by succesaivk exchanges ofprotons with heavy water at 60°C until the D/H ratio measured by NMR was greater than 100. Single crystals were grown-by slow cooling of saturated sollitions of malonic acid d4 in heavy water and malonic acid in water. &we wanted to correlate ESR results aid crystallographical data, it was necessary to irradiate samples with knotin o$entations with respect to the crystallographical axis . Samples .were irradiated at 4 K 0; ‘77 :‘K by 6oCo y-rays with doses lying between 1020-1022 ..eVg-I. me ESR spectra were recorded with a Jeol (JES IXj spectrometer at 4 K for some experiments and at temp&at&es Qiiq between 77 .K and room temperatures .fortheothers. -
,lOG,
.‘+&a . T:Orientatiokkd cubing were kin&i performed b; Mr. :&rtrand Ir&the,CNET l&oratory. .. :
:.
Fig. 3. -ESRspectrum of konic acid single crystal irradiated at 77 K. Specfxum recorded at 200 K.
M. Kikuchi et al./Radical reactiorls in .malonic acid single cvsral
171
3. Results After y-irradiation at 77 K, we observed the following transformations of the ESR spectra from 77 K to room temperature: - Fig. 2 shows a spectrum recorded at 90 K immediately after irradiation. Lines I and I’ correspond to the two different CH2COOH radicals already observed by Whiffen [la] . One of them results from the loss of the Cl 011012H12 group, the other one by the loss of the other carboxylic group. The other lines on this spectrum cannot be attributed at this stage because the resolution is not high enough. - When the sample is heated to 200 K a narrowing of these lines occurs (fig. 3). It is now possible to see that they correspond to a radical with two weakly anisotropic protons, the HOOCCH,C<~~
x 500
i?OG
radical. Two types of
this radical II and II’ exist and are better separated in a single crystal of perdeuterated malonic acid where the 13C couplings can be measured (fig. 4). - After a warming to 270 K, II and 11’disappear and a single line appears. The 13C spectrum of this radical si~ws that there are also two forms III and III’ (fig. 5). The 13C tensors of those radicals show that they are two different forms of the COOHCH,CO radical discovered by Kwiram [6].
Fig. 5. ESR spectrum of perdeuterated malonic acid single crystal irradiated at 77 K. Spectrum recorded at 270 K.
- Finally, at room temperature, these u radicals disappear in a few hours, giving (fig. 6) the well-known cH(COOH)2 radical (IV). The changes observed in the ESR spectra as a function of temperature show that: (1) CH~COOH is present at all temperatures. (2) The &-I(COOH)Z appears at room temperature and its formation seems to be completely independent from that of CH$OOH. In order to discuss the reactions steps leading to the formation of these radicals we have to determine more precisely the structure of radicals I, I’, II, II’, III, III’, IV and their orientation in ihe matrix.
.?.I. Structure of radicals
Xl000
Fig. 4. ESR sepctrum of perdeuterated malonic acid ti irradiated at 77 K. Spectrum recorded at 200 K.
single crys-
3.1.1. CH~COOH Theanisotropic coupling tensors of the CH2COOH (I and I’) radicals and their directions cosines in a crystalline reference frame are given in table 1. Since fat a protons, the intermediate values of the ardsotropic tensor give the orbital direction and the-lower values the C-H bond direction, we can determine the C,H,,l and C,,H,z directions for radicals I and I’. According to Whiffen [la] , radical 1 is formed by loss of the $021022822 carboxylic group so that the C,H,,H,2 plane is perpendicular to the remaining
M. Kihrchi et aL/Radical
reactions in malonic acid
single
crystal
C,0,,0,,H,2 and radical I’ to the toss of C~O,,O~~H~~. For both of these radicals, COH~~HO~ is in the plane of the remaining carboxylic group. So, our experimental results are consistent with the expected, more stable, configurations of these radicals.
II-
3.1.2. Radicals II and II’ Proton coupling tensors for radical 11and 13C coupling tensors for radicals II and II’ are reported in table 1. In various r-irradiated carboxylic acids, formation of the molecular anion has been postulated [7,8] on the basis of the 13C isotropic coupling constants. Recently ENDOR studies by Iwasaki [2] have shown that in fact these species are the protonated form of tbe molecular anion. Comparison of our experimental data with that of ref. [2] shows that radical II and II’ are certainly the protonated anion of malonic acid . ,OH COOHCH,C,oH . It is to be noted that the values of 13C isotropic coupling constants of radical II (86.2 G) and II’ (112.2 G) are quite different. One possibie explanation for this difference are the environmental effects upon the structure of these radicaIs. Such effects have been observed in~a;;;e~;fal;~i yj$;;;C = 89.7 G) and of suc-
Fig. 6. ESR spectrum of mdonic acid sinde crystal irradiated at 77 K. Spectrum recorded at room temperature.
carboxylic group. This conclusion is rather surprising since the more stable structure of eH,COOH ought to allow the mixing of the rr orbital of the unpaired elec.tron with the II system of the carboxylic group. Ifi order to elucidate this point, we have calculated two hypothetical conformations of these radicals in the crystal. The directions of CO&~, Co&z bonds and .of the unpaired electron orbital were calculated using crystallographicaj data and as&ning identical bond length and angles for the C-COOH part of the radical
and of,ihe molecule in the cjstal. The central atom wassupposed to de s$ hybridized. This calculation Wixsmade. for both types of CH2COOH radicals (loss of~Cz02,0,$Izz oi; C,0,20,,H,2 groups), supposing that,CO~1~2 is ei*er perpendicular or in.the plage ‘of the remaiqing caiboxyljc group, To compare with .$e ex&imerM r&ults we determined the angle 0 be-. tween me calculated and &e experimental directions. able 2 shtiws that radical.1 co&ponds to jhe less of ._ .: .. _.. :,
,_.:
1:
_.
