Temperature Study of a Glycine Radical in the Solid State Adopting a DFT Periodic Approach: Vibrational Analysis and Comparison with EPR Experiments

Abstract

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.