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

More documents

Recommendations

Info

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
Page 1: RESEARCH PAPERAllyl type radical fo
Page 5 and 6: Structure III is the allylic radica
Page 7: (e.g., succinimide, oxazolidinone 2

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?