TASAS DE VARIACIÓN (%) EN EL IMPUESTO SOBRE EL ALCOHOL Y BEBIDAS DERIVADAS

Another type of EMR studies has focused on the interpretation of the EPR spectrum of solid sucrose X irradiated and measured at different tempera- tures [130–134] (Table 5.1). The EPR spectrum of irradiated sucrose powder

Table 5.1: Overview of some experimental parameters in the most important EMR literature studies on radiation-induced defects in solid sucrose.

Reference [130, 131, 135] [132] [133] [134]

Irrad. temp. ∼4 K 285 K RT RT

Meas. temp. ≤4 K RT 60 K 5 K - RT

Micr. freq. (GHz) 9.5, 70 9.5, 35 9.15 9.73, 94, 190, 285 is quite difficult to interpret mainly due to its multicomposite character and the presence of various anisotropic hyperfine coupling (HFC) interactions for each of the radicals. In general, this is an undesirable feature for a dosimeter because the different radical species may exhibit different dose response, time stability, saturation behaviour, . . . Knowledge of the individual components allows interpreting observed spectral changes in terms of the individual components and can thus be helpful in developing dosimetric protocols. This was the primary aim in the studies by Vanhaelewyn et al. [133] and Georgieva et al. [134] Another motivation for studying the radiation-induced radicals in sucrose is to gain insight in the direct radiation effects in the deoxyribose moiety of DNA, as was the case in Refs. [130–132, 135]. Comparable EMR studies were performed on numerous other saccharides, e.g. rhamnose [103, 109, 130, 136–139], glucose [140, 141], methylglucose [8, 99, 142, 143], glucose 1-phosphate [108, 144, 145], fructose [146, 147], and trehalose [148, 149]. The primary aims of these studies were identification of the radiation-induced radicals and radical processes. In the current context, however, it makes more sense to group the studies on sucrose according to the temperature at which radiation was performed: approximately RT [132–134] or 4 K [130, 131, 135] (Table 5.1).

5.2.2.1 Irradiation at RT

After irradiation of sucrose single crystals at RT, composite EPR spectra are observed at conventional microwave frequencies (X and Q band) [132–134]. EPR and ENDOR measurements at RT on sucrose single crystals by Sagstuen et al. [132] allowed for the determination of five proton HFC tensors that were assigned to two different radicals. The g tensor of one of these species was

5.2. EMR results in the literature

extracted from Q-band EPR angular variations and possible radical models were proposed for both species. In the study by Vanhaelewyn et al. [133] EPR, ENDOR and EIE measurements at 60 K revealed nine proton HFC tensors that were assigned to three dominant species. Two of these were found to be very similar, but clearly distinguishable. The three radicals were shown to dominate the EPR spectrum, but an analysis using a multivariate statistical method called MLCFA7 (Maximum Likelihood Common Factor Analysis) revealed that there are at least three other minority radical species contributing to the EPR powder spectrum. The number and characteristics of the HFC tensors are clearly different from those reported by Sagstuen et al. , although several tensors are similar.

Finally, a multifrequency EPR study at different temperatures (Table 5.1) by Georgieva et al. confirmed the presence of at least three different stable radicals. By means of EPR spectrum simulations and using the proton HFC tensors reported by Vanhaelewyn et al. [133] the g tensors of these three radical species were derived.

As will be discussed in Section 5.3.1, we showed all three studies to be erroneous (to some extent) in a new, extensive EMR study (Paper I).

5.2.2.2 Irradiation at 4 K

EMR measurements at 4 K after X irradiation at 4 K or lower (see Table 5.1) reveal the presence of several radiation-induced radical species in sucrose single crystals [130, 131, 135]. Two species were characterised in detail and identified: an intermolecularly trapped electron [130, 131] and an O3’- centred alkoxy radical [135]. The results of these studies will be of interest in Section 5.3.3 and are discussed in more detail below. A significant part of the EPR absorption observed in these studies cannot be attributed to either of these species and are, based on the small g anisotropy around g≈2.0030, most likely due to carbon-centred radicals.

