Allyl type radical formation in X-irradiated glutarimide crystals ...

RESEARCH PAPERAllyl type radical formation in X-irradiated glutarimide crystalsstudied by ENDOR and ENDOR-induced EPROmer I. Eid,y ab A. Lund* a and E. Sagstuen cPCCPwww.rsc.org/pccpa Department of Physics and Measurement Technology, Linköping University, S-511 83,Linköping, Sweden. E-mail: alund@ifm.liu.se; Tel: þ46 13 282665b Department of Physics, Bayan College for Science and Technology, PO Box 210,Khartoum, Sudanc Department of Physics, University of Oslo, PO Box 1048, Blindern N-0316, Oslo, NorwayReceived15th January 2004, Accepted18th March 2004F|rst published as an Advance Article on the web 27th April 2004Glutarimide single crystals X-irradiated at room temperature were reinvestigated at 150 K with the purpose toobtain information about possible ring opening and other fragmentation processes involving free radicals in thiscompound, previously only observed to take place in aqueous solutions. In previous work, using EPR,ENDORand EIE spectroscopy, two H-abstraction radicals present in a 3:1 relative ratio in the irradiated crystals wereidentified. In the present work the detection of a third radical species, III, is reported. The g- and hyperfinecoupling tensors for all three radicals at 150 K were obtained. Based on simulations of the EPR spectra therelative abundance of the three radicals was estimated to be 60, 25 and 15% for radicals I, II and III,respectively. Radical III is proposed to be of the allyl radical type –CH=CH– CH– formed formally by aconcerted H 2 elimination from C2 and C3 of radical I and/or from C3 and C4 of radical II. This proposal andstructure is at variance with observations from aqueous solution studies of succinimide, where the CO–CH 2bond was susceptible for rupture. However, the assignment is consistent with density functional theory (DFT)calculations, predicting equivalent hyperfine interactions to the two a-type protons at C2 and C4 in excellentagreement with the experimentally determined hyperfine coupling tensors. EIE results confirmed that bothcouplings originate from the same radical species. Possible mechanisms for the formation of radical IIIare discussed.DOI: 10.1039/b400730a1. IntroductionNumerous studies have been performed to elucidate the freeradical chemistry of succinimide, see ref. 1 for a review. Muchinterest has been focused on the properties of the succinimidylradical. An interesting case of ring opening was found to takeplace in aqueous solutions. 2 However, fewer studies have beendevoted to related radicals formed in other imides. Kasai 3prepared the phthalimidyl and 1,8-naphthalimidyl radicals bychemical reduction, and verified their p-electron structure byEPR, in agreement with the result for succinimidyl. 4The radicals previously observed in X-irradiated glutarimidesingle crystals at room temperature were typical of C-centeredp-electron radicals, formally formed by scission of C–Hbonds. 5Ring opening of glutarimidyl radicals does not take place inliquid solution, contrary to the succinimidyl case. 6–8 The mainaim for the research reported in this work was to investigate if,y Permanent Address: Department of Physics, Faculty of Science,University of Khartoum, PO Box 321, 11115 Khartoum, Sudan.E-mail: omer_eid@hotmail.com.and eventually to what extent, ring opening and/or other fragmentationprocesses occur upon X-irradiation in the solidglutarimide matrix by studying the types of radicals that arestabilized using high-resolution variants of EPR spectroscopy,i.e. ENDOR and ENDOR-induced EPR (EIE). The basis forthis was the observation 9 that the ENDOR spectra obtainedat 150 K exhibited new resonance lines in addition to thosealready assigned for radicals I and II. An additional issueaddressed in our previous study 5 was to clarify the internaldynamics observed to occur in radical I. Similar dynamics,probably involving libration motions of C b –H bonds, wasobserved for radical II during the present work but requiresfurther studies.2. Experimental detailsSingle crystals of glutarimide were grown from aqueous solutions,the details of the crystal growing were as described previously.5 The crystal structure of glutarimide was determinedby X-diffraction. 