. .
It has already been noted that the two carboxylic groups of the malonic acid molecule are not equivalent in the crystal. As a consequence they will give two different protonated anions corresponding to the location of the unpaired electron on C, or C2. To show that these two radicals are in fact radicals II and II’, we have calculated the angle between the free electron orbital direction in II and 11’and COCl and C& directions in the crystal. For radical II, we find that the axis of the free elec&on orbital makes an angle of 99” with C&z and 18” with the perpendicular to the CzO2,022 plane. Thus the free electron is most probably located on carbon C2 and the C0C202iO22 group becomes pyramidal when the protonated anion ls formed. This conclusion is confirmed by an INDO calculation of the most stable structure of radical II, in excellent agreement with experimental results (table 3). As shown on fig.,7 the structure of radical II e be deduced from that of the. parent molecule by two succes&ve rotations of the carboxylic group: a first o’n&of 35” around a fine A per-
Table 1 Hyperfime tkors
for radicals observedin malonic acid single crystal. All the couplings are given in ~JUS.S
Radid
Principal value
Isotropic coupling ~--
----.-
33.42 Ha1
20.53
0.6172 22.06
12.23
31.87
22.11
12.77 &C00~ (1’)
11.21
21.80
20.82
32.59
26.13 31.32
8.7
74.54 ‘k
86.23
112.2
138.15 ‘3Cc,j 181.00
151.61
51.35
129.2
140.B
11.59 20.61
21.73
0.9964 0.7633
0.7750 -0.2934 0.6111
0.5755
0.520
0.821
-0.408
0.5456
0.7978 -0.5828
0.479 -0.272 0.6547 -0.1519
0.6029 -0.7407 -0.463 0.359 0.811
0.779 -0.273 0.565
0.1498
0.1856
0.9713
0.7691
0.6399
0.0206
0.7463
0.2381
0.8910 -0.3935
0.828
0.9988-
33.00 H(1
0.0631
0.5233
0.460
143.2 CH(COOHI2 (IV)
0.4682 -0.0799
0.753
0.893
0.0385 0.9988
0.8814 -0.0289
0.660
0.321
149.9 “Cc,)
0.777
0.4537
45.34 COOHCHZkO CIII’)
0.9992
0.0170 -0.4672
47.18 61.63
0.5610
-0.6211
135.67
‘3C{2j
-0.4715
-0.152
105.8
(11’) COOHCH2k0 (III)
142.1
-0.0295
0.423
88.7 ‘“c
0.9995 -0.0079 0.0090
0.3408
58.11
COOHCHIC;z;
0.8858
-0.232 9.4
13.6
126.04
-0.1427
-0.5428
6.0 Hp,
0.2526
0.7462
0.6249 -0.1637 21.63
0.7674
0.3894
0.0569
25.44 “0,
0.5748
0.7165 -0.6502
0.8800 21.53
20.20
COOOHCH,c<; (11)
0.5028 -0.6052 0.6456 yO.2118
0.0292 -0.0388
11.79 H=2
---
0.5399 -0.4417
33.38 H01
-0.7337 0.2842
21.68 H=2
Direction cosines
-0.0317 0.0385
0.7923. -0.661 0.545 -OS?7
0.8845 -0.2251 0.4090 0.679 O.138 -0.721
0.0306 -0.0394 0.999.1 ~0.0276 0.0288
0.9988
174
M. Kikuclri et aL/Radicaf
reacrims in malortic acid
single crystal
Table 2 Determinationof CH bond and orbital direction in radicals 1 and I’ e
Direction cosines Loss of C101101zH~z
CoHol and CoHoz coplanar with
CoHol
-0.3031 -0.1551
GO21022~~22
CoH22
-0.3284
0.7316 -0.6701
orbital CoHol and CoHol: in a plane perpendicular to C~OZI 022H22
W-h
-0.9642
CotIol and CoHoz coplanar with
COHOI
C1011’312H12
CoHo2
orbital CoHol and CoHoz in a plane per-
CO~OI
pendicular to C,OIIO~~H~Z
CoHo2
9” 5O
0.1253
5’
0.2622 -0.0412
70”
0.1848 0.9796
73” 85”
0.7149 0.6387 0.0903 -0.9816 -0.1474 -0.0131
0.1153 0.0005 0.9890
20” 11” 10”
orbital
pendiculq.to C& and located in the C202202t plane, a second _ne of 30” around A’ the bisector of the Oz,C20 angle. In radical II, the 021H21 and 022H22. onds are in the C202t022 plane. f For ra;iic‘al II’ the free electron is localised on carbon Cl r&er than C2. The distortion of the parent molecule when this radical is formed is more pronounced than for radical II. The free electron orbital makes an angle of 130”.with C,Co and 8.5” with the direction perpendicular to the C1O11O12 plane of the parent molecule. Since in this particular case the proton couplings are not sufficiently resolved to allow a determi-
0.9402 -0.7966
0.3324 -0.9249 0.0126 0.20b3
CoHo2
orbital Loss of C20z1022H22
-0.5075
0.3971 -0.0538 0.9160 0.5302 -0.2394 -0.8136 0.3766 -0.9221 0.0890
7s”
75” 82”
nation of the coupling tensors, it is not possible to go further in the study of this radical at the present time. 3.1.3. Radical 111 and III’ The experimental values of the 13C coupling tensors found for radical III (table 1) are in good agreement with the results reported by Kwiram [6] for the radical COOHCH#O. The structure
of radical III can be derived from that
Table 3 Comparison of our experimental results for the isotropic hyperfine coupling values wilh other experimental values [2] and INDO c;llculation. Au the couplings are given in gauss Resent work
‘hvomki [2]
INDO utcuhtion
bKxinic) 13c 86.2 k& .2;.; OH2 OH2
..
