Cage Geometry Reorganizes N···C Contacts While Preserving Nitrogen Spin Localization in N@C60 and N@C70: A DFT Study

Endohedral nitrogen fullerenes are promising candidates for molecular spin qubits and quantum sensing. We compare nitrogen spin localization and N···C contact topology in N@C60 and N@C70. Baseline structural, vibrational, electronic, population, spin density, noncovalent interaction (NCI), and quantum theory of atoms in molecules (QTAIM) analyses used UB3LYP/6-31G(d). Counterpoise-corrected interaction energies were calculated using UB3LYP-D3(BJ)/def2-TZVP, with def2-QZVP checks. QTAIM sensitivity was assessed at fixed geometries using UB3LYP/def2-TZVP and UωB97X-D/def2-TZVP. Encapsulation leaves both cages essentially unperturbed: selected vibrational shifts are ≤0.8 cm−1 and HOMO–LUMO gap changes are ≤0.007 eV. Frontier orbitals remain cage-dominated, whereas Mulliken and Hirshfeld analyses assign 94–99% of the spin population to nitrogen, without substantial charge transfer. Counterpoise-corrected def2-TZVP interaction energies are −9.09 and −8.55 kcal mol−1, respectively; def2-QZVP changes each by <0.05 kcal mol−1. NCI/QTAIM descriptors indicate weak, low-density N···C contacts. N@C60 retains 60 connections across all three tested levels; N@C70 has eight at baseline and the same six in both def2-TZVP calculations. Thus, extensive and restricted contact networks remain distinct, although the exact N@C70 count and local descriptors depend on computational level. Predominantly nitrogen-centered spin therefore accompanies different surrounding contact topologies in cages that differ in size and shape.