10 The crystals are monoclinic with spacegroup P2 l /c. Fig. 1a shows the molecular packing in the crystaland in particular depicts the asymmetry in the hydrogen bonding,whereas Fig. 1b shows the half-chair conformation of theglutarimide molecule. The crystals were elongated along hbi.The crystals were X-irradiated at room temperature using anX-ray tube operating at 70 kV and 20 mA. Aided by X-diffractiontechniques, each crystal was then mounted to a quartz rodbeing an integral part of an EPR goniometer with the axis ofrotation parallel with the crystallographic a*, b, and c-axes.EPR and ENDOR measurements were made by rotatingthe sample through 5 intervals over 180 about each of thethree axes.3604 Phys. Chem. Chem. Phys., 2004, 6, 3604–3610 This journal is Q The Owner Societies 2004

e determined. 13 The fit was done simultaneously for j ¼ 1to3to obtain the tensor. Spectrum simulations were made usingthe program KVASEC, as described previously. 14 Crystallographiccoordinates were calculated using a modified versionof the crystallographic data program ORFEE, 15 complementedby locally developed programs.The DFT calculations were performed using the GAUS-SIAN98 program package. 16 The single point calculationsused the B3LYP hybrid functional and the 6-311þG(2df,p)basis set based on an optimized structure evaluated usingthe 6-31þG(d) basis set using a starting geometry obtainedfrom the X-diffraction data. 10Fig. 1 (a) The glutarimide structure viewed down hci. Broken linesindicate hydrogen bonds. Only C3=O1 participates in hydrogen bonding,to N1–H of a neighboring molecule around a centre of symmetry.(b) A view of the glutarimide molecule, depicting the half chair conformationaround the C3 atom.The EPR, ENDOR and ENDOR-induced EPR (EIE) spectrawere recorded using an X-band Bruker ESP300E spectrometerequipped with a microwave frequency counter andNMR gaussmeter. The temperature was controlled using aBRUKER ER 4112 variable temperature control unit. ForENDOR experiments, BRUKER’s DICE technology for generationof the rf-field in combination with the BRUKERENDOR cavity (TM 110 ) was employed together with a 200W ENI rf-amplifier. The system was set to generate a rf-fieldsquare-wave-like frequency modulation of the rf-field at12.5 kHz with a modulation depth of typically 140 kHz.The ENDOR signal intensity was first investigated at differenttemperatures and it was noted that for the present systemthe ENDOR enhancement showed a maximum at 150 K.The measuring temperature was hence chosen to be 150 K.The ENDOR frequency n M depends on the orientation ofthe magnetic field l ¼ B/B and is given by 11n 2 M ¼ l M Mg gA n N1g Ag nN1l ð1ÞIt contains contributions from the hyperfine tensor A and fromthe nuclear Zeeman term n N ¼ g N m N B/h, both given in frequencyunits. 1 is the unit tensor, and M ¼ 1 2is the electronicquantum number. A QuickBasic program allowing the evaluationof g- and A-tensors from EPR single crystal measurementsby non-linear or linear least squares fits to eqns. (1) or (2), wasused to derive the proton hfc tensors from the ENDOR data.In general, a non-linear least squares fit has to be employed toextract the tensor elements of A, assumed to be symmetric, butdue to the small g-anisotropy a linear procedure was sufficient.The hyperfine tensor of each coupling was accordingly obtainableby the method of Schonland, 12 adopting the expression:n 2 M ¼ a j þ b j cos 2y þ c j sin 2yThe measured ENDOR frequencies depending on magneticfield orientation y in the three different planes ( j) wereemployed to calculate the values of a j , b j and c j that are linearfunctions of the elements of the hyperfine coupling tensor A toð2Þ3. ENDOR resultsThe EPR spectra obtained after irradiation at 295 K andrecorded at 150 K were difficult to interpret and analyze dueto a large number of overlapping resonance lines from severaldifferent radical species. In contrast, the ENDOR spectraobtained at 150 K revealed a number of well-resolved resonancelines with different intensities. This is illustrated inFig. 