84.9.
:
:
27.3
27.5
8.36 -0.5 0.85
9.0 2.7 3.2
Fig. 7. Modifications lading to the formation of anion II. The aboxylic group rotates around axes A and A’.
:
. . .
. .
175
M Kihuchi er af./Radical reoctiom in malmic acid singlecrystal
of the parent molecule by abstraction of the 011H12 group. From the values of the r3C couplings tensors and o&he direction cosines of their principal values, the CC0 angle can be estimated to 128”. For radical III’, only one of the carbon 13 tensors can be determined; the anisotropic part of this tensor (+9.1 G; -11.6 G; 2.4 Gj is weaker than that of the corresponding tensor for radical III. This value and the direction cosines are consistent with a radical COOHCH,CO obtained by a C1022 bond rupture and for which large librational motions of Ozl around the CuC, bond occurs. The small difference observed between the largest isotropic i3C coupling constants of radicals III and III’ is certainly due to environmental effects as in the case of radical II and II’. 3.1.4. &((cooH), This radical is one of the first organic free radicals studied [4] and its structure is now very well known. 3.2. Radiation damage process From crystallographical and ESR data, relative orientations of molecules and radicals in the crystal are now very well known; thus it may be possible to see if the reactions which occur in the malonic acid crystal are stereospecific. Radicals COOHCH2 as well as the protonated anions II and II’ are present at 77 K. When the samples are heated from 77 K to 273 K, the concentration of &I$OOH radical remains unchanged; but we observed transformations of radical II leading successively to the formation of radicals III and IV. This behaviour is an indication that CHzCOOH and CH(COOH)* which are relatively stable at room temperature in the T-irradiated malonic acid, do not have the same precursor. This result is consistent with the general scheme of radiation damage observed in other carboxylic acids. According to this scheme, the primary damage consists of anioncation radical pairs which undergo chemical reaction in the crystal; CH$OOH is formed from the cation and CH(COOH)z from the anion3.2.1_ cH$OOH formation Ayscough [9] and Iwasaki [lo] proposed the following scheme for the formation of CH$OOH from the primary cation COOHCH2COOH+:
COOHCH2C //O + ‘0. wQ
0
+
COOHCH,Cc
i
HO ‘CCH,COOH HO/
COOHCH,+CO,.
t I ,
(1)
(2)
0' P0 In order to study these reaction steps we have made an attempt to stabilize radical V or radical VI. Single crystals of malonic acid were irradiated at 4 K and their ESR spectra recorded at this temperature. Unfortunately the observed spectra are identical to those obtained at 77 K, i.e., r?H,COOH and the protonated anion II and II’ are the only radicals present at 4 K. Nevertheless this result does not conflict with the reaction mechanism proposed by Ayscough. The resulting reactions may occur at 4 K since step (1) consists in a proton transfer across a hydrogen bond and step (2) does not need the CO2 fragment to escape out of the cage, the reverse reaction being endothermic by 40 kcal. Another result that can be related to the cationic origin of CH$OOH is the relative proportion of radicals I and I’. ln the crystal we have seen that the two carboxylic groups are not equivalent. It follows that the relative ionisation probability on each of them must be different from unity..If I and I’ proceed from the cation, the ratio I/I’ will not be equal to 1. The experimental result I/I’ = 4 is in good agreement with this assumption. 3.2.2. cH(COOH)2 formation The experimental results strongly suggest that the CH(COOH), radical is formed from an anionic precursor through the following steps 0’
COOHCH#
+ COOHCH,COOH + ‘OH
CVIJ) . PH -0, COOJ-ICH&, + CCH,COOH, OH 0’ (II)
(3)
M
176
COOHCH,e
/OH
et aL/Radicai
Kikdri
+ COOHCH,@
reactions in molonic ocid single crystal
0 + H,O,
(4)
‘OH (II) 0 COOHCH,CT + HOOCCH,COOH + COOHCH,CHO + Hc(COOH)2. W)
(5)
Like step (I), step (3) is a proton transfer through a hydrogerrbond and is achieved at 4 K, so that radical VII.cannot be observed. Step (4) is a dehydration reaction of the protonated anion leading to the formation of an acyl radical. Step (5) is the only bimolecular reaction which is not a proton transfer through a hydrogen bond. The stereospeciflcity of this reaction may be discussed since the structure of radicals and their orientation with respect to molecules in the solid are known. Constructing a model of the crystal one can see which H atom of an adjacent molecule may be abstracted by each type of acyl radicals (III and 111’)we can then determine from the crystallography the distances and angles between the free electron orbital and the H atom. For radical lil, there is a hydrogen atom of a methylene group at 3.9 a from the radical center and the direc-
