• Raman spectroscopy;
  • paramagnetic properties;
  • spin;
  • intermolecular interactions;
  • carbon materials

Carbon materials typically have a high density of unpaired electronic spins but the exact nature of the defect sites that give rise to their magnetic properties are not yet well understood. In this work, the paramagnetic interactions between the unpaired electronic spins of carbon atoms and the nuclear spins of hydrogen molecules were probed with Raman spectroscopy by monitoring the relative population of H2 rotational states. For H2, the symmetries of nuclear spin and rotational wave functions are correlated. Because of the weak interactions between H2 nuclear spins, the transitions between odd and even rotational states are normally hindered. The magnetic field generated by unpaired electronic spins relaxes the selection rules and promotes transitions between H2 rotational levels of different symmetry. This affects the rotational levels' relaxation kinetics toward equilibrium and makes H2 molecules useful to study unpaired electrons in paramagnetic materials. It is suggested that simultaneous electron paramagnetic resonance and Raman measurements on carbon materials interacting with hydrogen molecules could result in a better understanding of the nature of paramagnetic defects in carbon materials, which could have a substantial impact on Li-ion batteries or for understanding the graphene electronic properties. Copyright © 2011 John Wiley & Sons, Ltd.