2, where the ENDOR lines accounting for the previouslyreported radicals I and II are clearly shown (a I , b I1 , b I2 anda II , b II1 , b II2 ).z This ENDOR spectrum is recorded with theexternal magnetic field parallel to ha*i. New resonance lines,not previously detected at room temperature were clearlyobservable in the spectrum at 150 K. Based on the experimentalresults presented below it has been concluded that theselines are due to a new radical species denoted III, and the resonancelines are given the symbols a III1 and a III2 . Additionallines were observed in the 15–22 MHz range. They were difficultto follow, and could not be unambiguously attributed to aparticular species. The significant line broadening observed forthe two b-protons of radical II may be due to the dynamicsinvolved in this system. Different dynamical models were discussedin our previous paper. 5 The dynamics is not discussedin the present work.Figs. 3a–cz show the dependence of the ENDOR resonancefrequencies measured at 150 K with the angle of rotation of theFig. 2 ENDOR spectrum of a glutarimide single crystal X-irradiatedat 295 K and recorded at 150 K with the static magnetic field parallelto ha*i. Lines a I , b I1 , b I2 belong to radical I; lines a II , b II1 , b II2 are dueto radical II; lines a III1 , a III1 have been assigned to radical III. Linemarked by arrow could not be analyzed.z An ENDOR line is superscripted with þ if it the transition originsfrom the M ¼ 1/2 manifold of states, and with if it origins fromthe M ¼ 1/2 manifold.This journal is Q The Owner Societies 2004 Phys. Chem. Chem. Phys., 2004, 6, 3604–3610 3605

Fig. 3 Angular variation of the ENDOR line positions for radial I(a), II(b) and III(c) from measurements in the a*b, bc and ca* planes of theglutarimide single crystal irradiated at 295 K and recorded at 150 K. a I , b I1 , b I2 lines are due to radical I, a II , b II1 , b II2 lines are due to radical II,and lines marked a III1 , a III2 due to radical III.crystal for each of the three orthogonal planes of rotation. Thelines belonging to radicals I and II were easily followed in thethree planes a*b, bc, and ca*, as indicated. The hyperfine couplingsof the two radicals I and II at 295 K were reported in aprevious paper, 5 but due to the different temperature of measurement,the couplings are significantly different in the presentcase and were re-analyzed to obtain sufficiently precise data forEPR spectrum simulations at this temperature. In addition,new lines originating from radical III were easily detected, asdemonstrated in Fig. 3c. The six ENDOR transitions labeleda I , b I1 , b I2 , a II , b II1 , b II2 , a III1 and a III2 , could be fairly wellfollowed in all three planes. The a-label indicates resonancelines exhibiting a large angular anisotropy, characteristic forthe interaction with a proton in a-position to the site of majorunpaired spin density. The b-label indicates a large hyperfineinteraction with small hyperfine anisotropy, typical for hyperfineinteraction with a proton in b-position to the major site ofunpaired spin density.The hyperfine tensor of each coupling was obtained asdescribed above, and the principal values and correspondingeigenvectors are given in Table 1.4. EIE resultsIn addition to the practice of using the EIE technique in differentiatingbetween different radicals contributing to a complexEPR resonance pattern, it can also be used to obtain theg-tensors for the various radicals. Calculations of the g-valueof an EPR pattern require knowledge of the value of the staticmagnetic field at the center of the resonance (as well as themicrowave frequency of the experiment). From the ENDORspectrum only the hyperfine coupling parameters may beobtained. In EIE, first the ENDOR spectrum for an EPR lineis recorded. Next, the intensity of one of these lines is observedby locking the rf oscillator to a particular ENDOR frequencywhile sweeping the magnetic field. For each ENDOR line monitored,an EIE spectrum is observed. For I ¼ 1/2, the sameEIE spectrum occurs for all monitored ENDOR lines belongingto the same radical. Alternatively, if the ENDOR lines donot belong to the same radical, different EIE spectra will result.Thus, measuring the EIE at a sufficient number of ENDORfrequencies at a sufficient number of orientations