tion defined by this radical center and the hydrogen atom makes an angle of 46O with the free electron orbital of III. For radical III’ we have seen that there are torsional oscillations or restricted rotations of the C=O group around the C-C bond. During this motion, the free electron orbital of III’ generates a cone, inside which an H atom of a methylene group of an adjacent molecule lies at a distance of 3.7 A from the radical center. Since the relative positions of radical centers and H atoms are different in these two symmetric reactions III --zIV and III’ + IV, the preexponential factors and possibly activation energy should be different. Wnfor.tunately it has not been possible to study the kinetics
_.. .:.-
I
of III and III’ disappearance because their ESR spectra are not resolved and overlap each other. Radicals LII and III’ have been separated and identified only on the basis of their 13C couplings and it is not possible to study their disappearance since the 13C lines have very low intensities. Another interesting feature is that the concentration ratios II/II’ and III/III’ are nearly equal to unity. If the formation of II and II’ proceeds from the anion, this may be due to a very high efficiency of electron attachment on the carboxylic groups leading to equal quantities of radicals II or II’.
References 11) (a) A. Horsfield, J.R. Morton and D.H. Whiffen, Mol. Phys. 4 (1961) 327. (b) J.W. Sinclair and M.W. Hanna, J. Phys. Chem. 71 (1967) 84. (c) H. hluto, T. fnoue and hl. lwasaki, 3. Chem. Phys. 57 (1972) 3220. [2] H. Muto, K. Nunone and M. Iwasaki, J. Chem. Phys. 61 (1974) 1075;5311,5315. [3] (a) DC. Strrwand G.C. Moulton, J. Chem. Phys. 60 (1974) 1223,123l. (b) N. Tamura, M.A. Collins and D.H. Whiffen, Trans.
Faraday Sot. 62 (196612434. (cl J. Sinclair and P. CodeLla, J. Chem. Phys. 59 (1973) 1569. 141 h1.M. McConneJf, C. Heller, T. Cole and R.W. Fessenden, J. Am. Chem. Sot. 8.2 (1960) 766. [5] J.A. Goedkoop and C.H. MacCiJlavry, Acta Cryst. 10 (1957) 12.5. [6] R.C. McCal.leyand A.L. Kwiram, J. Am. Chem. Sot. 92 (1970) 1441. [7] J. Sinclair and M.W. Hanna, J. Chem. Phys. 50 (1969) 2125. [8] H. Box, H. Freund and K. LiJga, J. Chem. Phys. 42 (1965) 1471. 19) P.B. Ayscough and J.P. Oversby, Trans. Faraday Sot. 67 (1971) 1369. jIO] B. Eda and M. Iwasalci, J. Chem. Phys. 55 (1971) 34.42.
RADICAL REACTIONS 1N MALONlC ACID SINGLE CRYSTAL BETWEEN 4 K AND 300 K
M. KIKUCHI*, N. LERAY, J. RONCIN Laboratoire
de Rhonance
Electronique
et Ioniqlde, Universite’de ParisSlrd,
Centre d’Orsay - 91405
Orsay, France
and
B. JOUKOFF CNETDepartment
PEC/CCM,
196 rue de Paris - 92 Bagneox. France
Received 21 July 1975
For three out of the four different radicals which are produced by y-irradiation in malonic a!id single crystals, two stereo. isomers have been observed. The structures ?nd orientations of these radicals were determined. CH2COOH proceeds from the cation and&already formed at 4 K; as to HC(COOH)z, it proceeds from the molecular form of the protonated anion COOHCHzC(OH)z.
1. Introduction
also concerned
Parent anions and cations are, with trapped electrons and excited molecules the primary species produced by ionizing radiations in organic solids. Usually the ionic species are very reactive and, even when they are trapped in the crystal, they can decompose or react with molecules of the matrix to give either other transient species, e.g., ions or free radicals, or stable final products. Since many of these primary or secondary transient species are paramagnetic, ESR spectroscopy is a very useful tool to study the reaction steps leading to stable products formation. ESR spectroscopy on single crystals associated to crystallographic data may also give useful inforrnations on the stereospecificity of solid state reactions. There has been much work done by ESR [l] or, more recently by ENDOR [2], on radiation damages in dicarboxylic acids and aminoacids. The main scope of these experiments was the identification and structural study of the free radicals produced in the solid by ionizing radiations. However some authors were _l
Resent address: hl. KiiuchiQawa, Radiation
Chemistry,
Department
Laboratory of Applied of Nuclear Engineering
-
University of Tokyo, 7-3-l Hongo Bunkyoku, Tokyo, Japan.,
with the determination
of reaction steps
[31. 1: the case of malonic acid, two radicals eH2COOH
and CH(COOH), [4] have already been dbserved to be stable at room temperature. Since there are crystallographical data available for this molecule, we shall discuss here the reaction steps leading to the formation of those tw6 relatively stable radicals. The crystallography of malonic acid was studied by Coedkoop [S] , who showed that the molecules are arranged in zigzag along the C-axis with the carboxylic groups linked through two hydrogen bonds. In the present work we use the same notation as Goedkoop for the atoms of the malonic acid molecule (fig. 1). In the crystal cell, this molecule has no element of symmetry and the two carboxylic groups are not equivalent: the C,0,,012H12 plane is nearly parallel to the C&C2 plane (13”), while the C,021022H,2 plane is perpendicular to it. This particularity leads us to expect they will play different roles in the various reactions.
