High-precision measurement of the electron spin g factor of trapped atomic nitrogen in the endohedral fullerene N@C60

The electronic g factor carries highly useful information about the electronic structure of a paramagnetic species, such as spin-orbit coupling and dia- or paramagnetic (de-)shielding due to local fields of surrounding electron pairs. However, in many cases, a near "spin-only" case is observed, in particular for light elements, necessitating accurate and precise measurement of the g factors. Such measurement is typically impeded by a "chicken and egg situation": internal or external reference standards are used for relative comparison of electron paramagnetic resonance (EPR) Larmor frequencies. However, the g factor of the standard itself usually is subject to a significant uncertainty which directly limits the precision and/or accuracy of the sought after sample g factor. Here, we apply an EPR reference-free approach for determining the g factor of atomic nitrogen trapped within the endohedral fullerene C-60:N@C-60 in its polycrystalline state by measuring the H-1 NMR resonance frequency of dispersing toluene at room temperature. We found a value of g = 2.00204(4) with a finally reached relative precision of 20 ppm. This accurate measurement allows us to directly compare the electronic properties of N@C-60 to those found in atomic nitrogen in the gas phase or trapped in other solid matrices at liquid helium temperature. We conclude that spin-orbit coupling in N@C-60 at room temperature is very similar in magnitude and of same sign as found in other inert solid matrices and that interactions between the quartet spin system and the C-60 molecular orbitals are thus negligible. (C) 2018 Elsevier Inc. All rights reserved.