will enableextracting the g-tensors for the radicals contributing to thecomplex, overlapping EPR pattern. This procedure wasrecently discussed in detail by Nelson et al. 17Fig. 4 shows the EIE spectra obtained at 150 K with theexternal magnetic field parallel to hbi. The spectra (I, II, II)were recorded upon locking the rf to the a I , a II , and a III resonancelines respectively. The EIE spectra obtained from thea III1 and a III2 lines were almost identical and very differentfrom the EIE spectra recorded when irradiating at the a I anda II positions. This supports our hypothesis that a differentradical species, III, causes these ENDOR transitions.In the present work, a number of EIE spectra were obtained asdescribed above for the three different radicals at different crystalorientations relative to the magnetic field. These data enabledthe calculation of the g-tensors for each radical using basicallythe same procedure as that described for hyperfine couplings. 7The g-tensors for the three radicals are listed in Table 2.From the ENDOR and EIE experiments it cannot be concludedif the substructure (see Fig. 4) is caused by hyperfineinteraction with H, N, or both. Lines from weakly coupledprotons did appear in the ENDOR experiments but couldnot be unambiguously assigned to radical III species.5. Radical identificationThe X-irradiated glutarimide single crystal studied at 150 Kyields three different radicals. Radicals I and II were identifiedpreviously to have the structure shown in the scheme below. 5The new radical, III, exhibiting anisotropic hyperfine couplingto two protons is the main issue of the present study.3606 Phys. Chem. Chem. Phys., 2004, 6, 3604–3610 This journal is Q The Owner Societies 2004

Table 1 The hyperfine coupling tensors (in MHz) obtained from theENDOR angular variation data for the X-irradiated glutarimide singlecrystals measured at 150 KTensorIsotropicvaluesPrincipalvaluesEigenvectors aa* b ca I 55.73 25.54 0.3696 0.9167 0.152155.34 0.4693 0.0429 0.882086.30 0.8020 0.3973 0.4460b I1 132.24 127.00 0.1196 0.9928 0.0098130.12 0.6827 0.1088 0.4938139.60 0.3913 0.0506 0.8695b I2 37.98 33.62 0.1491 0.4273 0.893434.44 0.4878 0.8174 0.306345.88 0.8601 0.3902 0.3286a II 54.40 24.42 0.9460 0.1628 0.280254.58 0.2941 0.0682 0.953384.20 0.1361 0.9843 0.1124b II1 59.64 56.18 0.9873 0.0000 0.158857.46 0.1588 0.0000 0.987365.28 0.0000 1.0000 0.0000b II2 110.37 104.68 0.7289 0.3785 0.5705110.98 0.6675 0.2077 0.7151115.46 0.1522 0.9020 0.4040a III1 33.59 15.15 0.9472 0.1133 0.300134.02 0.2990 0.0263 0.953951.60 0.1160 0.9932 0.0089a III2 34.61 16.07 0.3917 0.9093 0.140234.78 0.3233 0.0066 0.946352.98 0.8614 0.4160 0.2914Crystallographic directions:C4–C5 bond direction 0.6156 0.7762 0.1359N–C1 bond direction 0.3585 0.9075 0.2187N–C5 bond direction 0.9162 0.2186 0.3358Perpendicular to the N–C5–C4 plane 0.3212 0.1000 0.9417a Eigenvectors are given for one magnetic site in the unit cell, reversingthe sign of the b-component yields the other site.The two protons belonging to radical III show relativelylarge anisotropic hyperfine coupling behaviour with an angulardependency similar to those of a I and a II . These are spectralcharacteristics of protons directly bonded to carbon atom(s)carrying the main unpaired spin density, that is, a-protons.The eigenvectors associated with the intermediate principalcomponents of the coupling tensors a III1 , and a III2 are within1.8 of being parallel to each other, see Table 1. Similarly,the angle between the two eigenvectors associated with theminimum principal components of the two a-proton couplingtensors of radical III is 121.1 . The very similar isotropic couplingvalues both indicate a 2pp-spin density of about 0.48based on the McConnell relation 18 using a Q-value of 70MHz. The dipolar coupling elements likewise indicate a p-spindensity of 0.48 (ref. 18) and hence the available experimentaldata suggests a planar p-electron structure with trigonal hybridizationfor the a-carbon(s) to which both protons are bonded.According to the crystal structure data (see Table 1), theangle between the C4–C5 bond direction and the direction correspondingto the sum of the eigenvectors for the minimumprincipal tensor component of a III1 , and a III2 ( 0.5646,0.8091, 0.1625) is 3.6 . The angles between the normal to theN–C5–C4 plane and the intermediate principal componentsof the two a tensors were at most 5.4 i.e. almost parallel.Furthermore, the eigenvectors for the minimum principal tensorcomponents of a III1 , and a III2 make angles of 6.6 and 4.9 with the N–C5 and N–C1 bond directions, respectively. Onemay also note that the smallest g-factor component of radicalIII is near the free electron value and oriented approximatelyFig. 