Interplay of stereo-electronic, environmental, and dynamical effects in determining the EPR parameters of aromatic spin-probes: INDCO as a test case.

An integrated computational strategy for the evaluation of reliable structures and magneticproperties of spin probes and spin labels has been extended to aromatic species. From anelectronic point of view, delocalization of the unpaired electron density over aromatic moietiesreduces significantly the computed nitrogen isotropic hyperfine coupling constant (AN) withrespect to values characteristic of aliphatic nitroxides. Solvent effects in not too high polaritymedia are quite small, but not negligible. At this stage computed AN are lower than theirexperimental counterparts by more than 1 G. Inclusion of vibrational averaging effects bymolecular dynamics simulations with a new reliable force field restores full agreement withexperiment pointing out the limits of static approaches irrespective of the sophistication of theelectronic quantum mechanical method. The generality and computational effectiveness of theproposed integrated approach paves the route toward a reliable analysis of the interplay ofstereo-electronic, environmental, and dynamical effects in tuning the properties of large flexiblemagnetic systems of biological and technological interest.