2. Experimental The’ 13C hyperfime intera&&ris a sensitive probe ’ of the unpaired electron distiibution in the region of ..
170
hf. Kikuchi et al./Radical
reactiom
in ndonic
acid single crystal
Fig. 1. MaIonicacid molecule. the 13C nucleus. Unfortunately, with samples containing 13C in natural abundance, the 13C features are usually hidden in the Gngs of the spectrum of unlabelled spdcies. However, in perdeuterated molecules, hyperfme and superhyperfme couplings of protons are reduced and, comequently, in many cases, determination of 13C coupling tensors becomes possible. For -this reason, most of our experiments were performed
I
&
Ha Fig. 2. ESR spectrum oi malonic acid single crystal irradiated
at 77 K. Spectrum recorded at 90 K.
on perdeuterated malonic acid single crystals. MaIonic &id from Flub (99% purity) was used .without further purification. Perdeuterated malonic acid was obtdined by succesaivk exchanges ofprotons with heavy water at 60°C until the D/H ratio measured by NMR was greater than 100. Single crystals were grown-by slow cooling of saturated sollitions of malonic acid d4 in heavy water and malonic acid in water. &we wanted to correlate ESR results aid crystallographical data, it was necessary to irradiate samples with knotin o$entations with respect to the crystallographical axis . Samples .were irradiated at 4 K 0; ‘77 :‘K by 6oCo y-rays with doses lying between 1020-1022 ..eVg-I. me ESR spectra were recorded with a Jeol (JES IXj spectrometer at 4 K for some experiments and at temp&at&es Qiiq between 77 .K and room temperatures .fortheothers. -
,lOG,
.‘+&a . T:Orientatiokkd cubing were kin&i performed b; Mr. :&rtrand Ir&the,CNET l&oratory. .. :
:.
Fig. 3. -ESRspectrum of konic acid single crystal irradiated at 77 K. Specfxum recorded at 200 K.
M. Kikuchi et al./Radical reactiorls in .malonic acid single cvsral
171
3. Results After y-irradiation at 77 K, we observed the following transformations of the ESR spectra from 77 K to room temperature: - Fig. 2 shows a spectrum recorded at 90 K immediately after irradiation. Lines I and I’ correspond to the two different CH2COOH radicals already observed by Whiffen [la] . One of them results from the loss of the Cl 011012H12 group, the other one by the loss of the other carboxylic group. The other lines on this spectrum cannot be attributed at this stage because the resolution is not high enough. - When the sample is heated to 200 K a narrowing of these lines occurs (fig. 3). It is now possible to see that they correspond to a radical with two weakly anisotropic protons, the HOOCCH,C<~~
x 500
i?OG
radical. Two types of
this radical II and II’ exist and are better separated in a single crystal of perdeuterated malonic acid where the 13C couplings can be measured (fig. 4). - After a warming to 270 K, II and 11’disappear and a single line appears. The 13C spectrum of this radical si~ws that there are also two forms III and III’ (fig. 5). The 13C tensors of those radicals show that they are two different forms of the COOHCH,CO radical discovered by Kwiram [6].
Fig. 5. ESR spectrum of perdeuterated malonic acid single crystal irradiated at 77 K. Spectrum recorded at 270 K.
- Finally, at room temperature, these u radicals disappear in a few hours, giving (fig. 6) the well-known cH(COOH)2 radical (IV). The changes observed in the ESR spectra as a function of temperature show that: (1) CH~COOH is present at all temperatures. (2) The &-I(COOH)Z appears at room temperature and its formation seems to be completely independent from that of CH$OOH. In order to discuss the reactions steps leading to the formation of these radicals we have to determine more precisely the structure of radicals I, I’, II, II’, III, III’, IV and their orientation in ihe matrix.
.?.I. Structure of radicals
Xl000
Fig. 4. ESR sepctrum of perdeuterated malonic acid ti irradiated at 77 K. Spectrum recorded at 200 K.
single crys-
3.1.1. CH~COOH Theanisotropic coupling tensors of the CH2COOH (I and I’) radicals and their directions cosines in a crystalline reference frame are given in table 1. Since fat a protons, the intermediate values of the ardsotropic tensor give the orbital direction and the-lower values the C-H bond direction, we can determine the C,H,,l and C,,H,z directions for radicals I and I’. According to Whiffen [la] , radical 1 is formed by loss of the $021022822 carboxylic group so that the C,H,,H,2 plane is perpendicular to the remaining
M. Kihrchi et aL/Radical
reactions in malonic acid
single
crystal
C,0,,0,,H,2 and radical I’ to the toss of C~O,,O~~H~~. For both of these radicals, COH~~HO~ is in the plane of the remaining carboxylic group. So, our experimental results are consistent with the expected, more stable, configurations of these radicals.
