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

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