4 ENDOR-induced EPR spectra obtained at 150 K for anX-irradiated single crystal of glutarimide. The static magnetic field isparallel to hbi. Spectra (I), (II), and (III) are obtained upon lockingthe external magnetic field to the ENDOR lines a I , a II and a III lines,respectively.along the N–C1–C5 normal (deviation 15.3 ), as expected fora p-radical. Similarly, the eigenvector for the maximum g-value component is close to the N–H bond direction (deviation17.8 ).The experimental results reported above are in agreementwith three possible radical candidates, III, IVa and IVb below,as well as various tautomeric forms of these.Table 2 ENDOR-induced EPR determined g-tensors of the threeradicals I–III formed in X-irradiated glutarimide single crystalsrecorded at 150 KTensorPrincipal valuesEigenvectors aa* b cg I 2.0010 0.3820 0.1865 0.90522.0038 0.7420 0.6458 0.18012.0058 0.5509 0.7404 0.3851g II 2.0008 0.5088 0.4477 0.73532.0032 0.6183 0.4043 0.67392.0055 0.5990 0.7976 0.0711g III 2.0013 0.2612 0.3548 0.89772.0046 0.5099 0.7389 0.44042.0050 0.8196 0.5728 0.0121a Eigenvectors are given for one magnetic site in the unit cell, reversingthe sign of the b-component yields the other site.This journal is Q The Owner Societies 2004 Phys. Chem. Chem. Phys., 2004, 6, 3604–3610 3607

Structure III is the allylic radical 19 characterized by the twosimilar a-proton couplings, each associated with a little lessthan 50% spin density. The central carbon (C3), which isnow situated in the main molecular plane, is expected to carrya small, negative spin density yielding a third a-type couplingwith eigenvectors in directions somewhat different from thoseof an ordinary a-proton coupling. 13,19 The g-tensor will bevery dependent upon the tautomeric nature of the radical,whether it is protonated at one or both of the oxygens, eventuallyin combination with deprotonation of the nitrogen.The actual structure depicted above is expected to exhibit amaximum g-value in the C1–C5 direction, apparently in contradictionwith observations. However, the distinction betweenthe maximum and intermediate principal g-values is small andprobably within experimental uncertainties. Except for the failureof clearly observing the third a-proton interaction in theENDOR spectra (see also Fig. 3), all other reported spectralcharacteristics are compatible with this basic type of radicalstructure.The possibility for structures IVa (b) to be compatible withthe available data has to be considered. These a–CH 2 structureswith a C=O group attached to the methylene group arevery similar to those observed for radicals of the typeCH 2 COO stabilised, e.g., in irradiated malonic acid 20,21and zinc acetate. 22,23 The principal values of the proton tensorsa III1 and a III2 in Table 1 are, however, numerically much smallerthan those for the CH 2 COO radical, e.g. ( 15.15 34.0251.60) MHz as compared to ( 29.6, 58.0 92.4) MHz inzinc acetate 23 and similar values in malonic acid. 21 Unlessthe actual spin distribution will be much dependent on the tautomericnature of the radical as well as the nature of the fragmentR (being –CH=CH 2 if only a plain ring opening betweenC4(C2) and C3(C1) takes place) the assignment to radical IVappears less probable. The DFT calculations reported belowdid not provide evidence for such dependency. From this itis concluded that the a–CH 2 structures are not compatible withthe experimental data, and that a ring opening of the glutarimidering is not likely to occur in the solid state, in agreementwith observations in liquid solution. 