II-
3.1.2. Radicals II and II’ Proton coupling tensors for radical 11and 13C coupling tensors for radicals II and II’ are reported in table 1. In various r-irradiated carboxylic acids, formation of the molecular anion has been postulated [7,8] on the basis of the 13C isotropic coupling constants. Recently ENDOR studies by Iwasaki [2] have shown that in fact these species are the protonated form of tbe molecular anion. Comparison of our experimental data with that of ref. [2] shows that radical II and II’ are certainly the protonated anion of malonic acid . ,OH COOHCH,C,oH . It is to be noted that the values of 13C isotropic coupling constants of radical II (86.2 G) and II’ (112.2 G) are quite different. One possibie explanation for this difference are the environmental effects upon the structure of these radicaIs. Such effects have been observed in~a;;;e~;fal;~i yj$;;;C = 89.7 G) and of suc-
Fig. 6. ESR spectrum of mdonic acid sinde crystal irradiated at 77 K. Spectrum recorded at room temperature.
carboxylic group. This conclusion is rather surprising since the more stable structure of eH,COOH ought to allow the mixing of the rr orbital of the unpaired elec.tron with the II system of the carboxylic group. Ifi order to elucidate this point, we have calculated two hypothetical conformations of these radicals in the crystal. The directions of CO&~, Co&z bonds and .of the unpaired electron orbital were calculated using crystallographicaj data and as&ning identical bond length and angles for the C-COOH part of the radical
and of,ihe molecule in the cjstal. The central atom wassupposed to de s$ hybridized. This calculation Wixsmade. for both types of CH2COOH radicals (loss of~Cz02,0,$Izz oi; C,0,20,,H,2 groups), supposing that,CO~1~2 is ei*er perpendicular or in.the plage ‘of the remaiqing caiboxyljc group, To compare with .$e ex&imerM r&ults we determined the angle 0 be-. tween me calculated and &e experimental directions. able 2 shtiws that radical.1 co&ponds to jhe less of ._ .: .. _.. :,
,_.:
1:
_.
. .
It has already been noted that the two carboxylic groups of the malonic acid molecule are not equivalent in the crystal. As a consequence they will give two different protonated anions corresponding to the location of the unpaired electron on C, or C2. To show that these two radicals are in fact radicals II and II’, we have calculated the angle between the free electron orbital direction in II and 11’and COCl and C& directions in the crystal. For radical II, we find that the axis of the free elec&on orbital makes an angle of 99” with C&z and 18” with the perpendicular to the CzO2,022 plane. Thus the free electron is most probably located on carbon C2 and the C0C202iO22 group becomes pyramidal when the protonated anion ls formed. This conclusion is confirmed by an INDO calculation of the most stable structure of radical II, in excellent agreement with experimental results (table 3). As shown on fig.,7 the structure of radical II e be deduced from that of the. parent molecule by two succes&ve rotations of the carboxylic group: a first o’n&of 35” around a fine A per-
Table 1 Hyperfime tkors
for radicals observedin malonic acid single crystal. All the couplings are given in ~JUS.S
Radid
Principal value
Isotropic coupling ~--
----.-
33.42 Ha1
20.53
0.6172 22.06
12.23
31.87
22.11
12.77 &C00~ (1’)
11.21
21.80
20.82
32.59
26.13 31.32
8.7
74.54 ‘k
86.23
112.2
138.15 ‘3Cc,j 181.00
151.61
51.35
129.2
140.B
11.59 20.61
21.73
0.9964 0.7633
0.7750 -0.2934 0.6111
0.5755
0.520
0.821
-0.408
0.5456
0.7978 -0.5828
0.479 -0.272 0.6547 -0.1519
0.6029 -0.7407 -0.463 0.359 0.811
0.779 -0.273 0.565
0.1498
0.1856
0.9713
0.7691
0.6399
0.0206
0.7463
0.2381
0.8910 -0.3935
0.828
0.9988-
33.00 H(1
0.0631
0.5233
0.460
143.2 CH(COOHI2 (IV)
0.4682 -0.0799
0.753
0.893
0.0385 0.9988
0.8814 -0.0289
0.660
0.321
149.9 “Cc,)
0.777
0.4537
45.34 COOHCHZkO CIII’)
0.9992
0.0170 -0.4672
47.18 61.63
0.5610
-0.6211
135.67
‘3C{2j
-0.4715
-0.152
105.8
(11’) COOHCH2k0 (III)
142.1
-0.0295
0.423
88.7 ‘“c
0.9995 -0.0079 0.0090
0.3408
58.11
COOHCHIC;z;
0.8858
-0.232 9.4
13.6
126.04
-0.1427
-0.5428
6.0 Hp,
0.2526
0.7462
0.6249 -0.1637 21.63
0.7674
0.3894
0.0569
25.44 “0,
0.5748
0.7165 -0.6502
0.8800 21.53
20.20
COOOHCH,c<; (11)
0.5028 -0.6052 0.6456 yO.2118
0.0292 -0.0388
11.79 H=2
---
0.5399 -0.4417
33.38 H01
-0.7337 0.2842
21.68 H=2
Direction cosines
-0.0317 0.0385
0.7923. -0.661 0.545 -OS?7
0.8845 -0.2251 0.4090 0.679 O.138 -0.721
