Probing the Interaction of NO with C₆₀: Comparison between Endohedral and Exohedral Complexes

Recent advances in synthetic methodologies have opened new strategies for synthesizing stable metal-free electron spin systems based on fullerenes. Introducing nitric oxide (NO) inside a fullerene cage is one of the methods to attain this goal. In the present study, dispersion corrected density functional theory (B3LYP-D3) has been used to evaluate the structure, stability, and electronic properties of NO encapsulated fullerene NO@C₆₀ and compared those with its exohedral fullerene NO.C₆₀ analog. The calculated stabilization energy for NO@C₆₀ is appreciably higher than NO.C₆₀, and this difference is comprehended via the Quantum theory of atoms in molecules (QTAIM) and noncovalent interaction (NCI) topological analyses. The delocalization of electron density of NO and the C₆₀ cage in NO@C₆₀ is discussed using electrostatic potential analysis. In addition, an attempt has been made to understand the different locations and orientations involving the interaction of two NO radicals and the fullerene C₆₀. It is shown that the encapsulation of the NO dimer inside the C₆₀ cage is an energetically unfavorable process. On the other hand, stable structures are obtained upon the physisorption of other NO on the surface of NO@C₆₀ and NO.C₆₀. The present work provides an in-depth understanding of the interaction of NO and C₆₀ fullerene, its preferable position, and its orientation in both endohedral and exohedral complexes.