6–8A good test for the correctness of the analysis is using EPRspectrum simulations. Fig. 6 shows the result of adding thesimulated spectra based on the recorded g- and hyperfine couplingtensors from the three radicals in different amounts.Good agreement with the experimental spectrum was achievedwith the relative weights of the added spectra being 60, 25, and15% for radicals I, II, and III, respectively. One also notes thatupon ignoring radical III and only considering I and II withthe ratio 3:1 the simulated spectrum does not accurately fitwith the experimental one, yielding in particular a clear discrepancyin the centre of the spectrum.6. DFT calculationsThe experimental data referred to the above indicate that theradical center is planar, delocalised with a spin density eitheron an a–CH 2 as in structures IVa and b of about 0.5, or moreprobably in an allyl type radical III. This issue was furtheraddressed using quantum chemical calculations in the hopeof obtaining information about the nature and degree ofspin delocalisation at adjacent atoms. Gaussian98/DFTcalculations were performed for this purpose, as described inSection 2.a–CH 2 structures IVThe starting point for this part of the calculations was the conclusiondrawn experimentally that the a–CH 2 fragment had tobe at the C2 or C4 position. This conclusion was based on thecomparison of crystallographic coordinates for the moleculeFig. 5 (a) The fully optimized structure of the O1-protonated versionof radical structure IVa. (b) The fully optimized radical structure III.and ENDOR and EIE data for the radical. This seems difficultto accommodate in an intact glutarimide structure, and a ringopening to yield structure IV was therefore considered. If thering opening is to proceed from an imidyl precursor radical,then a b-(isocyanatocarbonyl) alkyl radical denoted PI in thework by Lind et al. has been inferred to form. 1,2 Since no largehyperfine couplings attributable to b–H at carbon positionsadjacent to the a–CH 2 are observed, this structure seemsexcluded and was not considered in the calculations. A structurelike IVa,b above is more in line with the experimentalobservations and was used as a starting point for the calculations.The structure of the terminating R group could not bededuced from the experiments, and was initially assumed tobe vinyl, –CH=CH 2 . Leaving all bond lengths and bondingangles free for optimization, the results shown in Table 3 wereobtained. The calculated isotropic couplings of the two a-protons,about 52 MHz, are smaller in magnitude than thoseexpected for a p-electron planar structure with unit spin densityand corresponds to a calculated spin density of ca. 0.89at C2, the remaining part residing mainly on the O4 oxygen(0.25). The magnitude of the couplings is larger by a factorof 1.5 than the experimental ones of ca. 34 MHz. Also thecalculated dipolar a-proton couplings in the principal axes systemare larger in magnitude than the experimental ones byapproximately the same factor. On the other hand, these calculatedresults are quite similar to the experimental data for theCH 2 COO commented on above.Different protonation states of the carbonyl oxygens and thenitrogen atom of structure IVa might influence the spin densitydistribution. However, DFT calculations on a variety of differentlyprotonated/deprotonated structures did not yield anyresults comparable to those obtained experimentally. Likewise,the possibility of a different spin localization depending on thenature of the terminating group R was also investigated.3608 Phys. Chem. Chem. Phys., 2004, 6, 3604–3610 This journal is Q The Owner Societies 2004

Table 3 Hyperfine couplings (in MHz) for radical structures III andIVa calculated using DFT methods, as compared with experimentaldataIII a IVa a ExperimentalAtomisotr. dipolar isotr. dipolar isotr. dipolarH a (C 2 ) 32.09 19.20 51.97 31.64 34.61 18.371.09 0.38 0.1720.29 34.40 18.54H b (C2) 32.09 19.20 51.74 31.64 33.59 18.011.09 1.81 0.4320.29 33.45 18.44N 3.14 2.26 1.69 0.84b0.98 0.351.28 1.19H(NH) 0.18 2.15 1.95 2.10b0.13 1.542.28 3.64H(C3) 9.75 4.11 — —b1.35 —5.45 —a Calculated using the completely optimised structures b Barelyresolved substructure shown in Fig. 4 in EIE spectrum of Radical III.Assuming R to be methyl, CH 3 , the calculated spin densitiesand