0.0306 -0.0394 0.999.1 ~0.0276 0.0288
0.9988
174
M. Kikuclri et aL/Radicaf
reacrims in malortic acid
single crystal
Table 2 Determinationof CH bond and orbital direction in radicals 1 and I’ e
Direction cosines Loss of C101101zH~z
CoHol and CoHoz coplanar with
CoHol
-0.3031 -0.1551
GO21022~~22
CoH22
-0.3284
0.7316 -0.6701
orbital CoHol and CoHol: in a plane perpendicular to C~OZI 022H22
W-h
-0.9642
CotIol and CoHoz coplanar with
COHOI
C1011’312H12
CoHo2
orbital CoHol and CoHoz in a plane per-
CO~OI
pendicular to C,OIIO~~H~Z
CoHo2
9” 5O
0.1253
5’
0.2622 -0.0412
70”
0.1848 0.9796
73” 85”
0.7149 0.6387 0.0903 -0.9816 -0.1474 -0.0131
0.1153 0.0005 0.9890
20” 11” 10”
orbital
pendiculq.to C& and located in the C202202t plane, a second _ne of 30” around A’ the bisector of the Oz,C20 angle. In radical II, the 021H21 and 022H22. onds are in the C202t022 plane. f For ra;iic‘al II’ the free electron is localised on carbon Cl r&er than C2. The distortion of the parent molecule when this radical is formed is more pronounced than for radical II. The free electron orbital makes an angle of 130”.with C,Co and 8.5” with the direction perpendicular to the C1O11O12 plane of the parent molecule. Since in this particular case the proton couplings are not sufficiently resolved to allow a determi-
0.9402 -0.7966
0.3324 -0.9249 0.0126 0.20b3
CoHo2
orbital Loss of C20z1022H22
-0.5075
0.3971 -0.0538 0.9160 0.5302 -0.2394 -0.8136 0.3766 -0.9221 0.0890
7s”
75” 82”
nation of the coupling tensors, it is not possible to go further in the study of this radical at the present time. 3.1.3. Radical 111 and III’ The experimental values of the 13C coupling tensors found for radical III (table 1) are in good agreement with the results reported by Kwiram [6] for the radical COOHCH#O. The structure
of radical III can be derived from that
Table 3 Comparison of our experimental results for the isotropic hyperfine coupling values wilh other experimental values [2] and INDO c;llculation. Au the couplings are given in gauss Resent work
‘hvomki [2]
INDO utcuhtion
bKxinic) 13c 86.2 k& .2;.; OH2 OH2
..
84.9.
:
:
27.3
27.5
8.36 -0.5 0.85
9.0 2.7 3.2
Fig. 7. Modifications lading to the formation of anion II. The aboxylic group rotates around axes A and A’.
:
. . .
. .
175
M Kihuchi er af./Radical reoctiom in malmic acid singlecrystal
of the parent molecule by abstraction of the 011H12 group. From the values of the r3C couplings tensors and o&he direction cosines of their principal values, the CC0 angle can be estimated to 128”. For radical III’, only one of the carbon 13 tensors can be determined; the anisotropic part of this tensor (+9.1 G; -11.6 G; 2.4 Gj is weaker than that of the corresponding tensor for radical III. This value and the direction cosines are consistent with a radical COOHCH,CO obtained by a C1022 bond rupture and for which large librational motions of Ozl around the CuC, bond occurs. The small difference observed between the largest isotropic i3C coupling constants of radicals III and III’ is certainly due to environmental effects as in the case of radical II and II’. 3.1.4. &((cooH), This radical is one of the first organic free radicals studied [4] and its structure is now very well known. 3.2. Radiation damage process From crystallographical and ESR data, relative orientations of molecules and radicals in the crystal are now very well known; thus it may be possible to see if the reactions which occur in the malonic acid crystal are stereospecific. Radicals COOHCH2 as well as the protonated anions II and II’ are present at 77 K. When the samples are heated from 77 K to 273 K, the concentration of &I$OOH radical remains unchanged; but we observed transformations of radical II leading successively to the formation of radicals III and IV. This behaviour is an indication that CHzCOOH and CH(COOH)* which are relatively stable at room temperature in the T-irradiated malonic acid, do not have the same precursor. This result is consistent with the general scheme of radiation damage observed in other carboxylic acids. According to this scheme, the primary damage consists of anioncation radical pairs which undergo chemical reaction in the crystal; CH$OOH is formed from the cation and CH(COOH)z from the anion3.2.1_ cH$OOH formation Ayscough [9] and Iwasaki [lo] proposed the following scheme for the formation of CH$OOH from the primary cation COOHCH2COOH+:
COOHCH2C //O + ‘0. wQ
0
+
COOHCH,Cc
i
HO ‘CCH,COOH HO/
COOHCH,+CO,.
