TASAS DE VARIACIÓN (%) EN EL IMPUESTO SOBRE PRODUCTOS INTERMEDIOS (ANUALES)

In order clear up the discrepancies between the studies performed by Van- haelewyn et al. [133] and Sagstuen et al. [132], we carried out an EMR study on RT X-irradiated sucrose single crystals. This included extensive EPR, ENDOR and EIE measurements in four different crystallographic planes at 110 K, both in X and Q band. The variation of EPR and ENDOR spectra with measuring temperature was also studied and advanced DFT calculations were performed to evaluate the radical model structures proposed by Sagstuen et al. [132]. The results of these studies are reported in Paper I. We summarise the main results/conclusions here.

• The proton HFC tensors reported in both studies [132, 133] are erroneous to some extent. Consequently, the g tensors reported by Georgieva et al. [134] (and determined based on the HFC tensors in Ref. [133]) are also incorrect, although the size of the g anisotropy probably is approximately correct.

• The study by Vanhaelewyn et al. [133] is qualitatively correct: there are three dominant radical species, two of them being very similar - but distinguishable - in all respects. These are labelled T1, T2 and T3, where T2 and T3 are the similar radicals. Each of the three radical species exhibits three HFC interactions with appreciable isotropic HFC values. • The difference in measuring temperature accounts for the difference

in number of radicals and HFC interactions between Refs. [132] (RT) and [133] (60 K): the ENDOR lines of some HFC interactions are not or hardly visible at RT.

• The difference in measuring temperature also has a small effect on the HFC tensors themselves. However, the maximum shift of the ENDOR lines observed between 60 K and RT are about 2 MHz, and the EPR spectrum is almost unaltered in the range 60 K - RT. This suggests that the HFC tensors most likely are essentially the same throughout this temperature range.

5.3. Own research results

• The correct tensors were determined accurately for eight of the nine HFC interactions. For one of the T1 HFC tensors, however, the Schonland ambiguity could not be eliminated with the available experimental data. Moreover, both sign choices were equally (im)probable because this tensor has somewhat peculiar properties.

• The most plausible radical models proposed in the literature [132] cannot account for the experimental data.

We eliminated the remaining ambiguities for that T1 HFC tensor (see next- to-last item above) by means of ENDOR measurements on a sucrose single crystal at 110 K in yet another, carefully selected skewed plane and RT hyperfine sublevel correlation spectroscopy (HYSCORE) measurements on sucrose powder (Paper III). The final set of the nine proton HFC tensors belonging to radical species T1, T2 and T3 is given in Table 5.3. Experimental ENDOR angular variations and corresponding simulations using the tensors in Table 5.3 are given in Papers I and III. Using the point-dipole approxi- mation and semi-empirical rules, a limited set of plausible radical models was inferred. Extensive DFT calculations (in a periodic approach for the geometry optimisation and in a cluster approach for the calculation of the EMR parameters) were employed to identify the chemical structure of all three radicals (T1 in Paper III, T2/T3 in Paper II). These are shown in Figure 5.5.10

The HFC tensors calculated for the radical models in Figure 5.5 are listed and compared to the experimental data in Table 5.4. From our DFT study on T2/T3 (Paper II) we concluded that in order for T2 and T3 to have such similar HFC tensors, they must have essentially the same chemical structure (T2M in

Figure 5.5) and that the differences most likely arise from a difference in the conformations of T2 and T3, which in turn probably is due to a difference in the conformation of their immediate surroundings. The DFT calculations also suggest that the T2 and T3 conformations must differ rather substantially to account for the observed discrepancies. Note that the calculated eigenvector directions of T2M match the experimental eigenvector directions of T2 better

than those of T3.

T1Mand T2Mshare three common features:

• a broken glycosidic linkage.

• a carbonyl group adjacent to the radical centre.

10_{Although this assignation is quite certain, we wish to make a clear distinction between the} real, physical radical species and the proposed radical models by labelling the models with a subscript M.

5.3. Own research results

Table 5.3: Proton HFC tensors ((an)isotropic values in MHz) of the three dominant radical species T1, T2 and T3 in sucrose single crystals X irradiated at RT, determined from ENDOR measurements at 110 K approximately two days after irradiation. The HFC tensors are taken from Paper I, except Hβ3(T1), which is taken from Paper III.

