E. Pauwels

The calculation of the g tensor of the main +NH3–˙CH–COO− radiation-induced radical in solid-state α-glycine presents a real challenge to computational methods. Density functional calculations of this spectroscopic property struggle with its small anisotropy and the zwitterionic nature of the amino acids in the crystal of this seemingly simple system. Here, several factors influencing the calculated g tensor are examined by comparing with experimental data. The extent of the molecular environment is varied in both a cluster and a periodic approach and dynamic calculations are performed to account for temperature effects. The latter does not necessarily lead to a better agreement with experiment than a static calculation. Application of a periodic approach is straightforward, but an all-electron scheme clearly is favorable. In a cluster approach, the selected basis set and density functional are of less importance, provided a hybrid functional is used to prevent cluster boundary effects. The applied spin–orbit coupling operators and proper treatment of the gauge origin of the magnetic vector potential also seem to be less critical than in other, similar molecular systems. But a careful selection of the cluster size proves to be essential for this glycine radical system. The calculated g tensor varies significantly with increasing cluster size, yielding only a good agreement with experiment when 5–7 glycine molecules in the immediate environment of the central glycine radical are incorporated. Further expansion of the cluster size can even lead to an essentially incorrect description of the radical in the condensed phase, indicating that bigger clusters can become unbalanced.

Recently, the chemical structure of two of the three major stable radicals (T2 and T3) produced in sucrose single crystals by X-irradiation at room temperature was identified by comparing Density Functional Theory (DFT) calculations of Electron Magnetic Resonance parameters with experimental results [H. De Cooman, E. Pauwels, H. Vrielinck, E. Sagstuen, F. Callens and M. Waroquier, J. Phys. Chem. B, 2008, 112, 7298–7307]. Ambiguities concerning an unusual proton hyperfine coupling (HFC) tensor prevented the identification of the third major stable radical (T1). In the present work, experimental results of continuous wave Electron Nuclear Double Resonance experiments on sucrose single crystals and Hyperfine Sublevel Correlation Spectroscopy experiments on sucrose powder are presented that lift these remaining ambiguities. Using the final set of experimental HFC tensors and employing advanced DFT calculations, the chemical structure of the T1 radical is established: an allylic-type radical with approximately half of the spin density localised on the C2′ carbon of the fructose unit, involving glycosidic bond cleavage at the fructose side and a concerted formation of a carbonyl group at the C1′ carbon. The electronic structure of the T1 radical is discussed in more detail by means of additional DFT calculations, yielding a better understanding of the peculiar properties of the unusual proton HFC tensor mentioned above.

Four radiation-induced carbon-centered radicals in dipotassium glucose-1-phosphate dihydrate single crystals are examined with DFT methods, consistently relying on a periodic computational scheme. Starting from a set of plausible radical models, EPR hyperfine coupling tensors are calculated for optimized structures and compared with data obtained from EPR/ENDOR measurements, which are described in part I of this work. In this way, an independent structural identification is made of all the radicals that were observed in the experiments (R1−R4) and tentative reaction schemes are proposed. Also, the first strong evidence for conformational freedom in sugar radicals is established: two species are found to have the same chemical composition but different conformations and consequently different hyperfine coupling tensors. Analysis of the calculated energies for all model compounds suggests that the radiation chemistry of sugars, in general, is kinetically and not necessarily thermodynamically controlled.

The major radiation-induced radical in crystalline glycine is examined using DFT calculations, in which both molecular environment and temperature are accounted for. This is achieved by molecular dynamics simulations of the radical embedded in a supercell under periodic boundary conditions. At 100 and 300 K, a vibrational analysis is performed based on Fourier transformation of the atomic velocity autocorrelation functions. By the use of a novel band-pass filtering approach, several vibrational modes are identified and associated with experimental infrared and Raman assignments. Decomposition of the calculated spectra in terms of radical motion reveals that several vibrational modes are unique to the radical, the most prominent one at 702 cm(-1) corresponding to out-of-plane motion of the paramagnetic center, inversely coupled with similar motion of the carboxyl carbon. A hybrid periodic/cluster scheme is used to evaluate the EPR properties of the glycine radical along the MD trajectories resulting in temperature dependent magnetic properties. These are compared with available experimental data conducted at 77 K and room temperature. Ground state or low temperature calculations yield very good agreement with 77 K experimental EPR properties. From the 300 K simulations, an important improvement is achieved on the isotropic hyperfine coupling of the (13)C tensor, which becomes closer to the value measured at room temperature. It is established that this is the result of a nonlinear relation between the planarity of the radical center and the isotropic couplings of the nuclei bound to it. Finally, a critical reevaluation of the experimental (14)N hyperfine tensor data strongly suggests that an erroneous tensor was reported in literature. It is convincingly shown that from the same experimental data set a different tensor can be derived, which is in substantially better agreement with all calculations.

The results are presented of an electron magnetic resonance analysis at 110 K of radiation-induced defects in sucrose single crystals X-irradiated at room temperature, yielding a total of nine 1H hyperfine coupling tensors assigned to three different radical species. Comparisons are made with results previously reported in the literature. By means of electron paramagnetic resonance and electron nuclear double resonance temperature variation scans, most of the discrepancies between the present 110 K study and a previous 295 K study by Sagstuen and co-workers are shown to originate from the temperature dependence of proton relaxation times and hyperfine coupling constants. Finally, radical models previously suggested in the literature are convincingly refuted by means of quantum chemical density functional theory calculations.