t I ,
(1)
(2)
0' P0 In order to study these reaction steps we have made an attempt to stabilize radical V or radical VI. Single crystals of malonic acid were irradiated at 4 K and their ESR spectra recorded at this temperature. Unfortunately the observed spectra are identical to those obtained at 77 K, i.e., r?H,COOH and the protonated anion II and II’ are the only radicals present at 4 K. Nevertheless this result does not conflict with the reaction mechanism proposed by Ayscough. The resulting reactions may occur at 4 K since step (1) consists in a proton transfer across a hydrogen bond and step (2) does not need the CO2 fragment to escape out of the cage, the reverse reaction being endothermic by 40 kcal. Another result that can be related to the cationic origin of CH$OOH is the relative proportion of radicals I and I’. ln the crystal we have seen that the two carboxylic groups are not equivalent. It follows that the relative ionisation probability on each of them must be different from unity..If I and I’ proceed from the cation, the ratio I/I’ will not be equal to 1. The experimental result I/I’ = 4 is in good agreement with this assumption. 3.2.2. cH(COOH)2 formation The experimental results strongly suggest that the CH(COOH), radical is formed from an anionic precursor through the following steps 0’
COOHCH#
+ COOHCH,COOH + ‘OH
CVIJ) . PH -0, COOJ-ICH&, + CCH,COOH, OH 0’ (II)
(3)
M
176
COOHCH,e
/OH
et aL/Radicai
Kikdri
+ COOHCH,@
reactions in molonic ocid single crystal
0 + H,O,
(4)
‘OH (II) 0 COOHCH,CT + HOOCCH,COOH + COOHCH,CHO + Hc(COOH)2. W)
(5)
Like step (I), step (3) is a proton transfer through a hydrogerrbond and is achieved at 4 K, so that radical VII.cannot be observed. Step (4) is a dehydration reaction of the protonated anion leading to the formation of an acyl radical. Step (5) is the only bimolecular reaction which is not a proton transfer through a hydrogen bond. The stereospeciflcity of this reaction may be discussed since the structure of radicals and their orientation with respect to molecules in the solid are known. Constructing a model of the crystal one can see which H atom of an adjacent molecule may be abstracted by each type of acyl radicals (III and 111’)we can then determine from the crystallography the distances and angles between the free electron orbital and the H atom. For radical lil, there is a hydrogen atom of a methylene group at 3.9 a from the radical center and the direc-
tion defined by this radical center and the hydrogen atom makes an angle of 46O with the free electron orbital of III. For radical III’ we have seen that there are torsional oscillations or restricted rotations of the C=O group around the C-C bond. During this motion, the free electron orbital of III’ generates a cone, inside which an H atom of a methylene group of an adjacent molecule lies at a distance of 3.7 A from the radical center. Since the relative positions of radical centers and H atoms are different in these two symmetric reactions III --zIV and III’ + IV, the preexponential factors and possibly activation energy should be different. Wnfor.tunately it has not been possible to study the kinetics
_.. .:.-
I
of III and III’ disappearance because their ESR spectra are not resolved and overlap each other. Radicals LII and III’ have been separated and identified only on the basis of their 13C couplings and it is not possible to study their disappearance since the 13C lines have very low intensities. Another interesting feature is that the concentration ratios II/II’ and III/III’ are nearly equal to unity. If the formation of II and II’ proceeds from the anion, this may be due to a very high efficiency of electron attachment on the carboxylic groups leading to equal quantities of radicals II or II’.
References 11) (a) A. Horsfield, J.R. Morton and D.H. Whiffen, Mol. Phys. 4 (1961) 327. (b) J.W. Sinclair and M.W. Hanna, J. Phys. Chem. 71 (1967) 84. (c) H. hluto, T. fnoue and hl. lwasaki, 3. Chem. Phys. 57 (1972) 3220. [2] H. Muto, K. Nunone and M. Iwasaki, J. Chem. Phys. 61 (1974) 1075;5311,5315. [3] (a) DC. Strrwand G.C. Moulton, J. Chem. Phys. 60 (1974) 1223,123l. (b) N. Tamura, M.A. Collins and D.H. Whiffen, Trans.
Faraday Sot. 62 (196612434. (cl J. Sinclair and P. CodeLla, J. Chem. Phys. 59 (1973) 1569. 141 h1.M. McConneJf, C. Heller, T. Cole and R.W. Fessenden, J. Am. Chem. Sot. 8.2 (1960) 766. [5] J.A. Goedkoop and C.H. MacCiJlavry, Acta Cryst. 10 (1957) 12.5. [6] R.C. McCal.leyand A.L. Kwiram, J. Am. Chem. Sot. 92 (1970) 1441. [7] J. Sinclair and M.W. Hanna, J. Chem. Phys. 50 (1969) 2125. [8] H. Box, H. Freund and K. LiJga, J. Chem. Phys. 42 (1965) 1471. 19) P.B. Ayscough and J.P. Oversby, Trans. Faraday Sot. 67 (1971) 1369. jIO] B. Eda and M. Iwasalci, J. Chem. Phys. 55 (1971) 34.42.