5.2.2.2.1 The trapped electron Sucrose is one of several organic single crystals in which electrons are trapped at intermolecular sites after low- temperature irradiation. Other examples include arabinose, rhamnose, glu- cose 1-phosphate, xylitol, sorbitol, dulcitol, hexanediol and octanediol [108, 130, 131, 151–157]. The electrons are trapped in between two or more neighbouring hydroxy groups and are assumedly stabilized by the (partially induced) dipolar fields of these hydroxy groups. Such trapped electrons are characterised by

5.2. EMR results in the literature

• strongly anisotropic HFC interactions with two or more exchangeable protons. The anisotropic part exhibits axial symmetry (in the case of sucrose: ∼(-15, -15, +30) MHz) while the isotropic part is positive and can be quite large (in the case of sucrose:∼+50 MHz).

• a nearly isotropic g tensor with all principal g values slightly smaller than the free-electron value (in the case of sucrose: g∼2.0020).

These features are quite specific and clearly different from those of carbon- or oxygen-centred radicals (cf. Sections 4.4.2.1 and 4.4.2.2).

Budzinski et al. used ENDOR angular variations to determine two HFC tensors due to exchangeable protons for the trapped electron in sucrose [130]. A more detailed study was undertaken by Box et al., in which four extra HFC tensors were determined: one exchangeable and three non-exchangeable [131]. Interestingly, the non-exchangeable couplings all have negative isotropic components, which was qualitatively explained by the authors in terms of a simple valence bond model. By calculating the electric potentials arising from hydroxy dipoles and by using the HFC tensors of the three non-exchangeable protons,8 the trapping site was identified: along (but not on) the C5-C6 and C6-O6 bonds (site I in Figure 5.3). Finally, it should be noted that the trapped electron in sucrose was reported to be stable up to 61 K.9

5.2.2.2.2 The alkoxy radical The alkoxy radical present in sucrose at 4 K after X irradiation at 4 K was thoroughly characterized by Box and Budzin- ski [135]. Six proton HFC tensors as well as the g tensor were determined from EPR and ENDOR angular variations. Because these will be of interest in Section 5.3.3, they are listed in Table 5.2. We note that the choice of sign of the HFC tensors is arbitrary. It will be shown in Section 5.3.3 that the correct sign was chosen by the authors with exception of A3. Box and Budzinski proposed the radical structure depicted in Figure 5.4: an O3’- centred alkoxy radical where the HO3’ proton has migrated (hopped) to the O4’ oxygen of a neighbouring molecule (to which it is hydrogen bound in the pristine lattice). The maximum principal g value of alkoxy radicals in other organic compounds typically varies between 2.020 and 2.110 (but are most often larger than 2.040), and the minimum principal g value between 1.970

8_{Using the point-dipole approximation, approximate values can be calculated from the} anisotropic HFC values for the distance between the electron and the interacting proton and for the orientation of the line connecting them, relative to the crystal lattice (cf. Section 4.3.1.1). The exchangeable protons may be expected to reorient when the electron is trapped, while the non-exchangeable protons are more distant and therefore more reliable for this purpose.

9_{Our own results, however, indicate it is not stable after in-situ X irradiation at 10 K. This is} discussed in Section 5.3.3.3, page 126.

5.2. EMR results in the literature

Figure 5.3: Possible sites for electron trapping (indicated with Roman numerals), based on calculations of the electric potentials arising from hydroxy dipoles in the sucrose lattice. Using the anisotropic part of the non-exchangeable proton HFC tensors, site I is found to be the most likely trapping site. The figure is taken from Ref. [131].

Figure 5.4: Left: the chemical structure of the alkoxy radical present in sucrose single crystals after in-situ X irradiation at 4 K, proposed by Box and Budzinski [135]. Right: attribution by Box and Budzinski of the HFC tensors A1-A6 (cf. Table 5.2) to the protons in this model.

and 2.002 [94, 103, 106–110]. For the present alkoxy radical the maximum value (2.027) is small but within the range, while the minimum value (2.0037) is markedly larger than any value observed so far. Furthermore, the HFC