hyperfine coupling constants were, however, similar tothe ones obtained with vinyl as a terminating group. It seemstherefore that the terminating group has little influence onthe local structure at the radical centre.These findings indicate that the assignment to an a–CH 2radical is not compatible with the calculations, and that a ringopening of the glutarimide ring appears less likely also from atheoretical point of view.Allyl-type radical IIITable 3 also shows the results from the DFT calculation onstructure III. It appears that the a-proton hyperfine couplingconstants are in excellent agreement with those experimentallyobserved, with regard to the isotropic as well as to the anisotropiccomponents. There is also a very good agreement withthe allyl-radical parameters determined using EPR spectroscopyof X-irradiated glutaconic acid crystals by Heller andCole. 19 The major argument against this structure is the lackof a clear ENDOR line due to the proton at C3. As commentedabove, there are lines in the 15–22 MHz range that weredifficult to follow (see, e.g., line marked with arrow in Fig.2). One might envisage several reasons for this and an importanttask for future experiments will also be a more detailedexamination of the ENDOR spectra in the weakly couplingregion. However, the EIE spectrum (III) in Fig. 4 shows clearfeatures that could, in part, be due to additional small couplings(other possible sources are satellite lines and forbiddentransitions). This spectrum is recorded with the externalmagnetic field along hbi, which is also in a direction wherethe C3–H coupling should be fairly large as the (numerically)minimum value will occur perpendicular to the molecularplane. The maximum value is predicted from the DFT calculationin Table 3 to be about 15 MHz or more than 0.5 mT.7. Radical formationThe formation of an allylic radical from the initially saturatedglutarimide molecule represents a mechanistically complexalthough structurally simple situation. The lack of knowledgeabout precursors for the radicals observed after X-irradiationat 295 K leaves only speculations to be made at this stage.Fig. 6 EPR spectra at 150 K from a single crystal of glutarimide withthe static magnetic field parallel to hci. Top: Simulated spectrumobtained using the parameters in Tables 1 and 2 and adding the threeradicals I, II, and III in relative amounts 60, 25 and 15%, respectively.Middle: Experimental spectrum. Bottom: Simulated spectrumobtained by adding radicals I and II only in relative amounts 75 and25%, respectively. Note the discrepancy in the centre of the spectrum.One might envisage that radicals I or II are formed either byremoving the axial or the equatorial hydrogens from C2 orC4, respectively (see Fig. 1b). If one of the axial hydrogens isabstracted, it appears that only small rearrangements wouldbe required for obtaining the stable structures I or II. However,if one of the equatorial hydrogens is abstracted, significantelectronic and geometric rearrangements would followin order to attain the most stable structure. It is possible in thissituation that it is energetically more favourable for the reactionrather to proceed by a concerted H 2 elimination towardsthe stable allylic-type radical as endpoint. This might well bean overall exothermic process considering the extensive delocalizationof the unpaired electron over the entire molecular skeletonand the associated gain in resonance stabilization energy.8. ConclusionThe study of the glutarimide single crystals at 150 K revealedthe existence of a new radical (III) in addition to those (I andII) already reported formed by X-irradiation at 295 K. Thepresent results did not provide evidence for ring opening processesto occur in glutarimide in the solid state. Radical III,most probably an allylic type radical, provides indirect evidencethat radiation induced reactions involving hydrogenelimination can take place in solid phase glutarimide. Theseresults suggest strongly that further studies should bemade of glutarimide and also other simple cyclic imidesThis journal is Q The Owner Societies 2004 Phys. Chem. Chem. Phys., 2004, 6, 3604–3610 3609

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