Radical Proton Iso Aniso Eigenvectors

a* b c T1 Hβ1 46.80 -3.99 0.616 0.121 -0.778 -2.38 0.072 0.975 0.209 6.37 0.784 -0.185 0.592 Hβ2 15.88 -2.69 0.106 0.825 0.555 -2.31 0.989 -0.144 0.025 5.00 0.101 0.546 -0.832 Hβ3 -11.07 -6.41 0.711 -0.241 -0.660 -3.73 0.380 0.922 0.073 10.14 0.591 -0.302 0.748 T2 Hα -38.69 -19.66 0.424 -0.163 -0.891 -2.11 0.886 0.280 0.371 21.77 0.189 -0.946 0.263 Hβ1 16.37 -2.32 0.869 -0.355 -0.344 -1.72 -0.209 0.368 -0.906 4.04 0.448 0.860 0.246 Hβ2 13.68 -3.09 0.718 -0.650 0.248 -2.17 0.638 0.473 -0.608 5.26 0.278 0.595 0.754 T3 Hα -35.81 -18.98 0.584 -0.184 -0.790 -2.11 0.755 0.481 0.446 21.09 0.298 -0.857 0.420 Hβ1 16.42 -2.10 0.840 -0.541 -0.034 -1.77 0.178 0.334 -0.926 3.87 0.512 0.772 0.377 Hβ2 12.24 -3.62 0.528 -0.822 0.214 -2.12 0.804 0.402 -0.439 5.74 0.275 0.403 0.873

5.3. Own research results

Table 5.4: DFT-calculated proton HFC tensors for the radical models T1M(taken from paper III) and T2M(= T3M) (taken from paper II) depicted in Figure 5.5. The geometry optimisations were performed periodically in an <ab2c> supercell, while the HFC tensors were calculated in a cluster cut out of the periodic structure, consisting of the radical surrounded by the ten sucrose molecules to which it is hydrogen bound in the lattice. δ is the angle between the calculated and the corresponding experimental eigenvector directions (cf. 5.3).

Mo- Pro- Iso Aniso Eigenvectors Pro- δ (◦)

del ton a* b c ton T1

T1M H3’ 43.47 -3.86 0.624 -0.220 -0.750 Hβ1 6 -2.27 -0.021 0.954 -0.298 6 6.13 0.781 0.202 0.590 1 H5’ 15.42 -2.37 -0.286 0.788 -0.546 Hβ2 11 -2.24 0.950 0.307 -0.055 11 4.61 -0.124 0.535 0.836 2 H1’ -10.58 -6.81 0.709 -0.136 -0.692 Hβ3 7 -4.62 0.362 0.912 0.193 7 11.43 0.605 -0.387 0.696 6 T2 T3 T2M H1 -34.60 -20.23 -0.451 0.155 0.879 Hα 2 9 -4.66 0.875 0.267 0.403 2 14 24.89 -0.173 0.951 -0.256 1 13 = H3 16.18 -2.16 0.874 0.462 0.154 Hβ1 30 12 -1.86 -0.249 0.152 0.957 30 11 4.02 -0.418 0.874 -0.248 2 11 T3M H5 12.59 -2.88 0.675 0.673 0.303 Hβ2 4 13 -2.06 0.698 -0.450 -0.557 5 10 4.95 0.239 -0.588 0.773 3 12

5.3. Own research results

Figure 5.5:The chemical structures of the models for the three dominant radical species (T1, T2 and T3) in sucrose single crystals X irradiated at RT, determined from DFT calculations as reported in Paper II (for T2M/T3M) and Paper III (for T1M).

• a ring oxygen adjacent to the radical centre.

Roughly half of the spin density is localised on the ring oxygen and the carbonyl group. This delocalisation likely contributes significantly to the stability of the radical, which may explain the dominant presence of these radicals in the stable state.

In the past, the smaller proton HFC interactions were always assumed to arise from β protons because the isotropic components were believed to be too large for γ protons. However, as discussed in papers II and III, the spin delocalisation onto the ring oxygen and the carbonyl group, and the specific orientation of the Cγ-Hγ bonds can indeed induce the observed isotropic

HFC’s in γ protons.11 This type of proton HFC tensors probably does not occur for other types of radical species and they may therefore be considered as fingerprints for radicals similar to T1Mand T2M. On the other hand, radical

structures may easily be conceived with features similar to those of T1M and

T2M (spin delocalisation onto a carbonyl group and/or a ring oxygen), but

e.g. without γ protons or with γ protons oriented differently with respect to the molecular orbitals carrying the spin density.

Our DFT calculations also provided an explanation for the peculiar Hβ3(T1)

11_{Because of the delocalisation, the discrimination between β and γ protons is in fact} somewhat vague.

5.3. Own research results

HFC tensor in terms of a change in the spin density on the carbonyl carbon. The latter originates mainly from the presence of the adjacent ring oxygen and from hydrogen bonding with a neighbouring molecule in the lattice. To the best of our knowledge, this type of proton HFC tensor was not reported before in the literature. Because of the unusual combination of a dipolar- type anisotropic coupling and a negative isotropic coupling, one could easily believe this tensor to be the wrong Schonland variant (when insufficient data are at hand to eliminate the ambiguity, cf. 2.4.2.2, page 44). Clearly, care has to be taken. A HFC tensor similar to Hβ3(T1) may be considered as an indication

of an allylic-type radical fragment like in T1M(Figure 5.5). But again, an allylic-

type radical like T1M does not necessarily exhibit a proton HFC exactly like

Hβ3(T1).

5.3.2 EPR-spectrum evolution during first hours after RT irradiation