Abstract
In this work we focus on the spin-rotational contribution to transverse relaxation rate 1/T2 for a nitronyl nitroxide radical (PTIO) in fluid solution of toluene. We recorded the X-band continuous-wave electron paramagnetic resonance spectra of the radical in a wide temperature range and compared them with the calculated spectra. The calculation was afforded in two steps: first, we calculated the spectral profiles in the same range of temperatures taking into account the hyperfine interaction with the two 14N nuclei, by integration of the stochastic Liouville equation with the E-SpiReS package in the presence of the tumbling motion of the molecule, which modulates the hyperfine and the g tensors. In the second step, we included the proton hyperfine structure by convoluting the spectrum with the pattern due to the 17 PTIO-coupled protons. A further Lorentzian broadening was added by a best fitting procedure to reproduce the experimental linewidths. The additional broadening is associated with a relaxation rate, Wγ, which varies linearly on kB; this trend is expected for the spin-rotational relaxation term modelled by the well-known Atkins–Kivelson expression. We found a discrepancy between the two parameters associated with the radii of the radical, obtained either from the rotational diffusion tensor in the framework of the Debye–Stokes–Einstein model or from the spin-rotational contribution. We discuss this issue in relation to the intrinsic approximations of the spin-rotational model and, in particular, the isotropic Brownian rotation.
Acknowledgements
We thank Prof. A. Polimeno of our department for fruitful discussions. The work has been supported by Università di Padova inside the project: Studio di Materiali e di Sistemi Biologici con Risonanze Magnetiche di Spin Elettronico (ex 60%).
Notes
The package is available at the website www.chimica.unipd.it/licc. Calculations were run on the HPC hardware of the ‘Laboratorio Interdipartimentale di Chimica Computazionale’ (LICC) hosted at the Department of Chemical Sciences of the University of Padova.